Elevator Phase Sequence Relay: Phase Reversal Detection, Phase Failure Protection and Motor Burnout Prevention

The phase sequence relay (also called a phase monitoring relay or phase failure relay) protects the three-phase AC induction motor and transformer from incorrect supply conditions. Functions: (1) Phase sequence detection — verifies that the three phases (L1, L2, L3) arrive in the correct rotational order (e.g. L1–L2–L3 for clockwise rotation); if the sequence is reversed (e.g. by accidental interchange of two supply cables), the motor would run in the wrong direction, potentially causing the car to move in the wrong direction with dangerous consequences; (2) Phase failure detection — detects the loss of any one of the three phases (single-phasing); running a three-phase motor on two phases causes very high current in the remaining windings, leading to rapid overheating and motor burnout; (3) Phase voltage imbalance — some modern relays also detect excessive voltage asymmetry between phases (typically >3–5% imbalance), which also causes motor heating. Installation: the phase sequence relay is typically wired in series with the main contactor coil circuit so that if it trips, the main contactor drops out and the elevator cannot run. EN 81-20 does not specify the phase sequence relay directly but requires that all equipment be protected against incorrect operation due to supply faults. Phase sequence relays are typically set to trip within <1 second of detecting a fault.

View All Terms