Induction Motors — Principle, Faults & Testing

1. Rotating Field, Chasing Rotor

Feed the stator three phases and their magnetic fields add into one field rotating at supply speed. That sweeping field cuts the shorted rotor bars, induces current in them, and the rotor's own field drags after the stator's — always slower, because zero relative motion would mean zero induction and zero torque. The lag is slip: small at healthy load, maximum at standstill (which is exactly why starting current explodes — see starters).

Most shipboard AC motors are this squirrel-cage type: copper conductors along the rotor axis joined by end rings into a cage. Skewing staggers each bar's field-cutting so torque flows smoothly instead of jerking — while defeating slot-to-slot magnetic cogging and quieting running hum. Healthy slip runs 1–5% from no-load to full-load: a near-constant-speed machine whose widening slip under load is torque answering the call.

2. Single Phasing — One Phase Lost

Running motor

Keeps turning on two phases but draws heavy excess current, hums louder, vibrates instead of rotating cleanly, overheats — and burns if protection is blind to it (plain heater overloads often are).

Stationary motor

Cannot start at all — two phases make a pulsating, not rotating, field. Hums, draws locked-rotor current, goes nowhere.

Causes live upstream of the windings: a blown backup fuse in one leg, an open conductor, loose wires, bad starter contacts, breaker or fuse failure, overload-relay faults. Test under load at the motor terminal box with meters on all three phases at once — phase-to-ground readings isolate the dead leg (healthy ≈ phase-to-phase ÷ 1.7). Beware the trap: pitted contacts pass meter voltage yet collapse under load current, so test loaded, then lock out and work upstream from contactors to supply.

Defence: three-phase loads deserve protection that drops all three legs together, plus dedicated single-phasing, over-current and negative-phase-sequence relays — larger motor relays arrive with single-phasing protection already fitted.

3. Fail to Start — Walk the Chain in Order

1. Supply: is power actually there — all three phases at the starter? A blown fuse or a dead incoming leg is the commonest silence of all.

2. Protection: has the overload tripped? Reset properly (off before on) and allow bi-metallic cool-down — then ask why it tripped before re-closing.

3. Control circuit: contactor coil burnt, contacts pitted, control relay faulty, stop switch open-circuit, start button not operating, hold-on (seal-in) contact not making so the starter drops the instant you release.

4. Motor itself: only now — continuity, insulation, and the burnt-winding verdict (run or rewind).

Warning — A motor that will not start cooks fastest.

Locked-rotor current with zero cooling airflow is the harshest heating a winding ever sees. Find the break quickly, and never hold the start button in hoping it catches.

4. Speed, Direction and the Wider Family

Wound-rotor — the heavy lifter

Rotor windings brought out to slip rings so external resistance shapes the starting torque. Lives on portable machines and small winches where breakaway torque matters most.

AC speed — frequency only

Speed follows supply frequency, so variable speed needs a 3-phase frequency converter (fast-switching power devices). Lowering volts alone only starves torque — it never commands speed.

AC direction — swap two lines

Interchange any two supply phases (R-Y-B to R-B-Y): the rotating field reverses, and the rotor follows it the other way.

DC motors (rotor and stator, field poles, commutator, brushes, fan, bearings, housing) answer differently: speed from a constant-voltage supply through a series pot or PWM chopper, direction by reversing armature or field polarity — never both (both flips cancel). And a DC motor fed with AC is a casualty in motion: low speed, brush sparking, eddy-current heat, then a burnt winding.

5. Megger Proof and the Three Fault Tests

A multimeter uses roughly 3 V and can show a false pass on weak insulation. A megger applies 500–1000 V DC to reveal leakage under electrical stress.

Ritual: self-test (probes shorted ≈ zero), isolate and lock out, prove frame earth continuity first, then log phase-to-phase (U-V, V-W, W-U) and phase-to-earth. Motors pass at 1 MΩ minimum, HV conductors at (kV+1) MΩ (440 V → 1.44 MΩ, 11 kV → 12 MΩ). Below that, high-voltage machinery stays off. Moisture, dust, oil films and vibration are what drag readings down.

Return to service — three checks before the load sees the motor: open, short, earth and IR clear; windings wired correctly (never a delta motor in star); rotation proved right before coupling the load. Dead after overhaul — check in this order, then verdict: run or rewind: terminal bars and star/delta links first, supply voltage (230/400) second, winding continuity and equal phase ohms third, IR fourth.

6. Temperature Class — Heat the Insulation May Take

Overload itself is just physics abused: slowdown draws excess current, excess current cooks windings — and temperature is what finally kills insulation, which is why heat rises and resistance falls in insulators while conductors grow more resistive with heat.