Lower supply-voltage limit monitor - protects asynchronous motor during starting from voltages too low to overcome loading torque

摘要

The voltage monitor protects the asynchronous motor during starting from overheating caused by voltages that are too low to start the motor. The a.c. suply is applied over a smoothing network toa discharge lamp coupled to the gate of a FET. When the lamp is pulsed only a capacitor coupled to the FET gate is charged. When the voltage falls and the lamp turns off, the capacitor is discharged through the FET causing the voltage to rise across its emitter resistor and to turn on a triac power switch in series with the supply line to the motor. If the voltage remains low the lamp remains off and the capacitor cannot recharge to turn on the triac again after it has turned itself off.