Power Electronics

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.