Bidirectional PWM DC Motor Drive with Regenerative Braking

Exercise 2 Bidirectional PWM DC Motor Drive with Regenerative Braking EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with two better types of PWM dc motor drives: the buck-boost chopper dc motor drive and the fourquadrant dc motor drive. You will understand the block diagrams and the modes of operation of those drives which both address a few of the drawbacks of the basic PWM dc motor drive. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Implementing regenerative braking Unidirectional PWM dc motor drive with regenerative braking Bidirectional PWM dc motor drive with regenerative braking One-quadrant, two-quadrant, and four-quadrant dc motor drives DISCUSSION Implementing regenerative braking As observed in Exercise 1, a basic PWM dc motor drive (i.e., a buck-chopper dc motor drive) does not have the capability to brake a dc motor. This is because the current can only flow in one direction (i.e., from the drive to the motor). The direction of the motor torque thus cannot be reversed to oppose the rotation of the load which must slow down on its own due to the friction of the system. In other words, the incapacity to reverse the motor torque makes the operation of the motor as a generator impossible, thereby making electromagnetic braking impossible. Electromagnetic braking is a force opposed to the rotation obtained through the interaction of magnetic fields. It helps in lessening the problem of the loss of speed control and the duration of motor coasting during the deceleration of mechanical loads with significant inertia and small friction torque. When the electrical energy produced through electromagnetic braking is reused immediately or stored in a battery, electromagnetic braking is commonly referred to as regenerative braking. To make regenerative braking possible in a PWM dc motor drive, the dc current through the motor must be able to flow in both directions (from the drive to the motor as before, and from the motor to the drive when required). This can be made possible by replacing the buck chopper in the basic PWM dc drive with a buck-boost chopper. Unidirectional PWM dc motor drive with regenerative braking As can be seen in Figure 8, the block diagram of a unidirectional dc motor drive capable of regenerative braking makes use of a buck-boost chopper (compared Festo Didactic 88553-00 19

Exercise 2 Bidirectional PWM DC Motor Drive with Regenerative Braking Discussion with the block diagram of the basic PWM dc motor drive shown in Figure 2 where a buck chopper was used). A battery is used as the dc power source. This allows the electrical energy recovered during the regenerative braking to be stored in the battery. It is important to notice that, contrary to a typical buck-boost chopper, no inductor is shown in this circuit. This is because the inductance of the motor armature ( ) provides the required inductor for boost-chopper operation. Buck-Boost Chopper Battery DC Motor Remember that is on when is off and viceversa and that a decrease in duty cycle implies that is on for shorter intervals. PWM Generator Duty Cycle Control Input DC Motor Equivalent Diagram Figure 8. Unidirectional PWM dc motor drive with regenerative braking. Regenerative braking is possible with the motor drive shown in Figure 8. When the duty cycle of the buck-boost chopper is decreased, the average dc voltage applied to the motor decreases as well. This causes the armature current to decrease to zero, as is the case when using a buck chopper. However, since the current can flow through a buck-boost chopper in either direction, the polarity of the armature current reverses as the motor makes the current flow through the motor in the opposite direction. This causes the motor to supply power to the buck-boost chopper and acts as a generator. This produces torque at the motor shaft that opposes rotation, thereby braking the motor. The power received by the buck-boost chopper from the motor during braking charges the battery. To sum up, when the dc motor accelerates or runs at a certain speed, the buckboost chopper operates as a buck chopper and dc current (power) flows from the battery to the motor. Conversely, when the dc motor decelerates, the buck-boost chopper operates as a boost chopper and dc current (power) flows in the opposite direction (i.e., from the motor to the battery). 20 Festo Didactic 88553-00

Exercise 2 Bidirectional PWM DC Motor Drive with Regenerative Braking Discussion Bidirectional PWM dc motor drive with regenerative braking A PWM dc motor drive implemented with either a buck chopper or a buck-boost chopper is unidirectional because neither of these devices can reverse the polarity of the voltage applied to the dc motor. To obtain a bidirectional PWM dc motor drive, a four-quadrant chopper can be used as shown below. Four-Quadrant Chopper Battery DC Motor Remember that and are on when and are off and vice-versa and that a decrease in duty cycle implies that and are on for shorter time intervals. PWM Generator Duty Cycle Control Input DC Motor Equivalent Diagram Figure 9. Bidirectional PWM dc motor drive with regenerative braking. The relationship between the dc voltage out of the four-quadrant chopper and the duty cycle is offset relative to that prevailing in the case of the buck chopper and the buck-boost chopper. In the case of the four-quadrant chopper, a duty cycle of 50% corresponds to a null dc voltage and, consequently, to a stopped motor. The formula describing the relation between the dc voltage and the duty cycle for a four-quadrant chopper is: (2) where is the dc voltage at the output of the four-quadrant chopper is the dc voltage at the input of the four-quadrant chopper is the duty cycle of the four-quadrant chopper (i.e., of switches and ) The dc output voltage of the four-quadrant chopper is null for a duty cycle of 50%. As the duty cycle increases from 50%, the dc output voltage becomes of Festo Didactic 88553-00 21

Exercise 2 Bidirectional PWM DC Motor Drive with Regenerative Braking Discussion positive polarity and increases. This causes the dc motor to rotate clockwise and the speed to increase. On the other hand, decreasing the duty cycle from 50% causes the dc output voltage to become of negative polarity and increase. This causes the dc motor to rotate counterclockwise and the speed to increase. Figure 10 summarizes this behavior. Motor rotates clockwise (%) Motor rotates counterclockwise Motor is stopped at 50% Figure 10. Output voltage of a four-quadrant chopper and motor speed and direction of rotation as a function of the duty cycle. With the four-quadrant chopper being a bidirectional device (i.e., current can flow through the device in either direction) like the buck-boost chopper, the bidirectional PWM dc motor drive shown above thus provides regenerative braking. Regenerative braking in the bidirectional PWM dc motor drive operates the same way as in the unidirectional PWM dc motor drive (buck-boost chopper dc motor drive). One-quadrant, two-quadrant, and four-quadrant dc motor drives DC motor drives are often classified according to their ability to operate in the various quadrants of the voltage-current (E-I) plane. The E-I plane is also referred to as the speed-torque (n-t) plane since the speed is proportional to the voltage and the torque is proportional to the current in a dc motor. The basic PWM dc motor drive (made with a buck chopper) is known as a onequadrant dc motor drive because it can operate in a single quadrant (either quadrant I or quadrant III) of the E-I plane. 22 Festo Didactic 88553-00

Exercise 2 Bidirectional PWM DC Motor Drive with Regenerative Braking Discussion I Buck Chopper Quadrant II Quadrant I Generator CCW Rotation Motor CW Rotation Quadrant III Quadrant IV E Motor CCW Rotation Generator CW Rotation Figure 11. Basic (unidirectional) PWM dc motor drive. The unidirectional PWM dc motor drive with regenerative braking (made with a buck-boost chopper) is known as a two-quadrant dc motor drive because it can operate in two quadrants of the E-I plane (either quadrants I and IV or quadrants II and III). I Buck-Boost Chopper Quadrant II Quadrant I Generator CCW Rotation Motor CW Rotation Quadrant III Quadrant IV E Motor CCW Rotation Generator CW Rotation Figure 12. Unidirectional PWM dc motor drive with regenerative braking. Festo Didactic 88553-00 23

Exercise 2 Bidirectional PWM DC Motor Drive with Regenerative Braking Discussion The bidirectional PWM dc motor drive with regenerative braking (made with a four-quadrant chopper) is known as a four-quadrant dc motor drive, as it can operate in all four quadrants of the E-I plane. Four-Quadrant Chopper Quadrant II I Quadrant I Generator CCW Rotation Motor CW Rotation Quadrant III Quadrant IV E Motor CCW Rotation Generator CW Rotation Figure 13. Bidirectional PWM dc motor drive with regenerative braking. The bidirectional PWM dc motor drive with regenerative braking eliminates two of the shortcomings of the basic PWM drive: It allows rotation of the motor in both directions and it prevents motor coasting during decelerations. However, it still exhibits poor speed regulation and it may be subject to overcurrents during accelerations. These two shortcomings will be studied further and solved in the next exercise of this manual. 24 Festo Didactic 88553-00

Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Regenerative braking in a PWM dc motor drive Bidirectional PWM dc motor drive with regenerative braking Shortcomings of the bidirectional PWM dc motor drive with regenerative braking PROCEDURE High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified. Set up and connections In this part of the exercise, you will set up and connect the equipment. 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform the exercise. Install the equipment in the Workstation. a a Make sure that the Permanent Magnet DC Motor is installed to the right of the Four-Quadrant Dynamometer/Power Supply. Before beginning this exercise, measure the open-circuit voltage across the Lead-Acid Battery Pack, Model 8802, using a multimeter. If the open-circuit voltage is lower than 51.2 V, ask your instructor for assistance as the Lead- Acid Battery Pack is probably not fully charged. Appendix D of this manual indicates how to prepare (fully charge) the Lead-Acid Battery Pack before each laboratory period. 2. Mechanically couple the Four-Quadrant Dynamometer/Power Supply to the Permanent Magnet DC Motor using a timing belt. Before coupling rotating machines or working on them, make absolutely sure that power is turned off to prevent any machine from starting inadvertently. 3. Make sure that the main power switch on the Four-Quadrant Dynamometer/Power Supply is set to the O (off) position, then connect its Power Input to an ac power wall outlet. 4. Connect the Power Input of the Data Acquisition and Control Interface (DACI) to a 24 V ac power supply. Connect the Low Power Input of the Chopper/Inverter to the Power Input of the DACI. Turn the 24 V ac power supply on. Festo Didactic 88553-00 25

5. Connect the USB port of the DACI to a USB port of the host computer. Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer. 6. Turn the Four-Quadrant Dynamometer/Power Supply on, then set the Operating Mode switch to Dynamometer. 7. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure that the DACI and the Four- Quadrant Dynamometer/Power Supply are detected. Make sure that the Computer-Based Instrumentation and Chopper/Inverter Control functions for the DACI are available. Also, select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window. 8. Connect the Digital Outputs of the DACI to the Switching Control Inputs of the Chopper/Inverter using a DB9 connector cable. On the Chopper/Inverter, set the Dumping switch to the O (off) position. The Dumping switch is used to prevent overvoltage on the dc bus of the Chopper/Inverter. It is not required in this exercise. 9. Set up the circuit shown in Figure 14, which is a unidirectional PWM dc motor drive with regenerative braking. Use the Lead-Acid Battery Pack as a fixedvoltage dc power source for the unidirectional PWM dc motor drive with regenerative braking. Make sure to use the 40 A terminal of current input I2 of the DACI. Set the range of current input I2 to High (40 A) in the Data Acquisition and Control Settings window of LVDAC-EMS. 26 Festo Didactic 88553-00

IGBT Chopper/Inverter 40 A Battery Pack (48 V) Permanent Magnet DC Motor Mechanical Load Switching Control Signals from the DACI Figure 14. Unidirectional PWM dc motor drive with regenerative braking (buck-boost chopper dc motor drive). 10. In LVDAC-EMS, open the Chopper/Inverter Control window, then make the following settings: Set the Function parameter to Buck/Boost Chopper. Set the Switching Frequency parameter to 5 khz. Make sure the Duty Cycle Control parameter is set to Knob. This allows the duty cycle of the buck-boost chopper to be adjusted manually using a control knob. a Make sure the Duty Cycle [Q1] parameter is set to 0%. A typical switching frequency for a buck-boost chopper is around 20 khz. The switching frequency is set to 5 khz in this exercise to allow the observation of the motor voltage and current waveforms using the Oscilloscope without aliasing effect and without having too much audible noise. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window. In the Tools menu of this window, select Friction Compensation Calibration, which will bring up the Friction Compensation Calibration dialog box. Click OK in this box to start the calibration process. Observe that the prime mover starts to rotate at high speed, thereby driving the permanent magnet dc motor. The prime mover speed is then automatically decreased by steps to perform the calibration process. Once the calibration process is completed (which takes about two minutes), the prime mover stops rotating, then the Friction Compensation Calibration dialog box indicates that the calibration process is finished. Click OK in the Friction Compensation Calibration dialog box to close this box. Restart the Four-Quadrant Dynamometer/Power Supply to apply the changes (i.e., the newly calibrated Festo Didactic 88553-00 27

friction compensation curve) by setting the main power switch of this module to O (off), and then I (on). 11. In the Four-Quadrant Dynamometer/Power Supply window, make the following settings: Set the Function parameter to Mechanical Load. This makes the Four-Quadrant Dynamometer/Power Supply operate like a configurable mechanical load. Set the Load Type parameter to Flywheel. This makes the mechanical load emulate a flywheel. Set the Inertia parameter to 0.050 kgm 2 (1.187 lbft 2 ). This sets the inertia of the emulated flywheel. Set the Friction Torque parameter to 0.06 Nm (0.53 lbfin). This sets the torque which opposes rotation of the emulated flywheel. a Set the Pulley Ratio parameter to 24:12. Note that the pulley ratio between the Four-Quadrant Power Supply/Dynamometer and the Permanent Magnet DC Motor is 24:12. Start the mechanical load. The dc motor is now coupled to a flywheel emulated by the mechanical load. Regenerative braking in a PWM dc motor drive In this part of the exercise, you will use the unidirectional PWM dc motor drive with regenerative braking to power the dc motor and you will observe its behavior as the duty cycle is changed. 12. In LVDAC-EMS, open the Metering window. Set six meters to measure the dc battery voltage (input E1), the dc battery current (input I1), the battery power (measured from inputs E1 and I1), the dc armature voltage (input E2), the dc armature current (input I2), and the electric power at the dc motor (measured from inputs E2 and I2). Click the Continuous Refresh button to enable continuous refresh of the values indicated by the various meters in the Metering window. Note that in the Four-Quadrant Dynamometer/Power Supply window you can observe the dc motor speed, torque, and mechanical power. Click the Continuous Refresh button to enable continuous refresh of the values indicated by the various meters in the Four-Quadrant Dynamometer/Power Supply window. 13. In LVDAC-EMS, open the Oscilloscope window. Make the appropriate settings to observe the waveforms of the motor armature voltage and current (inputs E2 and I2, respectively). 28 Festo Didactic 88553-00

Click the Continuous Refresh button to enable continuous display refresh of the waveforms shown in the Oscilloscope window. 14. In the Chopper/Inverter Control window, start the buck-boost chopper (i.e., the unidirectional PWM dc motor drive with regenerative braking) by clicking the Start/Stop button. Gradually increase the duty cycle of the buckboost chopper to 40% to make the motor rotate. a Increasing the duty cycle in large increments might cause an overcurrent condition to happen in the IGBT Chopper/Inverter module. If so, stop the drive, set the duty cycle to 0%, press the Overcurrent Reset button on the IGBT Chopper/Inverter module and start the manipulation over using smaller duty cycle increments. 15. In LVDAC-EMS, open the Data Table window. Set the timer to make 300 records with an interval of 1 second between each record. This corresponds to a 5 minute period. Set the data table to record the duty cycle of the buck-boost chopper, the dc armature voltage, the dc armature current, the electric power at the dc motor, as well as the dc motor speed torque and mechanical power. Also record the battery dc voltage, the battery dc current, and the battery power. Finally, set the data table to record the time associated with each record. Start the timer to begin recording data. 16. Suddenly increase the duty cycle of the buck-boost chopper from 40% to 80% and wait for the motor speed to stabilize. Once the motor speed has stabilized, suddenly decrease the duty cycle from 80% to 40%. Wait again for the motor speed to stabilize. In the Data Table window, stop the timer, then save the recorded data. In the Chopper / Inverter Control window, set the buck-boost chopper duty cycle to 0% then stop the unidirectional PWM dc motor drive. In the Four- Quadrant Dynamometer/Power Supply window, stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating. 17. Plot to graphs the evolution of the chopper duty cycle, motor speed, dc armature current, and electric power at the dc motor as a function of time using the data you saved to a file. Observe the evolution of the different parameters. What happens to the dc armature current as the dc motor decelerates? In which direction is electric power flowing during the deceleration? Festo Didactic 88553-00 29

18. Plot to another series of graphs the evolution of the chopper duty cycle, motor torque, and mechanical power as a function of time using the data you saved to a file. Observe the evolution of the different parameters. What happens to the motor torque and mechanical power as the dc motor decelerates? How is the motor operating during the deceleration? 19. Plot to another series of graphs the evolution of the chopper duty cycle, dc battery current, and battery power as a function of time using the data you saved to a file. Observe the evolution of the different parameters. What happens to the dc battery current and battery power as the dc motor decelerates? What does it imply for the charge of the battery? Based on your observations at steps 17, 18, and 19, explain how regenerative braking is achieved in a PWM dc motor drive. 20. Determine the motor deceleration time (i.e., the time required to reach a steady-state motor speed when the duty cycle is decreased to 40%) from the graphs you plotted in step 17. How does this deceleration time compare to the deceleration time obtained when using a basic PWM dc motor drive with the same load parameters (refer to your answer at step 22 in Exercise 1)? 30 Festo Didactic 88553-00

Bidirectional PWM dc motor drive with regenerative braking In this part of the exercise, you will use a bidirectional PWM dc motor drive with regenerative braking to power the dc motor and you will observe its behavior as the duty cycle is changed. 21. Make sure the chopper and mechanical load are stopped. 22. Set up the circuit shown in Figure 15. Use the Lead-Acid Battery Pack as a fixed-voltage dc power source for the bidirectional PWM dc motor drive with regenerative braking. Make sure to use the 40 A terminal of current input I2 of the DACI. Also, make sure that the range of current input I2 is set to High (40 A) in the Data Acquisition and Control Settings window of LVDAC-EMS. IGBT Chopper/Inverter 40 A Battery Pack (48 V) Mechanical Load Permanent Magnet DC Motor Switching Control Signals from the DACI Figure 15. Bidirectional PWM dc motor drive with regenerative braking (four-quadrant chopper dc motor drive). 23. In the Chopper/Inverter Control window, make the following settings: Set the Function parameter to Four-Quadrant Chopper. Set the Switching Frequency parameter to 5 khz. Make sure the Duty Cycle Control parameter is set to Knob. Make sure the Duty Cycle (Q1) parameter is set to 50%. Festo Didactic 88553-00 31

24. In the Oscilloscope window, make the appropriate settings to observe the operating point of the PWM dc motor drive in the E-I plane. To do so, set the X-Y and X-Y Average parameters to On in the Display section of the Oscilloscope Settings. This sets the oscilloscope in the X-Y mode. Then, assign the motor armature voltage (input E2) to channel X (i.e., channel 1) and the armature current (input I2) to channel Y (i.e., channel 2). Click the Continuous Refresh button to enable continuous display refresh of the waveforms shown in the Oscilloscope window. 25. In the Four-Quadrant Dynamometer/Power Supply window, start the mechanical load. The dc motor is now coupled to a flywheel emulated by the mechanical load. 26. Set the data table to record the duty cycle of the four-quadrant chopper, the dc armature voltage, the dc armature current, the dc motor speed, and the dc motor torque. 27. In the Chopper/Inverter Control window, start the four-quadrant chopper (i.e., the bidirectional PWM dc motor drive) by clicking the Start/Stop button. Increase the duty cycle of the four-quadrant chopper from 50% to 100% in 5% increments. At each step, observe the display of the oscilloscope and the values of the various parameters indicated by the meters. Record the data in the data table at each step. In which direction is the dc motor rotating when the duty cycle of the fourquadrant chopper goes from 50% to 100%? In which quadrant(s) is the bidirectional PWM dc motor drive then operating? 28. In the Chopper/Inverter Control window, decrease the duty cycle from 100% to 75% to decrease the dc motor speed. Observe the meters and the oscilloscope display as you do so. Does regenerative braking work with the bidirectional PWM dc motor drive? Yes No As the motor decelerates, in which quadrant does the dc motor drive operate? Decrease the duty cycle from 75% to 50% to stop the dc motor. 32 Festo Didactic 88553-00

29. In the Chopper/Inverter Control window, decrease the duty cycle of the fourquadrant chopper from 50% to 0% in 5% steps. At each step, observe the display of the oscilloscope and the values of the various parameters indicated by the meters. Record the data in the data table at each step. Save the recorded data (for the complete range of duty cycles ranging from 0% to 100%). In which direction is the dc motor rotating when the duty cycle of the fourquadrant chopper goes from 50% to 0%? In which quadrant(s) is the bidirectional PWM dc motor drive then operating? 30. In the Chopper/Inverter Control window, increase the duty cycle from 0% to 25% to decrease the dc motor speed. Observe the meters and the oscilloscope display as you do so. Does regenerative braking work with the bidirectional PWM dc motor drive no matter what the direction of rotation is? Yes No As the motor decelerates, in which quadrant does the dc motor drive operate? In the Chopper/Inverter Control window, set the four-quadrant chopper duty cycle to 50%, then stop the bidirectional PWM dc motor drive. In the Four- Quadrant Dynamometer/Power Supply window, stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating. 31. Plot to graphs the relationship between the dc armature voltage and the chopper duty cycle, as well as the relationship between the motor speed and the chopper duty cycle using the data you saved to a file. How does the motor speed evolve as the chopper duty cycle increases from 50% to 100%? In which direction does the dc motor turn? How does the motor speed evolve as the chopper duty cycle decreases from 50% to 0%? In which direction does the dc motor turn? Festo Didactic 88553-00 33

Is the four-quadrant chopper dc motor drive bidirectional? Yes No Shortcomings of the bidirectional PWM dc motor drive with regenerative braking In this part of the exercise, the maximum dc armature current drawn during the acceleration of the dc motor is observed, along with other parameters, for given conditions. The motor speed is also analyzed for different values of the load torque. 32. In the Four-Quadrant Dynamometer/Power Supply window, start the mechanical load (i.e., the emulated flywheel). In the Chopper/Inverter Control window, start the bidirectional PWM dc motor drive and slowly increase the four-quadrant chopper duty cycle from 50% to 70%. Let the motor speed stabilize. 33. In the Data Table window, make sure that the timer is set to make 300 records with an interval of 1 second between each record. This corresponds to a 5 minute period. Set the data table to record the duty cycle of the four-quadrant chopper, the dc armature voltage, the dc armature current, the dc motor speed, and the dc motor torque. Also, set the data table to record the time associated with each record. Start the timer to begin recording data. 34. Suddenly increase the duty cycle of the four-quadrant chopper from 70% to 90% and wait for the motor speed to stabilize. Once the motor speed has stabilized, suddenly decrease the duty cycle from 90% to 70%. Wait again for the motor speed to stabilize. In the Data Table window, stop the timer, then save the recorded data. In the Chopper/Inverter Control window, set the four-quadrant chopper duty cycle to 50%, then stop the bidirectional PWM dc motor drive. In the Four- Quadrant Dynamometer/Power Supply window, stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating. 35. Plot to graphs the evolution of the chopper duty cycle, motor speed, motor torque, and dc armature current as a function of time using the data you saved to a file. Observe the evolution of the different parameters. 34 Festo Didactic 88553-00

What value does the dc armature current ( ) reach during the motor acceleration? How does this value compare to the nominal armature current indicated on the front panel of the Permanent Magnet DC Motor? Is the bidirectional PWM dc motor drive susceptible to causing overcurrents? 36. In the Four-Quadrant Dynamometer/Power Supply window, set the inertia and the friction torque of the flywheel to 0.010 kgm 2 (0.237 lbft 2 ) and 0.1 Nm (0.89 lbfin). Start the mechanical load. In the Chopper/Inverter Control window, start the bidirectional PWM dc motor drive and progressively increase the duty cycle of the four-quadrant chopper to 75%. Wait for the motor speed to stabilize and note its value: Speed of the motor (torque = 0.1 Nm (0.89 lbfin)) r/min Increase the friction torque of the flywheel to 0.3 Nm. Wait for the motor speed to stabilize and note its value: Speed of the motor (torque = 0.3 Nm (2.66 lbfin)) r/min Increase the friction torque of the flywheel to 0.5 Nm. Wait for the motor speed to stabilize and note its value: Speed of the motor (torque = 0.5 Nm (4.43 lbfin)) r/min In the Chopper/Inverter Control window, set the four-quadrant chopper duty cycle to 50%, then stop the bidirectional PWM dc motor drive. In the Four- Quadrant Dynamometer/Power Supply window, stop the mechanical load (i.e., the emulated flywheel). Wait for the motor to stop rotating. Does the bidirectional PWM dc motor drive exhibit good speed regulation? Explain briefly. 37. In the Tools menu of the Four-Quadrant Dynamometer/Power Supply window, select Reset to Default Friction Compensation. This will bring up the Reset Friction Compensation dialog box. Click Yes in this window to reset the friction compensation to the factory default compensation. Festo Didactic 88553-00 35

Exercise 2 Bidirectional PWM DC Motor Drive with Regenerative Braking Conclusion 38. Close LVDAC-EMS, then turn off all equipment. Remove all leads and cables. Make sure the Lead-Acid Battery Pack is recharged promptly. CONCLUSION This exercise introduced two more sophisticated types of dc motor drives. The unidirectional PWM dc motor drive with regenerative braking is made with a buck-boost chopper and, as it name implies, allows regenerative braking on top of controlling the rotation speed of the dc motor. The bidirectional PWM dc motor drive with regenerative braking is made with a four-quadrant chopper and adds the possibility to rotate the dc motor in both the clockwise and counterclockwise directions to the features of the unidirectional PWM dc motor drive with regenerative braking. It was shown, however, that even the bidirectional PWM dc motor drive has drawbacks: It has poor speed regulation and it offers no protection against overcurrents at the motor armature. The next exercises will explore methods to circumvent the different drawbacks of the bidirectional PWM dc motor drive. REVIEW QUESTIONS 1. Which type of chopper is used in a unidirectional PWM dc motor drive with regenerative braking? 2. Which type of chopper is used in a bidirectional PWM dc motor drive with regenerative braking? 3. In which quadrant(s) can a basic PWM dc motor drive (buck-chopper dc motor drive) operate? 4. What is regenerative braking? 5. What is required of a chopper to obtain a bidirectional dc motor drive? 36 Festo Didactic 88553-00