AC Voltage Controllers

AC VOLTAGE CONTROLLERS

For many house hold and Industrial applications, Induction motors are preferred due to its self starting nature. The three phase induction motors are classified into two types.

1. Squirrel cage induction motor

2. slip ring or wound rotor Induction motor

The basic principle of Induction motor is a rotating magnetic field is created by the three phase stator winding, when three phase supply is given to it, the rotating magnetic field distribution sinusoidally in the air gap.

Synchronous speed N_s= 120*f/p

Slip of Induction motor S = ( N_s – N_r)/ N_r

Rotor speed N_r= N_s ( 1 – S)

AC voltage controllers:

The AC voltage controller is done to make stator voltage variable for controlling torque production and speed. The below circuit diagram consists of three phase thyristor circuit with star and delta connections.

* The performance of above circuit is good and percentage of increase in input current at 2/3^rd synchronous speed in only about 10-15% compare to sinusoidal voltage control.

* The star connected circuit gives better performance than Delta connected because delta connection produces circulating current results in harmonic production.

Advantage with stator voltage control:

1. Simple controlling circuit

2. Equipment weight is less and compact size

3. very quick response is possible

4. energy savings is possible

Disadvantages with stator voltage control:

1. Input power factor is poor

2. Under low speed running condition performance is poor

3. Due to high ohmic losses, operating efficiency is low

4. On decreasing the stator voltage the maximum torque of motor decreases.