EE 423 POWER ELECTRONICS
EXPERIMENT #4
AC VOLTAGE CONTROLLERS (AC choppers)
If a thyristor switch is connected between ac supply and load, the power flow can be
controlled by varying the rms value of ac voltage applied to the load; and this type of power
circuit is known as ac voltage controller. The circuit in Figure.1 uses two SCRs connected in
anti-parallel configuration. This circuit is useful for understanding the principle of operation.
The most common applications of ac voltage controllers are: industrial heating, on-load
transformer tap changing, light controls, speed control of polyphase induction motors, static
compensation, and ac magnet controls.
Fig.1 AC voltage controller
PURPOSE
The purpose of this experiment is to analyze output current and voltage of an ac voltage
controller connected to an inductive load. (RL load)
EQUIPMENT
AC power supply (Function generator; max 10V, 100 mA), Oscilloscope, board and other
circuit elements (SCR, resistor, capacitor, diode etc.)
PROCEDURE
Single Phase Controllers with Inductive Load
1- Set up the circuit shown in the Fig.2.
2- By varying α between 0< α <180, observe the change in the load current and voltage,
and relation with angle β.
3- Determine the value of load angle θ and power factor Cosφ. Record it in your report.
4- By varying α observe the relationship between α and β
5- Determine αmin and αmax. Record it in your report.
6- For any value of α (α≠0) draw the waveforms of the load current and voltage on the
same axes.
7- Indicate important points on your graphs (Vm, α, β and θ).
8- Calculate rms value of the load voltage you draw.
D1
150nF
D2
SCR2
220kΩ
VS
D3
1N4001
A
1kΩ
GND
SCR1
150nF
D4
L
CH2
D1,D2,D3,D4: 1N4001
SCR: BT169
CH1
Fig.2 Experimental setup for ac voltage controller
Vm
VLoad
φ
ILoad
π+ α
2π
π
α
β
Fig.3 Load Current and voltage waveforms for an RL load
2π+ α