IMO a large motor would tend to run less stressed and therefore less tendency to vibrate but I am not an expert on this subject.

No.
You need to be looking elsewhere for the source of vibration.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

Hi,
10 years is already a very long time! Did you check for bearings failure, Misalignment, foundation damage? What about erosion? NPSH a...?
Do you have a PM program in place with regular monitoring of the equipment?
Let us know your findings.
Pierre

I've seen your motor situation many times. The OEM installs a large frame motor wound for low horsepower because your requirement had very high duty cycle or operates in high ambient temperatures. When the motor eventually fails it gets replaced with the closest off the shelf unit which will have a horsepower that matches the frame. This case won't increase vibration but may result in shorter motor life.

Unlikely to be the cause of vibration.

Check coupling and coupling alignment first and holding down bolts on the motor, pump and base plate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

There exists PM plan, call up cards and condition monitoring.
@Tugboat : You may be right. It might have been kept large power motor intentionally.

@LittleInch: Alignment is okay. Bearing condition monitoring is performed on regular basis. There is flexible coupling. Overhauling is performed after two years. Don't you think that Low weight impeller, dia & change in flow characteristics can cause an issue ?

Daren!

A change of flow conditions could lead to vibration, you would need to understand the original hydraulic selection and pump build and the current pump build and flow conditions.
That means having the original pump data sheet and hydraulic requirements and comparing it to the current details.
The job for an experienced pump hydraulic engineer - not a hit and miss approach or supplying half clocked detail to a group of guys on a pump forum.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

@Artisi ! You are angry.:) But I have limitations on data. Your opinions are worth-working. Moreover, my question was simple that over capacity motor can contribute vibration, especially in motor. But any how many answers are satisfactory.

Daren!

@ Darin. No not angry at all, just trying to point out what is needed to solve such an enquiry. Problem such as this are usually easily solved when good factual data is supplied,

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

The fact that you have a 45kW motor where a 37kW motor would do is likely a distraction. The motor will only put out whatever power the pump is demanding of it.

It has had increased vibration for 10 years, I would suggest a vibration analysis on the motor and pump. It's possible that the motor has bad bearings, or that it has a loose rotor bar, for example. If it has had increased vibration for 10 years, it seems to be operating well enough.

If it was a larger motor, or recip engine or pump, especially if installed on a smaller machine's foundation, heavier components could cause increased vibration, but that would probably not be relevant here.

Not to say that it is totally not possible either. Many of these small size pumps and motors often have very light, minimal pad type foundations. Is the foundation pad vibrating as well? If so, you might want to think about a new and heavier foundation.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

Darin,

"Don't you think that Low weight impeller, diam & change in flow characteristics can cause an issue ?"

I have absolutely no idea because you have given us ZERO information about this. Has anything changed?

But 10 YEARS of this and only now you're wondering what could be wrong?

Was the pump ever balanced?

You need to measure this as all pumps vibrate to certain extent, especially at 35kW, but j=how are you noticing it?

What sort of baseplate / foundations do you have?
Does the pipework warm up or cool down and stress the pump in operation but then when it cools down it is all back in alignment?

Many things can casue vibration, but a "slightly" larger motor isn't one of them. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Induction motors are routinely oversized. There's no reason that in itself would cause increased vibration. An induction motor is perfectly happy to operate at a lower load than nameplate.

Quote:

This motor-pump unit has experienced increased vibration issues for previous 10 years.

I think you're saying previous 10 years from now. How long ago was commissioning? Did the increased vibration start immediately or later? (if later that contradicts the proposition that the motor size caused the vibration). Did the vibration increase all at once or slowly (if slowly that contradicts the proposition that the motor size caused the vibration).

It could be due to many different things. It could be that piping loads on the pump nozzles are too great. This causes pumps to vibrate. Could be an unbalanced impeller. Could be pump not leveled and shimmed properly. Some pumps required baseplate to be filled with concrete/grout after installation, larger pumps required epoxy grout sometimes. As someone mentioned could be motor/pump coupling misallignment.

What is the "change in flow characteristics" ?

Off hand, that flex coupling may be the bad actor here.

Post the pump curve and then tell us where on the curve the pump actually operates....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Kenneth, What is the frequency ?

If the discharge piping has victaulic couplings or something similar the internal liquid pressure will cause the piping to push against the pump and move the pump. That in turn will cause misalignment between the pump and motor and will cause vibration issues.

When using victaulic couplings they should have tie rods across the coupling to make it act like a rigid joint. People make the mistake that a victaulic coupling is rigid joint (i.e. a flanged joint) It is not. I see this mistake all the time