Losses in a Squirrel-Cage Induction Motor

When a squirrel-cage induction motor operates on load, part of the electrical input power is lost in the motor due to the following:

1. Stator Copper Loss (I²R Loss): Loss due to the resistance of the stator windings. It is proportional to the square of the stator current.
2. Rotor Copper Loss (I²R Loss): Loss occurring in the rotor bars and end rings due to their electrical resistance. It is proportional to the square of the rotor current.
3. Core (Iron) Losses: These occur mainly in the stator core and consist of:
   • Hysteresis Loss – caused by repeated magnetisation and demagnetisation of the iron core.
   • Eddy Current Loss – caused by circulating currents induced in the laminated iron core.
4. Rotor core losses also occur, but under normal operating conditions they are very small and are generally neglected because the rotor/slip frequency is low.
5. Mechanical Losses: These consist of:
   • Friction Loss – caused mainly by friction in the bearings and other rotating contact points.
   • Windage Loss – caused by air resistance acting on the rotating rotor and cooling fan.
6. Stray Load Losses: These are additional small losses caused by leakage flux, harmonic magnetic fields, and non-uniform current distribution in the stator and rotor.

(i) Losses independent of load current and speed

• Stator Core (Iron) Losses – Hysteresis and Eddy Current Losses

These losses depend mainly on the supply voltage and frequency. Therefore, when the supply voltage and frequency remain constant, they are approximately independent of load current and motor speed.

(ii) Losses dependent on load current

• Stator Copper Loss
• Rotor Copper Loss
• Stray Load Loss

The stator copper loss varies as I₁²R₁, while the rotor copper loss varies as I₂²R₂. Therefore, these losses increase as the motor load and current increase. Stray load losses also increase approximately with load current.

(iii) Losses dependent on speed

• Friction Loss
• Windage Loss

These mechanical losses vary with the speed of rotation. Friction loss generally increases with speed, while windage loss increases more rapidly with speed and is approximately proportional to the cube of speed.

Note: Rotor core loss also varies with rotor/slip frequency (fᵣ = s × f) and is therefore related to slip and rotor speed. However, under normal operating conditions it is very small and is generally neglected.