Integral Cycle Control of AC Voltage Controller

Integral Cycle Control is a strategy to control the output voltage of AC voltage controller. In this strategy, load is connected to the source for an integral number of cycle of input supply and then disconnected by switching off the supply for a further number of integral cycles. This strategy is also known as on-off control, burst firing, zero-voltage switching, cycle selection or cycle syncopation.

For regulating the power flow in an AC voltage controller, control strategy are of two types: Phase Control or Integral Cycle Control.

In Phase control, the phase relationship between the start of load current and the input supply voltage is controlled by controlling the firing angle of the thyristor. You may refer “AC Voltage Controller” to better understand the phase control method. In this article, we will discuss Integral Cycle Control in detail.

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.

Let us consider the circuit diagram of single phase full wave AC voltage controller.

Let us assume that i_g1 & i_g2 be the gate pulse of thyristors T1 & T2 respectively. Load is assumed resistive in the circuit diagram. Now, carefully observe the waveforms shown below.

Thyristor T1 is fired at every positive crossing of zero voltage till 3 cycles. Whereas, thyristor T2 is gated at every negative crossing of zero voltage till 3 cycles. This scheme of switching ON of T1 & T2 will result in continuous flow of power from source to load for three cycles of supply input. It should also be noted here that thyristor T1 & T2 are naturally commutated at every negative & positive crossing of zero voltage respectively. In the above waveform, the load is connected to the source for just three cycles. We may use this strategy to connect load to source for “n” number of cycles.

Now, after three cycles, none of the thyristors T1 & T2 are fired for two cycles. This will disconnect to load for two cycles. We may use this method to switch off the supply to the load for “m” number of cycles. During this period, no power flows from source to the load.

In this manner, process of turn-on and turn-off is repeated for the control of load power. By varying the number of “n” & “m” cycles, power delivered to the load can be regulated as desired. Thus, the average power delivered to the load is controlled in integral cycle control of AC voltage controller.

Integral Cycle Control introduces less harmonics into the supply system, therefore integral cycle control method is used for heating loads and for motor control drives.