Length of Air Gap ( Factors affecting selection of Air Gap of 3 phase induction motor

Length of Air Gap ( Factors affecting selection of Air Gap of 3 phase induction motor) 




  • Length of air gap is very important in the induction motor as it controls important characteristics of induction motor such as power factor, over load capacity etc. 

Following points should be considered while selecting length of air gap:

(1) Power factor:-

Power factor


  • In the performance characteristics of motor this is very important factor. The air gap consumes maximm amount of mmf out of various parts of magnetic circuit of induction mtor. Hence, larger the air gap, more wall be the mmf required to pass the flux through. it can be shown that mmf required to send the flux through air gap is proportional to the product of flux density and length of air gap. 
  • Thus if length of air gap is increased then mmf required I. e. In turm magnetizing current is  increased, which reduces the power factor. Fig  shows the phasor diagram of inductor motor. As length of air gap increases, magnetizing current increases  and hence power factor becomes poor. 

A designer should keep length of air gap (lg) as small as possible to get good power factor. 

(2) Overload capacity:-

  • The overload capacity of machine depends on the leakage reactance. If leakage reactance reduces then overload capacity of machine increases and vice versa. Now the zig zag leakage reactance, which forms the larger part of total leakage reactance of induction motor gets reduced if the length of air gap is increased. 

Thus large length of air gap increases the overload capacity of machine.

Zig zag flux:


  • In this flux passes from one tooth to another tooth in zig zag fashion, across the air gap.As length of air gap increases, reluctance increases and so zig zag flux is reduced which in turn reduces zig zag leakage reactance.

(3) Pulsation loss:-

  • Tooth pulsation losses are produced due to variation of reluctance of air gap. When air gap length is large, variation of reluctance due to slotting is small which reduces pulsation loss. 

Thus a designer can increase the length of air gap to reduce tooth putlsation loss.

(4) Unbalanced magnetic pull(UMP):-

  • in induction motor, stator and rotor both produce rotating magneac field, so opposite poles are formed on stator and rotor. Thus, there is always force of attraction between stator and rotor in radial direction (I. e. perpendicular to shaft axis). Now all these forces get balanced (or cancelled) due to concentricity of stator and rotor. But due to eccentricity of the shaft, these radial force are not balanced and we get unbalanced magnetic pull (UMP). 
  • If length of air gap is small, then the small eccentricity produces large irregularity in length of air gap and is responsible for production of large UMP. 

Thus to decrease the effect of UMP a designer should increase the length of air gap.

(5) Cooling:-

  • Wnen length of air gap is large then cooling conditions become better as there is more distance between stator and rotor. This will reduce the temperature of machine and increases life of insulation.

(6) Noise:- 

  • The noise in the motor is produced due to variation of reluctance of zig zag leakage flux path. This reluctance can be increased by increasing the length of air gap which reduces noise produced by the motor.









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