The Single-Phase PWM Inverter with Dual-Polarity DC Bus

Transcription

1 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the singlephase PWM inverter with dual-polarity dc bus. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Operation of a single-phase PWM inverter implemented with a dualpolarity dc bus DISCUSSION You have learned previously (in the manual Single-Phase AC Power Electronics, part number 86359) that a single-phase inverter can be implemented using a four-quadrant chopper (see Figure 16). The four electronic switches of the fourquadrant chopper are switched in pairs, that is, with and with. 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 four-quadrant 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. Four-quadrant chopper Filter + - Load Switching control signal generator Figure 16. Four-quadrant chopper used as a single-phase inverter. Festo Didactic

2 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Discussion The average (dc) output voltage of the four-quadrant chopper during an on-off cycle depends on the time each pair of electronic switches is on during this cycle. This, in turn, depends on the duty cycle of the switching control signals. Modulating the duty-cycle using a sine-wave signal allows a sine wave to be produced at the output of the four-quadrant chopper. Naturally, some filtering is required to obtain a sine wave at the chopper output (see Figure 19). Operation of a single-phase PWM inverter implemented with a dual-polarity dc bus Figure 17 shows a simplified diagram of a single-phase inverter implemented with a dual-polarity dc bus (i.e., a dual-polarity dc power supply). The singlephase PWM inverter consists of two electronic switches ( and ), two freewheeling diodes ( and ), and a switching control signal generator. The operation of the single-phase inverter implemented with a dual-polarity dc bus is quite similar to that of a single-phase inverter implemented using a four-quadrant chopper. However, the single-phase inverter implemented with a dual-polarity dcbus requires only two electronic switches to obtain ac voltage and current at the inverter output. Dual-polarity dc power supply Single-phase inverter 1 Filter 2 N Load Switching control signal generator Figure 17. Single-phase inverter implemented with a dual-polarity dc bus. 36 Festo Didactic

3 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Discussion In single-phase inverters implemented with a dual-polarity dc bus, the two switching control signals are complementary to ensure that when one electronic switch is on, the other is off, and vice versa. For instance, when electronic switch is on, a positive voltage ( ) is applied to the load and current flows in the load in the direction shown in Figure 18a. Conversely, when electronic switch is on, a negative voltage ( ) is applied to the load and current flows in the load in the direction shown in Figure 18b. 1 2 N Load Load current flow 1 2 N Load Load current flow Figure 18. Current flow in a single-phase inverter implemented with two electronic switches. The voltage waveform at the output of the single-phase inverter depends on the signal that modulates the duty cycle of the switching control signals. When a Festo Didactic

4 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Discussion sine-wave signal is used to modulate the duty cycle (i.e., and ) of the switching control signals, the voltage waveform at the single-phase inverter output is a train of rectangular bipolar pulses whose width varies in accordance with the instantaneous voltage of the sine-wave signal. The dashed line drawn over the train of rectangular bipolar pulses in Figure 19 shows the average voltage over each cycle of the rectangular bipolar pulse train at the inverter output. This voltage is an ac voltage having the same form (sinusoidal) as the signal applied to the duty-cycle control input of the single-phase inverter. The range over which the width of the rectangular bipolar pulses at the singlephase inverter output varies, depends on the sine-wave signal (duty-cycle control signal) amplitude. Increasing the sine-wave signal amplitude increases the range of variation of the pulse width, and therefore, the amplitude of the ac voltage at the single-phase inverter output. In other words, the amplitude of the ac voltage at the output of the single-phase inverter is proportional to the modulation index m. The amplitude of the voltage sine wave at the inverter output depends on both the input voltage, i.e.,, and the modulation index m. (3) where is the amplitude of the voltage sine wave at the single-phase PWM inverter output, expressed in V is half the average (dc) voltage at the single-phase PWM inverter input ( ), expressed in V m is the modulation index (pure number) is the average (dc) voltage at the single-phase PWM inverter input, expressed in V Equation (4) shows how the above equation can be modified to calculate the rms value of the voltage sine wave at the single-phase PWM inverter output. (4) The rate at which the pulse width varies at the single-phase inverter output depends on the frequency of the modulating sine-wave signal. Increasing the sine-wave signal frequency increases the rate at which the pulse width varies, and therefore, the frequency of the ac voltage at the single-phase inverter output. In many applications, a voltage whose waveform is a train of rectangular bipolar pulses (instead of a sine wave) can affect the operation of devices sensitive to electromagnetic interference (EMI). For this reason, a filter made of an inductor and a capacitor is usually added at the output of the single-phase inverter to smooth the current and voltage waveforms. This results in a sinusoidal voltage waveform (see Figure 19). The current waveform is similar to the voltage waveform when the load is purely resistive as shown in Figure Festo Didactic

5 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Discussion Duty-cycle control input signal Time Average value Output voltage before filtering Time Output voltage after filtering Time Output current Time Figure 19. Waveforms related to a single-phase PWM inverter implemented with a dual-polarity dc power supply. Festo Didactic

6 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Operation of a single-phase PWM inverter implemented with a dualpolarity dc bus Relationship between output voltage, input voltage, and modulation index Effect of a variation in frequency of the signal that modulates the duty cycle of the switching control signals on the amplitude and frequency of the load voltage. PROCEDURE High voltages are present in this laboratory exercise. Do not make of 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 the exercise. Install the equipment in the Workstation. 2. Make sure that the ac and dc power switches on the Power Supply are set to the O (off) position, then connect the Power Supply to a three-phase ac power outlet. 3. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply. Notice that the prefix IGBT has been left out in this manual when referring to the IGBT Chopper/Inverter module. Connect the Low Power Input of the Chopper/Inverter to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on. 4. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer. 5. 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 is detected. Make sure that the Computer-Based Instrumentation and Chopper/Inverter Control functions for the Data Acquisition and Control Interface are available. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac 40 Festo Didactic

7 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Procedure power network, then click the OK button to close the LVDAC-EMS Start-Up window. 6. Set up the circuit shown in Figure 20. Use the diodes and the capacitors in the Rectifier and Filtering Capacitors to implement the diode rectifier and capacitors and. Use the inductor and the capacitor of the Filtering Inductors/Capacitors module to implement inductor, and capacitor. Resistor is implemented with the Resistive Load module. The resistance value to be used for this resistor depends on your local ac power network voltage (see table in diagram). Dual-polarity dc power supply Single-phase inverter 1 Filter 2 N 3 Switching control signals from DACI Local ac power network Voltage (V) Frequency (Hz) () (mh) (µf) Figure 20. Single-phase PWM inverter implemented with a dual-polarity dc bus. 7. Connect the Digital Outputs of the Data Acquisition and Control Interface (DACI) to the Switching Control Inputs of the Chopper/Inverter using a DB9 connector cable. Festo Didactic

8 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Procedure Connect Switching Control Inputs 1 and 4 of the Chopper/Inverter to Analog Inputs 1 and 2 of the Data Acquisition and Control Interface using miniature banana plug leads. These connections allow the observation of the switching control signals of the electronic switches in the Chopper/Inverter. Connect the common (white) terminal of the Switching Control Inputs on the Chopper/Inverter to one of the two analog common (white) terminals on the Data Acquisition and Control Interface using a miniature banana plug lead. 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. Operation of a single-phase PWM inverter implemented with a dual-polarity dc bus In this part of the exercise, you will use the circuit shown in Figure 20 to observe the operation of a single-phase PWM inverter implemented with a dual-polarity dc bus. You will first observe the switching control signals. Then, you will vary the frequency and the amplitude of the sine-wave signal modulating the duty cycle of the switching control signals to observe the effects they have on the switching control signals as well as the waveforms of the voltage and current at the output of the single-phase PWM inverter. 8. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings: Set the Function parameter to Single-Phase, PWM Inverter. This setting allows the Data Acquisition and Control Interface to generate the switching control signals required by a single-phase PWM inverter implemented with a dual-polarity dc bus. Set the DC Bus parameter to Bipolar. This setting causes the switching control signal generator to produce signals for electronic switches and only. When the DC Bus parameter is set to Unipolar, the switching control signal generator produces signals for electronic switches,,, and as required by singlephase inverters implemented with a single-polarity dc bus. Set the Switching Frequency parameter to 4000 Hz. Set the Frequency parameter to the frequency of your local ac power network. This parameter sets the frequency of the signal that modulates the duty cycle of the switching control signals. Set the Peak Voltage parameter to 50%. This parameter sets the modulation index m, i.e., it sets the amplitude of the signal that modulates the duty cycle of the switching control signals. When the Peak Voltage parameter is set to 50%, the amplitude of the modulating signal is set to make the duty cycle vary over the span (25% to 75%) to obtain a peak output voltage corresponding to 50% of the half dc bus voltage (50% of ). 42 Festo Didactic

9 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Procedure Start the Single-Phase, PWM Inverter. 9. Turn the Power Supply on. 10. In LVDAC-EMS, open the Oscilloscope window and use channels 1 and 2 to display the switching control signals of electronic switches (AI-1) and (AI-2), channel 3 to display the dc bus voltage (E1), channel 4 to display the voltage at the output of the single-phase inverter before filtering (E2), channel 5 to display the voltage at the output of the single-phase inverter after filtering (E3), and channel 6 to display the current flowing through the load (I1). Select the Continuous Refresh mode, set the time base to 2 ms/div, and set the trigger controls so that the Oscilloscope triggers when the load voltage waveform (Ch5) passes through 0 V with a positive (rising) slope. Select convenient vertical scale and position settings to facilitate observation of the waveforms. Finally, set the Oscilloscope so that the waveforms are displayed on the screen using staircase steps (squared display mode). 11. Print or save the waveforms displayed on the Oscilloscope screen for future reference. It is suggested that you include these waveforms in your lab report. 12. Are the switching control signals of electronic switches and displayed by channels 1 and 2 on the Oscilloscope screen complementary? Yes No 13. Is the voltage at the output of the single-phase inverter before filtering (E2), displayed by channel 4 on the Oscilloscope screen, pulse-width modulated as shown in Figure 19? Yes No 14. Is the voltage at the output of the single-phase inverter after filtering (load voltage), displayed on channel 5, sinusoidal as shown in Figure 19? Yes No Festo Didactic

10 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Procedure Relationship between output voltage, input voltage, and modulation index In this step you will vary the modulation index (Peak Voltage parameter) and observe the effect on the amplitude of the voltage at the output of the singlephase PWM inverter. 15. Measure the average (dc) value of the dc bus voltage, i.e., the voltage between the positive and negative terminals of the dual-polarity dc power supply. DC bus voltage ( ): V 16. For each modulation index m in Table 2, calculate the amplitude of the voltage at the output of the single-phase PWM inverter ( ) using the dc bus voltage you measured in the previous step. Record your results in Table 2. Table 2. Relationship between the output voltage, input voltage, and modulation index. DC bus voltage (V) Modulation index m Amplitude of the inverter output voltage [calculated] (V) Amplitude of the inverter output voltage [measured] (V) In the Chopper/Inverter Control window, successively set the modulation index (Peak Voltage parameter) to each value shown in Table 2. For each value, measure the amplitude of the voltage at the output of the single-phase PWM inverter (after filtering) and record the values in the table. Are the amplitude values of the voltage measured at the output of the single phase PWM inverter ( ) approximately equal to the calculated values, confirming that? Yes No 44 Festo Didactic

11 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Conclusion Effect of a variation in frequency of the signal that modulates the duty cycle of the switching control signals on the amplitude and frequency of the load voltage. In this step, you will vary the Frequency parameter in the Chopper/Inverter Control window and observe the effect on the amplitude and frequency of the voltage at the output of the single-phase PWM inverter. 18. In the Chopper/Inverter Control window set the Peak Voltage parameter to 50%. Vary slowly the frequency of the sine-wave signal modulating the duty cycle of the switching control signals (Frequency parameter) from 10 Hz to 120 Hz and from 120 Hz to 10 Hz while observing the amplitude and frequency of the voltage at the output of the single-phase PWM inverter (after filtering). Describe the relationship between the frequency of the sine-wave signal modulating the duty cycle of the switching control signals (Frequency parameter) and the frequency of the voltage at the output of the single-phase PWM inverter (after filtering). 19. Does the frequency of the sine-wave signal modulating the duty cycle of the switching control signals (Frequency parameter) affect the amplitude of the voltage at the output of the single-phase PWM inverter (after filtering)? Yes No 20. From your observations, is it possible to convert dc power into ac power using a single-phase PWM inverter implemented with a dual-polarity dc bus and two electronic switches? Yes No 21. Stop the voltage source and the Single-Phase, PWM Inverter. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables. CONCLUSION In this exercise, you saw that a single-phase PWM inverter can be implemented with a dual-polarity dc bus and only two electronic switches. You observed that the frequency and amplitude of the voltage at the output of a single-phase PWM inverter are respectively proportional to the frequency and amplitude of the sinewave signal modulating the duty cycle. You verified the relationship between the amplitude of the voltage at the inverter output and the average voltage of the dual-polarity dc bus. Festo Didactic

12 Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus Review Questions REVIEW QUESTIONS 1. What type of power supply is required to implement a single-phase PWM inverter with two electronic switches? 2. How can the amplitude and frequency of the voltage at the output of a singlephase PWM inverter be varied? 3. What is the voltage waveform at the output of a single-phase PWM inverter when a sine-wave signal is used to modulate the duty cycle of the switching control signals? 4. What is the use of the filter made of an inductor and a capacitor that is usually added at the output of a single-phase PWM inverter? 5. A single-phase PWM inverter with a dual-polarity dc bus supplies power to a 230 V load. What modulation index is required knowing the voltage between the positive and negative terminals of the dual-polarity dc bus is 800 V? 46 Festo Didactic