Controlling Industrial Motors

Transcription

1 Study Unit Controlling Industrial Motors By Robert L. Cecci Technical Writer

2 There are many different types of electric motors used in today s industrial plants. A machine that s used to mill slots in a metal part will use stepper, DC servo, or brushless motors to drive the cutting tool. A conveyor system in the same plant may control the speed of that conveyor with an AC motor that s controlled by a frequency inverter. With the wide range of controller manufacturers and systems used in industry, it would be impossible to cover every type of controller in this text. However, this text will study the operation, electrical connections, and troubleshooting of generic controllers in detail, allowing you to easily translate your knowledge to most motor controllers used today. When you complete this study unit, you ll be able to Explain how stepper motors operate and how they re electronically controlled List the steps used to troubleshoot stepper motors and controllers Define how an AC motor rotates in synchronous speed to the AC line frequency Explain how a frequency inverter can alter the threephase output frequency and thereby control motor speed Identify proper troubleshooting procedures to use when working on AC inverter systems Describe how pulse width modulation is used to control a servo motor and how to find the causes of servo system problems such as inaccuracy and oscillation Explain how a brushless motor operates and how the controller commutates the motor to provide a precise positioning of the motor s shaft List the steps to use when troubleshooting brushless motor and controller systems P r e v i e w Remember to regularly check your student portal. Your instructor may post additional resources that you can access to enhance your learning experience. iii

3 INTRODUCTION TO CONTROL SYSTEM BASICS 1 The Mechanical Movement Components The Controller and Motor Drivers Control Loops Controller System Packaging Other Functions of Controllers STEPPER MOTOR CONTROLLERS 13 Stepper Motors Unipolar Stepper Motors Bipolar Stepper Motors Unipolar and Bipolar Stepper Motor Control Systems CNC Controllers Troubleshooting Stepper Motor Controllers DC SERVO MOTOR CONTROLLERS 28 Motor and System Basics Control System Basics DC Servo Driver Adjustments The H-Bridge Driver An Example Servo System Troubleshooting DC Servo Motor Systems BRUSHLESS DC MOTOR CONTROLLERS 43 Controller Basics Motor Commutation The Brushless Servo Motor Brushless Motor Commutation The Brushless Motor Controller Brushless Driver Adjustments Troubleshooting Brushless Motor Driver Systems AC FREQUENCY INVERTERS 60 Frequency Inverter Basics Principles of Operation Frequency Inverter Parameters Frequency Inverter Circuits Troubleshooting Frequency Inverters POWER CHECK ANSWERS 83 C o n t e n t s v

4 Controlling Industrial Motors INTRODUCTION TO CONTROL SYSTEM BASICS As long as there s been a need to move an object for a manufacturing process, there s been a need to control that motion. Before electrical and electronic controls became popular, the machine s operator would manually move the handles of the machine to provide the necessary motion. The dials near the handles of the machine identified the distances that were moved. For more accuracy, dial indicators were mounted on stops and the operator turned the machine s handles until the proper numbers appeared on the indicators, displaying that the machine moved the proper amount. In either case, the accuracy of the movement of the machine was entirely dependent upon the skill of the operator, and a skilled operator could produce parts with a high degree of accuracy. However this process is very time consuming and the repeatability of each dimension from part to part is always in question when the part is made manually. The first generation control systems were called indexers. A typical indexer is shown in Figure 1. The indexer has a front panel with five or six dials that are turned to define how far the machine is to move. For example, if you wanted a inch move from right to left, you would turn the dials to 0, 2, 0, 3, and 5 and set the direction to positive and press the X axis button. The stepper motor on the X axis would turn enough times to move the machine inches. Each movement of the machine had to be dialed in by hand, allowing for dial placement errors and 1

5 FIGURE 1 Early industrial motor controllers were called indexers. Imagine having to dial in each move of a complex program for a complicated part. therefore positioning errors on the manufactured part. This was a time-consuming process that did, however, produce very repeatable parts. The next improvement in control systems used an indexerstyle controller. But instead of turning the dials on the front of the controller, the program was fed to the indexer by means of punched paper tape. A small section of punched paper tape is shown in Figure 2. UP TO 8 HOLES WIDE (8 BITS) SPROCKET HOLES FIGURE 2 Paper tape was punched with holes to code a motion-control program. This method is very similar to the punched paper card systems that were used by early computers. The equally-spaced holes near the center of the tape were for the drive sprocket of the paper tape reader that was driven by a small stepper motor. The other holes in the tape were 2 Controlling Industrial Motors

6 used for the information such as X or Z This system worked well for its time. However, any program changes required that a new tape be punched and tested in the machine. The tapes also sometimes ripped or were caught in the reader or the reader itself would fail. Some readers used fine spring steel wires that made contact to a bottomgrounded plate to read each hole in the tape. Paper fibers would often keep the fingers from properly reading a hole, even when one was there. Some readers used optical light sources and phototransistors or solar cells. Paper fibers often blocked these too. By the time technology had surpassed the usefulness of the common indexer, paper tape had evolved to a polyester tape, a tape without the iron oxide coating similar to the base material used in today s VCR or cassette tapes. The introduction, development, and widespread use of the microprocessor was the next main improvement in industrial control systems. A microprocessor offered many advantages to the indexer-type controller. First, the manufacturing program could be stored in the computer s memory instead of on a paper tape. Early systems used audio cassette tapes for program storage. Later, the programs could be stored on floppy disks at the controller or at an off-line programmer. Second, on-line changes could be made in the computer program without the need to load a complete new program on tape or disk. Third, the computer could be preprogrammed with special canned cycles, such as a peck-drilling cycle or a bolt hole pattern cycle that made system programming very simple. The first microprocessors used were the 6502 and the Z80 series. Today s industrial control systems follow current trends of home personal computers and use the latest microprocessor chips. These powerful microprocessors offer such great speed and processing power that the control systems include features that were once only dreamed of by system developers. Motor and motor driver technology has also improved right along with the controller technology. At first, stepper motors were used for precise positioning of systems. These motors soon gave way to DC servo motors that were controlled by an analog signal from the computer controller. Next the use of digital drive control came about due to the precise tuning of Controlling Industrial Motors 3

7 the drives required for analog control and due to the thermal drift of the drives and the controller. Now the use of brushless motors and digital drive and controller packages is standard in all new industrial motor control systems. Let s begin our study of industrial motion control systems by looking at the components of these systems. The Mechanical Movement Components To precisely move an industrial machine, the drive motor of the control system must be coupled to a mechanical system on the machine. Figure 3 displays a typical mechanical system that s often used to provide linear or straight-line motion. The table, in most cases, is the work surface of the industrial machine. In a boring, drilling, or milling machine, this is the table on which a vise or fixture is mounted. The table is free to move back and forth on precision ground rods or specially TABLE ROD NUT LEAD SCREW BALL BUSHING SHORT SIDE VIEW TABLE ROD MOTOR BALL BUSHING NUT LEAD SCREW LONG SIDE VIEW BASE FIGURE 3 A mechanical movement can be precisely positioned by using an assembly such as the one shown here. 4 Controlling Industrial Motors

8 shaped and ground ways. Rods will support the table on special linear bearings called ball bushings. Ground ways allow for metal-to-metal surface contact with an adjustable device known as a gib between the way surfaces of the table and the base. This gib can be adjusted to take up for way surface wear. A lubricant known as way lube is usually pumped under low pressure to these surfaces to prevent excess friction and way surface wear. Beneath the table or movement is a set of devices known as a lead screw and nut. There are two types of lead screws and nuts. The Acme screw and nut assembly uses a ground threaded rod with somewhat squared-off threads and a set of brass alloy nuts. The lead screw turns in the nut assembly that s attached to the table or machine movement. This action pushes or pulls the table according to the direction of lead screw rotation and the amount of rotation. This system uses two nuts that can be tightened together or separated from each other to remove mechanical looseness called backlash. Backlash is a mechanical inefficiency that occurs when the lead screw rotates and the table or movement doesn t move. The second type of lead screw assembly is called a ball lead screw. Again a screw is ground with threads, but the threads are semicircular and can accept the balls of ball bearings. The nut contains a series of ball bearings that move in the internal channels of the nut as the lead screw is rotated. As with the Acme screw, when the ball lead screw is rotated, the nut and the table or movement are pulled or pushed a precise distance. Because this type of lead screw and nut assembly uses ball bearings in contact with the screw, there s very little wear and there can be high positional accuracy. Lead screws are ground to a dimension known as pitch that defines the number of threads per inch. Typically you ll see lead screws with five or ten threads per inch that therefore will require five or ten full turns of the lead screw to move the table or movement one inch. The motor can be coupled to the lead screw in one of many different ways. For linear motion, the motor is often connected to the lead screw using direct coupling or an alignment compensating coupling, or it can be belt coupled for further Controlling Industrial Motors 5

9 reduction. In some cases, a gearbox or a harmonic drive may be used. For rotary motion, a gearbox is used to decrease motor speed and increase torque. The Controller and Motor Drivers Figure 4 shows a basic block diagram of a three-axis control system. In this figure, the controller is attached to the motor drivers by a bidirectional signal buss. This bidirectional buss allows signal information to pass back and forth between the motor drivers and the controller. The type and the amount of information on this buss depends upon the type of control system, motor drivers, and motors used in the system. X DRIVER X AXIS MOTOR CONTROLLER Y DRIVER Z DRIVER Y AXIS MOTOR Z AXIS MOTOR FIGURE 4 This shows a basic block diagram for a controller with three axes. The controller is the component of the system that contains the main system microprocessor and computer circuits. This microprocessor is connected to the front panel display device, normally a cathode ray tube (CRT ), the manual data input keyboard and its related push buttons, the I/O (input/output) port or ports, the digital I/O system, and the motor drivers. The motor driver section of this system accepts signals such as drive enable and motor distance command signals from the main controller. Also many motor drivers accept inputs from external limit switches and/or external control switches. The outputs of the motor drivers are connected directly to the 6 Controlling Industrial Motors

10 motors. Some motor drivers are as simple as a basic electronic circuit section with power transistors or triacs for output devices. Other drivers are very complicated and can even have their own microprocessor. A standard motor control system for a machining center will have three motors or three axes of control. Industrial robots can have six or more axes of control. Control Loops The earliest industrial motion control systems used stepper motors on each axis of the machine. In these systems, a command to move was issued by the controller, and an axis logic circuit card issued pulses to the motor driver. These pulses were then translated into drive pulses to the motor. This is shown in Figure 5. MAX ACCEL DECEL CONTROLLER AXIS LOGIC DRIVER MOTOR FIGURE 5 This diagram shows how information, in the form of pulses, passes from the main computer controller to an axis logic circuit card, to the driver, and finally to the motor. From the axis logic card to the motor, all of the signals travel toward the motor in a unidirectional manner. There re no signals returning from the motor to the motor driver or controller. This is called an open-loop system. The controller issues the commands through the axis card; the driver sends the pulses to the motor. If the motor doesn t reach the proper position, the controller will have no idea that an error in positioning (mispositioning) has occurred. The controller will simply issue the next command. This can mean that a part is improperly machined or that a robot s gripper is out of position to pick up or release a part. Controlling Industrial Motors 7

11 To correct this situation most modern controllers operate in the closed-loop mode of operation. This mode is shown in Figure 6. POSITION FEEDBACK LOOP MOVE COMPUTER CONTROLLER AXIS LOGIC SERVO DRIVER MOTOR MOVE COMPLETED VELOCITY FEEDBACK LOOP FIGURE 6 This illustration displays a closed-loop servo system in block-diagram format. There are two different control loops in this system. The loop that comes from the motor and goes back to the axis logic card is called a position loop. A device known as an encoder or another type of device known as a resolver is connected to the motor and will send pulses back to the axis logic card. These pulses are counted to let the controller know that the motor has reached its final position. The second loop is used to feed back velocity information to the motor driver. The device used is a tachometer that produces a voltage that s dependent upon speed. The faster the motor turns, the faster the motor-coupled tachometer turns. The faster the tachometer turns, the greater the DC voltage produced by the tachometer. The control loops are often termed PID loops. These letters stand for P I D Proportional Integral Derivative This means that the control system monitors how far the system is out of position and then increases or decreases motor speed commands to compensate for wide or small position deviations (proportional). This also means that the control 8 Controlling Industrial Motors

12 system monitors how long the system hasn t been in position (integral). Finally, the control system monitors how fast the desired position to actual position information is changing (derivative). This PID control system is designed into the controller as a basic PID control loop. A part of installing and/or adjusting the control system involves what is known as tuning. This tuning involves modifying the parameters of the PID loop to match the capabilities and response of the servo system to the machine conditions. One term associated with servo systems is error. Error is the difference between actual position and the desired position of each axis. For example, if the motor for the X axis is at a standstill and is in the proper position, the error is zero. However, if the main controller issues a six-inch move command for the X axis, the error signal is now at six inches. What the PID control loop actually monitors is the error signal for each axis. In this example, the X axis motor driver command may be to run the X-axis motor at full speed. As the motor approaches the six-inch position, the error becomes smaller and therefore the controller begins to slow the motor until the error once again becomes zero. Another term is following error. Let s use the same six-inch move for this example of following error. The computer controller knows that a six-inch move command has just been issued and that the error is six inches (actually six inches worth of feedback pulses). The controller also knows that this move isn t going to be accomplished immediately. It s going to take a little time to get the machine to make this full move. The microprocessor will compute where the motor should be at various time increments along the path of the move. The feedback pulses will normally lag behind the computed quantity in what s known as following error. As long as this following error stays within a small range, the microprocessor allows the driver to keep moving the motor until the move is complete. However, if the following error gets too great, signifying that the motor has stalled or is having difficulty reaching final position, the controller can then shut off the drives to protect the motor and the machine from damage. This can also work to protect the motors and the machine if the following error is too small because the motor is moving too fast or if the motor is moving in the wrong direction. Controlling Industrial Motors 9

13 The PID loop also operates on the principle of gain. Gain describes how fast and how much the control system and the motor drivers respond to an error signal such as one that occurs when the controller issues a move command. A very high-gain system is normally desirable. This high-gain system will rapidly move each of its motors to a new position. However, too much gain can cause the motor to oscillate at a standstill or overshoot its position and require that the motor reverse itself to reach the final position. Controller System Packaging Normally, the entire motion-control system is housed in one cabinet. One section of the cabinet contains the positioning or motion-control computer circuit boards and other logic boards. The other section of the cabinet will house the power supply, transformers, and the motor drivers. Some controllers also contain lead-acid gel-cell batteries or other types of rechargeable cells near the power supply for memory battery backup. A typical layout of an industrial motion control system is shown in Figure 7. In this system the upper part of the cabinet contains the low-signal level devices such as the computer circuit boards and the interface boards. These electronic circuit boards are normally located in a rack where the boards FIGURE 7 A positioning system cabinet will usually have a lowlevel logic section and a high current section. CRT RACK LOGIC SECTION DRIVE SECTION X Y Z 10 Controlling Industrial Motors

14 plug into a common motherboard or backplane. The lower section of the cabinet contains the high-power devices such as the transformers, power supplies, and the motor drivers. A grounded metal separator is normally present between the low-and high-power devices to prevent electrical interference from affecting the low-level logic circuits. Most of today s machine tools are termed CNC, which stands for Computerized Numerical Control. These systems are normally programmed off line on a personal computer or workstation that is running a Computer Aided Design program (CAD). This program may be teamed up with a Computer Aided Manufac - turing program (CAM) to fully develop the CNC machine s program. This program can then be loaded into the motioncontrol computer by means of an I/O port. Robotics system motion-control computers can also have their motor-positioning programs loaded in from an off-line computer. However, many robot controllers are taught a series of moves using a teach pendent that is connected to the controller. An operator moves or jogs the robot to a position and that position is saved in the controller s memory. The robot is then moved to the next position and that position is also saved in memory. These actions continue until a complete set of moves is made and stored. A simple pick and place robot may have but a few move commands in its memory. A painting robot may have hundreds of these moves in its memory. Other Functions of Controllers Most industrial motion-control systems control more than just motors. For CNC machine tools there are also M functions, S functions, and T functions. The M functions are miscellaneous functions that start external devices such as motor starters for spindle motors or coolant pump motors, or energize solenoids for part ejection from fixtures or table clamps. The S functions normally control spindle speeds. The T functions are used with tool changers to select the proper tool from a magazine of tools. Robot controllers also often have T-function (tooling functions) for opening and closing grippers and other such operations. Controlling Industrial Motors 11

15 Power Check 1 At the end of each section of Controlling Industrial Motors, you ll be asked to pause and check your understanding of what you ve just read by completing a Power Check exercise. Writing the answers to these questions will help you to review what you ve studied so far. Please complete Power Check 1 now. 1. The first generation of electronic position controllers was called (indexers/path controllers). 2. A gib is a part of a a. stepper motor. b. table movement. c. motor driver. 3. The type of control system that does not feed back velocity or position information to the control system or driver is called a(n)-(open-loop/high-gain) system. 4. The difference between the actual position and the desired position is called (lag/error). Check your answers with those on page Controlling Industrial Motors

16 STEPPER MOTOR CONTROLLERS Stepper Motors For many years, stepper motors were used for positioning industrial machinery. These systems were open-loop positioning systems that were fairly low in cost and opened up CNC machining to large and small fabrication and machine and shops. With the introduction of closed-loop DC servo systems, stepper motor systems began to fade in popularity. DC servo systems offered closed-loop accuracy and greater speeds than the stepper motor systems. Stepper motors were still used in industry, mostly for computer disc drives, inexpensive singleaxis positioning systems, and for robot grippers. In recent years, closed-and open-loop stepper motor systems have been regaining popularity. The newer stepper motor systems operate off a three-phase AC driver system and have rotational resolution as small as degrees per step or 10,000 steps per revolution. This compares to the older openloop systems with a 1.8 degree-per-step resolution with a normal 200 steps per motor shaft revolution. Unipolar Stepper Motors Although there have been many special types of stepper motors developed, two of the most common open-loop motors are the unipolar and bipolar stepper motors. Figure 8 displays a simple schematic for a unipolar stepper motor. As shown, the windings of the unipolar stepping motor are center tapped. This center-tapped winding allows for the use of a simple electronics package to drive this type of motor. In operation, the center tap of each winding is typically connected to the positive or negative of the power supply of the stepper motor driver. The electronic control circuit in the driver then alternates the grounding or the supplying of voltage to each end of the winding to alternate the magnetic poles of the windings. These alternating magnetic fields cause the motor to rotate. To better see this in operation, look at the simple 30-degree motor of Figure 9. This motor has four field poles, and the permanent magnet rotor has six magnetic poles. Controlling Industrial Motors 13

17 1 A B A B 2 FIGURE 8 A unipolar stepper motor will have six wire leads due to its twin center-tapped windings. FIGURE 9 A simple four-pole stepper motor is shown here. 2 N S 1 N S N S 1 N S 2 Now let s look at how we can rotate this motor. Table 1 lists the steps for a few rotations of the motor s shaft. In this table, a 1 means that a positive voltage is being supplied to the motor terminal with a grounded center tap on each winding. A 0 means the circuit is open to that winding. 14 Controlling Industrial Motors

18 Table 1 MOTOR DRIVER OUTPUTS Winding 1A Winding 1B Winding 2A Winding 2B This chart reflects the motor moving 16 steps in a clockwise direction. The steps in Table 2 can also rotate the motor the same number of steps. However, the current draw of the motor will be doubled while the output torque will increase by about one and one-half times that of the motor driven by the pattern in Table 1. Table 2 FULL-STEP HIGH-CURRENT MODE Winding 1A Winding 1B Winding 2A Winding 2B Both of these stepper systems are full-step systems. A simple set of D flip-flops can provide the basic electronic signal translation needed to change an incoming pulse train from an indexer or computer controller into the pulse outputs of Table 2. Of course the outputs of the flip-flops would be boosted by applying the outputs of the flip-flops to the base terminals of power transistors. A sample D flip-flop circuit is shown in Figure 10. The unipolar stepper motor can also be operated in the halfstep mode to make the motor deliver a smaller step. For example, in our 30-degree full-step example the half-step mode delivers 15 degrees per step. Table 3 displays the switching pattern used for this type of operation. This sequence will result in clockwise rotation. To reverse rotation direction, reverse the rows 1A and 1B and then 2A and 2B. Controlling Industrial Motors 15

19 D Q D Q 1A PULSE IN C Q C Q 1B 2A 2B FIGURE 10 Flip-flops can be used to create the switching sequences needed for a unipolar stepper motor. Table 3 HALF-STEP MOTOR OPERATION Winding 1A Winding 1B Winding 2A Winding 2B The electronic circuit that s needed to create the half-step mode is somewhat more complex than that used for the full-step mode. To get finer resolution from a stepper motor, the number of magnetic poles on the rotor must be increased. As we mentioned earlier, a common figure for a unipolar stepper motor used for positioning was 1.8 degrees per step in full-step or 200 steps per revolution. If operated in half-step, the motor would produce 0.9 degrees per step with 400 steps per revolution. Bipolar Stepper Motors A simpler form of stepper motor is the bipolar stepper motor shown in Figure 11. At first glance the motor seems the same as the unipolar motor, but a closer inspection reveals that instead of being a six-lead motor, the bipolar motor is a fourlead motor. There s no center tap on the windings of the bipolar motor. 16 Controlling Industrial Motors

20 1A 1B 2A 2B FIGURE 11 A bipolar stepper motor has a simple internal winding system. In this example, we ll use the same 30-degree stepper motor of Figure 9. However, the motor will be wound as a bipolar motor with the winding arrangement of Figure 11. Because the bipolar motor has no center taps in the field windings, the current flow through the windings must be reversed to reverse the magnetic polarity of these field windings and to cause the motor to rotate. Table 4 displays the voltage patterns used to rotate a bipolar stepper motor. Where a ( ) symbol appears in the table, positive voltage is applied to that motor terminal. Where the ( ) symbol appears in the table, the negative of the power supply is applied to that terminal. Table 4 BIPOLAR STEPPER MOTOR SWITCHING Winding 1A Winding 1B Winding 2A Winding 2B By matching this table with Table 1, notice that this is the same switching sequence as is used for a unipolar motor at low current. Every ( ) can be changed to a (1); every ( ) to Controlling Industrial Motors 17

21 a (0). The major difference is that the motor terminals are sometimes positive and sometimes negative in polarity rather than simply turned on or off. This switching of polarity causes the complexity of the motor driver circuit to increase. Instead of a simple flip-flop circuit, a dedicated translator integrated circuit chip is used along with what is known as an H-bridge driver. We ll look more closely at the H-bridge drive later in this text when we re studying DC servo motors. Unipolar and Bipolar Stepper Motor Control Systems The earliest stepper motor controllers were simple indexers. These systems created pulses that were translated to stepper motor sequences by transistor circuits. These pulses were then applied to power transistors whose output was attached to the motor. Power resistors were often placed between the transistors and the motor to tailor the drive to the motor s size. Large motors used no resistors while small motors used large resistors. The distance the motor moved was defined by the number of pulses generated by the indexer. These pulses were counted down until the move was complete, at which time the down counter went to zero. For example, if a 200-step-per-revolution motor is directly attached to a five-pitch lead screw, it would take 1,000 pulses from the indexer to make the motor turn five turns and move the table one full inch. These 1,000 pulses would be loaded into the circuit and counted down from 1,000 to zero to make the lead screw move. CNC Controllers The next generation of motor controllers was the Computerized Numerical Control (CNC) controller. Instead of single commands, the CNC controller stored all the motion and function commands in its memory and issued these commands sequentially to the drivers as previous commands were completed. A block diagram of the open-loop stepper motor drive system is shown in Figure 12. The CPU part of this illustration reflects the main computer; the interpolator board that s used for 18 Controlling Industrial Motors

22 X X MOTOR INT CPU AXIS LOGIC Y Y MOTOR E STOP Z Z MOTOR HOLD M FUNCTION DRIVERS FIGURE 12 An open-loop three-axis stepper motor system is shown here. multi-axis motion; the M, S, and T function boards; and the axis logic board. The main component of the motor control section of this CNC controller is the axis logic electronic circuit board. In operation, the computer issues a command to move one of the three axis motors a certain distance in a certain direction. For example, an X axis enable signal in the computer will go to logic high. This action tells the axis logic board that the X axis is going to make a move by rotating its stepper motor. Likewise, a direction signal is also issued by the computer to tell the axis logic card, and ultimately the stepper motor driver, which direction the motor is to turn. On the axis logic card there are multiple circuits and adjustment potentiometers. The main circuit is a pulse generator that creates a pulse train at a frequency set by a MAX potentiometer. When an axis move command is issued, then the pulse train is both counted and sent along to the translator circuits in the driver to move the motor. No motor can go from stopped to full speed instantly. Ramp generator circuits are used to both accelerate and decelerate the motors. This is shown in Figure 13. In Figure 13A, you can see the speed profile of the motor as it accelerates, rotates at constant speed, then decelerates to a Controlling Industrial Motors 19

23 MAIN VELOCITY ACCEL DECEL TIME (A) TIME (B) FIGURE 13 This illustration shows how a motor ramps up to maximum speed and then decelerates. The lower illustration displays the pulse train on the axis logic card. stop. The slope of the two ramps (the acceleration and deceleration ramp) is controlled by the setting of two potentiometers on the axis logic circuit board. In a basic tuning of a stepper motor system, these ACCEL and DECEL potentiometers are turned to decrease the slope of the ramp until the motor stalls. The potentiometers are then adjusted in the opposite direction as much as one full turn. The main or maximum speed potentiometer is adjusted in a similar manner. The motor is run for a long-duration move at maximum speed and the MAX potentiometer is adjusted until the motor stalls. The MAX potentiometer is then turned in the opposite direction about one full turn. It should be noted that these three potentiometers adjust the ramps and maximum speed for all three axes of this machine. Usually this means tuning the axis with the most weight or drag and allowing the other axes to follow this setting. In Figure 13B, you can see how the pulses that are generated by the ramp generators and the maximum frequency generator look as the motor performs the move. 20 Controlling Industrial Motors

24 Many axis logic circuits also contain a scaling jumper or a scaling plug. This plug or jumper allows the fixed 200 step per revolution motor to be used with various belt or gear drive ratios. This is performed by taking the pulse train that s generated by the ramp and maximum speed oscillators and feeding it into a divide-by-n counter. The scaling jumper or plug is basically just programming the divide-by-n counter s division number. The output of this divide-by-n counter is sent to the drives as an output from the axis logic card. The axis logic card sends the pulses and the direction signals to the drives by means of shielded cables or ribbon cables. The drives are then responsible for translating the pulses into stepper motor drive signal sequences. These signals are then sent to the transistor output stages where the signal levels are boosted. The higher voltage and current outputs of the transistors are connected through fuses to the motors. Many stepper motor drives are current-output adjustable to allow the drive to be used with various sizes of motors. To adjust such a drive, you simply remove power and then one of the output fuses. Next, insert an ammeter across the fuse clips and step the motor, taking note of the current produced. To complete the adjustment, turn the current adjustment potentiometer until the value of output current matches the motor s nameplate current. Some stepper motor drivers are termed chopper drives. These chopper drives attempt to increase the motor s output torque by applying a short duration pulse of high voltage to the motor at the beginning of each step. For example, a 45 VDC pulse starts the step of Figure 14 while the remainder of the step is at 10 VDC. Also, many stepper drives hold the current to the coils of the stepper motor for a few seconds after the move is completed. This provides holding torque, or the motor s ability to resist being moved out of position, as long as this current is applied. There are very few interface signals from the machine to this type of controller. Typical interface signals include an emergency stop and feed hold switches that connect to the computer portion of the controller. Limit switches are normally connected to the drives. If a limit is actuated, the drive will stop and send a signal back to the computer section. This action then sets an error code that is displayed on the Controlling Industrial Motors 21

25 45V 10V 0V FIGURE 14 Some chopper-type stepper drivers apply a short pulse of high voltage electricity to increase the motor s torque and performance. controller s front panel or CRT display. As mentioned, there can also be other nonmotor driver signals involved with this type of controller such as M or machine codes, S or spindle codes, or T or tooling codes with their associated wiring to the computer section of the controller. A more modern single-axis stepper motor control system is shown in Figure 15. This system is a closed-loop system that has an encoder attached to the back of the motor. This CONTROLLER A B + _ EXTERNAL I/O STROBE ENABLE A1 B1 COM A2 B2 COM ENCODER MOTOR FIGURE 15 One type of single-axis stepper motor control system is shown here. 22 Controlling Industrial Motors

26 encoder supplies a pulse train back to the controller as the motor rotates. Typically, the encoder will have 100, 200, or 500 pulses per revolution that will identify the motor s position. When the motor is issued a run command, the translator section of the drive can be pulsed and the move can continue until the new position is nearly reached. Then, the controller will slow down the motor until the position is reached. After the move is complete, if there s any error between the desired and the actual positions, the controller will move the motor to the exact position. The controller shown is a single-axis controller that, like the three-axis controller shown previously, has an internal program stored within its computer memory. In fact, this controller can store multiple programs and perform move sequences that depend upon the binary-coded inputs to the controller. These inputs are the 1, 2, 4, and 8 inputs seen at the middle left of the controller. Table 5 displays a segment of a sample program, Program 6. Program 6 is identified in Step Number 2 as IN 6 (input 6). Program 6 is activated by inputs 4 and 2 being at logic high. Table 5 SAMPLE STEPPER CONTROLLER PROGRAM Step Number Command Description 1 F 100 Speed feedrate command (max frequency) 2 IN 6 Senses an input on both inputs 4 and 2 3 MOVE I Moves the axis motor steps positive 4 Wait 200 Delays 200 ms or 0.2 seconds 5 MOVE I 1000 Moves the axis motor 1000 steps negative 6 F 250 Changes the feedrate 7 MOVE I Moves the motor steps negative 8 Home Moves the motor to a preset position 9 End Stops this program The home position for this system is normally found by moving the motor at a certain speed until a limit or proximity switch is actuated. The motor s direction is then reversed Controlling Industrial Motors 23

27 until a special line within the encoder called a marker line is sensed at which time the motor is stopped. This position is always the same, allowing the programmer to use this special home position as a reference position for the moves for each internal program. Although program 6 is shown in this example, other programs can also be entered and executed by simply enabling the binary input commands to the controller. As with the three-axis system, there s normally an emergency stop and a feed hold input to the controller. There may also be other inputs and outputs, such as a remote start push button, an error signal output, and so forth. One of the newest types of stepper motor systems uses a three-phase system. This system closely resembles a brushless DC servo system and will therefore be covered in detail in that section. Troubleshooting Stepper Motor Controllers Stepper motor systems are very dependable and can operate for many years without service or repair work. The only maintenance that s normally required is cleaning the motor driver and voltage-regulator heat sinks of any collected dust, dirt, and oils or coolants. Some systems use fan assemblies and filters to make sure the heat sinks and therefore the transistors and regulators stay cool. These fans and filters should be cleaned on a normal maintenance schedule. Problems that normally develop with stepper motor systems usually fall into one of two categories. First, and most difficult to diagnose, is a mispositioning problem where the motor hasn t moved far enough. The second type of problem is where the motor doesn t work at all. In troubleshooting a mispositioning problem, you must first determine whether the system is an open- or closed-loop system. An open-loop system is much more prone to mispositioning due to its lack of feedback. If an open-loop system is mispositioning, you can try to detune the system by turning the MAX, ACCEL, and DECEL potentiometers. The motor should respond with a longer ramp time 24 Controlling Industrial Motors

28 and slower overall speed. If this cures the mispositioning problem, retune these potentiometers to peak the motor s performance without having the motor misposition. If there s still a mispositioning problem, it s possible that the motor is stalling due to too great of a load on the motor s shaft. Often you can easily disconnect the motor from the load and program a distance that s equal to one or more full turns of the motor. Mark the motor shaft, and make sure that the motor shaft repeats its original position after each move. If this doesn t occur, you may need to replace the axis logic circuit board or motor driver. Some multi-axis motor axis cards have sockets and use cables to tie each axis driver to the axis card. In this case, if one axis is mispositioning, you may be able to switch plugs on the motor axis card. Now you can run the X-axis motor on Y-axis logic, or vise versa, to see if the card or the driver is faulty. Gross mispositioning, where the motor is far off its intended position, is normally caused by motor stalling. This stalling is caused by excessive load or resonance. Excessive load can normally be determined by removing the motor from the load as previously described. Resonance is another matter. Resonance is caused by the combination of the motor and load reaching its natural frequency during operation. This natural frequency is similar to the note produced by a bell. The bell is struck and a wide range of frequencies is produced. We hear, however, only the strongest, natural or resonant frequency of the bell. In other words, the bell itself becomes a mechanical amplifier of that one main frequency. Stepper motor resonance occurs when the motor reaches that one tuned point in its operation where the rotor, like the bell, becomes resonant and amplifies a certain mechanical frequency. This frequency fights against further motion of the rotor and causes the rotor to vibrate back-and-forth rather than rotate. Various types of dampeners are available to help cure this problem and in some cases, simply lubricating the lead screw will provide enough dampening to remove the effects of resonance on the stepper motor. Total failure of a motor to rotate is typically a motor or motor driver fault. After removing power from the controller, measure across each fuse holder to check for an open fuse. There are typically two fuses on each drive, one for each motor Controlling Industrial Motors 25

29 winding. If one of these fuses is blown, it s a major problem: either one of the output transistors or one of the motor windings is shorted. You can measure the stepper motor winding resistance to ground (the reading should be infinity) and measure the resistance of both of the windings, and compare the readings. If the motor tests good, substitute the drive and recheck operation. Remember to always reset the current level on adjustable-current level drives. If more than one axis has failed, it s normally the fault of the power supply. Check the schematics for test points and voltage measurements to troubleshoot this problem. 26 Controlling Industrial Motors

30 Power Check 2 1. What type of circuit can be used to convert a pulse train to a switching sequence for a stepper motor? a. Counter b. Flip-flop circuit c. Regulator 2. If a motor has 1.8 degrees of motion per step, it would require (200/400) steps per revolution. 3. What potentiometer should be adjusted to make a stepper motor reach a faster or slower top speed than before an adjustment was made? a. ACCEL b. MAX c. DECEL 4. A problem with a blown driver fuse will normally cause the motor to a. misposition. b. not turn. c. turn in a reverse direction. Check your answers with those on page 83. Controlling Industrial Motors 27

31 DC SERVO MOTOR CONTROLLERS Motor and System Basics Closed-loop DC servo motors and control systems were introduced to eliminate the major problems of a stepper motor system. Since the system is a closed-loop system, the mispositioning problems of an open-loop system are solved with this newer system. Also, the speed and response of the system are greatly increased allowing speed increases from the typical 60 inches per minute of a stepper-motor system on a milling machine to speeds of 120 inches per minute or more on the same equipment. DC servo motors and drives could be made in greater sizes than stepper motors, allowing older hydraulic-powered equipment to be converted to the electric servo systems. Finally, the resolution of the system could be made finer than with a stepper motor system. Positioning accuracy in the millionths of an inch is available in a lowcost electronics and motor package. Control System Basics One of the first systems for controlling DC servo motors used an analog control loop between the axis logic card and the servo motor drivers. These control loops are shown in Figure 16. The motor has two feedback devices mounted on the back of the motor and driven by the motor s shaft. The first device is a tachometer. The tachometer produces a voltage that represents motor speed. For example, a typical tachometer is rated at 10.0 volts per thousand revolutions per minute (RPM). This means at 1000 RPM the output of the tachometer is 10.0 VDC. The output voltage produced by the tachometer is linear. At 2000 RPM the tachometer, or tach, will produce 20.0 VDC. In older servo systems, this voltage is fed back to the motor driver. Here, the voltage is used to electronically counteract the incoming reference signal level to keep the motor s speed under control. 28 Controlling Industrial Motors

32 CPU CONTROLLER AXIS LOGIC ANALOG DRIVE SIGNAL MOTOR DRIVER + _ MOTOR POWER + _ ANALOG TACH FEEDBACK TACHOMETER MOTOR DIGITAL FEEDBACK LOOP ENCODER OR RESOLVER FIGURE 16 A closed-loop servo system will have two feedback loops the velocity loop, and the position loop. The second device is the encoder which is part of the motor s position feedback loop. The device used in this feedback loop is usually one of two types. An encoder uses one or more internal disks that have lines photoengraved on the disk. Two light sources are placed on one side of the disk, and two light receivers are placed on the opposite side of the disk. As the motor turns, the disk or disks within the encoder also turn. This causes a series of pulses, the number of pulses depending upon the number of lines on the disk. Early encoders had 100 or 200 lines while newer encoders have thousands of lines. This type of encoder is called an incremental encoder. The outputs from this type of encoder are a series of pulses from the light receivers after the signals are amplified by internal amplifiers. The outputs are typically marked A and B. The encoder will also produce a third signal that is called the marker signal. One line is positioned at a special place on Controlling Industrial Motors 29

33 the encoder disk. A separate light source and receiver inside the encoder picks up this pulse once per revolution of the encoder. This marker signal is used to home the motor and the attached machine. Home is accomplished as the motor is moved into a limit or proximity sensor by the controller. The controller will then reverse and slowly move the motor until this marker pulse indicates where the motor will stop. A second type of encoder is the absolute encoder. This encoder consists of eight or sixteen outputs that present a unique digital number for each position of the shaft of the encoder. Inside this encoder are multiple disks that are moved by a geared system that rotates the disks at different rates. Another type of feedback device used in position loops is the resolver. A resolver requires an input of two electrical AC voltages spaced 90 degrees apart. These waves are called sine and cosine. These sine and cosine signals are fed to internal coils inside the resolver. An additional coil is placed on the rotor and brought to the outside of the case by means of slip rings and brushes. As the resolver s rotor rotates, the rotor will pick up, or receive through induction, the sine or the cosine winding s field signature. This signal is fed back to the controller s axis control card where the phase of the signal is checked, and the alternations of sine to cosine are counted to give position. The final control loop in our Figure 16 example connects the axis control card to each DC servo motor driver. In this example, an analog signal reflects the amount of position error that is present between the desired position and the actual position. If the error is small the output is small. If the position error is great, the output voltage will be higher. This signal, along with the tachometer s feedback signal, is shown in Figure 17 as they connect to the motor driver. DC Servo Driver Adjustments The DC motor servo driver has a series of adjustments that allow you to tune the performance of the driver/motor system. These adjustments are shown in Figure Controlling Industrial Motors

34 SERVO MOTOR DRIVER DRIVE SIGNAL FROM AXIS LOGIC CO M GND + _ SUMMING AMPLIFIER V/F VOLTAGE TO FREQUENCY CONVERTER COM GND H BRIDGE OUTPUT TACH MOTOR ENCODER FIGURE 17 This circuit shows how the inputs to a driver are converted to pulses that have a frequency that s proportional to the input signal s voltage level. SIGNAL IN SIG TACH C BAL TACH MOTOR FIGURE 18 A typical servo driver will have this basic layout with the adjustment potentiometers mounted in an accessible location. Controlling Industrial Motors 31

35 There are four potentiometers: The SIG (S) for signal input amplifier gain The TACH (T) for tachometer amplifier circuit gain The C for overall maximum driver frequency, also termed compensation The BAL (B) for driver system balance When tuning this type of motor driver, the signal and tachometer gain adjustments would be turned full counterclockwise. This would provide the least amount of gain from any circuit at the input of the drive. With power applied to the driver, you can watch the motor rotate and adjust the balance potentiometer until the motor stops turning. At this time the driver is in balance. Next, the signal and tachometer potentiometers are slowly turned clockwise. At some point the motor will lock in position as the position and velocity feedback loops become enabled by the increase of gain in their amplifier stages. Once the motor locks into position, tune the driver for response to input commands. Program maximum feed rate axis move commands, and look at the response of the motor. A good response curve is shown in Figure 19. The response of the driver is best seen on an oscilloscope that s attached to the signal or tachometer input amplifier circuits. Most servo motor drivers have these test points clearly labeled on the driver and on the driver s documentation. The newest servo drivers often include an on screen oscilloscope in their axis control software that makes tuning very simple. Figure 19A shows the results of a single high-speed move. This can reflect tach or signal voltage as opposed to time. Figure 19B shows an underdampened waveform. The motor actually oscillates around its final position until the oscillations, which get smaller and smaller, stop, and the motor comes to rest. Here either the signal gain is too high or the tachometer gain is too low. Figure 19C shows an overdampened waveform. The motor slowly comes to a stop. This is the result of too little reference or signal gain or too great of tachometer gain. Figure 19D shows a perfectly dampened, 32 Controlling Industrial Motors

36 (A) RINGING (B) UNDERDAMPED (C) OVERDAMPED (D) CRITICALLY DAMPED FIGURE 19 A critically dampened waveform is the ideal waveform to have after you have completed the axis driver adjustments. often termed a critically dampened, driver system. The motor decelerates and quickly comes to rest after the move is complete without oscillation or excessive time. In tuning a servo driver, it s best to program a series of short moves when performing the adjustments. When you feel that the driver is critically tuned, you can check the tuning by using a few long moves. The longer moves assure that the motor has reached maximum speed. It s possible that the process of making one adjustment will mess up all of the past adjustments. Many of the adjustments are interrelated. For example, if you adjust or reprogram the axis logic system for a greater acceleration or deceleration ramp, the system might become underdampened. Likewise, if you increase the signal gain, the driver will respond somewhat as if you ve lowered tachometer gain. Most system don t require exact adjustments when performed through poten- Controlling Industrial Motors 33

37 tiometers or programmed parameter values. Instead, a system is usually tuned toward the center of its critically tuned area and is considered in proper tune. The other potentiometer often found on older servo drives is the C potentiometer, which stands for compensation. This is used for adjusting the maximum frequency of the drive and prevents the drive from breaking into oscillation. Usually this is adjusted upward, normally clockwise, until a light buzzing is heard from the motor and then the potentiometer is reversed about one full turn. In the more modern programmable drives, this adjustment often falls under the simple term gain, which sets not only the maximum response of the system but also the maximum frequency of the system. The H-Bridge Driver A servo motor driver is often called an amplifier because the servo motor driver is a form of class D or switching amplifier. The driver itself is an H-bridge type. Let s look more closely at how a servo motor driver controls a motor. Figure 20 displays a typical H-bridge servo driver. + + A C 1 2 D B FIGURE 20 An H-bridge servo motor driver will have four bipolar or mosfet transistors. A diode, not shown, will normally be located across each transistor to protect the transistors from damage. 34 Controlling Industrial Motors

38 Notice in Figure 20 there are four transistors in the bridge. When transistors A and B are enabled by a base signal to each transistor, the number 1 side of the motor will be positive while the number 2 side of the motor will be negative. This may cause the motor to turn clockwise. Reverse this sequence by turning on transistors C and D. Now the number 2 terminal of the motor is positive while the number 1 terminal of the motor is negative. Of course the motor will reverse its direction. The base terminals of the transistors aren t actually supplied with a constant source of current. Instead, the base terminals are pulsed using a technique known as Pulse Width Modulation (PWM ). Typical PWM signals are shown in Figure 21. (A) (B) (C) FIGURE 21 This illustration displays a low-speed PWM signal in (A), and a high-speed signal in (B), and a PWM signal for an accelerating motor in (C). Controlling Industrial Motors 35

39 Assume that a logic high signal turns on the output transistor in the H-bridge circuit. Figure 21A shows the base signal that would result from a low-speed move. The off time of the signal is much greater than the on time of the signal resulting in a small overall voltage to the motor. This results in low motor speed. In Figure 21B, the opposite is true. The on time of the signal is much greater than the off time, resulting in a large output motor voltage. This causes the motor to turn very fast. Figure 21C shows what the PWM signals would look like as a motor accelerates. The direction the motor is turning determines which H-bridge transistor pairs receive the PWM signal. A logic circuit on the driver will receive the direction command and then apply the PWM signals to the proper transistors to get the proper direction from the motor. An Example Servo System Figure 22 shows a simplified diagram of a DC servo system. The first circuits on the driver consist of a signal and tachometer input and amplifier stage. These two signals are then POSITION VELOCITY SIGNAL INPUT CPU AXIS LOGIC DIRECTION ENABLE MOTOR DRIVER OUTPUT TRANSISTORS CONTROLLER MOTOR DIRECTION AND CONTROL LOGIC PWM CIRCUIT RELAY FIGURE 22 A single axis closed-loop servo system is shown here. For each additional axis, the driver, relay, and motor can easily PWM be added to the system. 36 Controlling Industrial Motors

40 compared with an operational amplifier, or op-amp, circuit, and the output of this circuit goes to the PWM stage. The stage creates the necessary signals to control the output transistors and is controlled by the direction and control logic circuits on the driver. The output of the PWM stage is fed to the output transistors. The input to the driver is sent from the axis logic card. In this example, the signal is an analog signal. Some systems use a unipolar analog signal that goes from zero volts (stopped) to 10 or more volts (maximum speed). A separate logic signal is used to provide direction information from the axis logic card to the servo motor drivers. A bipolar analog signal is used in other systems. For example, zero volts is still a stopped condition. However, if the signal goes to positive 10 volts, the motor should run full speed in one direction. A negative 10-volt signal would mean to run full speed in the opposite direction. Of course the analog signals can be any value of voltage from zero to the positive, or, if bipolar, to the positive and the negative maximum voltages to reflect the desired speed of the motor. The analog output signal is normally produced by a digital-to-analog converter circuit on the axis logic card. Early analog drive signal systems were very difficult to tune. The axis logic cards often had a series of potentiometers to adjust analog signal balance, converter gain, and often had adjustments for what was termed window width. This window width set how far the motor could be from its desired position before an analog output was produced by the axis logic card. Normally, after setting up this type of equipment you would replace the logic rack in the cabinet, close the cover, and then watch as the system began to drift. Close tuning of these systems requires many fine and superfine tuning attempts. The next generation of servo systems used digital axis logic control signals. The digital drive signals for each axis are created on the axis logic card and sent as digital data to the drivers. The driver converts the digital data to PWM transistor drive signals to operate the motors. Today, both analog and digital techniques can be used as the communication link between the axis logic card and the drivers. Today s electronic circuits are much more temperature stable than those used in the past, especially in the Controlling Industrial Motors 37

41 operational amplifier, filter, and digital-to-analog converter circuits where stability is especially required. Drifting is no longer a problem. There were a few problems associated with the early servo systems. We already mentioned drifting as one of the major problems. This drifting caused mispositioning and also caused the drive system and motor to sort of lean into machining moves. This caused offset circles, corner pockets, and so forth. Another problem that occurred with early servo systems was motor runaway. The velocity feedback loop consisted of a tachometer. The tachometer s output voltage was buffered by an operational amplifier and fed to the next stage of amplification as the inverting input. The noninverting input to this stage is the buffered reference signal command input from the axis logic card. If we were to lose the tachometer feedback signal, or if the tachometer s two wires were reversed at the driver, the motor would take off at high RPM. The motor would continue to accelerate until it either blew up internally from centrifugal force or until the machine struck a solid stop. More modern control systems monitor the following error we mentioned in the first section of this text. If the following error the difference between the planned and the actual position gets too great, the computer control section will disable the motor drivers to prevent a crash. Most DC servo motor systems prevented the servo motors from crashing by sensing problems and turning off or de-energizing a relay coil in the event of an emergency stop condition, computer system failure, driver fault, or a power down of the controller or drive. This circuit is shown in Figure 23. The relay is normally energized to close the contact sets and allow the output of the driver to be connected to the motor. there are two contact sets for each motor so that the motor is completely disconnected from the drive. Also note that when the coil is disconnected the motor is placed across a braking resistor. This resistor shorts out the motor and causes the motor to brake to a stop by means of its own generated electric current. This resistor is usually a very low value. Many of today s DC servo motor systems don t use a tachometer for speed control of a motor. Instead, a process called bit counting is used to calculate motor speed. In this system, the 38 Controlling Industrial Motors

42 E. STOP INPUT DRIVER OUTPUT COIL MOTOR FIGURE 23 A safety relay whose coil is controlled by an emergency stop and power control/fault circuit is shown here. Note that the normally-closed contacts are used to break the motor. pulses that are fed back by the encoder, or resolver, in some cases, are counted over a period of time. The number of pulses increases with speed so it s a simple matter for the logic control card or the main computer circuits to calculate speed and following error. The axis control card uses this information to tailor its output signal to the motor driver. Troubleshooting DC Servo Motor Systems Modern servo motor drivers and systems operate for long periods of time without failure. Mispositioning problems are normally caused by a failure of the position feedback device such as a faulty encoder or resolver. Often a coupling between the motor and the feedback device fails, causing a repeatable error each time the motor changes direction. Controlling Industrial Motors 39

43 The most common failure is normally reported by the control system as an excess following error type of failure. This is normally caused by a motor that hasn t performed a move due to faulty brushes. The brushes inside the motor will get shorter as they wear. Once the brushes reach their wear limit, they ll no longer make good contact with the commutator and will cause the motor not to respond to a move command and the voltage created by the driver. The only way to repair this problem is to replace the brushes or the motor. When replacing the motor, in some cases, you ll have to adjust the position of the feedback device. This is required if the motor is returned to a home position that reads the marker pulse from the encoder or reads a resolver zero. To adjust this device, remove the back cover of the motor and loosen the retaining screws that hold the encoder or resolver to the motor. Power up the system and issue a home command. Now look at the desired versus the actual position. If these positions are different, rotate the feedback device to the required position and tighten the screws. The troubleshooting of the electronic section of the axis logic system and driver will require proper system documentation and electronic test equipment. Look for signals such as power up enable and drives enable, and check to make sure that these signals are at their proper logic level. Ask yourself questions that can help identify the cause of the problem. For example, are all of the axes faulty or is just one axis in error? If it s only one axis, look to swap components that are only involved with that axis. A power supply module that s used for all axes won t normally cause a single axis failure. However, if all axes aren t operating, the power supply is a good place to begin testing. Match the voltages produced by the power supply with those listed on the schematics for your system. You can also use the many system indicators displayed on a CRT or internally in the cabinet as LEDs. You may see CRT errors displayed as Excess Following Error, Z Axis Drive Disabled, or other such message. LEDs are present on most logic boards and on the drivers to signify the health or a problem with the board or driver. 40 Controlling Industrial Motors

44 Of course, always remember to check the obvious. Look for a blown driver fuse or tripped circuit breaker, a cable or wire that has fallen off of a terminal, or a plug that s come out of a circuit board. A machine fault such as a faulty limit or proximity switch or a mechanically bound axis movement can also appear as a servo system failure. Controlling Industrial Motors 41

45 Power Check 3 1. The output voltage of a tachometer is 10.0 VDC per thousand RPM. What would be the output voltage at 500 RPM? a. 2.0 VDC b. 0.0 VDC c. 5.0 VDC 2. A type of feedback device that sends back a unique position information number for each position of the shaft is the a. resolver. b. incremental encoder. c. absolute encoder. 3. The first adjustment to make on a typical servo drive for a brushed DC servo motor is to adjust the a. balance. b. compensation. c. tach gain. 4. The type of driver used with a DC servo motor is the a. quantum driver. b. H-bridge driver. c. full-wave driver. Check your answers with those on page Controlling Industrial Motors

46 BRUSHLESS DC MOTOR CONTROLLERS Controller Basics Two of the newest types of positioning and motor control systems on the market are the brushless motor and the threephase stepper motors. One of the most common problems with the DC servo motor is that its brushes wear out. Using newer motors and controller systems eliminates this common cause of motor failure. The result is an industrial positioning system that will operate for years with repeatable accuracy and without failure. Motor Commutation The key to brushless motor control systems is the commutation system used with the motor. Without the use of modern high-speed electronic computer systems, the commutation of brushless motors would be all but impossible except in extremely coarse resolution low-speed systems. Commutation is the process of applying electric energy to a motor s field or armature windings at the proper time to cause the motor to rotate. In a standard DC servo motor, the segmented commutator and brushes provide the commutation needed by applying electric power to the armature s windings at just the right instant in the armature s rotation. To keep the motor s resolution small, there are four brushes and a large number of commutator segments within a brush-type DC servo motor. Because of the position of the brushes with respect to the rotating commutator segments, the motor is always commutating at just the right instant independent of motor speed. Since the brushless servo or stepper motor can t rely on the position of the brushes to commutate the motor, this property must be created electronically. The Brushless Servo Motor A brushless servo motor has two main sections as shown in Figure 24. The main section of the motor contains the rotor Controlling Industrial Motors 43

47 INTERNAL BRAKE (OPTIONAL) MOTOR FEEDBACK PACKAGE SHEET METAL COVER STEEL BODY MADE OF LAMINATIONS LABEL: 90 V THREE-PHASE BRUSHLESS INDUCTION MOTOR 8 POLE WITH 24 V DC BRAKE ENCODER 2048 LINES FIGURE 24 A typical brushless servo motor is shown here. and the field windings with the rear of the motor containing the feedback package. Typically there will be two connectors on the motor, one for the electrical connections to the stator windings and the second for connections to the feedback package. The feedback package for a basic brushless motor often consists of special metal detecting sensors called hall-effect switches. There are usually three or six hall-effect sensors mounted to the back casing of the feedback section of the motor. A metal vane that s attached to the permanent magnet rotor is used to trigger the hall-effect sensors and to tell the controller the rotor s exact angular position. If there are three hall-effect sensors, they re mounted at 120 degrees from each other. If there are six sensors, they re mounted at 60 degrees from each other. Some brushless servo or stepper motors will also contain a tachometer for velocity loop or speed feedback purposes. If the motor is to turn very slowly, a brush-type tachometer is often used instead of an optical or encoder pulse-counting technique. At very slow motor speeds, the spaces between the pulses of the optical tachometer can seem very large to 44 Controlling Industrial Motors

48 the control system. As it rotates, a brush-type tachometer will always produce a voltage that will be a function of rotational speed. Figure 25 displays the hall-effect sensor feedback for a three-sensor system with the motor turning at 60 RPM or one rotation per second. If these pulses are used for speed feedback purposes, their rate is just too slow to be very useful. However, many of today s microprocessor-based brushless motor drive systems use software to overcome this problem. If the motor is turning very slowly, the microprocessor stops counting pulses per time period and begins looking at and calculating speed from the period of the feedback pulses. This basically relates to how long the motor s rotor keeps its vane on each of the hall-effect sensors. If the time period is long, the motor is turning slower than if the time period is short. We used this example with the hall-effect sensor signals that are usually used for commutation. However, this same process of calculating period can also be used with the pulse feedback of an encoder. HALL-EFFECT SENSOR 1 HALL-EFFECT SENSOR 2 HALL-EFFECT SENSOR 3 1 SEC FIGURE 25 Shown here are the three signals from a slowly rotating motor that are fed back from the hall-effect sensors. Controlling Industrial Motors 45

49 The feedback from the hall-effect sensors will be close enough for very coarse positioning purposes or on systems intended to control the speed of the motor. However, many systems are using encoders or resolvers as position feedback devices. For example, a typical brushless servo system can have an encoder with 2,048 or 4,096 lines or pulses of feedback per revolution. Some very accurate stepper positioning systems are using encoders with up to 10,000 lines of feedback. Brushless Motor Commutation A typical PWM brushless servo or stepper motor driver is shown in simplified form in Figure 26. This motor driver differs from a standard DC motor servo driver in that there are three specific outputs. These outputs are normally marked with an R, S, and T, an A, B, and C, or with the industry +45 V DC +45 V DC +45 V DC U V W TO MOTOR FIGURE 26 A three-phase bridge is shown here. As with the DC servo H-bridge, a diode is normally placed across each transistor. 46 Controlling Industrial Motors

50 standard U, V, and W. The system consists of a three-phase AC input that s converted by the rectifiers into a DC buss voltage. The voltage used, usually under 100 volts, depends upon the manufacturer and motor type. This DC buss is then converted to a three-phase output signal by the modified H-bridge driver bipolar or MOSFET transistors. The trigger signal that s sent to the output transistors is a PWM signal. However, the switching frequency for this PWM signal is a set frequency. Typically this frequency is set to a value that s above the human hearing range of about 20 khz. Most drives operate in the 25 khz frequency range. The internal windings of brushless servo and stepper motors are arranged as if the motors were three-phase induction motors. Typically there ll be four, six, or eight poles in the motor with the coils of the windings connected as either a delta or a wye as shown in Figure 27. The rotor is a permanent magnet rotor with a higher-resolution motor having more poles than a low-resolution motor. These windings are directly connected to the output terminal block of the brushless servo motor driver. Often a resistor is also present on the driver circuit board that s connected in series with the motor and the output of the motor driver. This resistor measures the current produced on that motor phase. The current through the resistor is directly proportional to the voltage drop across the resistor. By measuring this voltage drop, the controller can calculate motor current. The measurement of motor current can be very useful at this final stage of the controller. First, the analog current signal from the resistor can be compared to a maximum current reference signal as inputs to an operational amplifier. If the measured analog signal exceeds the reference analog signal, the output of the op-amp can change logic state and trigger an error signal to the controller. The controller can then either shut down the driver or reduce the signal to the driver to possibly bring the current down to a reasonable level. A more modern scheme of operation is to compare the measured and reference signals. However, this time the overcurrent error output stays at the driver. Now the driver responds by reducing the PWM signal to the output transistors as shown in Figure 28. Controlling Industrial Motors 47

51 U V WYE CONNECTED W U DELTA CONNECTED V W FIGURE 27 Two types of three-phase brushless servo motors are the wye-connected and the delta-connected motors. In Figure 28A, the PWM signal is being delivered to the motor as a high-frequency 50 percent duty cycle signal. This would occur when the motor is to run at a certain speed. In Figure 28B, the driver has sensed an overcurrent condition and has shut off the PWM signal to the output transistors on a cycleby-cycle basis. By limiting the current on the driver instead of the microprocessor of the controller, it frees up microprocessor time for more important duties. Let s look at how the commutation of a motor occurs at the driver. Remember the stepper motor in an earlier section of this text? The brush-less servo and stepper motors work on much the same principles as a bipolar stepper motor. Table 6 48 Controlling Industrial Motors

52 NORMAL (A) NORMAL OVERCURRENT SENSED PERIOD SHORTENED (B) FIGURE 28 On some newer brushless servo systems, the PWM signal can be shut down each time the drive system senses an overcurrent condition. Table 6 CLOCKWISE COMMUTATION SEQUENCE FOR A BRUSHLESS SERVO MOTOR Shaft Angle Sensor 1 Sensor 2 Sensor 3 Phase U Phase V Phase W V GND OPEN V OPEN GND OPEN V GND GND V OPEN GND OPEN V OPEN GND V displays a typical commutation sequence for a three-hall-effect sensor three-phase servo motor. This table shows the condition of the hall-effect sensor and the condition of the output transistor for the three phases of the 150-degrees-ofrotation motor. To continue on to 180 degrees, and to further continue the rotation, simply repeat the table Controlling Industrial Motors 49

53 from the top. Where the table lists the sensor output as a 1, the sensor is turned on by the vane. The phase outputs can be in one of three states: connected to the positive power supply ( V), connected to ground (GND), or turned off and in a high impedance state (OPEN). Table 7 displays the commutation sequence for counterclockwise rotation of the motor. Table 7 COMMUTATION SEQUENCE FOR COUNTERCLOCKWISE ROTATION Shaft Angle Sensor 1 Sensor 2 Sensor 3 Phase U Phase V Phase W GND V OPEN OPEN V GND V OPEN GND V GND OPEN OPEN GND V GND OPEN V To understand how a controller can respond to speed reference signals and control the speed of the brushless or three-phase stepping motor, let s look a little closer at the previous two tables. Assume that the motor is stopped and the analog input signal, or reference signal, has now increased to a small value of 0.5 VDC ina0to10vdc system. The motor should turn very slowly with this input or reference signal at the driver s input. To see how the motor turns, run your finger down the table slowly. The right column of the table lists the output states of the transistors as set by the driver. Remember that one pass at the table is equal to one-half of a revolution of the motor. If you run your finger down the same table repeatedly, you would get complete motor rotations. Now let s assume that a 5.0 VDC command is issued as an analog reference signal by the controller to the driver. Move your finger rapidly down the repeated table. This would reflect the change in the rate that the outputs would be issued, and of course reflect an increase in motor speed since commuta- 50 Controlling Industrial Motors

54 tion is occurring at a much faster rate. The internal permanent rotor of the brushless DC or stepper would respond to follow the slow, then the faster, commutation sequence. If the input reference signal level reached 10.0 VDC, the maximum analog input voltage, the commutation sequence would operate very fast faster than you can run your finger down the tables. Electronic commutation will use the feedback signals from the hall-effect sensors to keep the motor commutation system in sync with the desired speed. Feedback from a tachometer or a bit-counting scheme can be used to track the desired speed very closely, independent of slight load variations. Also, many types of controllers and drivers will actually advance the timing of the commutation sequence for a motor that s accelerating and decrease it as the motor is decelerating. This acts to aid in the response of the motor as it speeds up and brakes to a stop. Some control systems will also use DC dynamic braking to rapidly stop a motor. More will be seen on DC braking in the next section of this text on AC inverters. The Brushless Motor Controller Brushless servo motors and controller systems are finding their way into all kinds of systems and applications. Some systems are as simple as a motor with an internal controller housed in the back section of the motor with the hall-effect sensors. This motor is supplied an AC input voltage of 120 or 240 VAC and is programmed for its desired speed by means of an I/O port. The motor will hold this speed as a constant, independent of physical load. Fans, air conditioners, refrigerators, conveyors, and other industrial and residential systems are now using a simplified brushless motor system. More complex brushless servo systems are now the standard for industrial positioning systems. A brushless servo or stepper system is much like a standard brush-type servo system in component types and layout. Figure 29 displays a block diagram of a typical single-axis brushless servo system. The analog input for this system is being delivered by the computer controller just as with the DC brushed servo system. This analog signal is normally a Controlling Industrial Motors 51

55 _ IN + IN + _ MOSFETS U U MOTOR CONTROLLER INPUT AMP V V PWM W W CPU ENABLE DIRECT CPU HALL LOGIC HALL EFFECT SENSORS + 5 U U U RESET + ENA _ ENA V W + 5 V W V W DRIVER FIGURE 29 A simplified view of a brushless DC system is shown here. 0 to 10 VDC unipolar DC signal. Most systems use zero volts to represent stopped and 10 volts to represent maximum speed. A separate direction signal and enable signal is also issued by the computer section of the control system. The other inputs to the drive section of the controller are the axis limit switches and the hall-effect switches. The output of the controller and driver is present at the U, V, and W terminals of the driver and is connected directly to these same terminals of the motor. The motor may be a wye-or delta-connected motor with four, six, or eight poles. Usually, a wire cable is manufactured for the motor to driver connection. The system shown is a single axis system. Adding additional axes is a simple matter, and most systems can have twelve or more axes. 52 Controlling Industrial Motors

56 Earlier in this section, we discussed some of the feedback devices used with brushless motor control systems. In addition to the encoder and resolver, the hall-effect sensors, and the brushed tachometer, there are two additional feedback devices often used for speed feedback. The first is an optical system that mimics the hall-effect sensors. Here, the tachometer has a spinning disk that s connected to the motor shaft. A set of lines on the disk interrupts light beams between the light source and the receivers just as occurs with an encoder. The feedback pulses are returned to the driver and are converted to a voltage by a digital-to-analog converter. This voltage is used in a manner similar to that of a standard brush tachometer. A second type of tachometer uses a miniature alternator as shown in Figure 30. ALTERNATOR (BRUSHLESS TACH) 3 O RECTIFIER + OUTPUT _ FIGURE 30 An alternator-type tachometer can be advantageous where the speed of the motor will be very slow. In this system, the three-phase output of the alternator-type tachometer is created by turning a set of three windings in a small magnetic field. The output is rectified by a diode system and is then passed on to the driver. The alternator acts much like an alternator in an automobile and is used for low-speed systems where an alternator can produce a relatively high and stable voltage. Brush-type tachometers are somewhat variable and unstable at extremely low speeds. Brushless Driver Adjustments The amount of adjustments and the procedures for making these adjustments vary widely from drive system to drive system. One type of drive system will have potentiometers that Controlling Industrial Motors 53

57 are similar in name and in operation to those used for DC closed-loop servo systems such as those presented in the last section of this text. On other systems where the controller, the motor/driver interface (axis logic), and the drivers themselves are integrated together by the same manufacturer, the adjustments are often made in software. In this case, a microprocessor is not only present in the main controller but also in the axis logic or each individual driver. Look at the manual tuning of a brushless servo driver. As with the DC servo motor driver, there are the following potentiometers on the driver: REF TACH LOOP BAL adjusts reference or input signal gain adjusts tachometer feedback gain adjusts the overall gain of the controller/driver adjusts driver balance Many brushless motor drivers are capable of providing commutation for a wide range of motor sizes. To adapt the driver to the motor, various components may need to be installed on a driver or controller card. For example, a driver may have a maximum output of 10 amps per phase. If the motor has a maximum current of five amps per phase, then a resistor may be added to the circuit board of the driver to set a maximum output level of five amps per phase. The driver may also have jumpers for identifying tachometer type and tachometer maximum voltage, hall-effect feedback sensor pulse quantity, the current level, and other such identities, sometimes called parameters. Often these components are mounted on a single scaling plug that s placed into a socket on the driver. A typical scaling plug is shown in Figure 31. The manufacturer s literature identifies the scaling plug s location. Be sure to remove the scaling plug and replace it into the proper socket when you re replacing a failed driver or drive card. To begin the adjustments, start with the REF and TACH potentiometers set to their full minimum position, normally full CCW, and the BAL potentiometer at its midpoint. The other adjustment for overall gain will be made later. 54 Controlling Industrial Motors

58 FIGURE 31 A header or scaling plug and socket will have resistors and capacitors soldered to the pins on the header or plug to custom tailor the drive to the motors and other components of the system. Power up the drive and controller. At this time, the driver will be in a torque mode because neither the reference signal nor the tach signal has any influence on the driver s operation. Begin by adjusting the balance of the drive. Turn the balance potentiometer slowly in one direction and then the other until you find the center point where the motor isn t drifting or has equal torque in both directions. Next, begin increasing the REF and TACH gain signals. After these adjustments have been increased a small amount, attempt to turn the shaft of the motor in one, then the other direction. The motor should pull backward against the motion. If the motor reverses direction and runs away quickly, remove power to prevent equipment damage. The problem will be that the tachometer wires are out of phase. Reverse the tachometer connections at the driver and reapply power. Once the drive has been phased and stabilized in a velocity or closed-loop mode, the finer details of the tuning can occur. Typically, the reference and tachometer gains are adjusted until there s no overshoot in the tachometer feedback or reference signal as viewed on an oscilloscope. The waveforms should appear as the critically-dampened waveforms seen earlier in this text. Controlling Industrial Motors 55

59 A special adjustment for overall system gain is sometimes used on some brushless and three-phase stepper drives. In performing this adjustment, you ll need a function generator and an oscilloscope. This adjustment is shown in Figure 32. FUNCTION GENERATOR MOTOR REFERENCE INPUT DRIVER TACH TACH OSCILLOSCOPE FIGURE 32 This is how to connect a function generator and an oscilloscope to a brushless servo driver to adjust overall system gain. The injected square wave must be a 10-volt, 50-percent duty cycle square wave at a frequency specified by the manufacturer, usually 1 2 Hz. This action will stop the drive at one half of its top speed. Use the oscilloscope to look at the tach signal and study the edges of the waveform. Increase the setting of the loop, or overall gain, until the edges distort. Back off this setting one turn to complete the adjustment. Many newer drive systems have a software package that allows you to tune the driver through a laptop or personal 56 Controlling Industrial Motors

60 computer (PC). The laptop or PC is attached to the drive via a serial port. A program that s installed in the laptop or PC communicates with the drive and allows you to begin by programming an autotune enable and pressing enter. The motor is then moved across its limits of motion and the autotuning is complete. Many software packages will allow you to select an on-screen oscilloscope to view the motor/driver response curves for the autotune values and to test modifications to those autotuned parameters. For example, to increase system gain beyond the conservative setting provided by autotuning, enter a tuning screen and select gain. By increasing the number and storing changes, you ve increased driver loop gain. Now return to the oscilloscope screen and check the results of your modifications. Troubleshooting Brushless Motor Driver Systems By eliminating the brushes in the motor and designing the motors with the stator windings cooled by their contact with the case, today s brushless motors last for years without maintenance or problems. In addition, the drivers and the computer systems used to control the drivers are designed for exceptionally long service life without a failure. Features such as open circuit and phase-to-phase and phase-to-ground short-circuit protection help prevent the output transistors or mosfets from damage. The computer system also diligently monitors system fault conditions and can quickly respond to fault conditions by shutting down the drives and displaying the cause of the error. The first place to begin checking for a system problem is at the front panel or CRT display on the control system. If one isn t present, and if the system can be connected to a laptop or PC, begin here. Now look for error codes or error messages that will identify the source or location of the problem. If no error codes are found, look for visual indicators on the controller cards and on the drives. Typically the drives will contain LEDs that can give you a quick indication of driver health. For example, a green LED is often used as a systems normal indicator. A red LED is Controlling Industrial Motors 57

61 mounted near the DC power supply section and monitors the DC power buss for an overvoltage or undervoltage condition. An undervoltage condition is rare and is usually caused by a blown input fuse or an otherwise missing input AC phase. An overvoltage condition is more common and is usually caused by the motor, making a large number of high speed moves. The motor absorbs current during the first part of the move but returns current as the motor brakes to a halt. To correct, lower the top speed of the driver slightly to keep the buss voltage at a lower level. Motor replacement is rare but can be neccessitated by abuse or by a manufacturing defect. Since a motor is connected by plugs on most systems, it s a simple matter to swap a motor. Simply disconnect the two electrical connectors, remove the motor from the coupling, install the new motor into the coupling, and replace the connectors. A shorted motor will usually shut off the drive before the driver is damaged. A red LED for this condition will often appear near the output transistors or module. You may also see another indicator such as an overtemperature LED in the same area of the drive. Another indicator, the Charged LED indicator, displays the condition of the DC buss voltage. Never attempt to service the drive if the Charged LED indicator is lit. Wait five to ten seconds for the DC buss voltage to discharge and this indicator to extinguish before working on or unhooking any component of the system. 58 Controlling Industrial Motors

62 Power Check 4 1. True or False? The motor in a brushless servo system can have a stator or field that s connected in a wye pattern. 2. The commutation of a brushless motor is kept at the desired timing by feedback from the hall-effect sensors as they re triggered by a (vane/pulse plate). 3. The first adjustment to make on a brushless servo drive is the (balance/ref). 4. Brushless motor drivers normally have adjustable limits selectable with jumpers mounted on a. 5. The industry standard designation of 3 phase motor control outputs is. Check your answers with those on page 83. Controlling Industrial Motors 59

63 AC FREQUENCY INVERTERS Frequency Inverter Basics For many years, the DC variable speed controller was the standard in industrial applications. Whenever the speed of a rotating machine had to be varied by more than two distinct speeds, a variable speed DC drive was used. This variable speed drive used transistors or SCRs to control the voltage and current to either the armature or the field windings. This variable speed system could smoothly vary the speed of the motor from a few RPMs up to the maximum RPM of the motor. This system suffered from two main problems. First, the DC motor contains brushes that are prone to wear and eventual failure. Second, the size of the motor being controlled is limited by the size of the transistors or SCRs used in the drive. As the motor size gets larger, the size of the drive gets excessive. Cooling by air or water becomes necessary to keep the solid state switching and rectification devices from overheating. The ideal situation would be to use an AC motor in variable speed applications. This would eliminate the brush wear problem of a DC motor. However, an AC motor is a somewhat synchronous motor. It will attempt to have its squirrel cage rotor lock on to the rotating magnetic field that is created in the stator. A small variance called slip is present between the synchronous speed and the actual speed of the motor. The number of poles of the AC motor is what specifies what speed the motor will turn. The two most common AC motors, the two pole and the four pole, turn at 3600 and 1800 RPM respectively. At first it might seem that you could vary the voltage to the motor with a simple device such as the rheostat circuit shown in Figure 33. Here, a rheostat is placed in series with each of the motor s stator windings. This circuit can vary the speed of the motor slightly with a great loss of torque and significant overheating of the stator windings. This is the same effect as when an electric tool burns up its universal motor windings because of a voltage drop due to too many or too small of a gauge of extension cords. 60 Controlling Industrial Motors

64 O 1 O 2 O 3 AC 3 O MOTOR FIGURE 33 The use of rheostats to control an AC motor s speed isn t very effective because it limits motor torque. The more practical method of varying the speed of an AC motor is to vary the frequency of the applied voltage to the motor. The nameplate of the motor reveals the RPM value at rated horsepower and at rated frequency. If the frequency changes, the RPM of the motor will follow suit. The rotor will still attempt to track the new frequency of the rotating magnetic fields within the stator. Early electronic systems that controlled the frequency of the AC power used high-power vacuum tubes where the grid of the tube was fed the desired AC frequency. The plate or cathode of the tube passed this frequency on to the motor at a much higher voltage and current level. It wasn t until high-current semiconductor devices were constructed that a non-vacuum tube inverter was developed. These high-current semiconductors could handle the currents involved in driving an induction motor. Principles of Operation Controlling the frequency of an AC induction motor rather than its voltage will control the rotating speed of that motor. This is true of single-phase, split-phase, and especially three-phase motors. Frequency changes aren t easy. Many complicated electronic circuits are used to perform this function. Let s begin by looking at a few simple waveforms such as the ones shown in Figure 34. The sine wave (A) is a very common waveform. It s normally created in an alternator as the windings of the rotor are passed through a stationary magnetic field. This is how the AC current that s supplied to our homes and businesses is developed. A sine waveform is a waveform that s a basic or primary waveform. For example, a 60 Hz sine wave is just Controlling Industrial Motors 61

65 O V (A) O V (B) O V (C) FIGURE 34 Three different types of waveforms are shown here. These waveforms are the sine, triangle, and square wave. that 60 Hz. There are no other frequencies involved. The triangle (B) and the square (C) waves shown in Figure 34 are rich in frequencies other than their primary frequencies. These frequencies are called harmonics. There are either even or odd harmonics of 60 Hz in the triangle and square waves. When the designers work on an AC variable-frequency driver, or simply an inverter, they re looking to duplicate the sine wave as close as possible. Today s most common inverters often produce square waves or modified square waves, such as the ones shown in Figure 35. The problem with the square wave system is that the square wave is made up of predominantly odd harmonics of the primary sine waveform. These odd harmonics create multiple 62 Controlling Industrial Motors

66 120 V FIGURE 35 The outputs from an inverter, one shown here, will have a distorted form due to the inductance of the motor load. eddy currents and have greater hysteresis losses than a standard sine wave. This, in turn, causes the motor to heat up much more than if it were operated solely on a sine wave. Various types of motors have been developed that limit the amount of iron or steel within the motor to limit the current flow and the heat generated by these stray currents. These motors are often termed Inverter Duty motors. The circuit that produces the three-phase square waves used to trigger the output transistors is shown in Figure 36. The three-phase input is placed on a diode bridge that converts it to DC voltage. This voltage ranges from 240 to 400 VDC, and care should always be exercised when working on or near the DC buss. A small sample of the incoming line voltage is sent to a reference circuit and to a low-voltage power supply. A main control section beneath the power supply is used to accept inputs from field devices such as start and stop push buttons, auxiliary contacts of safety circuits, analog inputs, potentiometer inputs, and various logic level and relay contact outputs. This section of the inverter will often contain a microprocessor that oversees the operation of the inverter and controls the output frequency to the motor. The oscillator section receives pulses from the control section of the Controlling Industrial Motors 63

67 1a 2a 3a U L 1 L 1 + V TO MOTOR L 2 L 2 DIODE BRIDGE 1b 2b 3b W L 3 L 3 _ 1a 1b 2a 2b 60 Hz REF POWER SUPPLY LOW VOLTAGE OSC 3a 3b I/O CONTROL AND TRIGGER FIGURE 36 A basic diagram for an AC inverter is shown here. inverter and converts these pulses, along with an enable signal, to three-phase square waves of the desired frequency. These three-phase square waves are then sent to the typical bipolar or mosfet output stage. Some modern inverters are designed to produce almost a true variable frequency sine wave at their output terminals. This sine wave is created in a very unique way. First, a sine wave form is digitized and stored in the computer s memory as shown in Figure 37. Next, the digitized waveform is scanned by the computer and the digital signal level is fed to the drive electronics and the output stage. The sine wave is therefore duplicated by the output bipolar or MOSFET transistors at a much higher voltage and current level. When the scan of one sine wave is complete, the scan restarts at the beginning of the waveform. The speed or rate of the scan will then set the frequency. If the memory is scanned at 60 times per second, you ll have an output that is at 60 Hz. If the scanning rate is 64 Controlling Industrial Motors

68 VOLTS T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 TI2 T13 SCAN FIGURE 37 In this illustration, the sine wave has been stored in computer memory and can be scanned by the computer and sent to the output transistors to create a single-, split-, or three-phase sine wave output to the motor. changed, the frequency of the output waveform will also change. The motor s rotational speed will follow these frequency changes. This system can be easily converted to a two-or three-phase system. The first scan is started to produce phase 1. After the correct number of degrees, the next scan is started to produce phase 2. Phase 3 will then start, if so assigned, at the next time interval. The type of drive we re discussing is the newest type of industrial drive at the time of this writing. This drive, in fact, can be used to control a brushed DC servo motor, a brushless DC servo motor, a three-phase stepper motor, and can also act as an AC frequency inverter. The computer on the driver is simply told what kind of motor is present, along with the drive parameters, and what kind of drive signals are present (analog, digital, current level). The computer will then use a commutation table for a DC servo, a stepper, or a brushless motor, or produce an AC sine wave, and send these signals to the output bipolar or mosfet transistor module, depending Controlling Industrial Motors 65

69 upon the programmed parameters. This is state-of-the-art flexibility provided by the computer circuits and computer programming of today. Frequency Inverter Parameters A frequency inverter is normally controlled by a microprocessor that monitors the status of the inputs, such as switches and push buttons, the output current, and any system faults. Its circuit outputs trigger the signals or waveforms that are boosted by the output stage to drive the motor. The microprocessor operates by scanning its internal program. This program is modified by means of identities called parameters. The parameters are used to tailor the inverter s operation to suit the motor and the machine to which the motor is coupled. Some inverters have only a few parameters. Other more complex inverters have hundreds of parameters. In this section, we ll look at the major parameters, the ones that directly affect inverter performance. These parameters can be accessed by entering the programming section of the inverter. To see how to enter this section and what kinds of parameter values are present, consult with the manual for the particular inverter that you re working on. The acceleration and deceleration rates of the inverter can be set using parameters. Usually these values program as a function of time. For example, a value of 1.5 may mean that the drive will ramp up its output frequency so that the maximum frequency will be present at one and one half seconds after a start command is issued. This is the same type of programming that s performed for the deceleration ramp. Another common parameter you ll often find on an inverter is the output shaping or a volts-to-hertz curve parameter. This parameter lets you select a linear curve (A) or an exponential curve (B) as shown in Figure 38. This parameter is normally set by using either a 0 or a 1. The most common curve is the linear curve. This linear curve setting provides a constantly increasing motor torque with increasing speed. Boost is a common inverter parameter. At low speeds, the torque supplied by the motor can be very low. If the inverter supplies an extra amount of voltage at low speeds, somewhat 66 Controlling Industrial Motors

70 VOLTS VOLTS HERTZ HERTZ (A) (B) FIGURE 38 Shown here are a linear volt-to-hertz curve in (A), and an exponential curve in (B). taking away from the linearity of the curve, the motor torque output will increase. Some inverters offer a selection called autoboost that tailors the amount of voltage that s added to the output. This tailoring is performed by monitoring the output current. Other inverters offer various stages of boost and can be programmed as values from one to ten. Electronic thermal overload is another common feature of an AC inverter that s accessible through parameters. This feature will often use more than one parameter. For example, in one parameter you might enter the motor s full load current value. The inverter s microprocessor will then constantly monitor the output current against this value. The second parameter will set up how you want the thermal overload system to work. Here, you may choose no thermal overload and let the inverter shut off automatically at a preset maximum current. You may also choose a quick trip of the overcurrent circuit to protect the motor. Finally, you can also select a parameter that will allow the inverter to provide excess current for a certain time interval such as 2.0 seconds. This will help prevent false trips of the electronic thermal overload circuit as the motor accelerates or decelerates. Controlling Industrial Motors 67

71 In reference to current, acceleration, and deceleration, many inverters offer a feature called frequency hold. When this feature is enabled, the inverter will monitor the current output of the inverter as the motor accelerates and decelerates. If the inverter senses that the current draw is too great, the inverter will hold off the increase or decrease in frequency, and therefore, the amount of acceleration or deceleration, until the current level comes back to a preset value. The inverter uses the preset motor nameplate current value entered previously. The ramps where this feature comes into play are shown in Figure 39. RPM FREQUENCY HOLD 0 0 TIME FIGURE 39 An inverter can provide a function called acceleration and deceleration frequency hold to prevent overcurrent trips of the electronic thermal overload that is internal to the inverter and is programmed by the parameters. As you can see the acceleration and deceleration ramps are held at some frequency until the inverter senses that the current has fallen back to normal values. Once this has occurred, the inverter will once again increase or decrease the frequency to finish the ramp. Braking is another feature of most inverters. A strange thing happens when an AC motor is supplied with a DC voltage. It stops and stops quickly. If you were to listen carefully to the 68 Controlling Industrial Motors

72 motor case when the DC is applied, you would actually hear a sort of groan from the motor. Inverters offer adjustable braking that can be used to make a deceleration ramp very short. Typically, up to three parameters may be programmed into the inverter to custom tune the braking of the motor. One of these parameters programs the braking strength, which is actually the amount of DC voltage that will be applied to the motor. Usually, this parameter will be programmed in values such as 1, 2, 3, and so forth. Sometimes it can be programmed as a percentage such as 10, 20, 30, and so forth. The next parameter programs the time the DC voltage will be present on the motor. This parameter usually is in seconds or decimal portions of a second. The final parameter is programmed to tell the inverter when to start the DC voltage application, often termed DC injection. This value is a whole number that relates to a frequency. For example, in Figure 40, the DC injection braking starts at 20 Hz Hz Hz 0 TIME STOPPING POINT DUE TO DC INJECTION NORMAL STOPPING POINT FIGURE 40 At 20 Hz on the deceleration ramp, the inverter will supply DC current to the motor to stop it quickly. Controlling Industrial Motors 69

73 Another useful parameter is maximum frequency. Typically, this value is set at 60 Hz but it can be modified upward or downward from this standard. Usually, the maximum frequency the motor can handle is 120 Hz, but most inverters can be programmed to 400 Hz or beyond. Most inverters will take the acceleration and deceleration rate as a time value from zero hertz to the maximum frequency value. Therefore, if you increase maximum frequency, you may also need to increase the acceleration and deceleration parameters. Base frequency is a parameter that describes the full voltage output frequency of the inverter. Normally, this is set to equal maximum frequency but the value may be lowered to apply more voltage at a lower frequency. Jog frequency and jog acceleration/deceleration time are two additional parameters on most inverters. Jog frequency sets the maximum output frequency of the drive when the jog input to the inverter is enabled. This value is usually a small value such as 15 or 20 Hz. The acceleration and deceleration times are normally kept long (about one or two seconds) because the machine is under manual control at this time. A special parameter to AC frequency inverters is called the resonance skip band or the skip frequency band. Here, a pair of frequencies can be programmed into the inverter s parameters. When the inverter accelerates or decelerates the motor and comes to the first frequency, the inverter will jump up to the second programmed frequency, skipping all the frequencies in between. This will prevent the motor from operating in a mechanical resonance zone. Of course the frequency band should be kept as small as possible to prevent large jumps in frequencies. A large jump is hard for the rotor to follow under mechanical load. Various other parameters are used to define how the control inputs are recognized, if the speed reference and stop/start circuits are local or external, how the motor stops, and how the logic level and relay outputs are configured. Some inverters have hundreds of parameters entered as bits in binary words. The best place to find the information on these parameters is in the system documentation for the inverter. 70 Controlling Industrial Motors

74 Frequency Inverter Circuits A basic inverter layout is shown in Figure 41. The inputs to this inverter have been kept simple at this time, and the only output is the motor. Note that a set of motor overloads is used in this circuit as is sometimes done when extra motor protection is desired. The internal thermal overload of the inverter is commonly used without having this extra layer of motor protection. The inverter input is also protected by a circuit breaker. Also note that there re no external start or stop buttons. Here, the front panel push buttons are used in place of external devices. DISPLAY KEYBOARD FRONT PANEL CONTROLS L 1 CONTROL I/O TERMINAL BLOCK L 2 L 3 L 1 L 2 L 3 UVW CIRCUIT BREAKER OVERLOADS 3O MOTOR FIGURE 41 A basic connection system for an AC inverter is shown here. This simple connection system is used to drive a motor using the front panel and keyboard controls. The front panel of the inverter will contain push buttons that allow the operator to start or stop the inverter and a rate potentiometer that s used to control the output frequency of Controlling Industrial Motors 71

75 the inverter and, therefore, the speed of the motor. This basic configuration is often selected by setting one or two parameters for what s termed local frequency and stop/start control. Because the inverter is usually mounted within an industrial enclosure where access to the front panel is limited, this mode of operation is normally only used for testing purposes. Figure 42 displays the use of a remote speed-control potentiometer that s connected to the control I/O terminal block inside the inverter. Also shown is the use of remote start and stop switches and a direction selection switch. At this time the parameters used to select remote or local control would be switched to the remote mode. For example, a zero (0) programmed into parameter locations 12 and 13 on one inverter may select local mode while a one (1) in these locations selects a remote mode. +V REF COM STR COM STP COM REV FWD REV START STOP SPEED CONTROL POTENTIOMETER FIGURE 42 This illustration shows how to connect to an inverter such basic devices as a speed potentiometer, a stop/start switch assembly, and a direction selector switch. With this selection of parameters and external equipment, an operator can now start and stop the inverter and reverse the direction of the motor. Also, the speed of the motor can be varied by increasing or decreasing the value of the resistance of the speed-control potentiometer. It should be noted that if the operator starts the drive and then increases the 72 Controlling Industrial Motors

76 speed control potentiometer setting which increases inverter output frequency, the inverter won t immediately increase the speed of the motor. Instead, the motor will ramp up or ramp down the frequency to the next speed setting according to the information stored in the acceleration and deceleration parameters. This ramping of the frequency will also take place if the direction of the motor is reversed. If the direction of the motor is reversed while it s turning, the inverter will ramp down the frequency, provide DC injection braking (if enabled), and then ramp up the frequency to the set amount, providing the desired speed in the reverse direction. Figure 43 displays two other methods of performing remote speed control of an AC inverter. In both systems, the inverter has its input to its signal reference terminals supplied by a digital-to-analog programmable controller module. These modules operate by receiving a digital number from the programmable controller logic and converting this number to an analog voltage or current output. Normally, the larger the programmed number, the greater the output voltage or current from the module. However, the programmable controller and the inverter are both intelligent enough to invert this rule and use a low output (inverter input) as high speed. In Figure 43A, the 24 VDC power supply is placed in series between the output of the module and the input of the inverter. Here, the output of the module will be a current that varies between 4 and 20 milliamps or 4-20 ma. Usually 4 ma is stopped and 20 ma is maximum speed. Again, the output value of current is dependent upon the number that s presented to the module by the programmable controller. In Figure 43B, the 24 VDC power supply is attached to the module and the output of the module is a variable voltage, usually from 0 to10 volts. This system sort of mimics the speed potentiometer seen earlier although the speed control is automatic rather than manual. In most cases, the variable output voltage is used for speed control of an inverter due to its simplicity in components and in wiring. However, the variable current 4-20 ma system is better at speed control because it s less susceptible to electrical noise spikes in a system s cabinets, wireways, and enclosures. Controlling Industrial Motors 73

77 4-20 ma + _ POWER SUPPLY INVERTER D/A REF (A) 0-10 V DC + _ + _ POWER SUPPLY INVERTER D/A REF (B) FIGURE 43 Shown here are two methods of controlling the speed of a motor by means of an inverter that s connected to a programmable controller s D/A module. The type of input signal or reference voltage or current must be identified by the inverter for it to perform properly. Usually, a parameter is modified with a value that describes the type of input that the inverter should expect. For example, a 0 might mean a speed potentiometer, a 1 means an analog voltage, and a 2 in this parameter means that the input is a 4-20 ma signal. 74 Controlling Industrial Motors

78 Some inverters have additional input terminals that can be programmed for a certain use. Typically, these terminals are inputs from switches or programmable controller output modules that select preset inverter frequencies. These inputs will completely eliminate any speed reference signal such as any of those just discussed. Figure 44 displays the connection of these switches to the control terminal block. COM 14 IN 1 15 IN 2 16 IN 3 17 IN 4 18 FIGURE 44 The four switches that are connected to the control terminal of the inverter can be used to select preset speeds and acceleration and deceleration ramps. SW1 SW2 SW3 SW4 In this system of motor speed control, each of the four switches, relay contacts, or programmable controller outputs, has its own terminal on the control terminal block with a single signal common. If any of these four inputs is closed to common, the inverter will identify the input and cause the output frequency to match the value programmed in parameter memory for that input. For example, say that switch 3 is set in the system parameters to call a frequency of 45.0 Hz. When the input from this switch is closed to common, the output of the inverter will be 45.0 Hz. There are usually other parameters that coincide with this parameter that define a special acceleration and deceleration ramp for this speed and additional parameters for these definitions at the other four speeds. Most frequency inverters also have inputs for resetting the inverter and for stopping the inverter if an external condition is at fault. These two inputs are shown in Figure 45. Controlling Industrial Motors 75

79 FIGURE 45 This illustration shows how a reset button and an auxiliary interlock circuit can be connected to an inverter. COM 16 RST COM AUX RESET E. STOP OVERLOADS LC As shown, the reset switch in usually a normally-open push button switch that s located inside the inverter enclosure. Pressing this switch will reset most inverter system faults and input conditions to a default state. This is especially useful if the inverter displays an operator or system fault that could otherwise only be reset by removing and reapplying power to the inverter. The auxiliary input terminal of the control terminal block inside the inverter is one of the most important control terminals. This input can be used as an external fault input to shut down the inverter in case the machine or system driven by the inverter is at fault. Shown in our sample illustration are an external machine emergency stop circuit relay contact, the motor s overload auxiliary contacts, and a relay contact from a light curtain as inputs to this terminal pair. If any of these contact sets open, the inverter will shut down and not restart until the circuit is closed again. In some cases, the inverter must be issued a stop and then a start signal to restart the system. In other cases, it can simply restart on its own or may restart after a time delay. All of these identities can be programmed into the parameters of the inverter. Also, the type of fault message that will be output when the auxiliary terminal pair circuit is opened can also sometimes be programmed into the parameters. One other set of input terminals is normally reserved for connection to a jog push button. Usually this push button is used in conjunction with the direction selector switch. 76 Controlling Industrial Motors

80 Some of the other terminals on the terminal block deal with inverter outputs. Many sets of terminals may be used to access the output of internal relays. One relay, for example, can be used to tell when the inverter has reached full frequency. Another contact set may contain an output signal that indicates the inverter is in a fault state. As with any piece of industrial equipment, the best place to look for system information is in the manufacturer s service manuals and in the machine s documentation. You can find the terminal names and numbers used for the control I/O and the exact number, name, and available options for each parameter. Some inverters can even have various parameter tiers, a normal tier for simple parameters, and then a second tier of engineering level parameters. Only the system documentation can accurately identify and describe these systems properly. Troubleshooting Frequency Inverters Although an inverter system may seem very complex, in reality it s very simple. If possible, the best way to learn about an inverter or any of the drive systems discussed in this text is to experiment with them. Place a motor in a restraint and connect it to the inverter or driver and change system values, adjust potentiometers, and modify parameters to see exactly what happens in each situation. Inverter system problems can show up in many different ways. Erratic frequency control is one of the most common problems. This can be caused by a variable driven load, a noisy signal or reference input, or a faulty motor or inverter. The inverter s display will normally allow you to read the frequency of the drive system. If this frequency is stable, and the motor speed is varying, the two most common causes of erratic speed are the mechanical load and the motor. In most cases, you can disconnect the motor from the mechanical load and check motor speed with a tachometer or strobe light. If the motor remains at speed, check the load for sticking bearings, mechanical interference, and so forth. If the motor is faulty, replace it. If the frequency display on the inverter shows a varying frequency, you can either have a signal or noise problem at the Controlling Industrial Motors 77

81 input or have a faulty inverter. If possible, the best test is to place the inverter in local frequency control by setting the proper parameter, and then use the front panel speed control. If the frequency still drifts, the inverter is faulty. If the frequency is stable using the internal speed reference potentiometer or programming pads, then you most likely have a signal input noise problem. Look for a loose shield connection or a condition known as a ground loop. A ground loop is shown in Figure 46. INVERTER D/A ISOLATE THE SHIELD HERE GROUND SHIELD HERE FIGURE 46 A ground loop can be avoided by grounding the shield at the inverter and isolating the shield at the output module or other source of signal output. A ground loop is caused by the grounding of the shield on a signal cable on both ends of the system (in our example the 0-10 VDC output module and the inverter). This attachment causes the shield to act as one or more turns of a transformer and induces electrical noise into the system. The general rule is to ground the shield of a signal cable at the inverter and to isolate or float it at the module. The remainder of the problems with AC frequency inverters are usually seen as a system that has no motion at the machine. The inverter may have no display, may be displaying normally, or be in a fault state. A dead inverter has no front panel display and no illuminated indicator LEDs. This situation is usually caused by an open circuit breaker or blown fuse on the input to the inverter. A quick visual check tells you the condition of the circuit breaker. A meter is necessary to check the fuses. Reset the breaker and replace one or more blown fuses, and try the 78 Controlling Industrial Motors

82 inverter again. If the problem reoccurs, consider replacing the inverter as it probably has a shorted rectifier diode or power output transistor. If the inverter seems to be in working order, check the frequency display. If there s a frequency shown, go to the motor and test the voltage. You should see a few volts. At low frequencies, the motor may be stalled, and the overcurrent circuits are keeping the frequency low until the motor begins to turn. This is especially true if acceleration frequency hold is enabled. Disconnect power and the motor from the mechanical load, and recheck operation. If the frequency displays read 00.0 Hz, the problem can be a missing input signal. Try pressing the reset button as a first attempt. Next use your meter and the prints to check the input signal levels on the control terminal board. Check to make sure that the start and stop inputs are closed and the auxiliary input is also closed. Some inverters will have different names such as latch and will have separate start forward and start reverse inputs. It s best to have the system documentation on hand to efficiently troubleshoot the system. Also don t forget to use your senses to troubleshoot a system. A burnt smell inside an inverter can mean that an electronic component has failed in the power stage. The rest of the inverter may be in good working order, but there s no DC voltage on the output stage, causing the inverter to seem good but not produce an output. One word of warning. You can t take valid current measurements on an inverter that doesn t produce a sine wave without special equipment. The square wave output, distorted as it is, disrupts the current measurement. You can take current readings on a working system and record them for comparison when the system goes down, but don t use current readings to diagnose a fault unless previous readings have been taken. The inverter itself can also tell what s wrong. Most inverters will display OC for an overcurrent condition, OL for an overload, and OV for an over-voltage condition. These are but a few of the most common inverter diagnostic outputs on the front panel display. The manufacturers manual and often the inside cover of the inverter will list these important trou- Controlling Industrial Motors 79

83 bleshooting error listings. Also, many inverters store these faults in memory as a parameter value. Some inverters have four or more parameters that store this information. If the equipment is running when you arrive on site, but has failed recently, look to these parameters for information as to what was wrong. Usually, the first parameter on the list is the most current. As new faults occur, the list bumps back in numerical order and the last fault is erased. Now, take a few moments to review what you ve learned by completing Power Check Controlling Industrial Motors

84 Power Check 5 1. A parameter for the acceleration rate of an inverter will normally program in units of (seconds/hertz). 2. An inverter doesn t have a front panel display or LED indicator lights. The first thing to check is the (motor phases/incoming power). 3. True or False? The auxiliary input terminals of a frequency inverter are rarely used. 4. The DC injection of an inverter is used to a. tune the drive. b. stop the motor. c. accelerate the motor slower. Check your answers with those on page 83. Controlling Industrial Motors 81