API 541 Unbalance Response Test

[Mike20]

My project has specified several 5 - 8 MW, 2-pole (3564 rpm) induction motors. The lateral analyses on these motors predict coupling-end vibration at the following:
- 4113 rpm, 3rd mode shape(exceeds run speed by 15.4%) - Motor A
- 7186 rpm, 4th mode (within 1% of 2X run speed and 2xLF) - Motor B
- 1843 rpm, 3rd mode shape (within 5% of 1/2 X run speed and 1/2 X LF) - Motor C

I plan to request an Unbalance Response Test (with weights placed at the coupling per API 541 4th Ed., Section 4.3.5.3) for Motor A. The test would be run to 120% of rated speed. In doing so, we would confirm the 15.4% separation margin.

Question - Would the Unbalance Response Test be worthwhile for Motors B and C if I'm concerned about 1/2X and 2X run speed and line frequency? Would the unbalance (1X) excite the 3rd and 4th modes (1/2X and 2X) of these motors and provide any meaningful information?

 

[GregLocock]

No. Unless you have really horrible mechanical problems adding unbalance can only excite at the run speed.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Mike20]

Thanks for your response.