On Off Control Controller: What is it? (Working Principle)

What is an On Off Controller?

Sometimes, the control element has only two positions either it is fully closed or fully open. This control element does not operate at any intermediate position, i.e. partly open or partly closed position. The control system made for controlling such elements is known as the on-off control theory. In this control system, when process variable changes and crosses a certain preset level, the output value of the system is suddenly fully opened and gives 100 % output.

Generally, in the on-off control system, the output causes a change in the process variable. Hence due to the effect of output, the process variable again starts changing but in the reverse direction.

During this change, when the process variable crosses a certain predetermined level, the output value of the system is immediately closed and output is suddenly reduced to 0%.

As there is no output, the process variable again starts changing in its normal direction. When it crosses the preset level, the output valve of the system is again fully open to give 100% output. This cycle of closing and opening of the output valve continues until the said on-off control system is in operation.

A very common example of on-off control theory is a fan controlling scheme of the transformer cooling system. When transformer runs with such a load, the temperature of the electrical power transformer rises beyond the preset value at which the cooling fans start rotating with their full capacity.

As the cooling fans run, the forced air (output of the cooling system) decreases the temperature of the transformer. When the temperature (process variable) comes down below a preset value, the control switch of fans trip and fans stop supplying forced air to the transformer.

on off control system

After that, as there is no cooling effect of fans, the temperature of the transformer again starts rising due to load. Again when during rising, the temperature crosses the preset value, the fans again start rotating to cool down the transformer.

Theoretically, we assume that there is no lag in the control equipment. That means, there is no time day for on and off operation of control equipment. With this assumption, if we draw a series of operations of an ideal on-off control system, we will get the graph given below.

But in practical on-off control, there is always a non zero time delay for closing and opening action of controller elements.

This time delay is known as dead time. Because of this time delay, the actual response curve differs from the above shown ideal response curve.

Let us try to draw the actual response curve of an on off control system.

on off control system


Say at time T O the temperature of the transformer starts rising. The measuring instrument of the temperature does not respond instantly, as it requires some time delay for heating up and expansion of mercury in temperature sensor bulb say from instant T1 the pointer of the temperature indicator starts rising.

This rising is exponential in nature. Let us at point A, the controller system starts actuating for switching on cooling fans, and finally, after the period of T2 the fans start delivering force air with its full capacity. Then the temperature of the transformer starts decreasing in an exponential manner.

At point B, the controller system starts actuating for switching off the cooling fans, and finally after a period of T3 the fans stop delivering force air. Then the temperature of the transformer again starts rising in the same exponential manner.

N.B.: Here during this operation, we have assumed that the loading condition of the electrical power transformer, ambient temperature and all other conditions of surrounding are fixed and constant.

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