Abstract

Method and apparatus for controlling the operation of a motor driven compressor which is automatically unloaded when the receiver pressure reaches a predetermined maximum level and loaded when the receiver pressure reaches a predetermined minimum level by means of pressure actuated electric control means which are time delayed to start and stop the compressor motor and the unloading and loading operation in accordance with predetermined mode of operation. The compressor driving motor is controlled to stop immediately upon the receiver pressure first reaching the maximum pressure level whereupon time delay circuit means starts to measure a time interval during which the motor upon having been restarted in response to a restart pulse is prevented from further stoppage each time the maximum receiver pressure level is reached subsequent to the first, whereby the compressor can be brought on load immediately in response to demand for working medium during the measured time interval and whereby overheating of the motor between subsequent stoppages and restarts is avoided. Upon expiration of the measured time interval, the motor will stop running until the minimum receiver pressure is reached, unless the compressor should be on load at such time, in which case the motor will continue running until the compressor again is unloaded upon reaching the maximum receiver pressure level, whereupon the aforementioned cycle is repeated.