This document discusses different types of AC voltage controllers. It begins by introducing AC voltage controllers and how they can control power flow into a load by varying the RMS value of the load voltage using thyristors. It then describes the main types of AC voltage controllers classified by input supply type and control method. Applications such as lighting, heating and motor speed control are also outlined. The document proceeds to explain the principles and techniques of on-off control and phase control. Circuit diagrams are provided to illustrate single phase and three phase controller configurations. The document concludes by briefly discussing cycloconverters which can provide a variable output voltage and frequency.

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.
AC
Voltage
Controller
V0(RMS)
fS
Variable AC
RMSO/PVoltage
AC
Input
Voltage
fs
Vs
fs

3. TYPE OF AC VOLTAGE CONTROLLERS
 Classification based on the type of input ac supply:
Single Phase AC Controllers
Three Phase AC Controllers
 Each type of controller may be sub divided into:
Unidirectional or half wave ac controller
Bi-directional or full wave ac controller
 In brief different types of ac voltage controllers are:
Single phase half wave ac voltage controller (uni-directional controller)
Single phase full wave ac voltage controller (bi-directional controller)
Three phase half wave ac voltage controller (uni-directional controller)
Three phase full wave ac voltage controller (bi-directional controller)

4. APPLICATIONS OF AC VOLTAGE CONTROLLERS
 Lighting / Illumination control in ac power circuits.
 Induction heating.
 Industrial heating & Domestic heating.
 Transformers tap changing (on load transformer tap
changing).
 Speed control of induction motors (single phase and
poly phase ac induction motor control).
 AC magnet controls.

5. AC VOLTAGE CONTROL TECHNIQUES
 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.

6. COMMUTATION
 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.

7. PRINCIPLE OF ON-OFF CONTROL
(INTEGRAL CYCLE CONTROL)

8.  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 motors.

9. Note that k is called the duty cycle, and the power
factor and output voltage vary with the square root of
k.
kV
nm
n
VV
tdtV
mn
n
V
ssrmso
srmso












 
2/1
2
0
22
)(sin2
)(2



If the input voltage is connected to load for n cycles and
is disconnected for m cycles, the output load voltage is
found from:

10. PRINCIPLE OF PHASE CONTROL
 The principle of phase control can be explained with
the following circuit.

11.  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.

12.  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.

13.  The rms value of the output voltage is :
 The average value of the output voltage is:
2/1
2/122
2
22
)]
2
2sin
2(
2
1
[
)]}(sin2)(sin2[
2
1
{










 
so
sso
VV
tdtVtdtVV
)1(cos
2
2
)](sin2)(sin2[
2
1 2

 








s
o
ssdc
V
V
tdtVtdtVV

14. SINGLE-PHASE BIDIRECTIONAL CONTROLLERS WITH
RESISTIVE LOADS
 The problem of dc input current can be prevented by
using bidirectional or full-wave control.

15.  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
22
2
2sin
(
1
)(sin2
2
2











 







so
so
VV
tdtVV

16. SINGLE-PHASE CONTROLLERS WITH INDUCTIVE LOAD
 In practice, most loads are inductive to a certain extent.

17.  The gating signals of thyristors could be short pulses for a
controller with a resistive load.
 However, they are not suitable for inductive loads.
 When thyristor T2 is fired, thyristor T1 is still conducting due
to the inductive load.
 By the time the current of T1 falls to zero and T1 is turned off,
the gate current of T2 has already ceased.
 Consequently, T2 will not be turned on.
 This difficulty can be resolved by using a continuous gate
signal with a duration of π - α.

18.  However a continuous gate pulse increases the
switching loss of thyristors.
 In practice a train of pulses with short duration are
used to overcome the loss problem.
 The rms value of the output load voltage is found
from:
2/1
2/1
22
2
2sin
2
2sin
(
1
)(sin2
2
2











 







so
so
VV
tdtVV

19. THREE-PHASE FULL-WAVE CONTROLLERS
 The unidirectional controllers, which contain dc input
current and higher harmonic content due to the
asymmetrical nature of the output voltage waveform,
are not normally used in ac motor drives.
 A three-phase bidirectional control is commonly used.

20. 30o

21.  For 0 < α < 60o:
 For 60o < α < 90o:
 For 90o < α < 150o:
2/1
)
8
2sin
46
(
1
6 








so VV
2/1
)
16
2cos3
16
2sin3
12
(
1
6 








so VV
2/1
)
16
2cos3
16
2sin
424
5
(
1
6 








so VV

22. THREE-PHASE BIDIRECTIONAL DELTA-CONNECTED
CONTROLLERS
 If the terminals of a three-phase system are accessible, the
control elements (SCRs) and load may be connected in delta.

23.  Since the phase current in a normal three-phase delta
system is only 1/√3 of the line current, the current ratings
of the thyristors are less.
 The following figure shows the waveforms for a delay angle
of 120 degrees.

24. 60o

25.  For resistive loads:
2/1
2
2sin
(
1










so VV

26. CYCLOCONVERTERS
 The ac voltage controllers provide a variable output
voltage, but the frequency of the output voltage is
fixed.
 In addition the harmonic content is high at low ac
voltages (high α).
 A variable output voltage at variable frequency can be
obtained from a two stage conversion.

27.  First the fixed ac is converted to a variable dc
(controlled rectifier), and then the variable dc is
converted to a variable ac at variable frequency
(inverter).
 However, the cycloconverter can eliminate the need of
one or more intermediate converters.
 A cycloconverter is a direct frequency changer that
converts ac power at one frequency to ac power at
another frequency by ac-ac conversion.

28.  Cycloconverters are naturally commutated and the
maximum output frequency is a fraction of the source
frequency.
 Therefore, cycloconverters are low speed ac motor
drives in ranges up to 15 MW with frequencies from 0 to
20 Hz.

29. SINGLE-PHASE CYCLOCONVERTERS
 The principle of operation of single-phase cycloconverters
can be explained with the following figure.
 First, two single-phase controlled converters are operated
as bridge rectifiers.
 Their delay angles are such that the output voltage of one
converter is equal and opposite to that of the other
converter.
 If αp is the delay angle of positive converter, the delay
angle of the negative converter is: αn = π – αp

31. THREE-PHASE CYCLOCONVERTER
 The circuit diagram of a three-phase/single phase
cycloconverter is shown next.
 The synthesis of output waveform for an output
frequency of 12 Hz is also in this figure.

33.  The cycloconverter of previous figure can be extended
to feed a three-phase load, by having six three-phase
converters.
 If six full-wave three-phase converters are used, 36
thyristors would be required.

35. REDUCTION OF OUTPUT HARMONICS
 The output voltage of cycloconverters is basically made up of segments
of input voltages.
 The average value of a segment depends on the delay angle for that
segment.
 If the delay angles of segments were varied in such a way that the
average values of segments corresponds as closely as possible to the
variations of desired sinusoidal output voltage, the harmonics on the
output voltage can be minimized.

36.  The delay angles for segments can be generated by
comparing a cosine signal at the source frequency with an
ideal sinusoidal reference voltage at the output frequency.
 The following figure shows generation of the gating signals
for the cycloconverter.