Electric motor dynamic analysis

[canwesteng]

Has anyone ever tried to high tune a motor base? In my experience these are balanced, and also basically impossibly to high tune for the motor operating frequency. A checker is trying to get me to do the check now and I need some backup for my position though

 

[WARose]

I'm not sure what you mean.....are you saying you are looking to have your base at a certain frequency ratio (relative to the motor)?

 

[canwesteng]

Exactly that. In this case, it's a 60 Hz motor, so the base needs to have fn>120 Hz

 

[WARose]

If this is sitting on the ground you are wasting your time because the spring constants you are likely working with are just guesses anyway. (I.e. they are always +/- X%.)

If this is sitting on steel framing, it should be a simple matter to do a dynamic analysis. (I.e. time-history.)

 

[canwesteng]

Agree, it's simple to do the analysis. But it isn't feasible to high tune a platform for a 60 hz motor, and IMO not necessary as the motor is balanced.

 

[WARose]

I wouldn't say it's balanced.....the unbalanced force is typically pretty low but its there (especially over time). For motors, I've normally checked it with about 1.5 mils of eccentricity (if I felt it was a concern; if it was sitting on a big chunk of concrete, I really don't worry about it).

 

[JoshPlumSE]

I'm more familiar with foundations supporting dynamic equipment than steel platforms. But, there are some good "rules of thumb" that are published for foundations that may help get this reviewer to accept your judgement.

I'm specifically referring to ACI 351.3R. This ACI document gives a number of criteria for calculating machine unbalance for equipment meeting standard ANSI / ISO / API criteria.

And, there are relatively simple hand calc methods where you can calculate an approximate dynamic amplification factor based on the ratio of the operating frequency of the equipment to the natural frequency of your structure.
Amplification = 1/(1-feq/fstructure)

Though your best bet is to show that the structure's natural frequencies are not within 20% of the operating frequency of the structure.

 

[Tmoose]

Is the motor and driven device going to be mounted on a subframe mounted directly to the floor?
Are there going to be occupied spaces adjacent to the motor or on the floor below?
Does the motor drive an airhandler?
The potential for transmitted noise and vibration from a well balanced motor is pretty high.
Add some interesting belt frequencies, and 120 Hz electrical hysteresis and it would not surprise me if the occupants will know whenever the device is running.

Careful isolation of the motor and auxilliearies is often necessary on upper stories.

http://www.mason-industries.com/masonind/_doc/pdf/ashlec.pdf

 

[canwesteng]

Electric motor on a subframe on a platform. Motor drives a crushes, this has well defined dynamic loads obviously. Noise doesn't matter, it's industrial

 

[SlideRuleEra]

canwesteng - Tuning of a foundation is based on the rotational speed, not the electrical power frequency. A 60Hz motor's rotational speed depends on the motor's basic design. For example, a 60 Hz two-pole machine has a nominal speed of 3600 rpm, a 60 Hz four-pole machine is 1800 rpm, 60 Hz six-pole machine is 1200 rpm, etc.

I have not been involved in tuning an electric motor pedestal, but 3600 rpm electric utility steam turbine / generator pedestals are routinely high-tuned. I would expect doing the same for an electric motor is possible, too.

Perhaps this paper will be of help:

http://yoga10.org/Documents/SPVKPSERCpaper.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[canwesteng]

Good paper - I've done foundation analysis for fans before, and those warrant their own separate foundation, but the issue is a small high rpm balanced motor. It would look ridiculous to put it on a large mass of concrete, not to mention the many cases where these motors are far enough from grade foundations aren't feasible.