and higher electric frequency f 2 which are both lower than the frequency f n of the power supply current 50 or 60 hz ie in the line conductor leads l and n it will be readily appreciated that a pre set speed of the motor 10 corresponds to each one of said frequencies eg a speed of 1 600 rpm may correspond to the frequency f 1 and a speed of 2 400 rpm may correspond to the frequency f 2 it actuates the normally open switch 23 to close on the third terminal contact 22 of the control unit 7 so as to short circuit the above cited first and second terminal contacts 18 and 19 of the same control unit 7 in this way the motor 10 is supplied at a frequency f t that is higher than the other two and may for instance be equal to the frequency f n of the power supply line so as to enable the motor 10 to rotate at a speed of 3 000 rpm it monitors via the line 9 both the absolute duration of operation of the compressor driving motor 10 and the utilization index generally known as operating percentage thereof which are two quantities that vary with time in a mutually independent manner it is of course also capable of calculating both the absolute values and the possible combinations of said variable quantities and or functions thereof such as for instance the consecutive number of operating cycles of.