Quantifying excessive mechanical vibration in a 1.5hp 3450 rpm motor?

[george55]

I have spoken to several local electrical motor service centers, but none have been able to indicate an actual method of simply quantifying vibration in small motors (with the viewpoint of establishing if a given motor is unacceptably unbalanced).

In my particular case, a brand new 1.5hp 2 pole 3450 rpm motor (for driving woodworking machinery) appears to vibrate (under no load) much more than any similar motor I have ever encountered. A second new motor vibrates similar to the first, thus I am inclined to believe that the problem lies in all thier units. The problem appears to be a mechanical imbalance since the vibration diminishes slowly as motor winds down.

I would assume that an unbalanced motor under no load conditions, and suitably mounted would oscillate by a quantifiable displacement, possibly as measured by a dial indicator against the housing. Is there any such or similar method by which I can gage this vibration?


I wish to press the manufacturer for some sort of further solution but I would prefere to have some kind of hard data (rather than a vague complaint about vibration) to back up my claim. Any replies would be greatly appreciated.

 

[Tmoose]

If its V-belt driven pulley runout and belt variations will tug on the belt very insistently.

I'd start by taping a half key in the naked shaft and run it to evaluate the balance.

 

[electricpete]

"I have spoken to several local electrical motor service centers, but none have been able to indicate an actual method of simply quantifying vibration in small motors (with the viewpoint of establishing if a given motor is unacceptably unbalanced)."

Balance is checked with the rotor removed on a balance machine. There are ISO standards addressing balance grades.

Vibration can be checked with the machine assembled and running. NEMA MG-1 calls out a limit on the order of 0.12 ips for rigidly mounted machines (new). About 0.15 ips for resiliently mounted. Higher limits generally apply to in service machine, and ISO addresses these as well. If you consider the level to be unacceptable, checking the frequency of vibration (among other things0 will help you beging to narrow down the cause.

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[george55]

Thanks for the two responses. The motor vibrates under no load, no pulley, key taped in as Tmoose indicated. I had read an article indicating that electrical imbalance can be ruled out by observing certain characteristics of the vibration on wind down, and running the motor at reduced voltages. As far as I am capable of determining, the problem remains mechanical in nature. The frequency appears to be of the same order as RPM. I assumed that imbalance was the primary cause but I now understand other mechanical factors may be at play.

I did attempt to find a clearer description of what NEMA mg-1 limit of 0.12 ips means but was unsuccessful. The information appeared available but at a significant and prohibitive cost for a one time purpose. Electricpete, if you have a moment, and if you can, could you please define .12 ips, or indicate anything else that might assist? Thank you.

 

[electricpete]

"Electricpete, if you have a moment, and if you can, could you please define .12 ips, or indicate anything else that might assist? Thank you."

I'm not sure what you mean. ips stands for inches per second (overall peak-0 unfiltered). It must be measured with a vibration monitoring instrument and anyone qualified to use the instrument will be familiar with the units and setup. I don't see any other way to quantify vibration.

I'm not sure if I have answered your question.

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[george55]

Thanks for the response Electricpete. According to a paper found on the Siemens website titled "An Analytical Approach to Solving Motor Vibration Problems", motor vibration can be quantified in three different ways. These are with units of acceleration, velocity (ips as per your indication), or displacement. The article indicates several NEMA and API standards for displacement measured in mils peak to peak, and this would appear to be the method I have been groping toward. The article contained all requisite information but I had inadequately read it prior to my post. My goal was to use some kind of standard more easily measured to ascertain whether the vibration in this motor was acceptable. A measure of motor displacement was not too difficult.