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Old 05-03-2012, 06:08 AM
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Default Question regarding AC Induction motor rotors

Hi, I posed this question on the Endless-Sphere board with one reply which didn't seem to answer my question, then nothing more. Could anyone here shed some light on this?

Quote:
Originally Posted by iti_uk
Hi all.

What is the significance of the number of induction bars on the rotor of an ACIM? Is there a general rule of thumb as there is with other PM motors? Do more bars reduce cogging? Are the limitations more down to manufacturability reasons?

It seems to me that with a slipping field, number of bars would not be critical, but I'm only guessing and would like to be corrected.

TIA

Chris
Quote:
Originally Posted by iti_uk
If I could further clarify my question, coming from another angle;

What would be the effect on efficiency, power, smoothness etc. if the rotor was created in the way shown in my quick CAD snapshot (attached). In this I have only shown what would be the aluminium (or copper) "bars", the void in the middle of the part being filled with laminated iron, as in a normal squirrel cage rotor. How would this work inside a 4 pole 3 phase stator? (p.s. I didn't bother drawing the bars skewed - it was a 2 min job to illustrate my question)
(Further note - I also did not draw the connecting "squirrel cage" rings at either end of the rotor. It's just an abstract illustration...)

TIA

Chris

edit:
A further thought on the idea, if I may. I have seen videos of AC motor stators spinning coke cans and the like. What I am suggesting here would effectively be a thick-walled, radially laminated can. Mad? Pointless?
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Last edited by iti_uk; 05-03-2012 at 06:12 AM.
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Old 05-03-2012, 07:17 AM
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Default Re: Question regarding AC Induction motor rotors

Quote:
Originally Posted by iti_uk View Post
Hi, I posed this question on the Endless-Sphere board with one reply which didn't seem to answer my question, then nothing more. Could anyone here shed some light on this?
The number of rotor "bars" (shorted turns, actually) is related to the number of stator poles. I don't remember the exact formula... Google turned up this forum post which goes into it some. I know of electricpete from other forums and he is very knowledgeable.


Quote:
Originally Posted by iti_uk View Post
...
A further thought on the idea, if I may. I have seen videos of AC motor stators spinning coke cans and the like. What I am suggesting here would effectively be a thick-walled, radially laminated can. Mad? Pointless?
That sounds like a hysteresis motor. Typically spin at synchronous speed but with very low torque output. Used almost exclusively for "mechanical timers" and clocks and such. Not something I'd want to use for an EV as the rotor losses are extremely high.
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Old 05-03-2012, 09:47 PM
JimDanielson JimDanielson is offline
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Default Re: Question regarding AC Induction motor rotors

starting at page 265 this book starts to describe how number of bars are determined-
http://books.google.com/books?id=4-K...page&q&f=false
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Old 05-04-2012, 02:23 AM
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Default Re: Question regarding AC Induction motor rotors

Thanks for the responses. Plenty of reading to do
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Old 05-09-2012, 11:27 AM
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Default Re: Question regarding AC Induction motor rotors

Good intentions, but Oh ooh, you are not too serious with this or are you?
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Old 05-10-2012, 04:39 AM
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Default Re: Question regarding AC Induction motor rotors

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Good intentions, but Oh ooh, you are not too serious with this or are you?
What are you saying? (that read very camp-ly btw ) I have a lot of time on my hands at the moment and I'm just throwing out ideas for solutions which I haven't seen before. Maybe one day, if I find something that works I can spend some proper time developing and making prototypes.

Chris
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Old 05-10-2012, 10:45 PM
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Default Re: Question regarding AC Induction motor rotors

I had researched this some time ago and discovered that it is (to me) insanely complicated, and somewhat of an "art". There are ways to use FEA but ultimately it requires experience and building prototypes and doing lots of testing. Here is a PPT from a seminar that goes into some of what is involved:
http://ceme.ece.illinois.edu/seminar...eorgiaTech.ppt

There's probably not much that has not already been tried and tested. But it is interesting and valuable to conceive of novel ideas and knock them around. As for the number of rotor induction bars, and the skew angle, it seems to be related to smoothness of operation, cogging, and physical noise. I have seen some charts of optimal and problematical numbers of rotor poles for various numbers of stator slots.
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