Three-Phase AC Power Electronics, 1 Power Diode Three ... · Exercise 1 – Power Diode Three-Phase Rectifiers Discussion © Festo Didactic 86362-00 5 Each diode conducts current

© Festo Didactic 86362-00 3

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 the 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. You will also be introduced to the dual-polarity dc power supply.

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

Dual-polarity dc power supply


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 2 shows.

Figure 2. Three-phase half-wave rectifier (positive-polarity output).

Figure 3 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, .

Power Diode Three-Phase Rectifiers

Exercise 1

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Line terminals 1, 2, and 3

Neutral terminal

Three-phaseac power source

Load

Exercise 1 – Power Diode Three-Phase Rectifiers Discussion

4 © Festo Didactic 86362-00

Figure 3. Waveforms of the voltages and currents in the three-phase half-wave rectifier (positive-polarity output).

Phase voltages

( )

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 90 210 150 270 330

(V)

(A)

(A)

(A)

(A)

(V)



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°.

The circuit operates as described below.

Initially, 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 waveform 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 value 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

(150 Hz in a 50-Hz ac power network), compared to 60 Hz (50 Hz in a 50 Hz

ac power network) for a single-phase half-wave rectifier, and 120 Hz (100 Hz in a

50 Hz ac power network) 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, which

means that 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 4 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 4. 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 positive terminal of load resistor is lower than the voltage at the negative

terminal of this resistor (e.g., when ).

Figure 5 shows the waveforms of the circuit voltages and currents in the three-phase half-wave rectifier of Figure 4. 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 ( or ) 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 value of the phase voltage ( )

and .

Line terminals


Neutral terminal



Figure 5. Waveforms of the 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

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

210 330 210 330

90 330 90 330

90 210 90 210

90 210 330 90 210 330

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

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

( )



The average value of the rectifier output voltage is equal

to or .

Note 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 6a shows. This circuit can be redrawn as shown in Figure 6b (usual representation of a three-phase full-wave rectifier). Terminal N is the neutral conductor of the source. Figure 7 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 towards 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 towards 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 6. Three-phase full-wave rectifier.


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 7. Waveforms of the 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 6 to assist in the explanation of circuit operation. Figure 8 shows the rectifier circuit diagram without the neutral

conductor. The two load resistors ( in Figure 6) have been replaced by a single resistor .

Figure 8. 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 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.

Note 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 9 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.

Three-phase ac power source

Line terminals



Figure 9. 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 1 – Power Diode Three-Phase Rectifiers Procedure Outline



The three-phase full-wave rectifier configuration in Figure 6b shows that a dual-polarity dc power supply can be obtained by simply using the positive ( ) and

negative ( ) voltages produced by the rectifier separately. This requires the use of the neutral conductor of the ac power source as it serves as the common

point of the dual-polarity dc power supply. The voltage waveforms and are shown in Figure 7. The average value of voltage , i.e., the voltage between the positive terminal and common (neutral) terminal of the dual-polarity dc power supply, is equal to 0.675 , rms. On the other hand, the average

value of voltage , i.e., the voltage between the negative terminal and common terminal of the dual-polarity dc power supply, is equal to

-0.675 , rms. The average value of the total voltage ( ) produced, i.e., the voltage between the positive and negative terminals of the dual-polarity dc power

supply is equal to 1.35 , rms (twice the value of or ).

Figure 10. Dual-polarity dc power supply.

The Procedure is divided into the following sections:

Setup and connections



Three-phase full-wave (bridge) rectifier


High voltages are present in this laboratory exercise. Do not make of modify any

banana jack connections with the power on unless otherwise specified.

PROCEDURE OUTLINE

PROCEDURE


Line terminals

Commonterminal

Negativeterminal

Positive terminal

Exercise 1 – Power Diode Three-Phase Rectifiers Procedure


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. 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 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 LVDAC-EMS Start-Up window.


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 the 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 that are obtained with a single-phase full-wave rectifier.

6. Set up the circuit shown in Figure 11. 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)

Frequency

(Hz)

120 60 171

220 50 629

240 50 686

220 60 629

Figure 11. 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).

The power rating of resistor in the circuit diagram of Figure 11 is exceeded significantly.

Do not leave the ac power source on for periods longer than 10 minutes to avoid

excessive heating of the resistors in the Resistive Load module.

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. Make sure that the time base is set to display at least two cycles of the sine waves.

N

1

2

3



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.

a The phase currents delivered by the source, which are respectively equal to diode currents , , and , are asymmetrical, which means that 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. Then,

compare this angle to the conduction angle of the diodes in a single-phase full-wave rectifier.

a The operation of the single-phase full-wave rectifier is covered in the manual Single-Phase AC Power Electronics (part number 86359). You can refer to this manual if necessary.

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 the ripple frequency at the output of 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 of 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 the 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 12. 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, hence the different numbering of the

diodes in the diagrams of Figure 11 and Figure 12). 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).


( ) Voltage

(V)

Frequency

(Hz)

120 60 171

220 50 629

240 50 686

220 60 629

Figure 12. Three-phase half-wave rectifier with a negative-polarity output (observation of voltage and current waveforms and measurement of parameters).

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

N

1

2

3



The power rating of resistor in the circuit diagram of Figure 12 is exceeded significantly.

Do not leave the ac power source on for periods longer than 10 minutes to avoid

excessive heating of the resistors in the Resistive Load module.

18. In 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. Make sure that the time base is set to display at least two cycles of the sine waves.


a Note 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 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 .


Average rectifier output 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 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.

26. 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 the 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.

27. Set up the circuit shown in Figure 13. 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).

a 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.




( ) Voltage

(V)

Frequency

(Hz)

120 60 171

220 50 629

240 50 686

220 60 629

Figure 13. Three-phase full-wave rectifier (observation of voltage and current waveforms and measurement of parameters).


The power rating of resistors and in the circuit diagram of Figure 13 is exceeded

significantly. Do not leave the ac power source on for periods longer than 10 minutes to

avoid excessive heating of the resistors in the Resistive Load module.

29. In 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. Make sure that the time base is set to display at least two cycles of the sine waves.

N

1

2

3




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.

31. 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?



32. 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 below.

Then, calculate the rectifier output power from the average value and

current .


Average rectifier output 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).

33. 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





In this part of the exercise, you will modify the three-phase full-wave rectifier circuit to obtain a dual-polarity dc power supply. You will determine the voltages at the output of the dual-polarity dc power supply.

35. Set up the circuit shown in Figure 14. In this circuit, is the three-phase ac power source of the Power Supply. E1 through E3 are voltage inputs of the Data Acquisition and Control Interface. The 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).


( ) Voltage

(V)

Frequency

(Hz)

120 60 300

220 50 1100

240 50 1200

220 60 1100

Figure 14. Dual-polarity dc power supply.

1

2

3

N







37. In the Oscilloscope, make the necessary settings to display phase

voltage (E1) and the voltages (E2 and E3) at the positive and negative outputs of the dual-polarity dc power supply on channels 1, 2, and 3, respectively. Make sure that the time base is set to display at least two cycles of the sine waves.

38. Describe the waveform of the voltage (input E2) at the positive output of the dual-polarity dc power supply.

39. Is it identical to the waveform of the voltage at the output of a three-phase half-wave rectifier with a positive-polarity output?

Yes No

40. Describe the waveform of the voltage (input E3) at the negative output of the dual-polarity dc power supply.

41. Is it identical to the waveform of the voltage at the output of a three-phase half-wave rectifier with a negative-polarity output?

Yes No

42. In the Metering window of LVDAC-EMS, set meters E2 and E3 to measure the average (dc) values of the voltage at the positive and negative outputs of the dual-polarity dc power supply. Record these values below.

Average voltage at the positive output: V

Average voltage at the negative output: V




Modify the circuit by adding capacitors in parallel with the resistors as shown in Figure 15. The capacitors are those in the Rectifier and Filtering Capacitors module.

Figure 15. Dual-polarity dc power supply with capacitors.





Display the voltage waveforms on the Oscilloscope screen. Describe how the voltage waveforms at the outputs of the dual-polarity dc power supply are affected by the insertion of the filtering capacitors.

45. In the Metering window of LVDAC-EMS, set meters E2 and E3 to measure the average (dc) values of the voltage at the positive and negative outputs of the dual-polarity dc power supply. Record these values below.

Average voltage at the positive output: V

Average voltage at the negative output: V

1

2

3

N

Exercise 1 – Power Diode Three-Phase Rectifiers Conclusion


46. Compare the average (dc) values of the voltage measured at the positive and negative outputs of the dual-polarity dc power supply with and without filtering capacitors. What is the effect of adding filtering capacitors?

47. Do your results confirm that splitting the output of a three-phase full-wave rectifier allows a dual-polarity dc power supply to be obtained?

Yes No

48. 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.

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 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 that 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. You also learned that a dual-polarity dc power supply can be obtained by simply using the positive and negative voltages produced by a three-phase full-wave rectifier separately. This requires the use of the neutral conductor of the ac power source as it serves as the common point of the dual-polarity dc power supply.

CONCLUSION

Exercise 1 – Power Diode Three-Phase Rectifiers Review Questions


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 by comparing the amplitude of the ripple in these voltages and the ripple frequency of these voltages.

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?

REVIEW QUESTIONS

Exercise 1 – Power Diode Three-Phase Rectifiers Review Questions


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.

Three-Phase AC Power Electronics, 1 Power Diode Three ... · Exercise 1 – Power Diode Three-Phase Rectifiers Discussion © Festo Didactic 86362-00 5 Each diode conducts current

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