The stator voltage is controlled in these speed control systems by means fo a power electronic controller. There are two methos of control as follows (a) on-off control (b) control.

In on-off xontrol, the thyristors qre employed as switches to connet the load circuit to the source for a few cycles of source and then, disconnect it for another few cycle conteol. In phase control, the thyristor are employed as switches that connect the load to the ac source for a portion of each cyxle of input differ in any manner. Normally, thyristors in phase conteol modes are used. The various 3-phase full wave ac voltage controller.

The ciruit of three phase half wave and full wave ac voltage controller for star connected stators. In half wave ac voltage controller consists of 3 CSRs and 3 diodes. Here one SCR and one dode in antiparalle are connected beteen the line and motor in a phase. The full wave ac voltage controller consists SCRs. Here two SCRs intiparalle are connected between the line and motor in phase. The main advantage of half wave conteoller is saving the cost of semiconductor devices and does not give rise to dc components in any part of the system. The disadvantage is that, it inteoduces more harmonics into the line current. The effective load voltage in three phase ac circuit ca be varied by varying the thyristor firing angles.

3-phase full wave ac voltage controller for delta connected load. It may be us and has the avdantage of reducing the current of the device. When the motor is delta connected, the third harmonic voltages produced by motor back emf causes circulating current theough the windings , which increases losses and thermal loading of the motor.

Three phase full ac voltage conteoller for delta connected load for low power rating motors anti-parallel SCR pair can be replaced by a teiac. It is ac voltage controller are also for soft start of motors.

The thyristor controller brings in two more sources of power losses. Power loss takes place in the power devices in the controller. In addition, harmonic losses take place in the motor due to harmonic current flowing in the winding due to phase control. These two additional loss conponents will make this speed controller further inefficient. Also, over heating of the motor on harmonic losses is another possibility. Harmonic currents can result in cogging etc, especially when attempts are made o turn the motor at very low speed.