Drive system for hoist or lift - has emergency braking device shorting together terminals of DC motor when supply fails

摘要

The drive system has a d.c. motor and an emergency braking device. The motor (2) is permanently excited. The emergency braking device (6,8) has a braking current path (6,7) that automatically bridges the motor's terminals when the motor's power supply fails. The motor is supplied from a controlled rectifier (3) connected to a mains supply (5). The rectifier is controlled (4) to give the motor the required speed and torque. The emergency braking device comprises a relay (8) that operates when either the mains or the rectifier fails. The relay's contacts (7) connect a load resistor (6) across the motor's supply terminals and disconnect these terminals from the output of the rectifier.