Electrical reason for big air gap of induction motor

[jaus tail]

in induction motor there is air gap between stator and rotor. the air gap needs to be as small as possible. but some air gap is needed for mechancial reasons.

i understand that a big air gap would reduce power factor as you'd need more magnetizing current. but are there any advantages of big air gap in induction motor or should the air gap be as small as possible(not giving any consideration to mechanical limitations)?

 

[Babadag]

In induction motors, where the air-gap is usually quite small, it is necessary to correct the air-gap

permeance for the effect of slot openings. These make the permeance of the air-gap slightly smaller

than calculated from the physical gap, effectively making the gap a bit bigger. The ratio of effective

to physical gap is Carter's coefficient.See [for instance]:

http://link.springer.com/article/10.1007/BF01578532#page-1

or:

https://www.physicsforums.com/file:///C:/Users/user/Downloads/chapter_3.pdf

On the other hand a small air-gap could conduit to enhancing harmonics due to slot presence.

In my opinion,the harmonic reactance of the conductor in slot will increase if the gap decreases so the harmonic voltage will increase.

 

[jim hardy]

from a 1915 book:
Specification and Design of Dynamo-electric Machinery
By Miles Walker

(In his day Direct Current was often called Constant Current , "cc" instead of our familiar "dc")
https://books.google.com/books?id=FYdPAAAAMAAJ&pg=PA62&lpg=PA62&dq=motor+air+gap+considerations&source=bl&ots=7-aRQupePy&sig=n4QgT6Fz50Tna6uE8j_LnZhCoW0&hl=en&sa=X&ved=0ahUKEwisvLHG4dXJAhUE6yYKHc-6B4EQ6AEIRDAH#v=onepage&q=motor air gap considerations&f=false

so i guess you might summarize thus:
armature reaction and fringing flux play into choice of airgap .

old jim

 

[jaus tail]

Thanks for the replies.

I thought of armature reaction but some websites from google said that there is no concept of armature reaction in induction motor??

 

[jim hardy]

Does flux change, or just the current to produce it ?

 

[jaus tail]

I didn't understand your question...

Induction motor--stator has distributed winding, rotating magnetic field...field is moving with respect to stationary rotor so currents flow in rotor...current oppose cause which is motion, so rotor moves and relative speed of stator and motor reduces.
armature reaction would be by flux produced by these rotor currents.
i guess these currents would produce rotor flux that would have a demagnetizing effect on stator flux??

 

[Babadag]

If we may consider an induction motor as a transformer in which the secondary winding-the rotor winding-frequency is variable [f2=s*f1] then the magnetic field excitation current will be constant and equals the sum [vectorial] of stator and rotor currents. That means-virtually-the armature current [rotor current] will reduce the stator current magnetic flux. Actually the flux it is the product of both currents together. Io*ks*ws=Is*ks*ws+Ir*kr*wr where ks and kr are winding factors in stator and respective in rotor, ws no.of stator winding turns, wr the same of the rotor.

 

[jaus tail]

Sorry but I'm still confused. in the induction motor the stator has rotating flux. the rotor currents also have flux that will try to align with the stator flux to reduce the relative motion(speed). if i increase the air gap wouldn't the torque reduce, since now the flux from rotor that can link with the stator has reduced. so the interaction between stator and rotor fluxes reduces.

over here: http://www.electrical4u.com/induction-motor-types-of-induction-motor/
working principle of induction motor, last 6 lines.

 

[jim hardy]

if i increase the air gap wouldn't the torque reduce, since now the flux from rotor that can link with the stator has reduced.

Quite so, for any given stator current.

I think you have the correct picture in your mind.

In an interconnected system it's difficult to just vary one thing and lock all others.
If i just swapped rotors in a squirrel cage induction motor, replacing a large one with one say 3/4 the diameter
seems to me
stator current would go up restoring flux to whatever is necessary to counter line voltage,
but more of that flux would be leakage flux that bypasses the rotor reducing rotor current and torque ,
and by simple mechanics , tangential force on rotor bars would act on a shorter radius reducing torque,

so yes, the same effect we call in synchronous machines 'armature reaction' is fundamental to induction machine operation.

Good Job, mr jaus !

old jim

 

[jaus tail]

so the main question remains unanswered...? any electrical reason that affects lower limit of air gap?

 

[jim hardy]

I think it was answered - at high flux level fringing flux overheats the corners

but perhaps that's a magnetic reason