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
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
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
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
