The signs
Any one of these is worth a proper check. Two or more together usually mean the windings:
- A burning smell — sharp, like hot varnish — from the motor body.
- Visible scorching or darkened windings through the end covers or vents.
- The motor hums but does not turn, or turns only with a push.
- It trips the breaker or blows fuses as soon as it starts.
- It runs far hotter than usual to the touch, even on a light load.
- Speed drops and it struggles on a load it used to manage.
- A megger (insulation) test shows low resistance to the frame.
What else it could be
Not every fault is a burnt winding. A three-phase motor that loses one phase will hum and overheat — the motor is fine; the supply is not. Worn bearings make a grinding or rumbling noise and can let the rotor rub. A jammed pump or gearbox will stall a healthy motor. These are cheaper to fix, so check them first.
Why windings fail
Winding insulation ages with heat. The usual causes of too much heat are:
- Overloading, or running a motor too small for the job.
- Low or unbalanced voltage, common at the end of long rural lines.
- Single-phasing: a three-phase motor running on two phases.
- Blocked cooling — dust, flour, or a guard wrapped over the fan.
- Moisture, especially in motors left idle through a damp winter.
What to do now
Switch off and do not keep restarting it. Each restart on a damaged winding adds heat and can spread the damage into the core, making a repair harder or impossible. Note the nameplate details — HP or kW, voltage, speed — and what happened just before it failed. That is what a rewinder will ask first.
General guidance only — every installation is different. For work on live equipment, use a licensed electrician. Last reviewed 28 September 2026.