Electrical frequencies found whilst recording vibration 1

[jda1]

My apologies for the long winded question. And many thanks in advance for any help you may be able to give me


When analysing vibration spectra on electric motors, I am finding responses @ 100-200 & 300 Hz 2-4&6 times UK AC line frequency. i know soft foot or sprung foot can cause this dur to the twisting of the stator. This is not so in these cases as all these motors are inverter driven. I know an insulated Motor NDE bearing is suggested for anything above 280 mm shaftt height. but would like to know of any in line gizmo could be fitted to eradicate the eddy currents from rotor to stator.

 

[bklauba]

Let's sort this out a little.

A soft or sprung foot will often cause a peak in the vib spectrum at 2X, i.e., 2 times the speed of rotation. This is due to impacting of the foot that occurs twice during each rev:

- when the top of the foot hits the underside of the bolt head, and

- when the underside of the foot hits the base.

A means of pinpointing where the foot is soft is by looking at the phase. It will change 180 degrees when going thru the soft transit.

This problem is a mechanical one, having little to do with the motor being powered by a VFD (except that hard starts might have caused the problem) or directly off the line.

This is only one of many examples of what you find when looking at a vib spectrum of a rotating system. Many of the peaks in such a spectrum show up as integer multiples of the motor speed, the soft foot being probably the simplest example. Others include ball or roller pass frequencies (BPF's or RPF's) that involve of rolling elements in the bearings multiplied by the speed.

Maybe you should take a course!

 

[testtech]

Soft foot will cause increased vibration at 2x field frequency due to the eccentric air gap that exists. I have astounded many by making it disappear by simply loosening one foot bolt to demonstrate the problem. (Make sure the other three are tight before attempting this!)

I believe that you are also referring to shaft currents induced by high carrier frequencies from VSDs. See the following excellent paper on the subject:

http://www.irispower.com/techpapers/Years/2002

Click on the paper Shaft Earthing Brush Comparison.

One case described here might fit your symptoms. However, if your drive is operating at less than 50 Hz, it is not yet clear why you would see these frequencies in the vibration spectrum. What are the output frequencies of the drive?

 

[jbartos]

Suggestion: Electrical machinery and sets are known to experience even vibration harmonics. When it comes to even number of phases, e.g. 2, 4, etc. the even vibration harmonics are somewhat link to the even number of phases. It is not clear from the original posting, how many phases are in the motor power supply.

 

[jda1]

jbartos....3 phase 415v 50 Hz inverter driven. two and four pole machines.

bklauba....first of all thank you for your reply, however I have to disagree with you somewhat, as my question relates to harmonics of uk line frequency not rotational speed. what you are describing is more of a looseness issue with loose mounting bolts rather than a sprung or soft foot. and yes you are correct that phase analysis is by far the best way of confirming this. as far as a course is concerned i spend considerable time training others on this subject.

testtech....thank you for the link. i have just read a paper from GAMBICA & REMA on this subject and all is becomming less muddy i think.

once again thank you all for your help but watch this space as you never know how confused i am going to get.

 

[electricpete]

One comment:
Mechanical looseness can be excited by any forcing function. If the forcing funciton is unbalance we will usually expect 1x, 2x, 3x etc. If the forcing function is magnetic force we will usually expect 2LF, 4LF, 6LF etc.

One question:
Is it being claimed that 2LF, 4LF, 6LF etc is caused by shaft currents? I don't know for sure but that doesn't sound right to me.

 

[jbartos]

Suggestion: Visit

http://www.dur.ac.uk/li.ran/pubs.html

for:
Reference 3 and 7

http://www.reo.co.uk/files/kbase/pq_pdf1.pdf

Page 11 for:
Such problems include excessive
heating in transformers and electric
motors, which can cause transformers
to be derated, and excessive noise
and damage from increased motor
vibration. Furthermore,
Page 15 for:
The problem
It is the rate of rise time in the dV/dt
curve - or to put it a simpler way, the
combination of rapid voltage and
current increase - that stresses the
conductors and the motor, both
electrically and mechanically.
Consequently, the insulation of the
motor windings is in turn stressed
by the internal, energy-laden,
components, as well as external
vibration.

Etc. for more info

 

[electricpete]

jb - what are you responding to? Was there anything there about bearing currents. I am trying to figure out what bearing currents have to do with 2LF, 4LF, 6LF vib. So far the only thing I know they have in common is they may both be caused by vfd's... but nothing I know suggests that they are related to each other. i.e. insulating a bearing may protect the bearing but is not going to change 2LF, 4LF, 6LF pattern vib in the least imho.

 

[jbartos]

Suggestion to electricpete (Electrical) Nov 13, 2003 marked ///\\jb - what are you responding to?
///I am essentially responding to the original posting, namely:
""...but would like to know of any in line gizmo could be fitted to eradicate the eddy currents from rotor to stator.""
If motors are inverter driven, it does not mean that the motor input has the pure voltage sinusoidal waveform. If harmonic content is present, which actually is on the VFD output even when properly filtered dv/dt, carrier frequency filter, reactor, etc., the vibration posted in my above links can be caused by that harmonic content (not only harmonics, but also acoustical noise).\\
Was there anything there about bearing currents.
///Why are you questioning bearing currents?\\ I am trying to figure out what bearing currents have to do with 2LF, 4LF, 6LF vib.
///Bearing currents are milliamps. They can hardly cause any vibration until they damage the bearings. Then the vibrations are noticeable. In fact, all kinds of vibrations if the motor is firmly connected to the load. The motor-load set can experience the system vibration.\\ So far the only thing I know they have in common is they may both be caused by vfd's... but nothing I know suggests that they are related to each other.
///Not directly. There is a relationship of "cause" and "effect."\\ i.e. insulating a bearing may protect the bearing but is not going to change 2LF, 4LF, 6LF pattern vib in the least imho.
///Agreed.
Furthemore, I copy your posting from above and provide a comment within:
"""electricpete (Electrical) Nov 12, 2003
One comment:
Mechanical looseness can be excited by any forcing function.
///True. The forces caused by the harmonic content in the motor input should not be ruled out.\\ If the forcing funciton is unbalance
///What kind of unbalance? Electrical or Mechanical or both?\\ we will usually expect 1x, 2x, 3x etc. If the forcing function is magnetic force we will usually expect 2LF, 4LF, 6LF etc.
///The magnetic force is a very complex issue, e.g. magnetic eccentricity, distortion of electromagnetic fields by harmonic content, etc.\\
One question:
Is it being claimed that 2LF, 4LF, 6LF etc is caused by shaft currents?
///This is what appears to be sought in the previous comment. See my comments above.\\ I don't know for sure but that doesn't sound right to me.
///There can be various origins of shaft currents. Some of shaft currents (not necessarily bearing currents if there are slip rings and brushes to treat the shaft currents) can be relatively large, e.g. 200A or so on the ship propulsion shaft.

Also, subharmonics and interhamonics tend to produce mechanical vibrations. However, these are entering the motor stator from the motor power supply.\\

 

[electricpete]

jb - Thx for clarifying that your "suggestion" was not directed to me. The rest is irrelevant to me.

 

[jbartos]

Comment: Any one's posting is left to interpretations to some extent.

 

[ScottyUK]

jb,

Postings should be written with sufficient clarity that interpretation is not necessary! Interpretation also allows for mis-interpretation - it is much better to present the facts in a clear and unambiguous manner.

 

[jbartos]

Suggestion to the previous posting:
Some postings are possible to present clearly. This depends on the posting subject.

Very few individuals mentioned ambiguities in my postings. I have posted estimated 7000 postings, more than 1000 have been removed by the Forum system, which apparently discards old postings. Stars are also discarded with discarded old postings.

I find ambiguities in postings all the time, often asking for clarifications. This is often the case, when the original posting misses some important information.

I tend to post references or provide equations or proofs.

 

[ScottyUK]

jB,

That was intended as a general comment for all of us, myself included. I wasn't having a crack at your posts specifically. Apologies if that was how you interpreted it - maybe I wasn't clear enough in my post!

 

[jbartos]

Comment: No apology necessary. Initially, I was very sensitive to various comments; however, as time passes by I mostly concentrate on the posting technical content calling for eng-tips that actually count.