Exercise 8 The Four-Quadrant Chopper EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the operation of the four-quadrant chopper. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: The Four-Quadrant Chopper DISCUSSION The Four-Quadrant Chopper The dc-to-dc converters discussed so far, that is, the buck chopper, the boost chopper, and the buck/boost chopper, allow a dc voltage to be converted into a dc voltage of the same polarity and having a lower or higher value. Furthermore, in the buck and boost choppers, the current can flow in one direction only, whereas in the buck/boost chopper, it can flow in either direction. Another type of dc-to-dc converter, the four-quadrant chopper, allows a dc voltage to be converted into a dc voltage having a lower voltage value as well as current flow in either direction. Furthermore, in the four-quadrant chopper, the polarity of the converted dc voltage can be reversed with respect to the polarity of the original voltage. Therefore, a four-quadrant chopper can provide a positive or a negative dc voltage regardless of the direction in which current flows. This feature allows the four-quadrant chopper to operate in any of the four quadrants of a voltage-current Cartesian plan as shown in Figure 53. Voltage (V) Quadrant II Quadrant I Current (A) Quadrant III Quadrant IV Figure 53. Operation in any of the four quadrants. Festo Didactic 86356-00 117
Exercise 8 The Four-Quadrant Chopper Discussion In Figure 53, four dots have been drawn, one in each quadrant. Each of these dots represents a possible operating point of the four-quadrant chopper. Drawing straight lines from any one of these dots to the horizontal and vertical axes (current and voltage axes, respectively) allows the dc current and voltage at the output of the four-quadrant chopper to be determined. For example, the dot in Quadrant 3 indicates that the current and voltage are equal to -2 A dc and -60 V dc, respectively. If the operating point of the four-quadrant chopper passes from the dot in Quadrant 3 to the dot in Quadrant 4, the current and voltage change to +2 A dc and -60 V dc, respectively. This indicates that the direction in which current flows has been reversed while the voltage remains the same. Figure 54 shows a four-quadrant chopper built with electronic switches and diodes, and some waveforms related to the circuit. The switching control signals show that the electronic switches are switched in pairs, that is, with and with, and that when one pair of electronic switches is on, the other pair is off. Therefore, the input voltage is alternately applied to the output of the fourquadrant chopper through either one of the two pairs of electronic switches. The instantaneous polarity of the output voltage depends on which pair of electronic switches is on. It is positive when electronic switches and are on, and negative when electronic switches and are on. The average (dc) output voltage of the four-quadrant chopper depends on the time each pair of electronic switches is on during each cycle. This, in turn, depends on the duty cycle of the switching control signals. The equation relating voltages and is written below. (8) where is the dc voltage at the four-quadrant chopper output. is the dc voltage at the four-quadrant chopper input. is the duty cycle expressed as a decimal (50% = 0.5). In this equation, is the duty cycle of the switching control signals applied to electronic switches and in the four-quadrant chopper shown in Figure 54. Since the duty cycle can vary from approximately 0 to 100%, the voltage can vary from approximately - to +. 118 Festo Didactic 86356-00
Exercise 8 The Four-Quadrant Chopper Discussion and Switching control signals and Switching control signals Output Voltage ( ) Output Current ( ) Amplitude Amplitude Amplitude Amplitude 5 0 5 0 0 - I 0 and on and off and off and on and on and off Time Time Time Time Figure 54. Operation of the four-quadrant chopper. Varying the frequency of the switching control signals while maintaining the duty cycle constant does not affect the dc voltage and current at the four-quadrant chopper output. However, the ripple magnitude decreases as the frequency of the switching control signals increases. Festo Didactic 86356-00 119
Exercise 8 The Four-Quadrant Chopper Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Switching control signals of a four-quadrant chopper Operation of a four-quadrant chopper Partially discharging the batteries in the Lead-Acid Battery Pack Demonstrating four-quadrant operation 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. Setup 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 this exercise. Make sure that the batteries in the Lead-Acid Battery Pack are fully charged by measuring the open-circuit voltage with a voltmeter. If the open-circuit voltage is lower than 51 V, ask your instructor for assistance as the battery pack is probably not fully-charged. Appendix C of this manual indicates how to prepare (charge) the Lead-Acid Battery Pack before each laboratory period. Install the required equipment in the Workstation. 2. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply. Connect the Low Power Input of the IGBT Chopper/Inverter to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on. 3. Connect the USB port of the Data Acquisition and Control Interface 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. 120 Festo Didactic 86356-00
Exercise 8 The Four-Quadrant Chopper Procedure 4. Make sure that the main power switch of the Four-Quadrant Dynamometer/ Power Supply is set to O (off), then connect the Power Input to an ac power outlet. Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power Supply to Power Supply. Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main power switch to I (on). 5. Connect the Digital Outputs of the Data Acquisition and Control Interface (DACI) to the Switching Control Inputs of the IGBT Chopper/Inverter using a DB9 connector cable. Connect Switching Control Inputs 1, 2, 4, and 5 of the IGBT Chopper/Inverter to Analog Inputs 1, 2, 4, and 5, respectively, of the Data Acquisition and Control Interface using miniature banana plug leads. Connect the common (white) terminal of the Switching Control Inputs on the IGBT Chopper/Inverter to one of the two analog common (white) terminals on the Data Acquisition and Control Interface using a miniature banana plug lead. 6. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition and Control Interface and the Four-Quadrant Dynamometer/Power Supply are detected. Make sure that the Computer-Based Instrumentation and Chopper/Inverter Control functions for the Data Acquisition and Control Interface are available, as well as the Standard Functions (C.B. control) for the Four-Quadrant Dynamometer/Power Supply. 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. 7. Set up the circuit shown in Figure 55. Use the 2 mh inductor of the Filtering Inductors/Capacitors module to implement. Festo Didactic 86356-00 121
Exercise 8 The Four-Quadrant Chopper Procedure Chopper/Inverter 2 mh 100 V 171 Switching control signals from digital outputs on DACI Figure 55. Four-quadrant chopper circuit. 8. Make the necessary connections and switch settings on the Resistive Load in order to obtain the resistance value required. Switching control signals of a four-quadrant chopper In this part of the exercise, you will observe the switching control signals and see that the electronic switches in a four-quadrant chopper are switched in pairs ( with and with ), and that when one pair of electronic switches is on, the other pair is off. 9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window and make the following settings: Select the Voltage Source (+) function. Set the voltage to 100 V. Start the voltage source. 122 Festo Didactic 86356-00
Exercise 8 The Four-Quadrant Chopper Procedure 10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings: Select the Four-Quadrant Chopper function. Set the switching frequency to 1000 Hz. Set the Duty Cycle Control to Knob. Set the duty cycle to 25%. Make sure that the acceleration time is set to 0.0 s. Make sure that the deceleration time is set to 0.0 s. Make sure that the,,, and parameters are set to PWM. Start the four-quadrant chopper. 11. In LVDAC-EMS, open the Oscilloscope window and display the four switching control signals (AI-1, AI-2, AI-4, and AI-5). Select the Continuous Refresh mode, set the time base to display two complete cycles, and set the trigger controls so that the Oscilloscope triggers when the rising edge of the switching control signal (AI-1) of electronic switch reaches 2 V. Select convenient vertical scale and position settings in the Oscilloscope to facilitate observation of the waveforms. 12. Referring to the waveforms of the switching control signals displayed on the Oscilloscope, describe the switching sequence of the electronic switches in the four-quadrant chopper. 13. Referring to the waveform of the switching control signals, does the duty cycle setting that you have made in the Chopper/Inverter Control window correspond to the duty cycle of the switching control signals of electronic switches and or to the duty cycle of the switching control signals of electronic switches and? Festo Didactic 86356-00 123
Exercise 8 The Four-Quadrant Chopper Procedure Operation of a four-quadrant chopper In this part of the exercise, you will calculate the average (dc) output voltage of a four-quadrant chopper for various duty cycles and compare the results with the measured average (dc) output voltages using the circuit in Figure 55. You will observe that the dc voltage can be either positive or negative and that the current can flow in both directions at the output of a four-quadrant chopper. You will also observe the effect of the switching frequency on the voltage conversion. 14. Considering an input voltage equal to 100 V and using the equation, calculate the dc voltage which should appear at the output of the four-quadrant chopper for the following duty cycles: 25%, 50%, and 75%. Table 14. Calculated dc output voltages of a four-quadrant chopper for various duty cycles. Duty cycle (%) DC output voltage (V) 25 50 75 15. In the Oscilloscope window, display the voltage and current (inputs E1 and I1) at the four-quadrant chopper input, the switching control signal (AI-1) of electronic switch, and the voltage and current (inputs E2 and I2) at the four-quadrant chopper output. 16. In the Data Acquisition and Control Settings of LVDAC-EMS, set the range of E4 to High. This corresponds to a 0 V to 800 V range. 17. In the Chopper/Inverter Control window, set the switching frequency to 15 000 Hz, and then successively set the duty cycle to each of the values in Table 15. For each value, measure the dc voltage and current at the input (inputs E1 and I1) and output (inputs E2 and I2) of the four-quadrant chopper. Record your results in the corresponding cells of Table 15. Table 15. DC voltages and currents in a four-quadrant chopper. DC input voltage (V) DC input current (A) Duty cycle (%) DC output voltage (V) DC output current (A) 25 50 75 124 Festo Didactic 86356-00
Exercise 8 The Four-Quadrant Chopper Procedure 18. Explain why the dc voltage and current at the four-quadrant chopper output are almost null when the duty cycle is 50%. 19. Does the polarity of the dc currents measured in step 16 confirm that the current can flow in both directions in a four-quadrant chopper? Yes No 20. Does the polarity of the dc voltages at the output of the four-quadrant chopper measured in step 16 confirm that the voltage can be either positive or negative? Yes No 21. Do the voltages calculated in step 14 and measured in step 16 confirm that in a four-quadrant chopper? Yes No 22. Determine the range of the dc output voltage of the four-quadrant chopper by varying the duty cycle from 0% to 100%. DC output voltage when the duty cycle is set to 0%: DC output voltage when the duty cycle is set to 100%: 23. In the Chopper/Inverter Control window, set the duty cycle to 80%. Vary the switching frequency from 15 000 Hz to 1000 Hz while observing the voltage and current waveforms at the output of the four-quadrant chopper. Does the switching frequency have a significant effect on the dc voltage and current the four-quadrant chopper supplies? If so, describe this effect. 24. Stop the four-quadrant chopper and the voltage source. Festo Didactic 86356-00 125
Exercise 8 The Four-Quadrant Chopper Procedure Partially discharging the batteries in the Lead-Acid Battery Pack In this part of the exercise, you will partially discharge the batteries in the Lead- Acid Battery Pack. The batteries must be partially discharged in the next part of the exercise to obtain the desired operating point and make the appropriate observations. 25. Perform procedure step 37 and step 38 in Exercise 5. Demonstrating four-quadrant operation You observed that the dc voltage can be either positive or negative and that the current can flow in both directions at the output of a four-quadrant chopper. However, you observed the operation of the four-quadrant chopper in quadrants 1 and 3 only. In this part of the exercise, you will use the circuit shown in Figure 56 to observe the operation of the chopper in the four quadrants. You will use the Oscilloscope in the X-Y mode to observe in which quadrant the fourquadrant chopper operates. 26. Set up the circuit shown in Figure 56. Chopper/Inverter 2 mh 100 V 50 V Switching control signals from digital outputs on DACI Figure 56. Circuit used to demonstrate four-quadrant operation. 27. In the Four-Quadrant Dynamometer/Power Supply window, make sure that the source voltage is set to 100 V, and start the voltage source. 126 Festo Didactic 86356-00
Exercise 8 The Four-Quadrant Chopper Procedure 28. In the Chopper/Inverter Control window, make the following settings: Make sure that the Four-Quadrant Chopper function is selected. Set the switching frequency to 20 000 Hz. Set the duty cycle to 75%. Make sure that the acceleration time is set to 0.0 s. Make sure that the deceleration time is set to 0.0 s. Make sure that the,,, and parameters are set to PWM. Start the four-quadrant chopper. 29. In the Oscilloscope window, make the following settings: Set Channel 1 (X) to display the four-quadrant chopper output current (input I2) and set the sensitivity to 1 A/div. The input selected in Channel 1 is used as the X-axis parameter in the X-Y display mode of the Oscilloscope. Set Channel 2 (Y) to display the four-quadrant chopper output voltage (input E2) and set the sensitivity to 20 V/div. The input selected in Channel 2 is used as the Y-axis parameter in the X-Y display mode of the Oscilloscope. Turn the X-Y and X-Y Average parameters on. Select the Continuous Refresh mode. 30. In LVDAC-EMS, open the Data Table window. Under the Options tab, open the Record Settings dialog box, select Oscilloscope in the Settings field, then select Channel 1 AVG and Channel 2 AVG as the parameters to be recorded. Close the Record Settings dialog box. 31. Make sure that the average (dc) output current (input I2) is null. Slightly readjust the voltage of the voltage source if necessary. In this operating condition, the average (dc) output voltage of the four-quadrant chopper is positive and equal to the battery voltage plus or minus the voltage drop across inductor. In the Chopper/Inverter Control window, vary the duty cycle between 70% and 80% by 1% increments. For each setting, observe the location of the dot in the Oscilloscope display and record the average (dc) output current (Channel 1 AVG) and average (dc) output voltage (Channel 2 AVG) of the four-quadrant chopper in the data table. Do not exceed 4.5 A at the output of the four-quadrant chopper. Manually record the duty cycle corresponding to each setting in the Data Table. Festo Didactic 86356-00 127
Exercise 8 The Four-Quadrant Chopper Procedure 32. Stop the four-quadrant chopper and the voltage source. 33. Reverse the battery connections in the circuit as shown in Figure 57. Chopper/Inverter 2 mh 100 V 50 V Switching control signals from digital outputs on DACI Figure 57. Circuit used to demonstrate four-quadrant operation (with reversed battery connections). 34. In the Chopper/Inverter Control window, set the duty cycle to 25% then start the four-quadrant chopper and the voltage source. Make sure that the average (dc) output current (input I2) is null. Slightly readjust the voltage of the voltage source if necessary. In this operating condition, the average (dc) output voltage of the four-quadrant chopper is negative and equal to the battery voltage plus or minus the voltage drop across inductor. In the Chopper/Inverter Control window, vary the duty cycle between 30% and 20% by 1% increments. For each setting, observe the location of the dot in the Oscilloscope display and record the average (dc) output current (Channel 1 AVG) and average (dc) voltage (Channel 2 AVG) of the fourquadrant chopper in the data table. Make sure not to exceed 4.5 A at the output of the four-quadrant chopper. Manually record the duty cycle corresponding to each setting in the data table. 35. Stop the four-quadrant chopper and the voltage source. 128 Festo Didactic 86356-00
Exercise 8 The Four-Quadrant Chopper Conclusion 36. Transfer your recorded data into a spreadsheet application, and plot on a same graph, a curve of the dc output voltage (Y axis) versus dc output current (X axis) when the duty cycle varies from 70% to 80%, and a curve of the dc output voltage (Y axis) versus dc output current (X axis) when the duty cycle varies from 30% to 20%. 37. Do the curves you plotted confirm that the four-quadrant chopper can operate in any of the four quadrants? Yes No 38. Referring to the curves you plotted, determine the duty cycle at which the current changes polarity. 39. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables. CONCLUSION In this exercise, you observed that the electronic switches are switched in pairs in a four-quadrant chopper, and that when one pair of electronic switches is on, the other pair is off. You observed that the voltage at the input of the four-quadrant chopper is alternately applied to its output through each pair of electronic switches and that the polarity of the output voltage alternates depending on which pair of electronic switches is on. You also observed that the four-quadrant chopper can provide a positive or a negative average (dc) voltage regardless of the direction in which current flows, thereby allowing four-quadrant operation. REVIEW QUESTIONS 1. Referring to Figure 54, is the voltage applied to the load positive or negative when electronic switches and are switched on? 2. Determine the output voltage variation that can be observed when the duty cycle of a four-quadrant chopper varies from 0% to 100%? Festo Didactic 86356-00 129
Exercise 8 The Four-Quadrant Chopper Review Questions 3. Referring to Figure 54, briefly describe the four-quadrant chopper operation. 4. A four-quadrant chopper, which is supplied by a +200 V dc power supply, provides -24 V dc at its output. What is the duty cycle? 5. What is the main feature which distinguishes the four-quadrant chopper from the buck/boost chopper? 130 Festo Didactic 86356-00