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Single-Phase ControlledRectifiers

Wan Zain Bin Wan Dollah

Electrical Engineering DepartmentKota Bharu Polytechnic

SINGLE PHASE AC TO DCCONVERTER

1. HALFWAVE CONTROLLEDRECTIFIER

2. FULLWAVE HALFCONTROLLEDRECTIFIER

3. FULLWAVE FULLY CONTROLLEDRECTIFIER

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Normal rectifiers are considered as uncontrolledrectifiers.

Once the source and load parameters areestablished, the dc level of the output and power transferred to the load are fixed quantities .

A way to control the output is to use SCR instead of

diode. Two condition must be met before SCR canconduct: The SCR must be forward biased (V SCR >0) Current must be applied to the gate of SCR

The Half-wave Controlled Rectifier

The simplest controlled rectifier uses a single device, such as athyristor, to produce variable voltage d.c. from fixed voltage a.c.mains. The circuit arrangement is shown below

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The thyristor is turned on in the positivehalf-cycle, some time after supply voltagezero, by the application of a gate pulsewith delay angle . In the negative half-cycle, the thyristor is reverse biased andcannot switch on. The larger the delayangle, the smaller is the average loadvoltage.

Voltage waveformsfor two delay angles are shown below

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Controlled, Half-wave R load

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Example

Design a circuit to produce an average voltageof 40V across 100 load resistor from a 120V rms60 Hz ac source. Determine the power absorbedby the resistor.

Briefly describe what happen if the circuit isreplaced by diode to produce the same averageoutput.

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Example (Cont) Solution

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In such that to achieved 40Vaverage voltage, the delay anglemust be

If an uncontrolled diode is used,the average voltage would be

That means, some reducingaverage resistor to the design mustbe made. A series resistor or inductor could be added to anuncontrolled rectifier , whilecontrolled rectifier has advantageof not altering the load or introducing the losses

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Half - Wave ControlledRectifier Circuit with an RL

Load

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Figure 1 : Half-wave controlledRectifier with RL Load

Let vs(t) be Vm sin ( t). At t =0, the current through thecircuit is zero. As t becomes >0, vs becomes positive. If adiode instead of an SCR hasbeen used, the diode wouldstart conduction at t = 0. Withan SCR, the conduction doesnot start till the SCR istriggered. Let the SCR betriggered when t = . Then is called the firing angle andthe SCR continues to conduct.

When t = , the sourcebecomes zero, but at this instant,the current through the circuit isnot zero and there is someenergy stored in the inductor.When vs becomes negative, thecurrent through the circuit wouldnot become zero suddenlybecause of the inductor. Theinductor acts as a source andkeeps the SCR forward-biased tillthe energy stored in the inductor becomes zero. Let the currentthrough the circuit become zeroat t = and the value of > .For < t < 2 , the currentthrough the circuit is zero

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With an Inductive (RL) Load

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With Inductive Load andFreewheeling Diode

Average output current, I dc

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FULL-WAVE CONTROLLEDCENTER-TAP RECTIFIERS

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With Resistive Load

Average output voltage, V dc

Average output current,

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Control Characteristics for center-tap rectifier

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With Freewheeling Diode



Example 6.4Explain with the help of waveforms theoperation of a full-wave center-taprectifier with RL load for the followingfiring angles:

(a) 0 (b) 45 (c) 90 (d) 135 (e) 180

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During positive-half cycle ofsource voltage, SCR1 is forwardbiased and SCR2 is reversebiased. During negative half-cycle,SCR2 is forward biased and SCR1is 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 forthis period

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Voltage and current waveforms for =0

Average DC output voltagedecreases.

If SCR1 is triggered at 45 ,SCR2 will conduct upto thatpoint, even though the sourcevoltage is zero, due to highly

inductive nature 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, sothere is no transfer of powerfrom AC source to DC load.

Each SCR remains inconduction for 180

As firing angle is increasedfrom 0 to 90 , the power

supplied to the DC loaddecreases, becoming zero at =90

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Average DC voltage isnegative.

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.

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Average output DC voltage is atits maximum negative value.

SCRs remain in conduction for180

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Example 6.5

Show direction of power flow andoperating mode (rectifying orinversion) of center-tap rectifiercircuit with following firing angles:

A) > 0B) < 90C) > 90D) < 180

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Solution

For firing angle in the range 0 < < 901. Average output voltage is positive.2. Converter operates in the rectifying mode.3. Power to the load is positive4. Power flow is from AC source to the DC load.

For firing angle in the range 90 < < 1801. Average output voltage is negative2. Converter operates in inversion mode

3. Power to the load is negative4. Power flow is from DC load to AC source

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Assignment # 2

In example 6.4, draw waveforms for voltage across thyristor 2. i.e., V SCR2

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FULL-WAVE CONTROLLEDBRIDGE RECTIFIER

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With Resistive Load



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Averag e output current,

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For L >>> R

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Control characteristics for bridge rectifier

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With RL load and freewheeling diode

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HALF-CONTROLLEDOR

SEMICONTROLLED BRIDGERECTIFIERS

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In fully-controlled rectifier, onlyrectification can be obtained byconnecting a freewheeling diodeacross the output terminals of therectifier.

Another method of obtainingrectification in bridge rectifiers isreplacing half of the SCRs withdiodes. These circuits are calledsemicontrolled bridge rectifiers.

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Full-wave semicontrolled bridge

rectifier circuit

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Semicontrolled bridge rectifier with FW D

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Dual Converter

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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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