Effect of too wide air gap in induction machine

The air gap is the main reluctance in the magnetic circuit of an induction motor, so its length strongly affects performance.

Effects of a too-wide air gap

• High magnetising current: the gap mmf $= 800000\,B_g K_g l_g$ rises in proportion to $l_g$, so the no-load current rises sharply.
• Low power factor: the larger magnetising (reactive) current lowers the power factor; this is the most serious effect.
• Higher copper loss and lower efficiency at light loads because of the large no-load current.
• On the good side: lower leakage reactance (higher overload capacity), less noise, better cooling and less unbalanced magnetic pull.

So a wide gap is used only when overload capacity or mechanical reasons are more important than power factor.

Factors in selecting the air gap length

1. Power factor: small gap for low magnetising current and high power factor.
2. Overload capacity: larger gap lowers zigzag leakage reactance and raises the maximum torque.
3. Pulsation loss and noise: larger gap smooths the tooth-ripple flux, reducing pulsation losses, noise and vibration.
4. Cooling: larger gap allows more cooling air.
5. Unbalanced magnetic pull: larger gap reduces the effect of rotor eccentricity.
6. Mechanical factors: shaft deflection, bearing wear, length of core and manufacturing tolerances set a minimum gap.
7. Harmonics: larger gap reduces slot harmonic fields and their parasitic torques (cogging, crawling).

Empirical formula

$$l_g = 0.2 + 2\sqrt{DL}\ \text{mm} \quad (D, L \text{ in m})$$

Example: $D = 0.25$ m, $L = 0.16$ m gives $l_g = 0.2 + 2\sqrt{0.04} = 0.6$ mm.