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#1 bob

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Posted 20 February 2006 - 11:05 AM

Hi ,

As far as electric motor constructions are concerned, is there any specific requirement on the air gap ? Does it vary from manufacturers ? Im looking for a tangible value for one 260 k W squirrel cage motor 400 V application. I think 2mm is too wide. Any appreciation ?

Bob

#2 marke

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Posted 20 February 2006 - 05:46 PM

Hi Bob

Smallest is best. The greater the airgap, the higher the leakage reatance. Lower airgap will help to improve efficiency.
Perhaps someone else can comment on the absolute figures.

Best regards,

#3 jraef

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Posted 20 February 2006 - 09:49 PM

From the course on motor efficiency design I recently took, the answer is not as clear cut as you might hope.

"...in the theoretical calculation of stray load loss for an induction motor, some component losses vary as the inverse square of magnetizing current. By increasing that current, a larger gap reduces those loss components. Some "energy efficient" motors have therefore used larger air gaps for the best overall effect. In this, as in many other design issues, the answer to the question "what's best?" is, "that depends.""

But there is some hope in spite of that.

"It is impossible at the present time to derive a satisfactory equation, directly from theoretical considerations, for determining the proper length of gap." This "empirical" (based on experience) equation is often considered accurate enough:

Air gap, inch = 0.005 + 0.0003D + 0.001 L + 0.003V

in which D = rotor O.D., inches

L = core stack length, inches

V = rotor peripheral velocity in thousands of ft/min.= D(RPM/12,000)"


You are on your own for metric conversions, sorry.
It was all a little beyond what I needed to know to be honest, but I hope that helps you. That would make the time I spent in class a little more worthwhile.
"He's not dead, he's just pinin' for the fjords!"

#4 bob

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Posted 21 February 2006 - 05:10 AM

Many thanks Marke and Jraef.

The beautiful about learning, Jraef, is that no one can take it away from you.

Kind regards.

Bob

#5 jraef

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Posted 21 February 2006 - 07:36 PM

Yes, but the bad part is that memory can be likened to a train track on a plateau. If you keep adding cars on one end, eventually the track fills up and older things begin falling off the other end of the plateau.

Now I've lost my girlfriend's phone number from 7th grade and the 2nd half of Lincoln's Gettysburg Address I was forced to memorize in the 5th grade. Luckily, nothing important as of yet.
"He's not dead, he's just pinin' for the fjords!"

#6 marke

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Posted 21 February 2006 - 08:15 PM

Ah yes, but it is surprising how elastic that track is. The more you cram on, the larger it gets, but as you get older, .....

Best regards,

#7 GGOSS

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Posted 21 February 2006 - 11:31 PM

I think the important things remain with us forever, those forgotten were probably not worth learning in the first place.

There are however some things we cannot afford to forget under any circumstances. These include but are not limited to the following; the wife's name, her birthday and your wedding anniversary.

Regards,
GGOSS

#8 marke

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Posted 22 February 2006 - 12:03 AM

The easiest way to avoid that problem is do dispense with the wife??




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