ROTOR WINDING CLEARANCE

[PUMPDESIGNER]

Clearance between rotor and windings is a mystery to me.
It appears to me that the clearances are too small for just the bearings and motor end plates to maintain properly. This is just a casual observation based on nothing specific.
I therefore suspect that magnetic field contributes to the stability of the rotor by holding it centered when rotating.
Is this correct? Richard Neff
Irrigation Craft

 

[UKpete]

I can assure you it isn't. The airgap is maintained by ensuring adequate concentricity, fit and rigidity of all the components concerned:

stator laminations
motor housing bore and endframe spigots
endframe bearing housings
rotor shaft
rotor laminations

- all the tolerances on these components are carefully defined so that even in the worst case tolerance build-up, the airgap is acceptable.

 

[THooper]

Induction motor's size may determine air gap. Is there a way to calculate the best air gap for a particular induction motor size? We know that too small a gap can mean rubbing and high stray loss, but too large a gap can raise the magnetizing current too much. Where is the best balance? Formulas do exist for approximating the minimum advisable air gap depending upon a motor's physical size and number of poles. These values aren't based on precise theory but reflect actual design and experience. Most motors use somewhat larger gaps for mechanical reasons.

Here's one commonly used formula used in the motor repair industry.

Gap,inch=0.005+0.0003D+0.001L+0.003V

In which D is the stator bore diameter in inches; V is the rotor peripheral speed in thousands of feet per minute; and L is the core stack length in inches. For example, assume an 1800 rpm motor with a 10 inch bore and 8 inch stack length. The minimum gap equals 0.005+0.003+0.008+0.014, or 0.03 inch on a side.

Manufactures try to hold air gap variation to -+10% of the average or nominal value. For an integral horsepower ball bearing motor, with (for example) a 0.05" gap, that means an approximate range of 0.45" to 0.055" which is very common air gap.

For sleeve bearing motors the gap will be larger.

Hope this helps,
Kind Regards
motorhead

 

[electricpete]

Good answers all around. The bearings act to keep the rotor on center. If the rotor becomes off-center, the magnetic force will actually act in a direction to pull it further off-center. That is usually not a problem.

 

[jbartos]

Suggestion: The air-gap length is often corrected to so-called effective air-gap length that is bigger than the air-gap. Generally, the smaller air-gap will require more accurate machining and smaller tolerances, which will make the machine more expensive. However, the smaller air-gap, g, will reduce the gap reluctance, Rg, since
Rg = g / (muo x Ap)
where Ap is the surface area of the pole face and muo is the permeability of the air (free space).
If the air-gap is very small, the Ap is reduced to the effective air-gap area, Ag = Ap(t/(t+d)), where t is the rotor tooth width and d is the rotor slot width.

 

[PUMPDESIGNER]

Thank you guys.
I will start another thread with my real question.
I had to ask that question first. Richard Neff
Irrigation Craft