Abstract: The winding temperature of an asynchronous motor is determined for control as regards overheating, in that a value representing the prevailing motor resistance is calculated on the basis of the amplitude of the voltage (2) impressed on the motor, the amplitude of the motor current (1), the phase angle (8) between the voltage and the current, as well as the slip (3), derived from overtones in the current flowing in the input lines to the motor. The resistance value is converted (15) to a corresponding temperature value which is used for updating a winding temperature value produced in parallel on the basis of the difference between the dissipated power and the cooling loss power, when the slip lies within predetermined limits.

Abstract: For determining the speed of an asynchronous motor use is made of the most manifest tone in the frequency spectrum beneath an overtone to the basic tone of the motor current. The frequency difference between the overtone and the tone in the subjacent frequency spectrum is determined, and thereby the slip and speed of the motor can also be established.

Abstract: A load indicator for A.C. motors has an analog multiplying circuit for producing a signal proportional to the product of the current of the A.C. motor and the voltage thereof. From this signal a signal is substracted which is proportional to the absolute value of the motor current. The average value of the differential signal is a measure of the load of the motor which is substantially independent of variations in the motor voltage, provided the proportionality constants are chosen in a suitable manner.

Abstract: Renewed starting of an intermittently operating pump is controlled in that the running time of the pump from the last start thereof is measured by means of a running time gauge the measuring value of which is maximized, and in that the pause time commencing at the end of the running time is controlled by start delaying means in dependence on the duration, measured by the running time gauge, of the preceding running time such that a long running time gives a short pause time and a short running time gives a long pause time.