Understanding magnetic balance in AC induction motors

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21
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I need to find out how calculate magnetic forces appearing in AC induction motor rotor when it is not magnetically centered inside the stator.
 
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These forces try to center the rotor inside the stator when the motor is started up. It is a problem in motors 500 kw and above and if they have journal bearings and lack of thrust bearings. Picture yourself a cut view of an AC induction motor. That is the only drawing needed.
 

jim hardy

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I never before contemplated calculating that.

I guess i'd start with search for articles about it then search on terms in them to expand my vocabulary. Then i could phrase well stated questions.
http://www.baldorprospec.com/assets/pdf/motorprimer_part1.pdf
Brief discussion in section 12.

The magnetic centering force is a function of the magnetizing current of the motor (basically the no load amperage of the motor), air gap flux density, air gap radial distance, number of aligned rotor and stator segments (ends of rotor, and stator and radial air ducts), voltage, air gap axial length and axial misalignment between the rotor and stator. [22, 23] The magnetic centering force increases from zero when the motor is operating on its magnetic center (magnetic equilibrium), while the rotor is displaced axially relative to the stator. See Figure 15. Typically, at a 0.125” axial displacement, non-ducted rotors may develop 50 to 150 pounds of axial centering force, whereas rotors with 12 radial ventilating ducts aligned with stator duct may develop up to several hundred pounds. At start-up, these motors will develop axial forces up to three times these steady state values.

upload_2018-5-25_19-51-19.png




old jim
 

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Probably you never have to calculate it because you did not have to design the thrust bearing for a gear increaser coupled to a 1600 kw AV induction motor.
I ask for advise to somebody with experience and not somebody advising me to "to expand my vocabulary and then phrase a well stated question" An expert in the matter (if there is one in this group) will understand my problem and would offer directions. The website jim hardy posted is vague and not very conclusive. OK for a begginer.

Reference https://www.physicsforums.com/threads/understanding-magnetic-balance-in-ac-induction-motors.948116/
 

jim hardy

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I ask for advise to somebody with experience and not somebody advising me to "to expand my vocabulary and then phrase a well stated question" An expert in the matter (if there is one in this group) will understand my problem and would offer directions. The website jim hardy posted is vague and not very conclusive. OK for a begginer.
Well, EXcuuuusse Me!
A question well stated is half answered. You didn't say what level of answer you sought.
 

jrmichler

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Probably you never have to calculate it because you did not have to design the thrust bearing for a gear increaser coupled to a 1600 kw AV induction motor.
I ask for advise to somebody with experience and not somebody advising me to "to expand my vocabulary and then phrase a well stated question" An expert in the matter (if there is one in this group) will understand my problem and would offer directions. The website jim hardy posted is vague and not very conclusive. OK for a begginer.
/
You should have read the paper that Jim Hardy linked. There is a bibliography at the end. Reference #22 has the title AXIAL MAGNETIC FORCES ON INDUCTION MACHINE ROTORS.
 
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I searched the references in Jim Hardy article. Unfortunately the best one is very old and could not get it. Also, since the unbalance comes from the diferencial pressures in the cooling system of the motor (fan, ducts, etc.) the best way to measure is the way it is done in real operation: Measuring the displacement of the rotor at full speed (could be at no load)
 
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Probably the closes thing I've looked at is the bearing forces due to magnetization tolerance in PMSM machines, and by "looking at it" I mean programs->ansys maxwell :)

On a side note if on journals, you don't build oil pressure before starting? Does that not trash your bearings?
 
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<Moderator edit>

Now then, your question relates to something I haven't been able to find much research on, mainly because even most sleeved bearing motors don't hunt for the center very long. If there is a lot of "magnetic play," it may be something as simple as spacers being out of place in the barrings. A good friend of mine repairs motors for a living and if there's anything he's unsure of he'll usually ask me to help him figure it out. Two mind are better than one, and all that. Calculating it would have to be a nightmare and I'm not sure how one would (or could) go about doing it meaningfully. I might be forgetting some details but, if memory serves, if a motor is hunting for center enough to cause vibration/loss of efficiency, replacing the bearings usually does the trick. I can ask my buddy what he thinks about this.

You're trying to design the thrust bearing? Are there no such bearings available in market?

Here's a 2006 IEEE paper... https://ieeexplore.ieee.org/document/4199057/
I haven't read it but it may give you some further details/ideas.

As is now obvious, I'm no expert. Only trying to help.
 
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jim hardy

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I'm no expert either. Only one i ever worked on was a Kingsbury on a 6000 hp vertical pump. Of course the magnetic force was small compared to the weight of rotating parts.

I'd enjoy learning the basics of the subject.
 
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The explanation of magnetic unbalance being caused by aerodynamic forces generated by cooling fans, makes a lot of sense. Then, in big motors, finding the magnetic unbalance seems to be only one way to find them: Testing. Running the motor at no load and measuring the displacement of the shaft at full speed, seems to be the only way to consider the magnetic unbalance. Have not find any theoretical way to calculate them, and if there is one, it is probably far too complex.
 

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