Vibration limits on motors

[npower0073]

On site we have a vertical motor 1090kW 6.3kV 593rpm that develops 7.2-7.7 mm/s velocity on the Non Drive End during our vibration measurements. The motor comply with EN 60034-14. Is it OK to run it? We also plan to clean the impeller of the motor from dust. Also the motor drives an axial flow pump used for the main cooling water system of a power plant.

 

[electricpete]

First, you don't say what is the frequency of the vibration.

I will assume it's all 1x running speed.

At low frequency (below about 1000cpm), displacement is more suitable to judging severity than velocity.

NEMA MG-1 limits for vibration on new motors shop tested on a rigid base:
displacement 2 mils pk/pk (Limiting below ~ 1000 cpm)
velocity 0.12 ips pk/0 (limiting ~1000cpm - 24000cpm)
acceleratio 0.8 g's pk/0 (limiting >24,000 cpm)
by the way a mil = 0.001" (sorry, I'm using US units)

Your measurement 7.5 mm/sec, I'll assume is rms (pretty common for mm/sec)

That corresponds to 13.5 mils pk/pk or 0.4 ips pk/0.

These level are higher than generally acceptable for most people. We generally see 2 mils or less on our large slow speed circulating water motors. More concerning also is if there is an increasing trend, or other frequencies beyond 1x.

The cause we can only begin to guess. Unbalance and misalignment are the most common causes of 1x.

If the vibration is directional (higher one horizontal direction than the other), often suggests resonance on a vertical pump.

Could certainly provide more specific guesses if you provide more details. For example the frequency content of the vibration. Vibration at motor top bearing, bottom bearing, stuffing box... each in the axial direction and the two horizontal directions. Trend of vibration and was there any recent work done. etc.

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[motorspert]

I'm in agreement with electricpete, 7mm/s is high. I havent the EN to hand, but I am surprised it allows that high level. As its a vertical machine the frequency is important, reed frequencies etc.

Clean it, check it and them naybe get your local repair shop to have a look.

 

[edison123]

How & where did you measure vibration velocity ? What sort of probe ?

At such low speeds, displacement is normally.

 

[npower0073]

edisoa123, we measured vibration velocity on motor non drive end vertical with a probe, overall rm velocity.