Protective system for inverter circuit used in driving AC motors

摘要

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. Also provided is protective circuitry with a current-limiting function adapted to suppress an increase in the output current of the inverter circuit and limit this current to a stable value without terminating the operation of the inverter circuit at such time that the current continuously flowing into the transistors of the inverter circuit is about to flow to its output side at an increased magnitude that could damage the transistors.