This is a 300 HP motor AC of a vertical pump, the plate data says 1,781 RPM. The colected data of vibration at the free end was 0.13 inch/seg 0-pk @ 1,800 RPM, the oposite side was 0.05 inch/seg 0-pk @ 1,800 RPM (whitout load), then i took another data whit load and the readings were: free end 0.33 inch/seg 0-pk @ 1,800 RPM and the other side was 0.18 inch/seg 0-pk @ 1,800.
it´s like the case of a inverted pendulum, but what´s happening whit the motor?, resonance or fan unbalance?

thank you for your comments.

Hello HV,

Looks like another one of those vertical applications where a very heavy rotating mass (the motor in this case) is mounted up high on top of a slender flanged pipe, I would presume.
There is some unbalance yet left in the motor and it increases with load since it is, for all practical purposes, unrestrained.

Resonance ? Could be a factor but I would wager more on the flimsy instalation.

Try stiffening the motor close to the top from adjacent walls with temporary struts and see if that does the trick. Try to add more weight onto the motor top housing. There are some calculations for determining the weight for cases like these somewhere on this list of postings.

I've always hated these types of vertical pump instalations. Just like a top resting on a pointy support.

Good luck,

MarkoLeo

Vertical machines have intense mobility. Any unbalance can generate high vibration levels.

Some simples test will define if is resonance or no.

Ricardo Góz from Brazil

If, and only if, resonance is confirmed in these - I have had better luck "loosening" up these systems to obtain the desired results. Try installing a heavy paper gasket between the pump housing and the sole plate or try installing thin .030" washers between the motor and pump. Sometimes you've got to get creative and this is certainly alot faster and less expensive than trying to add enough mass and/or stiffen.All your trying to do is shift the critical away from the running speed.
Hope this helps,
Pat

such type of problems are due to :

1. Low rigidity of foundation/civil structure
2. Less contact area between pump stool top & motor bottom flange.
3. Low stiffness of pump stool.
4. Levelling of entire structure.

You may check above & eliminate one by one.

DKSONI
INDIA

I have seen vibration often higher at the top of vertical motor even when not resonant. So you need to check some more.

Is the vibration directional? (higher in one radial direction than the other).

Bump test would be a logical way to check for resonance. Also coastdown test. And the other good suggestions above.

I'm not sure what to make of the variation with load, but that would be an interesting clue to follow up on. I don't think that is typical for 1x from a motor but maybe if it originates from the pump? (hydraulic unbalance?) Are you sure it was varying with load and not with time (either oscillating or gradually increasing as warmed up upon startup).