At every start the compressor motor draws several times its rated current and the winding heats up; for this heat to be removed the motor has to run for a while and be cooled by the suction gas. At start-up some oil is also thrown from the crankcase into the system and returns only with sufficient running time. This is why compressor manufacturers limit the number of starts: depending on the model, 6–12 starts an hour, at least 2–3 minutes of running and at least 2–3 minutes of waiting before restarting.
The causes fall into two groups. Those arising from the control: a thermostat differential set too narrow (it should be 2–3 K rather than 0.5 K), the temperature sensor placed in the evaporator’s discharge air, equipment selected with a capacity much greater than the demand. Those arising from the safeties: the low-pressure switch cutting the system out and in again because of low charge or a blocked filter, or the high-pressure switch doing so because of a dirty condenser or a stopped fan. Warm air hitting the sensor while the door is open produces the same result.
A special kind is seen in pump-down systems: if the liquid line solenoid valve or the compressor valves leak at standstill, the suction pressure rises slowly, the pressure switch starts the compressor, and the compressor collects the gas in a few seconds and stops again. The compressor running in short bursts every few minutes although the room is not calling for cooling is the sign of this. The solution is to replace the leaking valve and to select the single pump-down setting on the controller. This fault is easiest to notice at night, when the room is in balance.
For diagnosis one looks at the start counter of the controller or at the remote monitoring records; the number of starts per hour and the running times show the problem at once. It is established which element is doing the stopping: the thermostat, the low pressure, the high pressure or the overload. Lasting solutions include entering a minimum running and waiting time in the controller, widening the differential, and using capacity control or an inverter on large equipment. For the right capacity the cooling load calculator is a good starting point.