Transcript LECTURE 25 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics Introduction  In Chapter 3, we have seen that diode rectifiers provide a fixed output voltage  To obtain.

LECTURE 25
Controlled Rectifiers
Dr. Rostamkolai
ECE 452
Power Electronics
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Introduction

In Chapter 3, we have seen that diode rectifiers
provide a fixed output voltage

To obtain controlled output voltages, phasecontrol thyristors are used instead of diodes

A phase-control thyristor is turned on by
applying a short pulse to its gate, and is turned
off due to natural or line commutation
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
In case of a highly inductive load, it is turned
off by firing another thyristor of the rectifier

The phase-control rectifiers are cheap, and their
efficiency is above 95%

Since these rectifiers convert form ac to dc, they
are called ac to dc converters
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
They are used extensively in industrial
applications, especially in variable speed drives

The phase-control converters are classified as:
Single-phase converters
 Three-phase converters


Each type is divided into:
Semiconverter
 Full converter
 Dual converter

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
A semiconverter is a one-quadrant converter
and it has one polarity of output voltage and
current

A full converter is a two-quadrant converter and
the polarity of its output voltage can be either
positive or negative, but one direction of current

A dual converter can operate in four quadrants,
and both the output voltage and the output
current can be either positive or negative
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
In some applications, converters are connected
in series to operate at higher voltages, and to
improve the input power factor

The method of Fourier series can be applied to
analyze the performance of the phase controlled
converters
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Semiconverter Operation

Consider the following circuit, where α is the
delay or firing (ignition) angle
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
If the frequency of the supply is fs, the lowest
frequency of the output ripple voltage will be fs

Now:
1
Vdc 
2


Vm
 Vm sin t d (t )  2 (1  cos  )
The dc voltage can be varied from Vm/π to 0 by
varying from 0 to π
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
The average output voltage will be maximum
when α is 0

The rms output voltage is:

Vrms
1
[
2
Vrms
Vm 1
sin 2 1 / 2

[ (   
)]
2 
2

Vm2 sin 2 t d (t )]1 / 2
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Single-Phase Full Converters

The circuit for a single-phase full converter is
shown below

The load is assumed to be highly inductive, so
the load current is continuous and ripple free

The converter operates in the rectification and
inversion modes
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11

The average output voltage is:
2
Vdc 
2

 

Vm sin t d (t ) 
2Vm

cos 
The dc voltage can be varied from 2Vm/π to
-2Vm/π by varying α from 0 to π
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
The average output voltage will be maximum
when α is 0

The rms output voltage is:
Vrms
2   2 2
 [  Vm sin t d (t )]1/ 2
2 
Vrms
Vm

 Vs
2
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
During the period from α to π, the input voltage
and current are positive
Power flows from the supply to load
 Converter operates in rectification mode


During the period from π to α + π, the input
voltage is negative, and input current is still
positive
Power flows from the load to supply
 Converter operates in inversion mode

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
The operation of the converter can be divided
into two identical modes
Mode 1: T1 and T2 conduct
 Mode 2: T3 and T4 conduct


This converter is used in industrial applications
up to 15 kW
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Single-Phase Dual Converter

In previous section, we have seen that the
single-phase full converters with inductive loads
allow only two-quadrant operation

If two of these converters are connected back to
back, both the output voltage and the output
current can be reversed

This system will provide four-quadrant
operation and it is called a dual converter
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
Dual converters are used in high-power variablespeed drives

If α1 and α2 are the delay angles of converters 1
and 2, the corresponding average output
voltages will be Vdc1 and Vdc2
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
The delay angles are controlled such that one
converter operates as a rectifier and the other
converter operates as an inverter

However, both converters produce the same
average output voltage

Therefore,
Vdc1 
2Vm
cos1
Vdc 2 
2Vm
cos 2


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
One converter is rectifying and the other one is
inverting, therefore:
Vdc1   Vdc 2
cos 2   cos1  cos(  1 )
 2    1

Since the instantaneous output voltages of the
two converters are out of phase, there will be an
instantaneous voltage difference between the
two converters
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
This will result in a circulating current between
the two converters

The dual converters can be operated with or
without a circulating current

In case of operation without the circulating
current, only one converter operates at a time
and carries the load current

The other converter is completely blocked by
inhibiting gate pulses
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