Electronic relay to protect motors and thyristors against symmetric overload, phase unbalance and short-circuit

摘要

A protective electronic relay is disclosed of the type which includes a feed source with a one-phase transformer, rectifying bridge, filter condenser and voltage regulator. Pickups are provided where one- phase signals are generated, connected to some diodes, connected to some capacitors and to a zener diode for the purpose of obtaining rectified, filtered and limited signals with a voltage level proportional to the line intensity of the protected motor. A multiple microswitch connected to some resistors permits presetting of the voltage level and nominal triggering intensity of a relay. An R-C network that can be timed in various scales comprised of resistors a capacitor and another multiple microswitch allows adjustment of the triggering time constant and is applied to that voltage level at the non-inverting input of an operational amplifier whose inverting input is at a reference voltage. Upon the non-inverting input of the operational amplifier reaching the reference voltage, due to a symmetric overload, the output of the operational amplifier passes to logic state 1. This sends a positive signal to the gate of a thyristor, driving it into conduction and depolarizing the base of a transistor making it pass from saturation to cut-off. As a result a relay connected to the collector of the transistor is triggered, changing the state of its contacts and causing disconnection of the protected motor.