Thyristor Power Electronics, 2 Power Diode Three-Phase ... · Exercise 2 – Power Diode...

© Festo Didactic 86363-00 27

When you have completed this exercise, you will be familiar with three-phase half-wave and full-wave rectifiers. You will be familiar with the waveforms of voltages and currents present in these rectifiers. You will know how to calculate the average dc voltage provided by each type of rectifier. You will know the advantages of three-phase rectifiers over single-phase rectifiers.

The Discussion of this exercise covers the following points:

Three-phase half-wave rectifier (positive-polarity output)

Three-phase half-wave rectifier (negative-polarity output)

Three-phase full-wave rectifier


A three-phase half-wave rectifier with a positive-polarity output converts three-phase ac voltage into positive dc voltage. The rectifier consists of three diodes connected between a three-phase ac power source and a load

(resistor ), as Figure 18 shows.

Figure 18. Three-phase half-wave rectifier (positive-polarity output).

Figure 19 shows the waveforms of the circuit voltages and currents in the three-phase half-wave rectifier. The rectifier output voltage is the voltage measured at point X with respect to the neutral terminal N of the three-phase ac power source. Therefore, .

Each diode conducts current when the voltage at its anode is higher than the voltage at its cathode. Whenever a diode stops conducting, another diode immediately starts conducting. Thus, the forward current is interrupted and transferred from one diode to another. The sudden switchover from one diode to another is called natural commutation. Natural commutation occurs at the following phase angles: 30°, 150°, and 270°.

Power Diode Three-Phase Rectifiers

Exercise 2

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Line terminals 1, 2, and 3

Three-phase ac power source

Load

Neutral terminal

Exercise 2 – Power Diode Three-Phase Rectifiers Discussion

28 © Festo Didactic 86363-00

Figure 19. Waveforms of voltages and currents in the three-phase half-wave rectifier (positive-polarity output).

Phase voltages

( )

Diode current

( )

Diode current

( )

Diode current

( )

Rectifier output

current

( )

Rectifier output

voltage

( )

30

90 210 Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

150 270

330

30

90 210

150 270

330

30 150 30 150

150 270 150 270

30 270 30 270

30 150 270 30 150 270

30 210150 270 330 30 90 210150 270 330

(V)

(A)

(A)

(A)

(A)

(V)



The circuit operates as described below:

Initially, i.e., at phase angle 0°, diode is conducting and diodes and are blocked. When the phase angle reaches 30°, the voltage at the

anode of diode (phase voltage ) becomes higher than the voltage at its cathode (phase voltage ). Therefore, diode enters into conduction (this causes diode to stop conducting) and current starts flowing from point X toward the neutral terminal N via the load (resistor ). Consequently, the rectifier output voltage follows the

positive peak of phase voltage . This situation lasts until the phase angle reaches 150°. Between phase angles 30° and 150°, diodes and

are blocked since the voltages (phase voltages and , respectively) present at their anodes are both lower than the voltage

(phase voltage ) present at their cathodes.

When the phase angle reaches 150°, the voltage at the anode of

diode (phase voltage ) becomes higher than the voltage at its cathode (phase voltage ). Therefore, diode enters into conduction

(this causes diode to stop conducting) and current starts flowing from point X toward the neutral terminal N via the load (resistor ).

Consequently, the rectifier output voltage follows the positive peak of phase voltage . This situation lasts until the phase angle reaches 270°. Between phase angles 150° and 270°, diodes and are

blocked since the voltages (phase voltages and , respectively) present at their anodes are both lower than the voltage (phase

voltage ) present at their cathodes.

When the phase angle reaches 270°, the voltage at the anode of

diode (phase voltage ) becomes higher than the voltage at its cathode (phase voltage ). Therefore, diode enters into conduction

(this causes diode to stop conducting) and current starts flowing from point X toward the neutral terminal N via the load (resistor ).

Consequently, the rectifier output voltage follows the positive peak of phase voltage . This situation lasts until the phase angle reaches 30° of the subsequent cycle. Between phase angles 270° and

30°, diodes and are blocked since the voltages (phase voltages and , respectively) present at their anodes are both

lower than the voltage (phase voltage ) present at their cathodes.

Each diode allows current to flow through resistor during equal intervals

of 120°. Therefore, the waveforms of the rectifier output current and voltage (and ) are composed of three positive pulses of equal duration (120° phase interval each) per cycle of the ac source voltage. The rectifier output voltage

varies between the maximum positive values of the phase voltage ( ) and

( ) This implies that the ripple (amplitude of the pulses) in the output

voltage of a three-phase half-wave rectifier is 50% lower than the ripple in the output voltage of single-phase rectifiers. Furthermore, the ripple frequency of the three-phase half-wave rectifier output voltage is 180 Hz, compared to 60 Hz for a single-phase half-wave rectifier, and 120 Hz for a single-phase full-wave rectifier. The lower ripple amplitude and higher ripple frequency result in a smoother voltage at the output of the three-phase rectifier. A smoother voltage is an important advantage in high-power rectifier circuits because this permits the use of smaller semiconductor devices with lower power ratings. Neglecting the voltage drops across the diodes in the three-phase half-wave rectifier, the



amplitude of the rectifier output voltage is equal to the amplitude (positive

maximum value) of the phase voltage of the three-phase ac power

source. The average value of the rectifier output voltage is:

(1)

where is the amplitude of the phase voltage.

is the rms value of the line-to-line voltage.

To conclude, the three-phase half-wave rectifier acts like three single-phase half-wave rectifiers (one for each phase) operating one after another. The phase currents , , and delivered by the three-phase ac power source,

which are respectively equal to currents , , and , are asymmetrical, i.e., they have a non-null average (dc) value. This results in dc current flow through the ac power source, i.e., through the electrical power network, which is highly undesirable.




A three-phase half-wave rectifier with a negative-polarity output converts three-phase ac voltage into negative dc voltage. Figure 20 shows the circuit diagram of a three-phase half-wave rectifier with a negative-polarity output. The circuit is identical to that studied in the previous section of this discussion, except that the diodes are connected in the opposite direction. Its operation is thus very similar to that of the three-phase half-wave rectifier with a positive-polarity output.

Figure 20. Three-phase half-wave rectifier (negative-polarity output).

a The + and - signs next to voltage in the figure indicate the convention of measurement of this voltage. The value of is negative when the voltage at the + terminal of load resistor is lower than the voltage at the - terminal of this resistor (e.g., when voltage ).

Figure 21 shows the waveforms of the circuit voltages and currents in the three-

phase half-wave rectifier of Figure 20. Each diode allows current ( ) to flow from the neutral terminal N toward point Y through the load resistor during equal intervals of 120°. The waveform of the rectifier output voltage therefore follows the negative peaks of the phase voltages of the three-phase source. This voltage waveform is thus composed of three negative pulses (120° interval each) per cycle of the ac source voltage, the voltage varying between the maximum

negative values of the phase voltage ( ) and .

Line terminals


Neutral terminal



Figure 21. Waveforms of voltages and currents in the three-phase half-wave rectifier (negative-polarity output).

Phase voltages

( ) 30

90 210 Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

150 270

330

30

90 210

150 270

330

30 150 270 30 150 270

210 330 210 330

90 330 90 330

90 210 90 210

90 210 330 90 210 330

30 150 270 30 150 270

90 210 330 90 210 330

(V)

(A)

(A)

(A)

(A)

(V)

Diode current

( )

Diode current

( )

Diode current

( )

Rectifier output

current

( )

Rectifier output

voltage

( )

Waveform obtained with a three-phase half-wave rectifier with a positive-polarity output (shown for comparison)

Waveform obtained with a three-phase half-wave rectifier with a positive-polarity output (shown for comparison)



The average value of the rectifier output voltage is equal

to or .

Notice that the phase angle intervals during which the diodes conduct current in the three-phase half-wave rectifier with a negative-polarity output differ from the phase angle intervals during which the diodes conduct in the three-phase rectifier with a positive-polarity output. This causes the pulses in the output voltage of the three-phase half-wave rectifier with a negative-polarity output to be offset 60° with respect to the pulses in the output voltage of the three-phase half-wave rectifier with a positive-polarity output.

Three-phase full-wave rectifier

The three-phase full-wave rectifier, also called a three-phase bridge rectifier, is the most commonly used in industrial applications. The circuit can be viewed as a combination of a three-phase half-wave rectifier with a positive-polarity output and a three-phase half-wave rectifier with a negative-polarity output, as Figure 22a shows. This circuit can be redrawn as shown in Figure 22b (usual representation of a three-phase full-wave rectifier). Terminal N is the neutral conductor of the source. Figure 23 shows the waveforms of the circuit voltages and currents.

The diodes successively conduct current by pairs, one pair after another during equal intervals of 60°, as indicated in Table 1. During each interval, a diode ( ,

, or ) conducts current from X toward the neutral point N through resistor , while another diode ( , , or ) conducts current from the neutral point N towards Y through resistor .

Table 1. Conducting diodes for each 60° interval.

Angular interval Conducting diodes

30° - 90° and

90° - 150° and

150° - 210° and

210° - 270° and

270° - 330° and

330° - 30° and

For example, when the phase angle is between 30° and 90° diode conducts current since the voltage ( ) at its anode is higher than the voltages ( and

) at the anodes of diodes and . This current, , flows from X toward N

through resistor . On the other hand, diodes and are blocked since the

voltages ( and ) at their anodes are lower than the voltage ( ) at their

cathodes. The voltage between X and N ( ) therefore follows the positive

peak of phase voltage . Meanwhile, diode also conducts current since the

voltage ( ) at its cathode is lower than the voltages ( and ) at the

cathodes of diodes and . This current, , flows from N toward Y through resistor . On the other hand, diodes and are blocked since the voltages

( and ) at their cathodes are higher than the voltage ( ) at their



anodes. The voltage between Y and N ( ) therefore follows the negative peak

of phase voltage .

Figure 22. Three-phase full-wave rectifier.

Three-phase

ac power source

Line terminals


Line terminals

(a) A three-phase full-wave rectifier can be viewed as a combination of a three-phase half-wave rectifier with a positive-polarity output and a three-phase half-wave rectifier with a negative polarity output.

(b) Usual representation of a three-phase full-wave rectifier. Terminal N is the neutral conductor of the source.



Figure 23. Waveforms of voltages and currents in the three-phase full-wave rectifier.

The current resulting from the successive conduction of diodes , , and causes the waveform of the voltage between X and N ( ) to follow the positive peaks of the phase voltages of the three-phase source. The waveform of

voltage is therefore identical to that produced by a three-phase half-wave rectifier with a positive-polarity output.

Similarly, the current resulting from the successive conduction of diodes ,

, and causes the waveform of the voltage between Y and N ( ) to follow

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase voltages

( ) 30

90 210

150 270

330

30

90 210

150 270

330

Order of conduction of

the diodes

Voltages ,

and 30

90 210

150 270

330

30

90 210

150 270

330

30 90 210150 270 330 30 90 210 150 270 330

30 90 210150 270 330 30 90 210 150 270 330

Rectifier output

current

Rectifier output

voltage



the negative peaks of the phase voltages of the three-phase ac power source.

The waveform of voltage is therefore identical to that produced by a three-phase half-wave rectifier with a negative-polarity output.

The output voltage of the three-phase full-wave rectifier is equal to the sum of (or ). The rectifier output voltage waveform is, therefore,

a pulsating positive voltage made of six pulses per cycle. The average value of

the rectifier output voltage is:

(2)

where is the amplitude of the phase voltage.

is the rms value of the line-to-line voltage.

The average current flowing to or from the neutral terminal N is null.

Thus, .Therefore, the neutral (N) conductor of the three-

phase source is not necessary for proper operation of the three-phase full-wave rectifier. This conductor is shown in Figure 22 to assist in the explanation of circuit operation. Figure 24 shows the rectifier circuit diagram without the neutral conductor. The two load resistors ( in Figure 22) have been replaced by a

single resistor .

Figure 24. Three-phase full-wave rectifier without the neutral conductor.

The ripple amplitude in the output voltage and current of a three-phase full-wave rectifier is lower than that observed in a three-phase half-wave rectifier. Also, the ripple frequency (360 Hz) is twice that observed in a three-phase half-wave rectifier. Therefore, three-phase full-wave rectifiers are preferred to three-phase half-wave rectifiers because they provide a smoother output voltage and current.

Notice that in the three-phase full-wave rectifier, the currents delivered by the three-phase source are symmetrical, i.e., they have a null average (dc) value, which is the normally desired condition. Figure 25 shows the currents flowing through the diodes, the currents delivered by the source, and the rectifier output current. The average (dc) value of each of the currents delivered by the source

( , , and ) is null.


Line terminals



Figure 25. Waveforms of the diode currents, source currents, and rectifier output current.

Phase angle (°)30 90 210150 270 330 30 90 210 150 270 330

Rectifier output

current

Current

( )

Current

( )

Current

( )

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

30

90 210

270 30

90 210

270

90

150 270

330 90

150 270

330

30

210

150

330

30

210

150

330

90 210 90 210

90 330 90 330

210 330 210 330

30 270 30 270

150 270 150 270

30 150 30 150

Current

Current

Current

Current

Current

Current

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

Exercise 2 – Power Diode Three-Phase Rectifiers Procedure Outline


The Procedure is divided into the following sections:

Set up and connections



Three-phase full-wave (bridge) rectifier

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.

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. 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 LVDAM-EMS Start-Up window, make sure that the Data Acquisition and Control Interface is detected. Make sure that the Computer-Based Instrumentation function for the Data Acquisition and Control Interface is 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 LVDAM-EMS Start-Up window.

PROCEDURE OUTLINE

PROCEDURE

Exercise 2 – Power Diode Three-Phase Rectifiers Procedure



In this part of the exercise, you will set up a three-phase half-wave rectifier with a positive-polarity output. You will observe the waveforms of voltages and currents in the rectifier. You will measure the frequency (ripple) of the rectified voltage, the conduction angle of the diodes, as well as the average values of the rectified voltage, current, and power. You will compare your results to those previously obtained in Exercise 1 with a single-phase full-wave rectifier.

6. Set up the circuit shown in Figure 26. In this circuit, is the three-phase ac power source of the Power Supply (Model 8823). E1 through E4 and I1 through I4 are voltage and current inputs of the Data Acquisition and Control Interface. The three diodes are those in the Rectifier and Filtering Capacitors

module. 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).

Local ac power network voltage

(V)

( )

120 171

220 629

240 686

Figure 26. Three-phase half-wave rectifier with a positive-polarity output (observation of voltage and current waveforms and measurement of parameters).

7. Turn the Power Supply on by setting the ac power switch to I (on).

N

L1

L2

L3



8. In LVDAC-EMS, start the Oscilloscope and make the necessary settings to display the phase voltages (E1, E2, and E3) and phase currents (I1, I2, and I3) of the three-phase ac power source on channels 1, 2, 3, 4, 5, and 6, respectively. Also, display the rectifier output current (I4) and rectifier output voltage (E4) on channels 7 and 8, respectively. Set the time base to display at least two cycles of the sine waves.

9. Describe the waveforms of the rectifier output current and rectifier output voltage with respect to the waveforms of the source phase voltages, and explain.

Notice that the phase currents delivered by the source, which are

respectively equal to diode currents , , and , are asymmetrical, i.e.,

they have a non-null average (dc) value. This results in dc current flow through the ac power source, i.e., through the electrical power network, which is highly undesirable.

10. During the positive peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.



During the positive peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

During the positive peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

11. Evaluate the conduction angle of the diodes from the waveforms of

currents , , and . Record the conduction angle of the diodes in the

space provided. Then, compare this angle to that obtained in the previous exercise for a single-phase full-wave rectifier.

Conduction angle of the diodes °

12. Measure and record the ripple frequency at the output of the three-phase half-wave rectifier (positive polarity output). Then, compare this ripple frequency to that obtained in the previous exercise for a single-phase full-wave rectifier.

Ripple frequency Hz



13. In LVDAC-EMS, open the Metering window. Set meters E4 and I4 to

measure the average (dc) values of the rectifier output voltage and rectifier output current , respectively. Record these values below.

Then, calculate the rectifier output power from the average values of

voltage and current .

Average rectifier output voltage V

Average rectifier output current A

Rectifier output power W

Compare the average output voltage of the three-phase half-wave rectifier (positive polarity output) to that obtained in the previous exercise for a single-phase full-wave rectifier.

14. Set meter E1 to measure the rms value of phase voltage . Record this

value below.

V

Calculate the maximum positive value of phase voltage . Record this

value below.

V

Compare to the rectifier output voltage measured in the

previous step. Is approximately equal to ?

Yes No

Calculate the line-to-line voltage and record your result.

V



Yes No

15. On the Power Supply, turn the three-phase ac power source off by setting the corresponding switch to O (off).




In this part of the exercise, you will set up a three-phase half-wave rectifier with a negative polarity output. You will observe the waveforms of voltages and currents in the rectifier. You will measure the frequency (ripple) of the rectified voltage, the conduction angle of the diodes, as well as the average values of the rectified voltage, current, and power.

16. Set up the circuit shown in Figure 27. The circuit is identical to that studied in the previous section of the procedure, except that the diodes are connected in the opposite direction (i.e., the other three diodes in the Rectifier and

Filtering Capacitors module are used). Resistor is implemented with the Resistive Load module. The resistance value to be used for resistor depends on your local ac power network voltage (see table in diagram).

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


(V)

( )

120 171

220 629

240 686

Figure 27. Three-phase half-wave rectifier with a negative-polarity output (observation of voltage and current waveforms, and measurement of parameters).

18. On the Oscilloscope, make sure that the proper settings are made to display the phase voltages (E1, E2, and E3) and phase currents (I1, I2, and I3) of the three-phase ac power source on channels 1, 2, 3, 4, 5, and 6, respectively. Also, display the rectifier output current (I4) and rectifier output voltage (E4) on channels 7 and 8, respectively. Set the time base to display at least two cycles of the sine waves.

N

L1

L2

L3




Notice that the phase currents delivered by the source, which are

respectively equal to diode currents , , and , are asymmetrical,

i.e., they have a non-null average (dc) value. This results in dc current flow through the ac power source, i.e., through the electrical power network, which is highly undesirable.

20. During the negative peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

During the negative peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.



During the negative peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

21. Evaluate the conduction angle of the diodes from the waveforms of

currents , , and . Record below the conduction angle of the diodes.

Conduction angle of the diodes °

Compare this angle to that previously obtained for a three-phase half-wave rectifier with a positive-polarity output (recorded in step 11). Are they the same?

Yes No

22. Measure and record the ripple frequency at the output of the three-phase half-wave rectifier (negative polarity output).

Ripple frequency = Hz

Compare this ripple frequency to that previously obtained for a three-phase half-wave rectifier with a positive-polarity output (recorded in step 12). Are they the same?

Yes No

23. In the Metering window, make sure meters E4 and I4 are set to measure the

average (dc) values of the rectifier output voltage and rectifier output current , respectively. Record these values below.

Then, calculate the rectifier output power from the average values of voltage and current .






Compare the average output voltage of the three-phase half-wave rectifier (negative-polarity output) to that previously obtained for a three-phase half-wave rectifier with a positive-polarity output (recorded in step 13). Are they approximately equal (neglect the voltage polarity)?

Yes No

24. Make sure meter E1 is set to measure the rms value of phase voltage . Record this value below.

V

Calculate the maximum negative value of phase voltage . Record this value below.

V



Yes No


V

Compare to the rectifier output voltage measured in the previous

step. Is approximately equal to ?

Yes No

25. Is the operation of the three-phase half-wave rectifier with a negative-polarity output very similar to that of the three-phase half-wave rectifier with a positive-polarity output? Do these rectifiers have the same conduction angles, ripple frequencies, and average output voltages? Explain.

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



Three-phase full-wave (bridge) rectifier

In this part of the exercise, you will set up a three-phase full-wave rectifier. You will observe the waveforms of voltages and currents in the rectifier. You will measure the frequency (ripple) of the rectified voltage, the conduction angle of the diodes, as well as the average values of the rectified voltage, current, and power. You will compare your results to those previously obtained with three-phase half-wave rectifiers.

26. Set up the circuit shown in Figure 28. In this circuit, is the three-phase ac power source of the Power Supply (Model 8823). E1 through E4 and I1 through I4 are voltage and current inputs of the Data Acquisition and Control Interface. The six diodes are those in the Rectifier and Filtering Capacitors

module. Resistors and are implemented with the Resistive Load module. The resistance values to be used for these resistors depend on your local ac power network voltage (see table in diagram).

Use two resistors in series for the rectifier output load. (The resistance value to be used

for each resistor is indicated in the table). If a single resistor is used, the nominal voltage

of the resistor will be greatly exceeded.




(V)

,

( )

120 171

220 629

240 686

Figure 28. Three-phase full-wave rectifier (observation of voltage and current waveforms and measurement of parameters).

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

28. On the Oscilloscope, make the necessary settings to display the phase voltages (E1, E2, and E3) and phase currents (I1, I2, and I3) of the three-phase ac power source on channels 1, 2, 3, 4, 5, and 6, respectively. Also, display the rectifier output current (I4) and rectifier output voltage (E4) on channels 7 and 8, respectively. Set the time base to display at least two cycles of the sine waves.

N

L1

L2

L3




Observe the waveforms of the phase currents delivered by the source, i.e.,

, , and . These currents are respectively equal to ,

and . These currents are symmetrical, i.e., they have a null

average (dc) value, which is the normal operating condition desired.

30. Measure and record the ripple frequency at the output of the three-phase full-wave rectifier.

Ripple frequency Hz

Compare this ripple frequency to that previously obtained for a three-phase half-wave rectifier with a positive- or negative-polarity output. Is the ripple frequency of a three-phase full-wave rectifier twice that of a three-phase half-wave rectifier, resulting in a smoother voltage at the output of the three-phase full-wave rectifier?

31. In the Metering window of LVDAC-EMS, make sure meters E4 and I4 are set to measure the average (dc) values of the rectifier output voltage and

rectifier output current , respectively. Record these values in the following blanks.

Then, calculate the rectifier output power from the average output voltage and current .






Compare the average output voltage of the three-phase full-wave rectifier to that previously obtained for a three-phase half-wave rectifier with a positive- or negative-polarity output (recorded in steps 13 and 23, respectively).

32. Set meter E1 to measure the rms value of phase voltage . Record this value below.

V

Calculate the maximum positive value of phase voltage . Record this value below.

V



Yes No


V

Compare to the rectifier output voltage measured in the previous

step. Is approximately equal to ?

Yes No

33. On the Power Supply, turn the three-phase ac power source off. Close LVDAC-EMS. Disconnect all leads and return them to their storage location.

Exercise 2 – Power Diode Three-Phase Rectifiers Conclusion


In this exercise, you studied the operation of three-phase half-wave and full-wave rectifiers. You learned that a three-phase half-wave rectifier uses three diodes to provide a dc voltage composed of three pulses of equal duration per cycle of the ac source voltage. This voltage can be either positive or negative, depending on the direction in which the diodes are connected. This rectifier has a narrower conduction angle (120°) and a higher ripple frequency (three times the network frequency) than single-phase half-wave and full-wave rectifiers, and thus, provides a smoother output voltage. However, the three-phase half-wave rectifier has the following drawback: the phase currents delivered by the source have a non-null average (dc) value, which results in a flow of dc current through the electrical load, but also through the electrical power network, which is highly undesirable. This drawback is eliminated with the use of a three-phase full-wave rectifier. This rectifier uses six diodes to provide a dc voltage composed of six pulses of equal duration per cycle of the ac source voltage. This rectifier provides twice the average voltage of a three-phase half-wave rectifier. Furthermore, this voltage is smoother than of a three-phase half-wave rectifier and the phase currents delivered by the source have a null average (dc) value, which is the normal operating condition desired.

1. What is a three-phase half-wave rectifier with a positive-polarity output? How does it work? Describe the waveform of the rectifier output voltage with respect to the waveform of the source voltage waveform.

2. Is the output voltage of three-phase half-wave rectifiers smoother than the output voltage of single-phase rectifiers? Explain why by comparing the amplitude of the ripple in these voltages, and the ripple frequency of these voltages.

CONCLUSION

REVIEW QUESTIONS

Exercise 2 – Power Diode Three-Phase Rectifiers Review Questions


3. Compare the operation of a three-phase half-wave rectifier with a negative-polarity output to that of a three-phase half-wave rectifier with a positive-polarity output. Do these rectifiers have the same conduction angles, ripple frequencies, and average output voltages?

4. What is a three-phase full-wave rectifier? How does it work? Describe the waveform of the rectifier output voltage with respect to the waveform of the source voltage waveform and explain.

5. Give three advantages of three-phase full-wave rectifiers over three-phase half-wave rectifiers.

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