motor lamination problem 1

[NIDinc]

After months of hard work, we tested our second prototype motor today with disappointing results. The motor amp draw is too high and the motor shaft quickly overheated. The problem was determined to be the stator laminations with a
c5 coating not providing enough insulation. We will try spraying a varnish on each lamination and building a new rotor for testing tomorrow. It's a squirrel cage rotor.

The motor is a small 2 hp. induction two pole, three phase, 208 volt AC motor that runs at about 12,000 rpms off a VFD at about 200 Hz.

We tried a C3 lamination but it didn't laser cut well. Question is does anyone have a recommendation for a better lamination material or a suggestion how to build the best rotor??

Thanks.

 

[electricpete]

how did you determine the lamination insulation is the cause of the overheating?

did you try simple core loop test with thermography to look for hot spots ?

what is the lamination thickness?



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(2B)+(2B)' ?

 

[NIDinc]

The rotor laminations are .025" thick. A test rotor with thicker C3 coated lams was tried and resulted in better performance, though it had air gap that was too big. Will try the varnish coated lams tomorrow with the proper air gap to see if this solves it. No other testing available at this time.
Thanks.

 

[NIDinc]

PROBLEM SOLVED!
The problem ended up being that we were using a non-magnetic 303 stainless steel motor shaft (need corrosion resistance). Using the same rotor, if we switch to a 1018 steel shaft, the amps and temp drop dramatically (about 70%).

 

[electricpete]

Thanks for feeding that back.

At first glance, I wouldn't have thought shaft permeability would be important. We certainly don't want homopolar flux.

Then I looked up this excerpt from EASA's Tech Note 43 (Making and Replacing Motor Shafts):

Considerations for stainless steel shafts
The use of stainless steels for shafts presents a unique
problem for electric machines–the reduction of magnetic
permeability of the rotor back iron. This is generally more
tolerable for motors with higher numbers of poles. In 2 pole
machines, however, a nonmagnetic or weakly magnetic
stainless steel shaft can cause variations in motor current at
slip speed due to the deeper flux paths. For a design with
high flux levels and a relatively large rotor inner diameter, a
nonmagnetic shaft could possibly result in a measurable
difference in motor performance.

It makes sense that for 2-pole motor, the flux path goes directly between 180-opposite poles and can pass right thru shaft... for 4-poles the flux flows more circumferentially in the rotor backiron and should not be a problem. For this hi-speed application I'm sure you've got a 2-pole

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(2B)+(2B)' ?

 

[NIDinc]

Thanks much for that explanation. We figured it was something like that, being a 2 pole motor. Now the question is whether or not using a 400 series stainless steel with slightly less magnetic properties than say 1018 carbon steel, will have an adverse effect. Sounds like it may. I need the corrosion resistance but can't afford to sacrifice motor efficiency.
Will keep you posted.

 

[waross]

Good information Pete. Thanks

Bill
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"Why not the best?"
Jimmy Carter