| 摘要 |
<p>Overload protection for an electric motor is provided by two RC temperature-simulating networks each charged from a voltage representing the square of the motor current and each comprising a resistor 1301, 1700, a switch 16, 17 controlled by recurrent pulses, and a capacitor 24, 25. The voltage across each capacitor 24, 25 is compared (29, 30) with a respective reference voltage in order to detect overload. The time constant of the second network 1700, 17, 25 represents the conventional cooling of the motor, indicated by the manufacturer. The time constant of the first network corresponds to the cold starting time for a current equal to six times the nominal current. The second reference voltage corresponds to a overload of a few percent and the first reference voltage is greater than this. A cross-coupling network 27, 2701, 2702 prevents the voltage across the first capacitor 24 falling below that across the second (25). The pulses supplied to switch 17 are varied depending on whether the motor is being energised or not. <IMAGE></p> |
