Possible faults on poly phase induction motors induction motors
Symptoms Possible causes Test and/or Rectification
Fuses or overcurrent trips operate at start Premature operation of protective gear.


Reversed phase of staticwinding.winding.

Short circuit of earth fault on stator circuit.

Short circuit of earth fault on rotorcircuitcircuit
Motor will not start Faulty supply or control gear.

Overload or low starting torque.

Open circuit in one stator phase.

Reversed phase of stator winding.

Open corcuit in rotar circuit.
Overheating bearing or noisy operation Bearing or mechanical defects.  
Periodical growl Reversed stator coil or coils.  
Humming of squirrel-cage motor Loose joints on rotor conductors.  
Fluctuating stator motor Open circuit in rotor circuit.  
General overheating of case Faulty ventilation, mechanical or electrical overload.

Rotor core not fully in stator tunnel.

Open circuit in one of two parallel stator circuits.
Reassemble motor correctly.
Overheating and overloading, two phases of star-connected stator or one phase of delta winding hotter than the rest. Reduced speed. Single phasing owing to open-circuited supply line.

Open circuit in one phase of stator circuit.

Mechanical overload, low volts or low frequency.

Open circuit in rotor circuit.
Reduced speed of slip-ring motor Rotor starter not fully operated.

Slip rings not short-circuited.

Voltage drop on cables to rotor starter.
Overhaul protective gear to ensure correct operation.

Use slip ring short-circuiting gear.

Fit rotor starter nearer motor, or use larger rotor circuit cables.