Rotor broken bars?

Dear Experts,

We have a squirrel cage inductance motor, 400V VFD, 220KW. The load is Roll which driven by Gear-box in Paper factory. We carried out MCSA and Vibration analysis for this motor. The opposition of MCSA and Vibration result was making me confuse so much.

- MCSA indicates rotor broken bars problem with high peaks at pole pass sidebands.

- There is no symptom in vibration spectrum.

We can see result in my attachment.

Please give me your oppinion!

Thank you!


Lampard Nguyen


Original Post

To diagnose rotor bar problems in vibration spectrum:

Identify actual running speed and subtract it from synchronous speed 1800-1775=25 rpm of slip, now multiply the slip rpm X the number of poles in the motor 4 poles x 25=100 cpm pole pass sidebands

IF the motor truly has a rotor bar problem, then you should see the pole pass sidebands around 1x 2x 3x 4x 4x 5x rpm or some combination of the harmonics. Keep in mind your bin spacing on spectrum, you must have good resolution to see these sidebands.

You should also see higher vibration at one time rpm and at one and two times rotor bar passing frequency, the motor will make an audible modulating growling noise, the motor will run hot, the motor will loose hp, the motor amps will constantly drift up and down, the speed of the motor will continually fluctuate.

MCSA data is valuable but these mechanical indications will also confirm the presence of broken rotor bars, these symptoms are present in copper bar or aluminum rotors

Hope this helps



like Dave said you must cut spectrum or Fmax. should be at 5-6 speed of motor to see good patern and good haystaks .

At figure 6 it should be rotor bar pass frequency pick at 1470 Hz and one pick left like twice line frequency sideband .

Good luck .

Dear all,

Thank you so much for your recommendations. I have just performed RIC (rotor influence check) for this motor. RIC profile indicates some erratic relate to rotor broken bar issue.

I think that there is problem of rotor. It's a interesting case-study!

Thank you!

Best regards,


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