University of Technology Lecture Note 09

Electrical Engineering Department AC-AC Converters: AC Chopper

Electrical Engineering Division Page 1 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

AC-AC Converters

AC Choppers
A power electronic ac–ac converter, in generic form, accepts electric power from one system
and converts it for delivery to another ac system with waveforms of different amplitude,
frequency, and phase. They may be single-phase or three-phase types depending on their power
ratings.

AC-AC converter can be categorised into Three topologies (see Fig.1):

 AC-AC Voltage Controller Converter (AC Choppers)
 AC Cycloconverter
 Matrix Converter
 Indirect Matrix Converter (AC-DC-AC Converter)
 Direct Matrix Converter

Fig.1

The circuit diagrams for the above converters can be depicted below. However, many
traditional and advance AC-AC converters are developed to improve their performance based
on their applications.
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 2 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

In this PE course the first, second, and third converter systems will be studied in details. The
first topology, which is AC voltage controller or called AC choppers will be presented in this
lecture.

AC Choppers
AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value of the
alternating voltage applied to a load circuit by introducing Thyristors between the load and a
constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is
controlled by controlling the triggering angle of the Thyristors/Triac in the ac voltage
controller circuits.
There are two different types of thyristor control used in practice to control the ac power
flow
On-Off control
Phase control
In On-Off control technique Thyristors are used as switches to connect the load circuit to the
ac supply (source) for a few cycles of the input ac supply and then to disconnect it for few
input cycles. The Thyristors thus act as a high speed contactor (or high speed ac switch). Only
Phase control will be considered in this lecture.
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 3 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

AC Phase Control Choppers

In phase control the Thyristors are used as switches to connect the load circuit to the input ac
supply, for a part of every input cycle. That is the ac supply voltage is chopped using
Thyristors/Triac during a part of each input cycle. The switch is turned on for a part of every
half cycle, so that input supply voltage appears across the load and then turned off during the
remaining part of input half cycle to disconnect the ac supply from the load.
By controlling the phase angle or the trigger angle ‘α’ (delay angle), the output RMS voltage
across the load can be controlled.

The trigger delay angle ‘α’ is defined as the phase angle (the value of ωt) at which the
thyristor turns on and the load current begins to flow.

Type of Ac Voltage Controllers

The ac voltage controllers are classified into two types based on the type of input ac supply
applied to the circuit.

Single Phase AC Controllers.

Three Phase AC Controllers.

Each type of controller may be sub divided into

Uni-directional or half wave ac controller.

Bi-directional or full wave ac controller.

University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 4 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

Applications of Ac Voltage Controllers

Lighting / Illumination control in ac power circuits.
Induction heating.
Industrial heating & Domestic heating.
Transformer tap changing (on load transformer tap changing).
Speed control of induction motors (single phase and poly phase ac induction motor control).

Principle of AC Phase Control

Half wave AC phase controller (Unidirectional Controller)

RMS Output Voltage

University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 5 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

A typical control characteristic of single phase half-wave phase controlled ac voltage controller
is as shown below
We can observe from the control characteristics
and that he range of RMS output voltage control
is from 100% of VS to 70.7% of VS when we vary
the α from zero to 180 degrees. Thus the half
wave ac controller has the drawback of limited
range RMS output voltage control.

Example 1: Derive an expression for DC voltage in a single-phase AC voltage controller.

Solution:
Based on the output voltage waveforms as shown aside
the Vodc can be found as derived below

Disadvantages of single phase half wave ac voltage controller

 The output load voltage has a DC component which can result in the problem of core
saturation of the input supply transformer.
 ac power flow to the load can be controlled only in one half cycle.
 Half wave ac voltage controller gives limited range of RMS output voltage control.
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 6 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

Single Phase Full Wave Ac Voltage Controller

With R-Load

Back-back SCR or instead Triac can be used for bidirectional full-wave AC chopper.
The main waveforms are shown below:

RMS Output Voltage

After simplification:

The maximum voltage appears across the load at α=00

lead to:

A typical control characteristic of single phase full-

wave phase controlled ac voltage controller is as
shown below
We can notice from the figure, that we get a full range
output voltage control by using a single phase full
wave ac voltage controller.
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 7 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

Need For Isolation

In the single phase full wave ac voltage controller

circuit using two SCRs in parallel, the gating
circuits (gate trigger pulse generating circuits) of
Thyristors must be isolated. Figure shows a pulse
transformer with two separate windings to provide
isolation between the gating signals.

Performance Parameters
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 8 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

Note: In the case of a single phase full wave ac voltage controller circuit using a Triac with
resistive load, the average thyristor current =0. Because the Triac conducts in both the half
cycles and the thyristor current is alternating and we obtain a symmetrical thyristor current
waveform which gives an average value of zero on integration.

Single Phase Full Wave Ac Voltage Controller

With RL-Load
In practice most of the loads are of RL type. For example if we consider a single phase full
wave ac voltage controller controlling the speed of a single phase ac induction motor, the load
which is the induction motor winding is an RL type of load, where R represents the motor
winding resistance and L represents the motor winding inductance.

Due to the inductance in the load, the load current io flowing through T1 would not fall to zero
at ωt=π when the input supply voltage starts to become negative. T1 will continue to conduct
the load current until all the inductive energy stored in the load inductor L is completely
utilized and the load current through T1 falls to zero at ωt=β, β where is referred to as the
Extinction angle
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 9 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

T1 conducts from ωt=α to ωt=β, T1 then

conduction angle δ=( β- α). δ depends on α and
the load impedance angle ϕ.

Discontinuous load current operation occurs

for α>ϕ and β< π+ α

 This circuit can be used to regulate the RMS voltage across the terminals of an ac motor
(induction motor). It can be used to control the temperature of a furnace by varying
the RMS output voltage.
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 10 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

 For very large load inductance ‘L’ the SCR may fail to commutate and the load voltage
will be a full sine wave.

A continuous load current and the output

voltage waveform appears as a continuous
sine wave identical to the input supply voltage
waveform for trigger angle α≤ϕ. We lose the
control on the output voltage and thus we
obtain:

RMS Output Voltage

for α>ϕ the load current and load voltage waveforms become discontinuous.

Output Current for (Inductive Load)

during T1 conduction:

The solution of the above differential

equation gives the general expression for the
output load current which is of the form

At ωt=α, io=0Amp, hence,

University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 11 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

By substituting ωt=α: then,

which results in the instantaneous

output current equal to:

Calculate Extinction Angle β

At ωt=β, io=0Amp, hence,

 β can be determined from this transcendental equation by using the iterative method of
solution (trial and error method).
 For δ<π, for β< π+ α he load current waveform appears as a discontinuous current
waveform.
 When is α decreased and made equal to the load impedance angle α=ϕ, we obtain from
the expression for
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 12 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

Hence we obtain a continuous load current and the output voltage waveform appears as a
continuous sine wave identical to the input supply voltage waveform.

Performance Parameters

Load Current

The Average Thyristor Current

University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 13 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

Example 2: A single phase full wave ac voltage controller supplies an RL load. The input
supply voltage is 230V, RMS at 50Hz. The load has L = 10mH, R = 10Ω, the delay angle of
SCR 1 and 2 are equal, where α1= α2=π/3
Determine
a. Conduction angle of the thyristor .
b. RMS output voltage.
c. The input power factor.
Comment on the type of operation.

Solution
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 14 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed
University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 15 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

a)

b)

c)

Exercise1: A single phase full wave controller has an input voltage of 120 V (RMS) and a load
resistance of 6 ohm. The firing angle of thyristor is 2π . Find
a) RMS output voltage (84.85 Volts)
b) Power output (1200 watts)
c) Input power factor (0.707 lag)
d) Average and RMS thyristor current. (4.5 A, 10 Amps)

Exercise2: A single phase half wave ac regulator using one SCR in anti-parallel with a diode
feeds 1 kW, 230 V heater. Find load power for a firing angle of π/4
Ans: 954.56 Watts
 University of Technology Lecture Note 09
Electrical Engineering Department AC-AC Converters: AC Chopper
Electrical Engineering Division Page 16 of 16
EG 405: Power Electronics Dr. Oday A. Ahmed

Exercise3: A single phase voltage controller is employed for controlling the power flow from
220 V, 50 Hz source into a load circuit consisting of R = 4 Ω and L = 6 mH. Calculate the
following
a. Control range of firing angle
b. Maximum value of RMS load current 30.5085 Amps
c. Maximum power and power factor 3723.077 W, 0.5547
d. Maximum value of average and RMS thyristor current. 13.7336 Amps, 21.57277 Amps

Self-assessments:
a) What phase angle control is as applied to single phase controllers? Highlight the
advantages and disadvantages of such a method of control. Draw all the wave forms.

b) What are unidirectional controllers? Explain the operation of the same with the help of
waveforms and obtain the expression for the RMS value of the output voltage. What
are the advantage and disadvantages of unidirectional controllers?