Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
What it is
An induction motor converts electrical energy into rotation. It has a stationary part (the stator, with windings) and a rotating part (the rotor). It's robust, cheap, and everywhere.
How it works
Three-phase current in the stator windings creates a magnetic field that rotates around the stator. This moving field induces currents in the rotor, which create their own field — and the rotor is dragged along, chasing the rotating field.
The rotor always runs slightly slower than the field (that difference is 'slip'), which is what lets it induce rotor current and produce torque. Single-phase motors need a start winding/capacitor to get the field rotating in the first place.
Where it's used
Pumps, fans, compressors, conveyors, machine tools — almost any rotating load. Direction reverses by swapping any two of the three phases.
Common faults: won't start (no supply, single-phasing, seized load), runs hot (overload, cooling, imbalance), tripping overloads, bearing noise, and low insulation to earth.
What it looks like when it fails
Windings fail three ways: open (one phase reads OL — the motor hums or won't start), shorted turns (runs hot, draws unbalanced current, trips the overload), or broken down to earth (trips the RCD or breaker; megger reads low). Single-phasing looks like a winding fault but usually starts upstream — a blown fuse, a burnt contactor pole, a loose termination.
Mechanically, bearings dry out and rumble before they seize — and a seized or jammed load trips on locked-rotor current with a perfectly good motor on the end of it. Blocked fan cowls cook motors slowly.
How to test it
Isolated and proven dead, at the motor terminals: winding resistance between each phase pair should be low and near-identical — the balance between pairs matters more than the exact number (a few ohms on small motors, less as they get bigger). One pair reading OL is an open winding; one pair noticeably lower than the others suggests shorted turns.
Insulation resistance: 500 V DC megger from each winding to earth — commonly expected to be well above 1 MΩ (verify the acceptance value you work to; a damp or dirty motor reads low and may recover after drying out). Running: clamp each phase — currents should be close to balanced, at or under nameplate FLC.
Gotchas
Disconnect the VSD or soft starter before you megger — 500 V into drive electronics kills them. Same goes for thermistor/PTC sensor leads: insulation-test the power windings only, never the sensor circuit.
On six-terminal motors, note the link arrangement (star or delta) before measuring or you'll misread the winding resistance. A motor that megs fine cold can still break down hot or damp — test as found where you can. And on dual-voltage nameplates, use the FLC for the connection you actually have.
Safety first
Rotating machinery can start unexpectedly when protection resets. Isolate, lock off, and prove dead; allow hot motors to cool.
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 won't start and makes no sound at all
Press start and nothing happens — no hum, no movement, no attempt to turn. The motor is completely dead rather than struggling.
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.
Motor running hot / overheating
The motor runs but gets excessively hot — too hot to touch, smell of hot insulation, or thermal protection cutting in after a while.
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.
Related definitions
VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.
Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Forward / reverse circuit
Two contactors run a motor in either direction; reverse swaps two phases, and an interlock prevents both closing at once.