Magnetic Motor Wedges 2

[Yooper]

Could someone explain to me what magnetic motor wedges are and what they are used for? Also, how could you tell if a motor has them by visual.

Thanks

 

[electricpete]

They are installed above the coils instead of mechanical wedges.

The way I look at it from an intuitive (probably incorrect) viewpoint, they reduce the effective air gap, which might tend to improve power factor.

I have heard someone say they reduce zig-zag leakage flux, which decreases leakage impedance.

 

[electricpete]

also the zig-zag flux causes higher core losses due to higher frequency. Reducing zig-zag flux improves effiiency.

Please take all the above with a grain of salt... a little out of my area.

 

[electricpete]

yooper - do you work for a government-owned utility with three letter name by any chance?

 

[edison123]

yooper,

As pete says, magnetic wedges (fitted in stator slots) are used to reduce zig zag reactance losses and especially pole face losses in salient pole generators.

You can identify these wedges by using an ordinary magnet. The wedges will attract to the magnet.

 

[Yooper]

pete,

i dont work for a government owned utility. why do you ask?

 

[electricpete]

I saw a similar question pop up at the exact same time in an epri mailing list posted by someone from tva... thought they might be related. Must have been a coincidence.

 

[Yooper]

As a matter of fact that is why I asked the question. I dont work for TVA though.

 

[vanstoja]

Stator slot wedges atop the winding slots in large electric motors restrain winding coil displacements from vibration and thermal expansion to prevent insulation chaffing wear and dielectric breakdown failures. They are usually inserted axially into broached keyway-like slots in the stator punching teeth. Materials used are ceramics, non-magnetic metals and (rarely)magnetic metals. To reduce eddy currents, stator wedges are often segmented lengthwise and driven in piece-by-piece. The use of magnetic wedges is primarily prompted by electromagnetic noise reduction considerations. Finley (1990)recommends their use to reduce tangential forces applied to the stator punching teeth. Magnetic wedges serve to reduce permeance variations associated with non-magnetic materials or airspace in the motor's magnetic fluxfield. A side benefit of magnetic wedges may be zigzag loss reduction and possibly reduced saturation of stator punching teeth tips under high loads. These effects may improve motor efficiency as well as reducing magnetic noise. Rhudy (1971) noted that usage of magnetic wedges was more prevalent in Europe than in the US because of loosening problems encountered in American motors during the 1920's. Siemen's has long been a proponent of magnetic wedge usage as indicated in papers by Gahleithner (1968), Keuth (1971)and Finley (1990).
Gahleithner,A., "A New Range of High Power 3-Phase Induction Motors", Siemans Review, Vol.35, No.8, 1968, pp.323-330.
Keuth,H., "PROTOFER Magnetic Slot Wedges for Electrical Machines", Siemans Review, Vol.38, No.5, 1971, pp.222-225
Rhudy,R.G., "Magnetic Stator Slot Wedges in Induction Motors", General Electric TIS Report No. 71MMG3, 9/7/71
Finley,W.R., "Noise in Induction Motors - Causes and Treatments", IEEE Paper No. PCIC-90-06, 1990, 9pp.

 

[electricpete]

You get a star for that John V.

 

[Yooper]

Thanks for the help

 

[jbartos]

Suggestion: Visit

http://www.camawi.com/four.html

http://www.camawi.com/

for: Magnetic wedges
and

http://www.iet.auc.dk/~por/papers/Paper13.pdf

for: Non-magnetic wedges

 

[jbartos]

Suggestion: Visit

http://www.astrosyn.co.uk/docs/specacdcmotors.pdf

for:
A joint project with Cambridge University to reduce electrical noise using a novel stator design incorporating magnetic wedges