Transcript LECTURE 28 AC Voltage Controllers Dr. Rostamkolai ECE 452 Power Electronics Introduction  The power flow into a load can be controlled by varying the rms value.

LECTURE 28
AC Voltage Controllers
Dr. Rostamkolai
ECE 452
Power 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
2

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
3

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
4

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

5

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
6
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
7
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

9

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

10
Principle of On-Off Control

The principle of on-off control can be explained
with the following single-phase full-wave
controller
11
12

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:
13

n
Vo rms  

 2 (n  m) 0
2
Vo rms  Vs


2Vs2 sin 2  t d ( t )

1/ 2
n
 Vs k
mn
Note that k is called the duty cycle, and the
power factor and output voltage vary with the
square root of k
14
Principle of Phase Control

The principle of phase control can be explained
with the following circuit
15

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:
2
1 
2
2
Vo  { [  2 Vs sin  t d ( t )   2 Vs2 sin 2  t d ( t )]}1/ 2

2 
1
sin 2 1/ 2
Vo  Vs [ (2   
)]
2
2

The average value of the output voltage is:
2
1 
Vdc 
[  2 Vs sin  t d ( t )  

2 
2 Vs
Vdc 
(cos  1)
2
2 Vs sin  t d ( t )]
18
Single-Phase Full-Wave
Controllers with Resistive Loads

The problem of dc input current can be
prevented by using bidirectional or full-wave
controller
19
20

The firing pulse of T1 and T2 are 180 degrees
apart

The rms value of the output voltage is:
 2
Vo  
 2



2 Vs2 sin 2  t d ( t )

sin 2 
1
Vo  Vs  (   
2 


1/ 2
1/ 2
By varying α from 0 to π, Vo can be varied from
Vs to 0
21