Abstract: A discharging circuit for a power source device with a rectifier circuit used in an power source for driving a motor in which when the alternating current power source is turned off and the load is electrically separated, the voltage of a time constant selection circuit (22, 23) falls rapidly, the voltage across a diode (25) rises, a Schmitt circuit (24) is activated, the circuit consisted of a transistor (27) and a resistor (26) is conducted, and a smoothing capacitor (28) is discharged, and hence the heat generation at the resistor is decreased and the concern over unsafety due to delayed discharge of the capacitor is eliminated.

Abstract: The invention relates to a speed control system which includes an inverter circuit for converting a direct current into a polyphase alternating current in the form of a rectangular wave by the switching action of transistors, and an inverter drive circuit adapted to apply control signals to the inverter circuit to bring its output frequency to a desired value, an AC motor being rotated by the alternating current supplied by the inverter circuit. The system features current detection means for detecting the current that flows thrugh the input side of the inverter circuit, and a circuit adapted to terminate the operation of the inverter circuit when the value detected by the current means approaches a value large enough to damage the inverter circuit transistors as at such time that a large momentary current is subjected thereto.