Exercise 4 Ripple in Choppers EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with ripple in choppers. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Ripple Ripple versus inductance / capacitance Ripple versus switching frequency DISCUSSION Ripple The residual current or voltage variation at the output of a chopper is called ripple. Figure 34 shows a typical current waveform with ripple at a buck chopper output. Because of ripple, the maximum current value is higher than the average current value. Without ripple, the maximum current and the average current are the same. Output current Max (peak) Average Ripple Time Figure 34. Current ripple at the output of a buck chopper. Ripple voltage is commonly expressed as a peak-to-peak value because it is easy to measure on an oscilloscope and simple to calculate theoretically. Filter circuits intended for the reduction of ripple are usually called smoothing or filtering circuits. The presence of ripple is unwanted because it has many undesirable effects. Some of the effects are: High current ripple increases the maximum current in a circuit. High currents may reduce the life of some electric and electronic components. For instance, high currents reduce the life of the brushes in dc motors, and require high capacity components. Festo Didactic 86356-00 55
Exercise 4 Ripple in Choppers Discussion High current ripple produces electromagnetic interference (disturbance) that can affect the operation of other electrical and electronic circuits. The disturbance can cause interruptions, or limit the performance of some circuits for instance. When the ripple frequency and its harmonics are within the audio band, they may be audible. High voltage ripple increases the maximum voltage in a circuit. High voltages may reduce the life of some electric and electronic components. For instance, high voltages also require components having high breakdown voltages. Ripple versus inductance / capacitance You have seen in the previous exercise that inductors oppose current variations and that capacitors oppose voltage variations. When the electronic switch shown in Figure 35 closes, current starts to flow through inductor and a magnetic field builds up around the coil of the inductor (energy is stored in the magnetic field). While the magnetic field is building up, the current in inductance increases gradually at a rate inversely proportional to the opposition to the current flow produced by inductor (this opposition is proportional to the inductance of the inductor). When electronic switch opens, the magnetic field around the coil collapses gradually. This provides electrical energy that keeps current flowing in inductor until the magnetic field collapsed completely. This explains why current continues to flow even after switch closes, as shown in Figure 35. Larger inductors have a larger inductance and build larger magnetic fields (store more energy), and thus the time taken to build the magnetic field (and to make the magnetic field collapse) is also longer. For this reason, large inductors oppose current variations more than small inductors as shown in Figure 35. The opposition to current flow causes a reduction in ripple. Without inductor in the circuit of Figure 35, the output current waveform and the switching control signal waveform would be similar. In a complete cycle, there would be periods when the current is null and other periods when current is high, two operating conditions resulting in a maximal amount of ripple (this is not desirable). 56 Festo Didactic 86356-00
Exercise 4 Ripple in Choppers Discussion Buck chopper Switching control signal Switching control signal Amplitude (V) 5 on off 0 Time Output current No inductor Small inductor Large inductor Time Figure 35. Opposition to current variations caused by inductors. Similarly, capacitors oppose voltage variations by storing electrical energy. Inside a capacitor, the terminals connect to two metal plates separated by a dielectric (non-conducting material). When a voltage is applied across the plates of a capacitor, an electric field builds up in the dielectric (electrical energy is stored). When the voltage applied to the capacitor is removed, the energy stored in the capacitor supplies the circuit for a period of time. The amount of electrical energy a capacitor can store is proportional to the capacitance. Large capacitors store more energy (i.e., have a higher capacity) than small capacitors and oppose more voltage variations. Increasing the size of capacitors is a means of reducing voltage ripple. Festo Didactic 86356-00 57
Exercise 4 Ripple in Choppers Procedure Outline Ripple versus switching frequency It is not always possible and economical to increase the size of inductors and capacitors to reduce the ripple. As you will observe in the procedure of this exercise, increasing the size of an inductor requires more space and adds weight to a device. Instead of increasing the size of inductors and capacitors, the ripple can also be reduced by increasing the switching frequency. Figure 36 shows a waveform having the same average value as the one shown in Figure 34 but with less ripple. The difference in ripple is caused by an increase in frequency of the switching control signal: doubling the switching frequency reduced the ripple by 50% and the maximum voltage reached by approximately 20%. Increasing the switching frequency is a good way of reducing both the current and voltage ripples. In modern equipment, high switching frequency (10 000 Hz and more) is common practice to minimize ripple. Output current Max (peak) Average Ripple Time Figure 36. Increasing the switching frequency reduces ripple. PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Current ripple versus switching frequency and inductance Voltage ripple versus switching frequency and capacitance 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. 58 Festo Didactic 86356-00
Exercise 4 Ripple in Choppers Procedure 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise. Before installing the Filtering Inductors/Capacitors module in the Workstation, take a look inside the module and observe the difference in size between the two inductors. It is quite evident that the biggest inductor (50 mh) is heavier and requires more space. The difference in size and weight between the two capacitors is also evident. 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. 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 Input 1 of the IGBT Chopper/Inverter to Analog Input 1 of the Data Acquisition and Control Interface using a miniature banana plug lead. 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. Festo Didactic 86356-00 59
Exercise 4 Ripple in Choppers Procedure 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 37. Use the 50 mh inductor in the Filtering Inductors/Capacitors module to implement. In this circuit, resistor is a resistive load and is a smoothing inductor. Chopper/Inverter 50 mh 100 V 57 Switching control signals from digital outputs on DACI Figure 37. Buck chopper circuit with a resistor load and a smoothing inductor. 8. Make the necessary connections and switch settings on the Resistive Load in order to obtain the resistance value required. Current ripple versus switching frequency and inductance In this part of the exercise, you will observe the ripple on the output current of a buck chopper. You will first observe how the ripple varies when the switching frequency is changed and then when the size of the inductor is changed. 60 Festo Didactic 86356-00
Exercise 4 Ripple in Choppers Procedure Current ripple versus frequency 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. 10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings: Select the Buck Chopper (high-side switching) function. Set the switching frequency to 400 Hz. Set the Duty Cycle Control parameter to Knob. Set the duty cycle to 50%. Make sure that the acceleration time is set to 0.0 s. Make sure that the deceleration time is set to 0.0 s. Set the parameter to PWM. Start the buck chopper. 11. In LVDAC-EMS, open the Oscilloscope window and display the following parameters: the voltage at the buck chopper input (input E1), the switching control signal (AI-1), and the load voltage and current (inputs E2 and I2). Select the Continuous Refresh mode, then 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) reaches 2 V. Select convenient vertical scale and position settings in the Oscilloscope to facilitate observation of the waveforms. Figure 38 shows an example of what the Oscilloscope should display. Festo Didactic 86356-00 61
Exercise 4 Ripple in Choppers Procedure Oscilloscope Setting Channel-1 Input... E1 Channel-1 Scale... 100 V/div Channel-1 Coupling... DC Channel-2 Input... AI-1 Channel-2 Scale... 5 V/div Channel-2 Coupling... DC Channel-3 Input... E-2 Channel-3 Scale... 50 V/div Channel-3 Coupling... DC Channel-4 Input... I-2 Channel-4 Scale... 1 A/div Channel-4 Coupling... DC Time Base... 0.5 ms/div Trigger Source... Ch2 Trigger Level... 2 V Trigger Slope... Rising Figure 38. Voltage and current waveforms of a buck chopper operating at 400 Hz with a 50 mh inductor. 12. Determine the maximum (peak) value and the ripple magnitude (peak-topeak value) of the current at the buck chopper output (input I2) using the Oscilloscope. Record your results in the corresponding cells of Table 6. a Use the horizontal cursors of the Oscilloscope to obtain good measuring accuracy. Table 6. Maximum current and current ripple magnitude at the buck chopper output. Inductance (mh) Switching frequency (Hz) Maximum current (A) Current ripple magnitude [peak-to-peak value] (A) 50 400 2000 2 2000 10 000 13. Slowly vary the switching frequency from 400 Hz to 5000 Hz using the Switching Frequency knob in the Chopper/Inverter Control window. If the ambient noise in your laboratory is low, you should be able to hear a tone associated with the switching frequency. 62 Festo Didactic 86356-00
Exercise 4 Ripple in Choppers Procedure 14. In the Chopper/Inverter Control window, set the switching frequency to 2000 Hz. Determine the maximum value and the ripple magnitude of the current at the buck chopper output (input I2) using the Oscilloscope. Record your results in the corresponding cells of Table 6. Change the time base setting as necessary. 15. Compare the ripple magnitude measured at 400 Hz and 2000 Hz (shown in Table 6). Do your results confirm that the ripple decreases when the switching frequency is increased? Yes No 16. What is the effect of a ripple reduction on the maximum value of current at the output of the buck chopper? 17. Stop the buck chopper and the voltage source. Current ripple versus inductance 18. Replace the 50 mh inductor in your circuit with the 2 mh inductor of the Filtering Inductors/Capacitors module to implement. Do not modify the parameters set in LVDAC-EMS. Start the buck chopper and the voltage source. 19. In the Oscilloscope, make sure that convenient vertical scales and positions are set to facilitate observation of the waveforms. Determine the maximum value and the ripple magnitude of the current at the buck chopper output (input I2) using the Oscilloscope. Record your results in the corresponding cells of Table 6. Festo Didactic 86356-00 63
Exercise 4 Ripple in Choppers Procedure 20. Referring to the values in Table 6, compare the magnitude of the ripple measured at 2000 Hz with both inductors. Is the magnitude of the ripple measured using the 2 mh inductor lower or higher than that measured using the 50 mh inductor? 21. Do your results confirm that a large inductor is more effective to smooth the current ripple than a small inductor? Yes No 22. Increase the switching frequency from 2000 Hz to 10 000 Hz. Determine the maximum value and the ripple magnitude of the current at the buck chopper output (input I2) using the Oscilloscope. Record your results in the corresponding cells of Table 6. 23. Compare the magnitude of the ripple measured at 2000 Hz with the 50 mh inductor to that measured at 10 000 Hz with the 2 mh inductor. They should be rather similar. Vary the switching frequency until the magnitude of the current ripple measured using the 2 mh inductor equals that measured using the 50 mh inductor at 2000 Hz. Record the switching frequency at which the current ripple magnitude equals that measured using the 50 mh inductor at 2000 Hz. Switching frequency: 24. Do your measurements confirm that a small inductor can replace a large inductor to reduce the current ripple when the switching frequency is increased? Yes No 25. Stop the buck chopper and the voltage source. 64 Festo Didactic 86356-00
Exercise 4 Ripple in Choppers Procedure Voltage ripple versus switching frequency and capacitance In this part of the exercise, you will observe the ripple on the output voltage of a buck chopper. You will observe how the voltage ripple varies when the switching frequency is changed and when the size of the capacitor is changed. 26. Modify your circuit as shown in Figure 39. Use the 210 µf capacitor in the Filtering Inductors/Capacitors module to implement. In this circuit, resistor is used as a current limiter, resistor is a resistive load, and is a filtering capacitor. Make sure that you connect the positive (+) terminal of the 210 µf capacitor to the junction of resistors and. Chopper/Inverter 86 100 V 210 µf 171 Switching control signals from digital outputs on DACI Figure 39. Buck chopper circuit with a resistive load and a filtering capacitor. 27. Make the necessary connections and switch settings on the Resistive Load in order to obtain the resistance values required. 28. In the Four-Quadrant Dynamometer/Power Supply window, make the following settings: Make sure that the Voltage Source (+) function is selected. Make sure that the voltage is set to 100 V. Start the voltage source. Festo Didactic 86356-00 65
Exercise 4 Ripple in Choppers Procedure 29. In the Chopper/Inverter Control window, make the following settings: Make sure the Buck Chopper (high-side switching) function is selected. Set the switching frequency to 400 Hz. Make sure that the duty cycle is set to 50%. 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 parameter is set to PWM. Start the buck chopper. 30. Make sure that the Oscilloscope is set to display the following parameters: the voltage at the buck chopper input (input E1), the switching control signal (AI-1), the voltage across the resistive load (input E2), and the load current (input I2). Select the Continuous Refresh mode, then set the time base to display at least two complete cycles. Set the trigger controls so that the Oscilloscope triggers when the rising edge of the switching control signal (AI-1) reaches 2 V. Select convenient vertical scale and position settings in the Oscilloscope to facilitate observation of all waveforms. Notice that the ripple magnitude is very low in this part of the exercise. 31. Determine the maximum (peak) value and the ripple magnitude (peak-topeak value) of the voltage across resistive load (input E2) using the Oscilloscope. Record your results in the corresponding cells of Table 7. Table 7. Maximum voltage and voltage ripple magnitude at the buck chopper output. Capacitance (µf) Switching frequency (Hz) Maximum voltage (V) Voltage ripple magnitude [peak-to-peak value] (V) 210 400 2000 5 2000 10 000 66 Festo Didactic 86356-00
Exercise 4 Ripple in Choppers Procedure 32. In the Chopper/Inverter Control window, set the switching frequency to 2000 Hz. Determine the maximum value and the ripple magnitude of the voltage across resistive load (input E2) using the Oscilloscope. Record your results in the corresponding cells of Table 7. 33. Stop the buck chopper and the voltage source. 34. Replace the 210 µf capacitor in your circuit by the 5 µf capacitor of the Filtering Inductors/Capacitors module to implement. In the Chopper/Inverter Control window, Make sure that the switching frequency is set to 2000 Hz. Do not modify the other parameters set in LVDAC-EMS. Start the buck chopper and the voltage source. 35. In the Oscilloscope, make sure that convenient vertical scales and positions are set to facilitate observation of the waveforms. Determine the maximum value and the ripple magnitude of the voltage across resistive load (input E2) using the Oscilloscope. Record your results in the corresponding cells of Table 7. 36. Referring to the values in Table 7, compare the magnitude of the ripple measured using both capacitors at 2000 Hz. Is the magnitude of the ripple measured using the 210 µf capacitor lower or higher than that measured using the 5 µf capacitor? 37. Do your results confirm that a large capacitor is more effective for smoothing the voltage ripple than a small capacitor? Yes No Festo Didactic 86356-00 67
Exercise 4 Ripple in Choppers Conclusion 38. Increase the switching frequency from 2000 Hz to 10 000 Hz. Determine the maximum value and the ripple magnitude of the voltage across resistive load (input E2) using the Oscilloscope. Record your results in the corresponding cells of Table 7. 39. Compare the ripple magnitude measured at 2000 Hz and 10 000 Hz (shown in Table 7) with the 5 µf capacitor. Do your results confirm that the magnitude of the ripple decreases when the switching frequency is increased? Yes No 40. Compare the maximum value of voltage measured at 2000 Hz and 10 000 Hz with the 5 µf capacitor to the magnitude of the ripple measured at each frequency (shown in Table 7). Does your comparison confirm that the maximum value of voltage decreases when the magnitude of the ripple decreases? Yes No 41. Compare the magnitude of the voltage ripple measured at 400 Hz with the 210 µf capacitor to that measured at 10 000 Hz with the 5 µf capacitor. They should be similar. Do your measurements confirm that a small capacitor can replace a large capacitor to smooth the voltage ripple when the switching frequency is increased? Yes No 42. Stop the buck chopper and the voltage source. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables. CONCLUSION In this exercise, you learned that voltage ripple and current ripple are undesired. For instance, high levels of current ripple are associated with high circuit currents that require high capacity components and can also cause premature wear of components. Furthermore, ripple produces disturbances that can affect the operation of electrical circuits. You saw that because inductors oppose current variations, they are widely used to smooth current waveforms. You observed that increasing the size of inductors and increasing the switching frequency are two means of reducing the current ripple. 68 Festo Didactic 86356-00
Exercise 4 Ripple in Choppers Review Questions You also saw that because capacitors oppose voltage variations, they are widely used to smooth (filter) voltage waveforms. You observed that increasing the size of capacitors and increasing the switching frequency are two means of reducing the voltage ripple. REVIEW QUESTIONS 1. Explain why it is important to reduce the ripple in chopper as much as possible? 2. What is a smoothing inductor used for? 3. What can be done to limit the voltage ripple? 4. Should a smoothing capacitor be connected in series or in parallel with the load in a circuit? 5. What can be done to improve the smoothing capability of an inductor? Festo Didactic 86356-00 69