In Induction Motors the EMF induced in the Rotor Winding is Mutually Induced EMF.Induction Motor can be treated as a Rotating Transformer as the E.M.F 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.

In Synchronous Machine the Magnetic Flux is set up separately by Field Winding.The Emf induced in the Stator Armature Winding is not by Mutual Induction.It is 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 will be much more.

P.S.

Induction motor air gap is less:

To avoide the noise.

To improve the power factor.

To avoide the leakage flux.

Induction motor is operated in induction principal so

distance between rotor and stator is less.

by: allinterview,com

and it is ok accroding to me

AIR GAP in induction motor

The main source of low power factor at which induction motor operates is the air gap between the stator and the rotor. This air gap increases the reluctance between the stator and the rotor, which enhances the magnetizing current for production of the given mutual flux between the stator and the rotor for a given supply voltage. Therefore, the no-load current of an induction motor becomes 30 to 40 per cent of full-load current, which is remarkably larger than the no-load current of a transformer for a given kVA rating. The air gap in an induction motor should be made small so that the induction motor gives better performance. The small air gap may result mechanical problems in addition to the noise and losses at the slot tooth faces.

S.M. AIR GAP IS NOT A GOVERNING FACTOR OF P.F.

main points are mention by Mr. Malik

Thanks for your valuable responses, but why air gap in synchronous machines is large with respect to induction ones?

Dear Iman

It is well known that due to being self-excited a synchronous machine is a source of reactive power. This reactive power is a function of the air-gap and a larger air-gap or low synchronous / magnetising reactance allows the synchronous machine to deliver and absorb higher levels of reactive power. This is very important as reactive power is required for establishing and maintaining the EM fields in a power system network and voltage stability. Also as power transfer along a transmission line is inversely proportional to the synchronous reactance it is desirable to have a low value to allow for higher active power transfer and a larger air-gap also allows for this.

The IM machine on the other hand is self excited and must draw its reactive power from the supply. To limit the reactive power drawn from the supply one requires a high magnetising reactance which is achieved by using a smaller air-gap.

That is why transformers draw considerably less reactive power than IM's as Akhilesh points out above.

Thus the SM has a large air gap for low magnetising reactance to allow higher reactive and active power to be delivered whereas the IM has a small air gap for higher magnetising reactance to limit the reactive power drawn.

It certainly is true and a very good, interesting and important question to have asked.

Kind regards

Greg Diana

Dear Diana very thanks for your comprehensive response.

In Induction Motors the EMF induced in the Rotor Winding is Mutually Induced EMF.Induction Motor can be treated as a Rotating Transformer as the E.M.F 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.

In Synchronous Machine the Magnetic Flux is set up separately by Field Winding.The Emf induced in the Stator Armature Winding is not by Mutual Induction.It is 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 will be much more.

P.S.

Simply put. The magnetizing field in an IM has to jump the gap, the bigger the gap, the harder it is to get the flux induced in the rotor cage, and the more current it takes to get the job done i.e. loss of efficiency. Synchronous machines have a separately excited field in the rotor, it is fed through other means and does not have to jump across the air gap, so the gap is bigger to facilitate manufacturing.

According to me the stability of synchronous machine is indirectly related to air gap length. So to improve stability air gap is more for sync machines. Checkout the relations between short circuit ratio, airgap length, sync power coeficient..

And there is no problem of power factor in sync machines ,we can overexcite the machine

I think that short circuit protection is a factor .In the case of generator I suppose it is

clear that synchronous reactance must be high if subtransient reactance is ignored

for a synchronous generator. Even as a sync motor an external fault would be fed

till 1/2 * H*w^2 goes to zero . In an induction motor the rotor resistance being high

the transient would die down fast. Stability in sync m/c can be addressed with field over excitation . other points are the design of damper bars , field shaping , noise etc.

Also Synchronous m/c operate generally at high speeds (2pole) so mechanically

they need to be stiff viz UMP..and CF

In addition to the justifications above, which take into account that in induction machine model the magnetizing reactance is in parallel, but in synchronous machines the synchronous reactance is in series, another reason for larger airgaps in synchronous (and DC) machines in comparison to induction machines is the effect of armature reaction. It is desirable to avoid distortion of the main flux (created by rotor in synchronous machines and by stator in DC machines), this can be achieved by a larger airgap. Of course, this will lead to higher field current requirements and thus higher field circuit copper losses.