Phase sequence & rotation
What 'phase rotation' means on a three-phase supply, and why getting it wrong reverses motors.
Order, not just presence
A three-phase supply has three lives whose voltages peak one after another, evenly spaced through the cycle. The order in which they peak — the phase sequence — is as much a property of the supply as the voltage itself. Swap any two phases and you reverse that order.
A motor cares about the order, not the labels: the rotating field it sets up turns in the direction the phases arrive. Connect the three phases in the standard sequence and it turns one way; swap two and the field — and the shaft — turn the other way.
Why it matters on site
Sequence matters anywhere direction matters: pumps, fans, conveyors, compressors. A pump running backwards may still move a little liquid and look like it's working while delivering almost nothing; a fan can move air the wrong way; a compressor can be damaged. After any supply work — a new connection, a board change, a generator changeover — the sequence is checked before motors are run.
It's verified with a rotation tester (or by proving direction on a known load), and corrected by swapping two phases. Three-phase outlets and connectors are wired to a consistent sequence so portable equipment runs the right way wherever it's plugged in.
Safety first
After any supply or board work, prove phase sequence before starting motors — a reversed pump or compressor can run backwards undetected or be damaged.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Motor runs in the wrong direction
The motor runs but the wrong way — pump runs backwards, fan blows the wrong way, conveyor reverses. Usually a phase-rotation issue after wiring or supply work.
Three-phase outlet / appliance not working (one phase missing)
A three-phase outlet or appliance (welder, large machine, commercial kitchen gear) isn't working right — a missing phase at the socket leaves it under-powered or not starting.
Motor goes one way but won't go the other (e.g. down but not up)
A reversing drive works in one direction only. One command (say, down) runs fine; the other (up) does nothing, or just hums/trips. Common on hoists, doors, and conveyors.
Related definitions
Three-phase power
Three AC supplies offset in time, giving smooth power and a rotating field for motors.
Forward / reverse circuit
Two contactors run a motor in either direction; reverse swaps two phases, and an interlock prevents both closing at once.
Phase failure / monitoring relay
Watches a three-phase supply and disconnects the load if a phase is lost, unbalanced, or in the wrong sequence.