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Universiti Sains MalaysiaElectrical Engineering DepartmentAdvanced Power Electronic Laboratory EEK471

The Three-Phase Rectifier

OBJECTIVE

To become familiar with three-phase three-pulse and three-phase six pulse dioderectifiers

To observe the waveforms and compare the characteristics of these rectifiers

INTRODUCTION

Three-phase, three-pulse rectifier

The three-phase rectifier circuit offers several advantages over a single-phase rectifier. UsingFigure 1, we will analyze the operation of such a circuit in order to fully understand theseadvantages. You will notice in Figure 1 that the circuit consists simply of three diodes (one foreach of the three phases), the three-phase source, and the load. The waveforms of the three linevoltages and the load voltage are as shown in Figure 2. The output voltage EO is that of point X,measured with respect to the neutral line N.

Figure 1: A three-phase, three pulse rectifier circuit

The voltage at X contains pulsations or ripple as shown in Figure 2. Notice however that theamount of ripple is less than the ripple obtained with a single-phase rectifier. Also the on time of

each diode is 120, compared to 180 for single-phase circuits. These two differences allow usingsmaller and less powerful semiconductor devices.

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Figure 2: Inputs and output voltages

In summary, a diode turns ON only when a forward voltage is applied. It remains ON until thecurrent stop flowing. The characteristic curve shows the current flows through the diode versusthe voltage across it. The curve shows that virtually no current flows when the diode is reversedbiased, but that the current increases very rapidly when it is forward biased.

Three-phase, six-pulse rectifier

Figure 3 shows a three-phase, six-pulse rectifier, also called a three-phase bridge rectifier whichuses diodes as the rectifying device.

Figure 3: A three-phase, six-pulse rectifier using diodes.

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The rectified output voltage EO is equal to EXN + ENY or EXN - EYN. Note that reversal of thesubscripts makes ENY = - EYN. N is the neutral line of the three phase source. This circuit can beconsidered to be composed of two three-pulse rectifiers. EXN is the output voltage of the three-pulse rectifier formed by D1, D2 and D3. EYN is of opposite polarity and is the output voltage of thethree-pulse rectifier formed by D4, D5 and D6. The flow of current through R1 is from X towards N.Current flows through R2 from N towards Y. Since the average current flowing to or from N iszero. The N terminal of the three-phase source is not necessary for operation. It is shown hereonly to simplify the explanation of circuit operation. Figure 4 shows the output voltage waveform.

Figure 4: Voltage waveform of the three-phase six-pulse rectifier

As shown in Figure 4, the maximum value of the output voltage is equal to the peak line to linevoltage. Both the three-phase three-pulse and the three-phase six-pulse rectifiers can be used tosupply power to an active load, as in a battery charger. They provide no means for electronicallycontrolling the current.

EQUIPMENTS

EMS 8821 Enclosure Power Supply

EMS 8840-0A PE Power Supply

EMS 8842-1A Power Diodes

EMS 8412-05 Lab-Volt DC Voltmeter/Ammeter

EMS 8311 Variable Resistance

EMS 8325 - 15 Smoothing Inductor

EMS 9056-15 and EMS 9056-05 Voltage/Current Isolator

Textronic Oscilloscope

Connection Leads

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WARNINGS

The voltages and currents that are used during this lab are larger and rated at 415VACLine-to-Lin e, 240VACLine-to-Neutraland 700VDCwith current as high as 20 amps (or higher if circuitsare improperly connected). Please take the proper precautions and use your head beforetouching any circuitry. NEVERchange any circuit connections while the power supply is turnedon. Ask the demonstrator to check your connections before turning on the switches. And followthe rating of voltmeters and ammeters given to prevent equipments from damaged.

Experiment: Part IThree-phase three-pulse rectifier

1. Make sure that the main power switch of the Power Supply is set to the O (OFF) position.Set the voltage control knob to 0 %. Make sure that the toggle switches on the Resistiveand Inductive Load are all set to the O (open) position.

2. In this part, you will set up a three-phase three-pulse rectifier, observe the waveformsand measure the output parameters. Set up the circuit of Figure 1a using the resistiveload Z1(a). Note that the incoming supply is three-phase four-wire system with neutralconnection.

Figure 1a: Three-phase, three-pulse rectifier circuit.

Table 1a: Parameters setting

Line To LineVoltage (V)

I1 dc (A) i1 (A) E1 dc (V) e1, e2 (V) Z1(a) Z1(b)

0 415(

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Figure 1b: Voltage and current waveforms for three-phase three-pulse rectifier.

4. Record the output voltage, current and power of the rectifier circuit in the first row ofTable 1b, Enter the conduction angle of the diode. To determine the diode conductionangle, connect the Current lsolator in series with diode D1. Before changing anyconnections, set the voltage control knob on the Power Supply to 0 %, then set the mainpower switch to 0 (OFF).

Table 1b: Results

Load Z1 E1 dc (V) I1 dc (A) PO = E1 X I1(watt)

ConductionAngle ()

(a) Resistive

(b) Inductive

5. Set the voltage control knob to the 0 % position then set the main power switch to the Oposition.

6. Change the load in the circuit to the inductive load Z1(b) as in Figure 1c. Repeat theprocedure steps necessary to complete Table 1b and Figure 1b. Explain the effect of aninductive load on the voltage and current waveforms and on the conduction angle.

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Figure 1c: Z1(b) connection.

7. Set the voltage control knob to the 0 position then set the main power switch to the 0position.

8. Compare the following characteristics of a three-phase three-pulse rectifier to those of asingle-phase bridge rectifier in terms of diode conduction angle, ripple frequency andaverage output voltage and power.

9. Compare the output voltage of the circuit to the theoretical value.

Theoretical value, E1 = ____________ V dcMeasured value, E1 = ____________ V dc

Experiment: Part IIThree-phase six-pulse rectifier

1. Make sure that the main power switch of the Power Supply is set to the O (OFF) position.Set the voltage control knob to 0 %. Make sure that the toggle switches on the Resistiveand Inductive Load are all set to the O (open) position.

2. In this part, you will set up a three-phase six-pulse rectifier circuit. Set up the circuit of

Figure 2a using the resistive load Z1(a). Set S1 of power diodes to 1 (ON) to form the six-pulse rectifier circuit, thus reduces additional cables. Refer a circuit on the left of S1 to getan idea the way it is built.

3. Note that the incoming supply is three-phase three-wire system without neutralconnection. In obtaining the reference waveform e1, the voltage isolator e1 should beconnected to 4-N position.

Figure 2a: Three-phase six-pulse rectifier circuit

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Table 2a: Parameters setting

Line To LineVoltage (V)

LineVoltage (V)

I1 dc (A) i1 (A) E1 dc (V) e1, e2 (V) Z1(a) Z1(b)

0 415(

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Table 2b: Results

Load Z1 E1 dc (V) I1 dc (A) PO = E1 X I1(watt)

ConductionAngle ()

(b) Resistive

(b) Inductive

6. Change the load in the circuit to the inductive load Z1(b) as in Figure 1c. Repeat theprocedure steps necessary to complete Table 2b and Figure 2b. Explain the effect of aninductive load on the voltage and current waveforms and on the conduction angle.

7. Set the voltage control knob to the 0 % position then set the main power switch to the O(OFF) position. Return S1 of power diodes to O (OFF) position.

8. Compare the following characteristics of a three-phase six-pulse rectifier to those of athree-phase three-pulse rectifier in terms of diode conduction angle, ripple frequency andaverage output voltage and power.

9. Compare the output voltage of the circuit to the theoretical value.

Theoretical value, E1 = ____________ V dcMeasured value, E1 = ____________ V dc

REVIEW QUESTIONS

1. What is the diode conduction angle in a three-phase three-pulse rectifier and in a three-phase, six-pulse rectifier?

2. What is the average output voltage of a three-phase three-pulse rectifier operating on aline to line voltage of 220 V?

3. What is the average output voltage of a three-phase six-pulse rectifier operating on a lineto line voltage of 240 V?

4 What are the advantages of a three-phase rectifier over a single-phase rectifier?

5. What are the advantages of a three-phase six-pulse rectifier over a three phase three-pulse rectifier?