Capacity control of a refrigeration or air conditioning plant can be defined as the arrangement which monitors and controls the output of the plant as per the required load. For large scale applications, the load requirement is high and varies due to factors like lighting, occupancy and ambient weather conditions. In order to obtain optimum system performance, compressor capacity control is a necessity.

The 3 traditionally used methods of capacity control are:

i)On/off Control: It is used when the power demand is irregular and the compressor wastes power if it runs continuously.

ii)By-pass or spill-back method: It is the most popular method of control owing to its smoothness, low initial cost and ease of application. Moreover, it can control the capacity from 0 to 100%.

iii)Constant speed step control: In this method the variation of capacity is done by unloading the cylinder ends. Commonly used schemes are 3 step and 5 step Power is saved with the increase in the number of steps.