Designing With Motion Handbook Chapter IV Brush There are many different types of systems that can use manyy different types of motor such as BLDC, Brush, Stepper, Hollow Core, etc. But for this write-up, I will be concentrating on Brush and BLDC only. The difference between a Brush and Brushless motors is exactly as is stated... one style of motor uses Brushes and the other does not. Exactly what a Brush is and is used for will be discussed along with each motors theory of operation. The important part of this paper will be to show how the magnetic fields inside a motor actually make it move. This paper will discuss: 1. Motor Control Loops 2. The Principle of Commutation 3. Brush Motor Commutation 4. BLDC Commutation 5. BLDC and Brush comparison Dynamics of Motor Operation * ********** ***** The Motor Control Loops Starting from the most inner loop is the commutation loop. Althoughh not actually part of a control loop, it is required to make the motor move. How that happens will be discussed in the section discussing the principle of commutation. But how commutation takes place is dependent on the type of motor which can make it a hardware or software operation. Next there is the Current Loop which can also be considered a Flux control loop. It is the Ki (Torque constant) of a motor thatt produces the applied Torque in a rotary motor or the Force in a Linear motor. By applying a current to a motor, theree is a Flux generated. This flux produces an electromagnetic force that works against fixed North and South Pole magnets built into the motor. The applied force can be varied by changing the current level in the motor windings. More discussion on this will be presented on upcoming sections of this paper. Moving outward, we then have the Voltage Loop, whichh in many cases is referred to as the Speed or Velocity loop. The voltage loop is considered a Speed Loop only. Although the voltage across the motor terminals does produce a current in a motor, you must remember that it does not take a lot of voltage to produce a lot of current in a motor that has a winding resistance of less than 1 Ohm. So the current in the voltage loop is not used as a control device, but only as a result of the torque needed to reach the Speed that the application is demanding. If we look at a Brush motor only at this moment, as its Terminal voltage is varied,, the speed of the motor will adjust based on the motors Ke (voltage constant). More discussion on this will be presented on upcoming sections of this paper. 1 of 9
Moving outward we come to other types of control loops. There can be a Position Loop, pressure loop, temperature loop, light sensitive loop, andd a host of other loops that may be required. To be able to accommodate all of these loops, a timing chart must be employed to insure alll control loops do not interfere with the operation of the motor being controlled. The Principle of Commutation The principle of commutation is fairly simple to explain. By looking as the following figure, you can see that in the case of two fixed magnets overlayingg each other, they will come to rest in a balanced position. The only way to generatee a horizontal force between them is to move or offset one of the magnets (the top one in this case) to the left or right of the bottom magnet. That is exactly what commutatio on does. As is shown in the figure, iff we take a small piece of the left side of the upper magnet and move it to the right side, an unbalanced situation arises. The magnets will then create a horizontal force betweenn them trying to get them to rebalance. As the top magnet moves to the left, commutation will sense the motion, and at the properr time, willl again take a piece of the left side of the top magnet and move it to its right side. By continually doing this, a linear motion is accomplished. And if the magnets are oriented in a circular fashion, the movement will be circular. 2 of 9
The following figure shows the forces imparted on tow magnets when moving horizontally or vertically with respect to each other. These forces are by design when a motor is developed. * ********** ***** Brush Motor Commutation In order to commutate a Brush motor, the motor windingg is split up into many sections, with each section attached to a copper bar. There is an electrical, i.e. magnetic offset produced by the shorting action of the commutation brushes. This is the action that maintains the shift of the magnet field within the motor. The following figure shows the Brush motor commutation system. Fixed Magnets Windings Commutation Bars Brushes 3 of 9
The next two figures show the CW and CCW rotation interaction of the Brush motor operation when being commutated. Note the RED and BLUE magnetic offset that will produce the forces to move the armature, also known as the rotor. * ********** ***** BLDC Motor Commutation Knowing that a BLDC motor must be externally commutated, the question which always pops up is; HOW is that done? Based on the earlier discussions about commutation, it is known that the BLDC motor requires a software commutation methodology. To do this, the position of the BLDC rotor must be known. The next section will discuss several ways of sensing and, therefore, being able to commutate BLDC motor windings. Sensing the BLDC Rotor Position There are many available methods to sense a BLDC rotor position... 1. Mechanically Commutation style bars 2. Magnetics Hall Effects via Halls or ic-haus IC s 3. Voltage Sensing winding BackEMF 4. Current Along with Voltage for FOC operation 5. Frequency Injecting frequencies to sense Response 6. Optics Visual Sensors 7. Others Only left to the imagination... For this write-up, however, only Voltage will be discussed Sensorless BLDC Commutation by Voltage Sensorless commutation is a method of changingg the appliedd power in a motor winding without the use of any external feedback device such as an encoder, resolver or Halls. To do this, the motor controller must monitor the motor winding BEMF to determine when itt is at the proper angular position to commutate. Once the motor windings are commutated, the winding BEMF will be monitored for the 4 of 9
new Zero-Crossoveplace. If it is seen thatt the motor is lagging behind the desired speed, in a speed controlled application, more motor current (power/torque) can be applied by increasing the PWM Duty-Cycle forcing the motor point will again indicate the correct rotor angular position for commutation to take to catch up and maintain the desired speed. By continuouslyy monitoring the time difference between successivee BEMF Zero-Crossover points, corrections for velocity lead or lag can be handled. 1. BackEMF Zero-Crossover Detection As mentioned, in order to close a commutation loop,, some form of true rotational (angular) reference point needs to be determined. As a motors armature iss rotated without applying external power, it will act as a generatorr developing a sinusoidal voltage acrosss its windings. The following figure shows the BEMF of a typical Sin BLDC motor. The Zero crossover points are pointed to by the red arrows... Zero-Crossover Level However, when operating as a SPEED controller using PWM control, the actual waveform is a combination of the motors BackEMF and the controllers PWM signal. The following figure shows an actual BLDC PWM winding waveform. Zero-Crossover Level The Orange line indicates the Zero-Crossover point of the BEMF with inductive PWM noise from the other two driven windings. In order to filter out the PWM noise in the BEMF area, shown in the RED squares, a combinationn of hardware and software filtering can be used. The method derived to 5 of 9
determinee the BEMF Zero-Crossove er point for the sample motor was done predominantly by the hardware circuit shown in the following diagram. By properly tuning the time constants of the circuit, a balanced and repeatable relationship between the actual Zero-Crossover point and the Exclusive OR Logic output (XOR)(i.e. indicated Crossover point) can be realized as shown in the following screenshot. Zero-Crossover Level In this case the pulses are leading the zero-crossover point by several degrees. Depending on the speed of the motor, this angle can increase or decrease shifting the commutation point. An analogy to this is the shifting of the firing point of a spark plug in a car engine. The idea of this is to get power into the commutated winding at the proper time to produce the most power possible. 6 of 9
2. Trapezoidal BLDC Operation The design for simple one speed pump or fan does not have to be very involved, since a one speed BLDC motor can act like a DC Brush motor. The idea in this design is to have the motor engineer develop a motor that will run at the proper speed, with the proper load, at the proper voltage. When running in this fashion, the control waveform can be set to 100% PWM, which will have several benefits. First, there will be no PWM pulsations as shown in the figure to the right... The Trapezoidal waveform in both pictures was generated by the same motor. However, the one on the left was developed using speed control, while the one on the right was running 100% PWM allowing the motor to act as a Brush motor. Both figures weree being operated using a base frequency of 20KHz. The figure on the right was generated using a 100% PWM rate using a 20KHz base frequency. If the motor is being commandedd to run at its design ratee of 12,000 RPM and the Trapezoidal switch is occurring at each Zero-Crossover point, then each commutation switch is being generated at a rate of < 10KHz.... 12000RPM / 60sec = 200 RPS 1sec / 200RPS = 5msec/Rev For an 8 Pole Motor (4 Pole Pair)... One magnetic 360 waveform is generated every... 5msec / 4 = 1.25msec Using a Trapezoidal generator, there are 6-phase Steps per 360 magnetic degrees, so... 1.25msec / 6 = 208 usec / Trapezoidal Step Since there is a discharge pulse generated between each Trapezoidall Step... 208 usec / 2 = 104usec / Trapezoidal Pulse Set 1 / 104usec = 9,615 Hz. It is true that there might be more incidental pulses beingg generated during the operation, but the base frequency and harmonic content will not be generated into the MHz range allowing for a lighter EMI filter saving weight and size. The figure on the left, however, was generated using a less than 100% PWM, but still using a 20KHz base frequency. If the motor is being commanded to run at less than its design rate, say 9,000 RPM then the Trapezoidal frequency being generated is... 7 of 9
1sec / 20,000 Hz = 50usec 50usec / 4096 PPS = 12nsec But the average speed control runs at a 15nsec to 16nsecc rate. Therefore: 1 / 16nsec ~= 66MHz. It quickly becomes apparent that at that rate, there wouldd be more filtering required to insure conducted and radiated emissions are suppressed. This then added parts, size, weight and cost to the project design. 3. Sinusoidal BLDC Operation The main difference between the Trapezoidal waveform and the Sinusoidal waveform is what is known as the INTERLACING of PWM Signals. Trapezoidal and Sinusoidal waveform outlines are shown in the following images... High-A High-B High-C Low-C Low-A Sinusoidal is a 3-Phase Operation Looking closely at the waveforms, note that the Sinusoidal patterns are ON and interlacing at all times, whereas the Trapezoidal waveforms are only ON sequentially..... Even though the High and Low Legs cannot be ON at the same time in either PWM situation, there is always one winding OFF when using Trapezoidal while the three Sinusoidal legs are pulsed ON at all times throughout the 360 degree motor rotation. A closer lookk at the following operation shows how Sinusoidal interlacing takes place... Low-B Trapezoidal is a 2-Phase Operation High-A Low-A High-B Low-B High C Low-C 8 of 9
* * * * * * * * * * * * * * * * Can a BLDC motor Operate as a Brush Motor As previously mentioned, a BLDC motor can only operate as a Brush motor IF the motor is designed for a one speed, one load situation running at 100% PWM. I have BLDC motor designs that will start a running operation at 3vdc and run all the way up to 48vdc with DO160 Voltage Spike capability up to 100vdc. This allows a pump or fan motor to run at it rated 28vdc, while allowing the load to increase forcing the motor to run at lower speeds without an increase in PWM content. For other requirements, speed control would be necessary, but the hardware design would have to change to accommodate the increase in EMI content. * * * * * * * * * * * * * * * * BLDC BRUSH A Short BLDC & BRUSH Motor Comparison Reduced Sensitivity to Motor Construction Direct Flus and Torque Control High Starting Torque Capability Good to Excellent Speed Regulation at High Speeds and Loads Fast Dynamic Response High Efficiency Low internal heating for the same output power due to windings on the Stator Can be driven by a Linear Sinusoidal controller Lighter than the equivalent Brush motor (does not require a hardware commutator) Must have a Drive Control, Sensored or Sensorless Can be run on DC or PWM voltages Can be run Trapezoidal or Sinusoidal voltages Limited service life of Bearings Low Cost Very Low Speed capability Does not require a Drive Control Commutation by Hardware Heavier than the equivalent BLDC motor Lower Efficiency that the equivalent BLDC Higher internal heating due to windings on the Armature Can be run on DC or PWM voltages Medium Starting Torque requirements Limited service life of Commutation Brushes & Bearings 9 of 9