Single-phasing
What happens when a three-phase load loses one phase — and why it's so damaging to motors.
What it is
Single-phasing is when a three-phase load loses one of its three phases and tries to run on the remaining two. It's a fault condition, not a mode of operation.
Why it's damaging
A three-phase motor running on two phases can't develop a proper rotating field. If it's already turning it keeps going but draws much higher current on the remaining phases, overheating quickly. If stopped, it usually just hums and won't start — drawing locked-rotor current and cooking itself.
Total silence usually means all three phases are gone; a hum with no rotation is the classic single-phasing signature.
Finding it
Measure phase-to-phase voltages and find which pair is down, then work back from the load toward the supply — a blown fuse, a non-making contactor pole, a burnt connection, or an upstream supply loss. A phase-failure relay is fitted specifically to detect this and protect the motor.
Safety first
A 'dead' phase at one point can be live at another. Single-phased motors overheat fast — isolate promptly. Prove dead carefully.
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
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
Motor hums but won't start to turn
On start the motor hums or buzzes but won't rotate — it may draw heavy current and trip protection. Often a sign it's only getting two phases or is mechanically locked.
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Phase failure / monitoring relay
Watches a three-phase supply and disconnects the load if a phase is lost, unbalanced, or in the wrong sequence.
Three-phase power
Three AC supplies offset in time, giving smooth power and a rotating field for motors.
Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.