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Eng r. A ffifa A deeb

The Islamia University of BahawalpurUniversity College of Engineering & Technology

EEN-324Power Electronics

Single-Phase ControlledRectifiers

Chp#6 1


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INTRODUCTION Diodes of rectifier circuits in chp5 are replaced by thyristors.

In thyristor based rectifiers, output voltage can be controlled.So they are termed as controlled rectifiers .

Controlled rectifiers produce variable DC output, whosemagnitude is varied by Phase control.Phase Contro l

DC output from rectifier is controlled by controlling duration ofthe conduction period by varying the point at which gate signal

is applied to SCR.

Main drawback of phase control is Radio FrequencyInterference (RFI)

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Controlled rectifiers are of two types,

1- Fully Controlled rectifiersDC current is unidirectional, but DC voltage has eitherpolarity. With one polarity, flow of power is from ACsource to DC load---Rectification.With the reversal of DC voltage by the load, flow of

power is from DC load to AC source---Inversion.

2- Half controlled rectifiersHalf of SCRs are replaced by diodes.

DC output current and voltage are unidirectional. i.e.,flow of power is from AC source to DC load.

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HALF -WAVE C ONTROLLED RECTIFIERS

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With Resis t ive Lo ad

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Contro l ch aracter is t ics o f half -wave rect i f ier

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With an Indu ct ive (RL) Load

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With Indu c t ive Load andFreewheel ing Diode


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FULL -WAVE C ONTROLLED CENTER -TAP R ECTIFIERS

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With Resis t ive Load


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Contro l Character is t ics for cen ter-tap rect i f ier


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With Freewh eel ing Diode


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EXAMPLE 6.4

Explain with the help of waveforms the operation ofa full-wave center-tap rectifier with RL load for thefollowing firing angles:(a) 0 (b) 45 (c) 90 (d) 135

(e) 180

Assume high ly Indu ct ive Load14


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During positive-half cycle of sourcevoltage, SCR1 is forward biasedand SCR2 is reverse biased. Duringnegative half-cycle, SCR2 isforward biased and SCR1 is

reverse biased. In either casevoltage across the load is Vs.

Output is similar to uncontrolledrectifier.

Each SCR conducts for 180 andsupplies current to the load for thisperiod

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Voltage and c ur ren t waveform s for =0


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Average DC output voltagedecreases.

If SCR1 is triggered at 45 , SCR2will conduct upto that point,even though the source voltageis zero, due to highly inductivenature of load.

When SCR1 is turned on, SCR2is turned off.

Current to the load is suppliedby SCR1 and SCR2, eachconducting for 180

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Average DC voltage is zero,so there is no transfer ofpower from AC source to DCload.

Each SCR remains inconduction for 180

As firing angle isincreased from 0 to 90 ,the power supplied to theDC load decreases,becoming zero at =90

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Average DC voltage is negative.

Load current still flows in eachSCR for 180 in its originaldirection.

Load voltage has changedpolarity.

Power now flows from DC loadto AC source .

Circuit acts like an inverter.18



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Average output DC voltage isat its maximum negativevalue.

SCRs remain in conductionfor 180

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EXAMPLE 6.5

Show direction of power flow and operating mode(rectifying or inversion) of center-tap rectifiercircuit with following firing angles:

A) > 0 B) < 90 C) > 90

D) < 180

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S OLUTION

For firing angle in the range 0 < < 90 1. Average output voltage is positive.

2. Converter operates in the rectifying mode.

3. Power to the load is positive

4. Power flow is from AC source to the DC load.

For firing angle in the range 90 < < 180 1. Average output voltage is negative

2. Converter operates in inversion mode3. Power to the load is negative

4. Power flow is from DC load to AC source

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ASSIGNMENT # 2

In example 6.4, draw waveforms for voltage acrossthyristor 2. i.e., VSCR2

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FULL-WAVE C ONTROLLED

BRIDGE R ECTIFIER

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For L >>> R


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With RL load and f reewh eel ing d iod e


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HALF -C ONTROLLED OR

S EMICONTROLLED B RIDGE R ECTIFIERS

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Full -wave sem icon tro l led br idgerect i f ier c i rcu i t


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Sem icon tro l led br idg e rect i f ier wi th FWD


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DUAL C ONVERTER

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Exercise Problems

6.1, 6.5, 6.6, 6.9, 6.12, 6.21, 6.22, 6.24 Also give analysis of waveforms in each case of all

above exercise problems

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