LECTURE 28 AC Voltage Controllers

Dr. Rostamkolai

ECE 452Power Electronics

1

Introduction

The power flow into a load can be controlled by varying the rms value of the load voltage

This can be accomplished by thyristors, and this type of power circuit is known as ac voltage controllers

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The most application of ac voltage controllers are:

Industrial heating

On-load transformer tap changing

Light controls

Speed control of induction motors

AC magnet controls

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For power transfer, two types of control are normally used: On-off Control Phase angle control

In on-off control, thyristor switches connect the load to the ac source for a few cycles of the input voltage and then disconnected for a few cycles

In phase control, thyristor switches connect the load to the ac source for a portion of each cycle

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The ac voltage controllers can be classified into two types:

Single-Phase Controllers Three-Phase Controllers

Each type can be subdivided into:

Unidirectional or Half-Wave Control Bidirectional or Full-Wave Control

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Since the input voltage is ac, thyristors are line commutated

Typically phase control thyristors which are cheaper are used

For applications up to 400 Hz, TRIACs are used

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Performance Parameters

An ac voltage controller produces a variable ac voltage at a fixed or variable frequency

Input source is a fixed voltage and frequency ac supply

120 or 240 V

50 or 60 Hz

The output should ideally be a pure sine-wave

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8

From the input side, the performance parameters are similar to those of diode rectifiers Input power, Pi

Rms input current, Is

Total harmonic distortion of the input current, THDi

Crest factor of the input current, CFi

Harmonic factor of the input current, HFi

Form factor of the input current, FFi

Input transformer utilization factor, TUFi

Ripple factor of the input current, RFi

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From the output side, the performance parameters are similar to those of inverters Output power, Po

Rms output current, Io

Output frequency, fo

Total harmonic distortion of the output voltage, THDv

Crest factor of the output voltage, CFv

Harmonic factor of the output voltage, HFv

Form factor of the output voltage, FFv

Ripple factor of the output voltage, RFv

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Principle of On-Off Control

The principle of on-off control can be explained with the following single-phase full-wave controller

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This type of control is applied in applications which have high mechanical inertia and high thermal time constant

Typical examples are industrial heating and speed control of large motors

If the input voltage is connected to load for n cycles and is disconnected for m cycles, the output load voltage is found from:

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Note that k is called the duty cycle, and the power factor and output voltage vary with the square root of k

kVnm

nVV

tdtVmn

nV

ssrmso

srmso

2/1

2

0

22 )(sin2)(2

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Principle of Phase Control

The principle of phase control can be explained with the following circuit

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Due to the presence of diode D1, the control range is limited

The rms output voltage can only be varied between 70.7 to 100%

The output voltage and input current are asymmetrical and contain a dc component

16

This circuit is a single-phase half-wave controller and is suitable only for low power resistive loads, such as heating and lighting

Since the power flow is controlled during the positive half-cycle of input voltage, this type of controller is also known as unidirectional controller 17

The rms value of the output voltage is found from:

The average value of the output voltage is:

2/1

2/122222

)]2

2sin2(

2

1[

)]}(sin2)(sin2[2

1{

so

sso

VV

tdtVtdtVV

)1(cos2

2

)](sin2)(sin2[2

1 2

sdc

ssdc

VV

tdtVtdtVV

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Single-Phase Full-Wave Controllers with Resistive Loads

The problem of dc input current can be prevented by using bidirectional or full-wave controller

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20

The firing pulse of T1 and T2 are 180 degrees apart

The rms value of the output voltage is:

By varying α from 0 to π, Vo can be varied from Vs to 0

2/1

2/122

2

2sin(

1

)(sin22

2

so

so

VV

tdtVV

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