Factors for air gap length in induction motor

The air gap of an induction motor is kept as small as mechanically possible, because the gap needs most of the magnetising mmf. Its final length is a compromise between the following factors.

1. Power factor (magnetising current)

The gap needs the largest share of magnetising ampere-turns. A larger gap increases the magnetising current and lowers the power factor. This is the main reason to keep the gap small.

2. Overload capacity

A larger gap reduces the zigzag and other leakage reactance. Lower leakage reactance gives a larger maximum (breakdown) torque, so the overload capacity improves with a larger gap.

3. Pulsation (tooth) losses and noise

Slot openings cause flux pulsations in the tooth tips. With a larger gap these pulsations are smoothed, so pulsation losses, magnetic noise and vibration are reduced.

4. Cooling

A larger gap allows more air to pass and gives better cooling of the rotor and stator surfaces.

5. Unbalanced magnetic pull

Any eccentricity of the rotor produces a one-sided pull. Its effect is relatively smaller with a larger gap because the percentage change in gap is smaller.

6. Mechanical considerations

The gap must allow for shaft deflection, bearing wear, and manufacturing tolerances of stator and rotor. Machines with long cores or large diameter need larger gaps.

7. Harmonics

A larger gap reduces the effect of slot harmonics, so harmful harmonic torques (cogging, crawling) are reduced.

Empirical formula

For small and medium machines (Sawhney):

$$l_g = 0.2 + 2\sqrt{DL}\ \text{mm}$$

with $D$ and $L$ in metres. Typical values are 0.25 mm to 2 mm; for example, a motor with $D = 0.16$ m and $L = 0.085$ m has $l_g = 0.2 + 2\sqrt{0.0136} = 0.43$ mm.