Discussion: Motors and Generators lab

Discussion: Motors and Generators lab
Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure Outline

Figure 9. Voltage waveform at the output of a PWM inverter with and without overmodulation (after filtering).

The Procedure is divided into the following sections:

? Setup and connections ? Measurement of the rms value of the fundamental-frequency component

in the voltage (unfiltered) at the output of a three-phase PWM inverter ? Overmodulation ? Effect of the frequency on the induction motor speed and magnetizing

current Operation at motor nominal frequency. Operation at 4/3 the motor nominal frequency. Operation at 1/2 the motor nominal frequency.

? Saturation curves

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 the exercise.

Install the equipment in the Workstation.

Mechanically couple the Four-Pole Squirrel Cage Induction Motor to the Four-Quadrant Dynamometer/Power Supply using a timing belt.

PROCEDURE OUTLINE

PROCEDURE

Voltage waveform without overmodulation

Voltage waveform with overmodulation

Sine wave modulating the duty cycle (with overmodulation)

/2 duty cycle = 100%

/2 duty cycle = 0%

Time

V o

lta g

e a

t th

e o

u tp

u t

o f

a t

h re

e -p

h a

se P

W M

in ve

rt e

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

12 © Festo Didactic 86368-00

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.

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 outlet.

3. 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 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.

Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer.

5. Turn the Four-Quadrant Dynamometer/Power Supply on, then set the Operating Mode switch to Dynamometer.

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. 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. 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.

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.

8. Set up the circuit shown in Figure 10. Use the diodes in the Rectifier and Filtering Capacitors to implement the three-phase full-wave rectifier. Use the Chopper/Inverter to implement the Three-phase inverter. Connect the Thermistor Output of the Four-Pole Squirrel Cage Induction Motor to the Thermistor Input of the Four-Quadrant Dynamometer/Power Supply.

Notice that the prefix IGBT has been left out in this manual when referring to the IGBT Chopper/Inverter module.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

Figure 10. Three-phase, variable-frequency induction-motor drive (with three-phase filter).

Measurement of the rms value of the fundamental-frequency component in the voltage (unfiltered) at the output of a three-phase PWM inverter

In this part of the exercise, you will use the Harmonic Analyzer in LVDAC-EMS to measure the rms value of the fundamental-frequency component in the unfiltered voltage at the output of a three-phase PWM inverter. You will also observe that when the load connected to the three-phase PWM inverter output is inductive (like a motor), the current waveform at the output of the three-phase PWM inverter remains sinusoidal even with no filter.

a The Four-Quadrant Dynamometer/Power Supply is not used in this part of the exercise although it is mechanically coupled to the Four-Pole Squirrel Cage Induction Motor.

9. In LVDAC-EMS, open the Chopper/Inverter Control window, then make the following settings:

? Set the Function parameter to Three-Phase PWM inverter.

? Set the Switching Frequency parameter to 3000 Hz.

? Make sure the Phase Sequence parameter is set to Fwd (1-2-3).

? Set the Frequency parameter to the motor nominal frequency indicated on the Four-Pole Squirrel Cage Induction Motor front panel.

Three-phase filter Three-phase

inverter Three-phase

rectifier

Three- phase

induction machine

Switching control signals from DACI

Brake

2 mH

5 F 5 F 5 F

5 F

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

14 © Festo Didactic 86368-00

? Set the Peak Voltage parameter to 100%. This parameter sets the modulation index m, i.e., it sets the amplitude of the sine-wave signal that modulates the duty cycle of the switching control signals. When the Peak Voltage parameter is set to 100%, the amplitude of the modulating signal is set to obtain a peak output voltage corresponding to 100% of half the dc bus voltage (100% of /2). In other words, this sets the value of the modulation index to 1 (m = 1).

? Make sure the Modulation Type parameter is set to Sinusoidal Pulse- Width Modulation.

? Start the Three-Phase PWM Inverter.

10. On the Power Supply, turn the three-phase ac power source on. The motor should start to rotate.

11. In LVDAC-EMS, open the Oscilloscope. Use channel 1 to display the dc bus voltage (input E1), channels 2 and 3 to display the line voltage at the output of the three-phase PWM inverter before and after filtering (inputs E2 and E3, respectively), and channel 4 to display the current flowing in the motor stator windings (input I1).

Select the Continuous Refresh mode, set the time base to display only one complete cycle of the voltage and current waveforms (this provides more precision for observing the trains of rectangular bipolar pulses produced by the three-phase PWM inverter), and set the trigger controls so that the Oscilloscope triggers when the waveform of the motor stator current passes through 0 A with a positive slope.

Select convenient vertical scale and position settings to facilitate observation of the waveforms.

12. 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.

13. Describe the waveform of the voltage at the output of the three-phase PWM inverter before and after filtering.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

14. Measure and record the rms value of the line voltage at the output of the three-phase PWM inverter before and after filtering.

RMS value of the line voltage at the output of the three-phase PWM inverter before filtering: V

RMS value of the line voltage at the output of the three-phase PWM inverter after filtering: V

15. Are the rms values of the line voltage at the output of the three-phase PWM inverter measured before and after filtering using the Oscilloscope equal? Explain briefly.

16. Describe the waveform of the current flowing in each stator winding of the three-phase motor (input I1).

17. In LVDAC-EMS, open the Harmonic Analyzer window, then make the following settings:

? Set the Fundamental Frequency Type parameter to User.

? Set the Fundamental Frequency parameter to the motor nominal frequency indicated on the Four-Pole Squirrel Cage Induction Motor front panel.

? Make sure the Number of Harmonics parameter is set to 40.

? Set the Input parameter to E3.

? Set the Scale Type parameter to V (voltage).

? Make sure the Scale Setting parameter is set to 50 V/div.

? Select the Continuous Refresh mode.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

16 © Festo Didactic 86368-00

18. Measure and record the rms value of the fundamental-frequency component (1f) in the line voltage at the output of the three-phase PWM inverter after filtering (input E3), indicated in the Harmonic Analyzer display.

RMS value of the fundamental-frequency component in the line voltage at the output of the three-phase PWM inverter after filtering: V

a Compare the rms value of the fundamental-frequency component in the line voltage at the output of the three-phase PWM inverter after filtering measured using the Harmonic Analyzer with the rms value of the line voltage measured previously using the Oscilloscope. The values should be identical because the voltage waveform is sinusoidal.

19. In the Harmonic Analyzer window, set the Input parameter to E2 to measure the line voltage at the output of the three-phase PWM inverter before filtering.

Measure and record the rms value of the fundamental-frequency component in the line voltage at the output of the three-phase PWM inverter before filtering (input E2), indicated in the Harmonic Analyzer display.

RMS value of the fundamental-frequency component in the line voltage at the output of the three-phase PWM inverter before filtering: V

a The rms value of the fundamental-frequency component in the line voltage at the output of the three-phase PWM inverter before filtering should be slightly higher than that after filtering because of the voltage drop across the three- phase filter.

20. Do your measurements confirm that the rms value of the fundamental- frequency component in the line voltage at the outputs of the three-phase PWM inverter before filtering can be measured using the Harmonic Analyzer? Explain briefly.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

21. In the Chopper/Inverter Control window, stop the Three-Phase PWM Inverter.

On the Power Supply, turn the three-phase ac power source off.

Remove the three-phase filter from the circuit and disconnect voltage input E3. The circuit should be as shown in Figure 11.

Figure 11. Three-phase, variable-frequency induction-motor drive (without three-phase filter).

22. In the Chopper/Inverter Control window, start the Three-Phase PWM Inverter.

On the Power Supply, turn the three-phase ac power source on.

Measure and record the rms value of the fundamental-frequency component in the line voltage at the output of the three-phase PWM inverter indicated in the Harmonic Analyzer display.

RMS value of the fundamental-frequency component in the line voltage at the output of the three-phase PWM inverter: V

a Compare the rms value of the fundamental-frequency component in the line voltage measured without filter to that measured in step 19 when the filter was in the circuit.

Three-phase inverter

Three-phase full-wave rectifier

Three- phase

induction machine

Switching control signals from DACI

Brake

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

18 © Festo Didactic 86368-00

23. On the Oscilloscope, use channel 1 to display the dc bus voltage (input E1), channel 2 to display the voltage at the output of the three-phase PWM inverter (input E2), and channel 3 to display the current flowing in one of the motor stator windings (input I1).

Select convenient vertical scale and position settings to facilitate observation of the waveforms.

24. 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.

25. Explain why the waveform of the motor stator current displayed on the Oscilloscope screen is sinusoidal even if the filter is removed.

Overmodulation

In this part of the exercise, you will increase the rms value of the line voltage at the output of a three-phase PWM inverter by increasing the amplitude of the sine wave that modulates the duty cycle of the switching transistors of the PWM inverter to a level that causes overmodulation.

a For the remaining of this exercise, the rms value of the line voltage at the output of the three-phase PWM inverter always refers to the rms value of the fundamental-frequency component in the line voltage (i.e., the rms value of the 1f component measured with the Harmonic Analyzer).

26. Compare the rms value of the line voltage at the output of the three-phase PWM inverter measured in step 22 with the nominal line voltage of the Four-Pole Squirrel Cage Induction Motor (indicated on its front panel). Does the maximum line voltage which can be obtained at the outputs of the three- phase PWM inverter without overmodulation (i.e., with modulation index set to 1) allow the induction motor to be operated at its nominal voltage?

? Yes ? No

27. In order to increase the voltage at the output of the three-phase PWM inverter (i.e., the voltage applied to the motor windings), set the Peak Voltage parameter in the Chopper/Inverter Control window to 117% of the dc bus voltage/2 (this corresponds to the maximum Peak Voltage value available in the Chopper/Inverter Control window). This setting increases the amplitude of the sine-wave signal that modulates the duty cycles of the switching control signals to a level that causes overmodulation.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

28. Using the Harmonic Analyzer, measure and record the rms value of the line voltage at the output of the three-phase PWM inverter obtained when overmodulation is used.

RMS value of the line voltage at the output of the three-phase PWM inverter (with overmodulation): V

29. Is the value measured in the previous step close to the nominal line voltage of the Four-Pole Squirrel Cage Induction Motor?

? Yes ? No

30. 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.

31. Compare the voltage waveforms at the output of the three-phase PWM inverter obtained with and without overmodulation (i.e., obtained in step 30 and step 24, respectively). Do you observe that the voltage waveform obtained with overmodulation contains less rectangular pulses, indicating that overmodulation occurs?

? Yes ? No

32. Do your observations confirm that overmodulation allows the rms value of the maximum voltage at the output of a three-phase PWM inverter to be increased?

? Yes ? No

33. In the Chopper/Inverter Control window, stop the Three-Phase PWM Inverter.

Effect of the frequency on the induction motor speed and magnetizing current

In this part of the exercise, you will use the same circuit as in the previous section (see Figure 11) to observe the effect of the frequency on the speed and magnetizing current of the three-phase induction motor.

34. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window, then make the following settings:

? Set the Function parameter to Positive Constant-Torque Prime Mover/Brake.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

20 © Festo Didactic 86368-00

? Make sure that the Torque Control parameter is set to Knob. This allows the torque command of the prime mover/brake to be controlled manually.

? Set the Torque parameter to 0.00 N·m (0.0 lbf·in), thereby ensuring that the Four-Pole Squirrel Cage Induction Motor operates with no mechanical load.

? Set the Pulley Ratio parameter to 24:24. The first and second numbers in this parameter specify the number of teeth on the pulley of the Four-Quadrant Dynamometer/Power Supply and the number of teeth on the pulley of the machine under test, respectively. The pulley ratio between the Four-Quadrant Dynamometer/Power Supply and the Four-Pole Squirrel Cage Induction Motor is 24:24.

? Set the Thermistor Type parameter to LV Type 2. This setting is required to match the characteristics of the thermistor in the Four- Pole Squirrel Cage Induction Motor with the Thermistor Input in the Four-Quadrant Dynamometer/Power Supply.

? Start the Positive Constant-Torque Primer Mover/Brake by setting the Status parameter to Started or by clicking the Start/Stop button.

? Select the Continuous Refresh mode of the meters by clicking on the corresponding button.

Operation at motor nominal frequency

35. In the Chopper/Inverter Control window, make the following settings:

? Make sure the Function parameter is set to Three-Phase PWM inverter.

? Set the Switching Frequency parameter to 2000 Hz.

? Make sure the Phase Sequence parameter is set to Fwd (1-2-3).

? Make sure the Frequency parameter is set to the motor nominal frequency indicated on the Four-Pole Squirrel Cage Induction Motor front panel.

? Set the Peak Voltage parameter to 100%.

? Make sure the Modulation Type parameter is set to Sinusoidal Pulse- Width Modulation.

? Start the Three-Phase PWM Inverter.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

36. The induction motor should rotate at a speed slightly below the synchronous speed of the three-phase induction motor. Record the motor speed of rotation indicated by the Speed meter in the Four-Quadrant Dynamometer/Power Supply window in Table 2.

a The synchronous speed of the Four-Pole Squirrel Cage Induction Motor is 1500 r/min at a local ac power network frequency of 50 Hz and 1800 r/min at a local ac power network frequency of 60 Hz.

Table 2. Effect of the frequency on the induction motor speed and magnetizing current.

Frequency Motor speed of rotation

(r/min)

DC bus voltage

(V)

RMS value of the motor magnetizing

current

(A)

Amplitude of the motor magnetizing

current (peak value)

(A)

Nominal frequency

4/3 the nominal frequency

1/2 the nominal frequency

37. On the Oscilloscope, use channel 1 to display the dc bus voltage (input E1), and channel 2 to display the motor stator current (input I1).

Select convenient vertical scale and position settings to facilitate observation of the waveforms.

38. Measure and record the dc bus voltage as well as the rms value and amplitude of the motor magnetizing current in Table 2.

Operation at 4/3 the motor nominal frequency

39. In the Chopper/Inverter Control window, gradually increase the frequency of the three-phase PWM inverter up to about 4/3 the motor nominal frequency while observing the motor speed and magnetizing current.

Measure and record the motor speed, the dc bus voltage, as well as the rms value and amplitude of the motor magnetizing current in Table 2.

40. Gradually decrease the frequency of the three-phase PWM inverter to the motor nominal frequency.

The stator current meas- ured when an induction motor operates with no mechanical load is virtually equal to the motor magnet- izing current.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

41. Compare the motor speed measured at 4/3 the nominal frequency to that measured at the motor nominal frequency. Does the motor speed increase in direct proportion with the increase in frequency?

? Yes ? No

42. Compare the motor magnetizing current measured at 4/3 the motor nominal frequency to that measured at the motor nominal frequency. How does the motor magnetizing current vary when the frequency is increased? Explain why.

43. How does the motor magnetizing current variation affect the maximum magnetic flux density in the motor?

Operation at 1/2 the motor nominal frequency

In the next steps, the current in the motor stator windings will exceed the nominal value. Perform your manipulations rapidly to prevent motor overheating. Also, make sure that the motor peak current never exceeds four times the nominal current shown on the Four-Pole Squirrel Cage Induction Motor front panel or the motor temperature never exceeds 80°C (176°F).

44. Gradually decrease the frequency of the three-phase PWM inverter down to about 1/2 the motor nominal frequency while observing the motor speed and magnetizing current.

Measure and record the motor speed, the dc bus voltage, as well as the rms value and amplitude of the motor magnetizing current in Table 2.

45. In the Chopper/Inverter Control window, stop the Three-Phase PWM Inverter.

On the Power Supply, turn the three-phase ac power source off.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

46. Compare the motor speed measured at 1/2 the motor nominal frequency to that measured at the motor nominal frequency. Does the motor speed decrease in direct proportion with the decrease in frequency?

? Yes ? No

47. How does the motor magnetizing current vary when the frequency is decreased? Explain why.

48. Compare the magnetizing current measured at 1/2 the motor nominal frequency to that measured at the motor nominal frequency as well as to the motor full-load current rating indicated on the Four-Pole Squirrel Cage Induction Motor front panel. What are the consequences of decreasing the frequency to 1/2 the motor nominal frequency while keeping the voltage constant?

Saturation curves

In this part of the exercise, you will plot the saturation curves of the three-phase induction motor at the motor nominal frequency, 2/3 the nominal frequency, 1/2 the nominal frequency, and 1/3 the nominal frequency.

49. Remove the timing belt that mechanically couples the Four-Pole Squirrel Cage Induction Motor to the Four-Quadrant Dynamometer/Power Supply.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

50. In the Chopper/Inverter Control window, make the following settings.

? Make sure the Function parameter is set to Three-Phase PWM inverter.

? Make sure the Switching Frequency parameter is set to 2000 Hz.

? Make sure the Phase Sequence parameter is set to Fwd (1-2-3).

? Set the Frequency parameter to the motor nominal operating frequency indicated on the Four-Pole Squirrel Cage Induction Motor front panel.

? Set the Peak Voltage parameter to 117%.

? Start the Three-Phase PWM Inverter.

51. On the Power Supply, turn the three-phase ac power source on.

For each value of the Peak Voltage parameter shown in Table 3, measure the rms value of the line voltage applied to the motor stator windings using the Harmonic Analyzer and the peak value of the current flowing in the motor stator windings (peak magnetizing current) using the Oscilloscope (use the horizontal cursors to make the measurement). To obtain optimal results, begin your measurements with the highest Peak Voltage setting as shown in Table 3.

To reduce the data acquisition time and to prevent the motor from overheating, it is strongly recommended to use the Data Table in LVDAC-EMS to record the circuit parameters measured.

Table 3. RMS value of the motor stator winding voltage (at the fundamental frequency) and peak magnetizing current of the three-phase induction motor at various frequencies.

Peak voltage (% of dc bus/2)

(%)

/ / /

Motor line voltage

(V)

Peak magnetizing

current (A)

Motor line voltage

(V)

Peak magnetizing

current (A)

Motor line voltage

(V)

Peak magnetizing

current (A)

Motor line voltage

(V)

Peak magnetizing

current (A)

117

110

100

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Procedure

52. Repeat the previous step at the following three other frequencies: 2 3? , 1/2 , and 1/3 . As mentioned earlier, make sure that the motor peak current never exceeds four times the nominal current indicated on the Four-Pole Squirrel Cage Induction Motor front panel or the temperature never exceeds 80°C (176°F), particularly at 1/2 and 1/3 the nominal motor operating frequency.

a Use the 40 A terminal of input I1 on the DACI and set the Range of input I1 to High in the Data Acquisition and Control Settings window of LVDAC-EMS as soon as the motor stator current reaches 4 A. Temporarily turn the ac power source off before making these changes.

53. Plot on the same graph the saturation curves (rms value of the motor stator winding voltage as a function of the peak magnetizing current) of the Four- Pole Squirrel Cage Induction Motor at each of the frequencies in Table 3.

a You will probably not observe any saturation at nominal frequency because the voltage applied to the motor windings cannot be increased sufficiently.

54. Do the saturation curves you plotted in the previous step show that, when the motor stator winding voltage is increased, the peak magnetizing current increases in similar proportions until the voltage reaches a value at which saturation in the stator core of the induction motor starts to occur, causing the peak magnetizing current to increase much more rapidly than the motor stator winding voltage?

? Yes ? No

55. Do your observations confirm that increasing the frequency allows higher voltages to be applied to the motor stator windings before saturation starts to occur?

? Yes ? No

56. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Conclusion

In this exercise, you learned that a three-phase, variable-frequency induction- motor drive consists of a power diode, three-phase full-wave rectifier and a three- phase PWM inverter. You learned that varying the operating frequency of the PWM inverter varies the frequency of the ac voltage applied to the induction motor, and thus, the motor speed. You also learned that varying the modulation index of the PWM inverter allows the voltage applied to the motor stator windings to be adjusted. You were introduced to the use of overmodulation in the PWM inverter to increase the maximum voltage that can be applied to the motor stator windings. You saw that for a given frequency, the maximum flux density ( .) in the stator of the motor is directly proportional to the rms value of voltage applied to the motor stator windings. You also saw that when the voltage increases and reaches a certain value, saturation occurs in the stator causing the motor magnetizing current to start increasing at a rate which largely exceeds that of the motor voltage. You learned that for a given voltage, the maximum flux density is inversely proportional to the frequency of the ac voltage applied to the motor windings, and consequently, the magnetizing current of the motor decreases when the frequency increases and vice versa. You were introduced to the use of a harmonic analyzer to measure the rms value of the fundamental- frequency component of a non-sinusoidal signal (e.g., the unfiltered voltage at the output of a three-phase PWM inverter).

1. How can the ac voltage at the output of a three-phase PWM inverter be varied?

2. How does the magnetizing current vary when saturation starts to occur in the stator of an induction motor?

3. What should be done for an induction motor to be able to produce the highest possible torque?

4. How do the maximum flux density ( .) and peak magnetizing current of an induction motor vary when the PWM inverter frequency decreases and the voltage at the PWM inverter output (motor stator voltage) remains constant?

CONCLUSION

REVIEW QUESTIONS

Exercise 1 Three-Phase, Variable-Frequency Induction-Motor Drive ? Review Questions

5. Explain why the rms value of the fundamental-frequency component in the voltage (unfiltered) at the output of a three-phase PWM inverter cannot be measured using a conventional voltmeter.

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Discussion: Motors and Generators lab

Discussion: Motors and Generators lab

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