The air gap in three-phase induction motors impacts the performance of the machine. Air gap length is usually abbreviated by l with subscript g. That is lg
Formula to find the air gap length
- In case of smaller machines: lg = 0.2+2√(DL) mm
- In case of larger machines: lg = 1.6√(D)-0.25 mm
D = Diameter in meters
L = Length in meters
The air gap and power factors are inversely related to each other. For good power factor the air gap should be small. Of all rotating parts of machine, the air gap partakes maximum MMF in order to allow flux through it. The greater length of air gap will demand greater number of ampere turns to magnetize the air gap as a result of which the magnetizing current will be increased and power factor will be reduced.
A larger air gap reduces the zigzag leakage reactance which results in larger overload capacity.
Zigzag leakage flux results in pulsation losses and noise. A smaller air gap has increased zigzag leakage flux, while on the other hand a larger air gap reduces zigzag leakage flux and henceforth pulsation losses.
In case of small air gap, the unbalance magnetic pull is visible, however if the air gap is larger, the unbalanced magnetic pull is not noticeable.
Noise level is inversely related to air gap. Larger air gap has less noise and vice versa.
From above points it is well understood that smaller air gap is recommended if good power factor is required, while larger air gap is desired if less noise, less pulsation losses, larger overload capacity, and less unbalanced magnetic pull is desired.
For most cases, electrical design engineers are often interested in power factor which plays a predominant criteria in fixing the length of air gap.