Stiff-enough stator design for 6 pole motor - Modal frequency analysis

[MS0000]

Hi,

I am engineering a 6-pole motor. Now, I am making FEA modal analysis of the stator frame. From literature and experiment I know that in 2 pole machines the stator frame oval shaped modal frequency is kept sub-critical compared to 2 x supply frequency. That is because the 2-pole excitation does have the same oval shape as that oval mode shape, therefore, the resonance would be for real for sure. How is it with machines with more poles: 4, 6, 8... and so on. Electro-magnetic excitation is divided into smaller forces per pole(6-merlon star in 6-pole machine) so probably the vibration response is smaller and so on. Shall I keep stator oval shape modal frequency still sub-critical compared to 2 x supply frequency in 6-pole machine? All the experiences about the design of 4-pole and on machines are really appreciated! I hope you got my point.

Thanks,

S

 

[electricpete]

I am not a designer. But I'll throw in my 2 cents worth what you paid for them ;-)

The force wave from the fundamental has P poles and time frequency 2*LF.

If you have eccentricity, the spatial order of the force wave can get shifted by one compared to the mmf wave... so 4-pole motor may have 3-pole and 5-pole force shape at twice line frequency. But not 2-pole.

You can also get shapes that have P+/-|R-S|, but those are not at 2*LF frequency (they are near RBPF).

I don't see any reason why the 2-pole mode shape would need to be separated below twice line frequency on a 4-pole or 6-pole motor.

=====================================
(2B)+(2B)' ?

 

[electricpete]

By the way, can you share: what is the refernce that suggests the 2-pole mode shape resonant frequency is kept below (not above) twice line frequency on a 2-pole motor?

=====================================
(2B)+(2B)' ?

 

[MS0000]

Thanks pete for your answer. You are encouraging me for more lightweight design which I was hoping to be real. Yet still comments from others are also appreciated.


Sorry, I did confuse mode freq and excitation. For sure 2xLF excitation is kept below mode shape - not another way Especially because this is variable speed machine.

I am not fluent in those shortcuts, can you explain: |R-S| and RBPF ?

 

[electricpete]

Sorry I should have defined my abbreviations:
R = Number of rotor bars
S = Number of stator slots
RBPF = Rotor Bar Pass Frequency

=====================================
(2B)+(2B)' ?