Electric hoisting machine safety system - detects imbalance of three=phase supply using magnetic ring around each conductor

摘要

A three-phase motor (1), causing movement of the hoist is fed by three-phase conductors (2, 3, 4). Each conductor is encircled by a magnetic torus (5, 6, 7) which transmits a signal to a flowmeter (8), the output of which is fed to the coil of a relay (9), activating a switch (10). At the same time, each conductor (2, 3, 4) feeds a rectifying bridge (11), charging a capacitor (12) of high capacitance. Normally, the capacitor is charged, and the switch is open. If one of the three-phase conductors is open-circuited, the imbalance created in the flowmeter results in the switch closing, and thence the capacitor discharging into the control coil (13). The control coil then activates the safety function by, e.g. cutting off the supply to the motor. Instead of a relay, a transistor or thyristor may be used.