A protective relay that continuously monitors the mains supply for undervoltage, overvoltage, phase failure, phase sequence error, and phase asymmetry, disconnecting the drive on detection of any fault condition.
Full Definition
The voltage monitoring relay (phase monitoring relay, supply quality relay) protects the elevator drive system and motor from damage caused by mains supply faults. A three-phase monitor detects: undervoltage (typically set at 85% of nominal), overvoltage (115%), phase failure (loss of any phase), incorrect phase sequence (which would cause the motor to run in reverse), and phase asymmetry (voltage unbalance exceeding 2–3%). On fault detection, the relay opens its output contact within 100–500 ms, de-energising the main contactor in the elevator control circuit. The relay automatically resets when supply quality returns to normal and a programmable delay (5–60 s) has elapsed, allowing the motor to stop before restart. EN 81-20 requires protection against phase failure and phase reversal for three-phase motors; the voltage monitor fulfils this requirement when incorporated in the safety circuit. Single-phase monitors provide under/overvoltage protection for single-phase auxiliary circuits. Modern frequency converters (VVVF drives) incorporate internal mains monitoring, but a dedicated external relay is still required as the safety circuit interlock.
