Why air gap length is small in induction motor than synchronous motor?
An Induction Motors (IM) works on the principle of Electromagnetic Induction. The induced EMF in the rotor winding or rotor bars is due to mutually induction caused from the rotating magnetic field produced be the stator winding. Thus, an IM can be treated as a Rotating Transformer as the EMF induced in the rotor is by Mutual Induction. If the air gap is more the leakage flux will be more and the mutual flux gets reduced, reducing rotor EMF, current and torque. Also, power factor of motor will decrease if air gap is more due to increase in leakage current and noise also increases.
But, in a Synchronous Motor (SM) the Magnetic Flux is set up separately by the field winding.The EMF induced in the stator armature winding is not by mutual induction, but it’s a dynamically induced EMF due to relative motion between the field and conductors. Hence, air gap is not the consideration, particularly for salient pole machines, in the region between the poles. the air gap is much more. Thus, air gap is not a governing factor of machine’s power factor.
Hence air gap length should be smaller in IM but can be larger in a SM.