vibration/alignment/balance

Hello Everyone,

We are in the process of doing acceptance testing for a serviced Motor (stator re-winding done). The Motor details are 831 HP, 6600 V, 50 Hz, 3000 RPM.

During No-load testing in Motor Service shop, the 2XLF amplitude recorded was about 3.3 mm/sec RMS. Overall Vibration was about 3.8 mm/sec RMS.

Is there any acceptance guideline available for 2XLF amplitude for New/Serviced Motor. I kindly request for feedback from the members.

Thanks and Regards

Maha

I see this as an unacceptable level. Who are they suppose to satisify? Open a dialogue with them and ask why and confirmination of electrical integrity. Sorry I don't have quick reference publication to cite.

If this motor was serviced in an agency that received a scope of work, the vibration levels, in the scope of work, must be used as the acceptance criteria. Based on what I frequently see, many of the proponents fail to mention this criteria in the scope of work.

A very good reference you can refer to on this subject is NEMA MG1. It has a section for the vibration acceptance limits with enough details.
Please notice that certain motor brands are inherently sensitive to 2LF in their nature and they run and can run for normal lives without problems (but this is another issue).

Anyhow, in general I tend to agree with Sam. The mentioned level (about 0.13 ips rms) is higher than a typical acceptable vibration level for a serviced motor (tested under solo run conditions).

Was the test performed bolted down or on a compressible pad? If bolted down, I recommend verifying that a good effort at correcting softfoot was attempted prior to the test. As a check, ask that one or two foot bolts be backed off and reverify the vibe condition.

Your vibration value of 3.8 mm/sec rms corresponds to 0.21 ips pk/0.

I second George's recommendation to investigate the foot mounting condition with off-line and running soft foot checks.

As a side note - I haven't personally seen any foot-sensitive 2*LF on any of our large 2-pole motors, but very often see it on smaller/medium (NEMA frame) 2-pole motors 250hp and below. I am curious if anyone else shares the same experience? Do you see foot sensitive 2*LF vibration on large machines also?

If the checks do not reveal foot sensitivity, you might ask the shop to check air-gap measurements. Also check voltage and current balance during the run.

quote:

Is there any acceptance guideline available for 2XLF amplitude for New/Serviced Motor. I kindly request for feedback from the members.

Of course, the limit that you put in your specification is very relevant. But if you want some other standard info:

http://www.easa.com/indus/AR100_0406.pdf
This is an ANSI standard for repair of induction motors.
See section 4.5.6 and table 4-5.

Now, one note of caution. In section 4.5.6 they correctly describe the conversion factor from ips pk/0 to mm/sec rms (multiply ips pk/0 by 18~25.4/sqrt2 to come up with mm/sec rms). But ironically in table 4-5 they forgot what they were doing and multiplied by 25.4, which is incorrect (unless you were dealing with mm/sec peak which would not a very common unit).

So for a 3600 rpm machine, resiliently mounted: 0.15 ips pk/0 gives (when multiplied by 18) gives 2.7 mm/sec rms. But your test was probably done rigidly mounted (you should insistn on rigid mounting imo, especially important for 2-pole motors so foot conditions that are on the motor side of the foot will be discovered by the repair shop, not by you when you install the motor in the plant), in which case the limit is multiplied by 0.8 which gives ~ 2.2 mm/sec rms limit.

NEMA MG-1 does have quite a bit of discussion of 2*LF, including a procedure to evaluate effective value of vibration when 2*LF is beating with 2*Running speed on 2-pole motors. But imo for all practical purposes, this is a limit on the overall vibration, including 2*LF.

Pete,
I'm glad you mentioned that you have noticed that you don't see softfoot sensitivity on larger 2-pole motors. I had never considered this...but now that I think of it I believe my experience is similar. I'm gonna stop short of saying this is some kind of rule...as my experience is limited to just my plant, which makes it real possible that the cause is from some other reason. I know you have experience at a Westinghouse nuke...so I thought you might find it interesting that the motors we find most softfoot sensitive are our 2-pole 100 HP stator cooling pump motors, and our 4-pole Heater 2 Drip Pump motors. (I'm writing from home...but I think the Heater 2 Drip Pump motor is 600 HP??) So, for us...your observation holds true.

Hello Everyone,

Thank you for a very valuable feedback and information. The No-Load test was done with out mounting the Motor rigidly. From the feedback from Electripete, I understand that it is important for us to do the testing by bolting it down in the Motor shop, so that soft foot conditions if any will be detected in the Motor shop itself.

Thanks again,

Regards

Maha

Maha
There is the ISO 2373 where the limits are: 4.5, 2.8 and 1.8 mm/s RMS for motors grades Normal, Reduzed and Special.

Pete,

In the chart prvided by NEMA MG1 for the vibration limits, the standard calls for using filtered amplitudes as a limit at each frequency, including the 2LF. Well, at least this is what could be understood from the write up.

I'm copying and pasting the below para from the standard:

For filtered vibration the velocity level at each component frequency of the spectrum analysis shall not exceed the value for the appropriate curve in Figure 7-6 at that frequency.

Interested to learn from you, as alwyas i'm.

It's a good point to mentioned filtered vibration which I didn't mention at all.

NEMA MG-1 requires us to meet the curve 7-6 for both filtered (spectral) and unfiltered (overall).

NEMA MG1 – 2003, Section 7.8.1

quote:

Figure 7-6 establishes the limits for bearing housing vibration levels of machines resiliently mounted for both unfiltered and filtered measurements.

For unfiltered vibration the measured velocity level shall not exceed the limit for the appropriate curve on Figure 7-6 corresponding to the rotational frequency.

For filtered vibration the velocity level at each component frequency of the spectrum analysis shall not exceed the value for the appropriate curve in Figure 7-6 at that frequency.

Unfiltered measurements of velocity, displacement, and acceleration may be used in place of a spectrum analysis to determine that the filtered vibration levels over the frequency range do not exceed the limits of the appropriate curve in Figure 7-6. For example, for the top curve in Figure 7-6 the unfiltered velocity should not exceed 0.15 in/s peak (3.8 mm/s), the displacement should not exceed 0.0025 inch (p-p) (63.5 microns), and the acceleration should not exceed 1g (peak).

NOTE—International Standards specify vibration velocity as rms in mm/s. To obtain an approximate metric rms equivalent, multiply the peak vibration in in/s by 18.

Maybe it seems to some people a little redundant to have limits of both filtered and unfiltered. But you can of course come up with situations where we need both to cover different situations. Let's say I have a 2-pole machine, and my unfiltered limit is 0.15 ips resiliently mounted (we'll stick with resiliently mounted for simplicity, although should be rigidly mounted).

A situation that is caught by the unfiltered (overall) but not by the filtered (spectral): Let's say I have a 0.14 ips peak at 60hz and a 014 ips peak at 120 hz. It meets my spectral limits, but the overall would be around 0.22ips and would exceed the overall.

A situation that is caught by the filtered (spectral), but not by a single unfiltered (overall) check: Let's say I have a 0.1 ips peak way up at 1000hz, it meets my velocity overall limit, but doesn't meet the filtered limit at that frequency which is around 0.06.

An alternate approach mentioned above is to check the overall in all three units: mils, velocity and g's and compare to those listed on the curve. That would also catch the last scenario since g's would exceed the limit. This approach would capture any overall or spectral deviation from those limits.

Again as I mentioned there is additional discussion in section 7.8.5 specifically related to 2*LF vibration and the only time it gives any different limit than the previous section is when there is beating between 2*LF and 2*running speed on 2-pole motor. In that case they allow you to compute an RMS-type of average of max and min vibration over a slip cycle and if that is below 80% of the curve (0.15 ips), you're still ok.

It looks like NEMA has the same unit conversion error that EASA did. They give the correct conversion factor of 18 conversion from ips pk/0 to mmps rms, but then they put in parantheses mmps numbers that are 25.4 times as large as the ips number. A little misleading imo.

Hi
Concern shoud be given to the motors
whose vib under load 60 to 70% induction motors exceeds 0.25in/sec peak at 2X line freq on new or re built motors
or .070 in/sec peak on in service motors having a stator pb. (50 HP to 1000HP)

Regards
Zaheerjiskani

http://www.easa.com/indus/AR100_0406.pdf
This is an ANSI standard for repair of induction motors.
See section 4.5.6 and table 4-5.

Mr. Pete,

I didnt find it like your above mention. Is the url not publish anymore?

Regards,
jono banten