4 rectifier
TRANSCRIPT
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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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3 phase Bridge rectifier
Sequence of conduction:
D1-D2, D3-D2, D3-D4, D5-D4, D5-D6
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3 phase Bridge rectifier
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