1.
LECTURE 28
AC Voltage Controllers
Dr. Rostamkolai
ECE 452
Power Electronics
1

2.
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

3.
 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

4.
 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

5.
 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

6.
 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

7.
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.
8

9.
 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

10.
 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

11.
Principle of On-Off Control
 The principle of on-off control can be explained
with the following single-phase full-wave
controller
11

12.
12

13.
 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

14.
 Note that k is called the duty cycle, and the
power factor and output voltage vary with the
square root of k
k
V
n
m
n
V
V
t
d
t
V
m
n
n
V
s
s
rms
o
s
rms
o












 
2
/
1
2
0
2
2
)
(
sin
2
)
(
2




14

15.
Principle of Phase Control
 The principle of phase control can be explained
with the following circuit
15

16.
 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

17.
 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

18.
 The rms value of the output voltage is found
from:
 The average value of the output voltage is:
2
/
1
2
/
1
2
2
2
2
2
)]
2
2
sin
2
(
2
1
[
)]}
(
sin
2
)
(
sin
2
[
2
1
{

















 

s
o
s
s
o
V
V
t
d
t
V
t
d
t
V
V
)
1
(cos
2
2
)]
(
sin
2
)
(
sin
2
[
2
1 2



 












s
dc
s
s
dc
V
V
t
d
t
V
t
d
t
V
V
18

19.
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.
20

21.
 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
/
1
2
2
2
2
sin
(
1
)
(
sin
2
2
2















 









s
o
s
o
V
V
t
d
t
V
V
21