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

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

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

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

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

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

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

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

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

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:

14. Note that k is called the duty cycle, and the power factor and output voltage vary with the square root of k

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

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

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

18. The rms value of the output voltage is found from:
The average value of the output voltage is:

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

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