What is AC Voltage Controller? – Definition, Working and Application


AC Voltage Controller is a thyristor based device which converts fixed alternating voltage directly to variable alternating voltage without a change in frequency.

AC Voltage Controller is a phase-controlled device and hence no force commutation circuitry is required.  Natural or line commutation is used. Phase control means that the phase relationship between the start of load current and supply voltage is controlled by varying the firing angle of thyristor used in the circuit of ac voltage controller.

Working Principle of AC Voltage Controller:

The working principle of AC Voltage Controller is based on either of two methods: Phase Control or Integral Cycle Control.

In Phase Control method, the phase relationship between the start of load current and the input supply voltage is controlled by controlling the firing angle of the thyristor.

In Integral Cycle Control, the AC input supply is switched ON for some integral cycles and turned OFF for further number of integral cycles. Integral cycle control is mainly used for applications where the mechanical time constant or thermal time constant is quite high of the order of some seconds. For example, mechanical time constant for many of the speed control drives, or the thermal time constant of the heating loads is usually quite high. For such applications, almost no variation in speed or temperature will be noticed if the control is achieved by connection the load to the source for some on-cycles and then disconnecting the load for some off-cycles. This form of power control is the integral cycle control.

Figure below shows the circuit diagram of a single phase full wave AC voltage controller:


The circuit comprises of two thyristors connected in anti-parallel. Anti-parallel connection is done so that thyristor (T1) is forward biased for the positive half of the input supply voltage whereas thyristor (T2) is forward biased for the negative half cycle of the input AC supply. Method of phase control is used to achieve output voltage control.

When T1 is forward biased, it may be fired to turn it ON. The firing angle of thyristor may be chosen based on the required output voltage. If the output voltage requirement is more, the firing angle (α) should be less.

Refer the figure below. This figure shows the method to control the voltage in this controller.

Explanation of AC Voltage Controller

Suppose, T1 is fired at an angle α. As soon as T1 is fired, it connects the load to the source for positive half cycle of input. If the load is resistive in nature, the load output voltage follows the envelop of AC supply input. The load current at once becomes (Vmsinα / R) and is in phase with the load voltage.

At ωt = π, the load voltage becomes zero and current, also, becomes zero. Since, thyristor T1 is reversed biased after ωt = π and current through it is zero, it gets naturally commutated.

At ωt = (π+α), forward biased thyristor T2 is gated. Hence, it conducts and connected load to the source. The load voltage now follows the negative envelop of the AC input supply and the load current does the same.

Thus, the root mean square voltage may be controlled by having a control of firing angle. In this way, voltage control is achieved in AC voltage Controller.

Application of AC Voltage Controller:

Some of the main application of AC Voltage Controller are for the following:

Earlier the devices were used for the above applications were auto-transformer, tap-changing transformers, magnetic amplifiers, saturable reactors etc. But these devices are now replaced by thyristor and TRIAC based AC Voltage Controller because of their high efficiency, flexibility in control, compact size and less maintenance requirement.  AC voltage controllers are also adaptable for closed-loop control system.

The main disadvantage of AC voltage controller is the introduction of objectionable harmonics in the supply current and load voltage waveform, particularly at reduced output voltage level.

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