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coolyaar
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High Speed Induction Motor

08/23/2011 2:51 PM

according to formula : P =120f /N , the no of poles of induction motor are inversely proportional to its rpm speed N. now according to this :

1500 rpm motors will run at 2 poles and 3000 rpm motors will run at 4 pole. i have seen induction motors running at 6000rpm without any external frequency drive at 50HZ,400V. how is it possible? is there any trick used in the internal stator winding of the motor because at 6000 rpm , motor pole will be (in theory ) = 120 x 50 / 6000 =1pole which is not possible . again, no frequency drive was used with motor . 400V,50Hz , 3phase was directly given to its stator. do any one have any idea about this ? ( because i was a bit confused when a technician asked me about it ) ?
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Anonymous Poster #1
#1

Re: High Speed Induction Motor

08/23/2011 4:11 PM

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SimpleMind
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#2

Re: High Speed Induction Motor

08/23/2011 4:27 PM

Yep, it can be done (with not very good power efficiency due to the long magnetic path). You simply need three windings on the circle, of 180 deg width each, overlapping at roughly 30 deg. S.M.

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coolyaar
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#3
In reply to #2

Re: High Speed Induction Motor

08/23/2011 4:54 PM

can you please explain it a bit or send me some literature link where i can study this concept and clear my ideas.

regards,
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SimpleMind
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#5
In reply to #3

Re: High Speed Induction Motor

08/23/2011 5:32 PM

No rocket science here. You have just three windings (for simplicity,actually you divide them into neigboring slots) at 120° phase difference, but for sinusoidal corformance I gave you the winding width of 180°, and the resulting overlap of 30°, It could be also done with 240° width windings and the overlap will go to 60° accordingly. Imagine that windings don't form classic pole pairs, each phase winding has the other two windings symmetrically on the oposite stator side, and this is almost equivalent to an opposite pole. Notice that the field rotation speed is double than the classic two pole 3-phase motor. The squirrel cage doesn't need a real pole on the other side, the combination of the other two phases will induce just the same. This config is not widely used because of it's inherent inefficiency and the too high speed per frequency, except for special cases. S.M.

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GW
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#7
In reply to #5

Re: High Speed Induction Motor

08/24/2011 1:24 AM

I am not familiar with this design. Can you please provide some references for study?

I am familiar with standard induction motors, DC motors, universal AC/DC motors, synchronous machines, wound rotor etc, but have never run across the design you mention.
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JRaef
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United States - Member - New Member Engineering Fields - Power Engineering - New Member

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#4

Re: High Speed Induction Motor

08/23/2011 5:25 PM

Most of those that you see are actually permanent magnet DC servo motors, often used as spindle drives on high speed machine tools. The drive system is built into the motor itself, so you only THINK it's an AC motor because all you see is the AC going into it.

By the way you have an error in your statement:

"P =120f /NP =120f /N , the no of poles of induction motor are inversely proportional to its rpm speed N. now according to this :

1500 rpm motors will run at 2 poles and 3000 rpm motors will run at 4 pole." (which in and of itself is a bit odd because you have swapped it around to find the number of poles, not usually used that way).

For a 3000RPM motor then, P = 120 x 50 / 3000 so P = 6000/3000 = 2 poles. You said 3000PRM was 4 poles. I think you may have just transposed your figures.

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leveles
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#6

Re: High Speed Induction Motor

08/23/2011 11:05 PM

Yep, Raef is right.
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hithuanand
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#8

Re: High Speed Induction Motor

08/24/2011 9:48 AM

not more than synchronous speed .
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