4 rectifier

8/12/2019 4 Rectifier

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Power Electronics

Uncontrolled Rectifiers


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Contents

Single-phase half-wave rectifiers, Performance parameters,

Single-phase full-wave rectifiers,

Single-phase full-wave rectifier with RLload,

Multiphase star rectifiers,

Three-phase bridge rectifiers,

Three-phase bridge rectifier with RL load.


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Performance parameters

Efficiency Ripple factor

Form factor

Transformer utilization factor

Distortion factor

Harmonic factor


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With R-L Load

Without Freewheeling diode


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RL load

With freewheeling diode


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Half wave rectifier: RC Load


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Role of the Capacitor

C is initially uncharged. Switch is turned on at wt=0.Diode becomes forward biased as the source become

positive.

When diode is ON the output is the same as source

voltage. C charges until Vm.

After wt=p/2, C discharges into load R.

The source becomes less than the output Voltage.

The output voltage decays exponentially.


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Full wave rectifier

Center-tapped (CT) rectifierrequires center-tap

transformer and 2 diodes.


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Bridge rectifier


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With RL load

If the load current is continuous


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With RL load

If the load current is discontinuous


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Multiphase Rectifier

A multi-phase rectifier increases the dc component and

lowers the harmonic components.


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Output of Multiphase Rectifier


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3 phase Bridge rectifier

Sequence of conduction:

D1-D2, D3-D2, D3-D4, D5-D4, D5-D6


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Sequence of conduction:

D1-D2, D3-D2, D3-D4, D5-D4, D5-D6


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Wave shape with R load


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Explanation using phase voltage


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Three-phase waveforms

Top group: diode with its anode at the highest

potential will conduct. The other two will be reversed.Bottom group: diode with the its cathode at the lowest

potential will conduct. The other two will be reversed.

For example, if D1 (of the top group) conducts, vp

(+)is connected to van.If D6 (of the bottom group)conducts, vn(-) connects to vbn. All other diodes areoff.

The resulting output waveform is given as: vo

=vp

-vn

For peak of the output voltage is equal to the peak ofthe line to line voltage vab.


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Comparisons of Diode Rectifiers

1- phase

Half wave

1- phase

Full wave

3- phase

star

3- phase

bridge

efficiency 40.5% 81% 96.77% 99.83%

RF 121% 48.2% 18.24% 4%

TUF 28.6% 57.32% 66.43% 0.9542

PIV Vm 2Vm/ Vm 3Vm 3Vm

FF 157% 111% 101.65% 100.08%


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Summary

An inductive load can make the load current

continuous.

With highly inductive load the input current of

a rectifier becomes an ac square wave.

With high value of capacitance the output

voltage remains almost constant.

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Exercise: Example

Assignment #2: Problem 2.9, 2.10, 3.11, 3.1

4 rectifier

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