Source of 2*Line Frequency Vibration in motor/transformers
Source of 2*Line Frequency Vibration in motor/transformers
(OP)
Can anyone explain what is the source of 2*Line Frequency vibration in motors and transformers.
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
Source of 2*Line Frequency Vibration in motor/transformers
|
Source of 2*Line Frequency Vibration in motor/transformersSource of 2*Line Frequency Vibration in motor/transformers(OP)
Can anyone explain what is the source of 2*Line Frequency vibration in motors and transformers.
Red Flag SubmittedThank you for helping keep Eng-Tips Forums free from inappropriate posts. Reply To This ThreadPosting in the Eng-Tips forums is a member-only feature.Click Here to join Eng-Tips and talk with other members! |
ResourcesWhat is rapid injection molding? For engineers working with tight product design timelines, rapid injection molding can be a critical tool for prototyping and testing functional models. Download Now
The world has changed considerably since the 1980s, when CAD first started displacing drafting tables. Download Now
Prototyping has always been a critical part of product development. Download Now
As the cloud is increasingly adopted for product development, questions remain as to just how cloud software tools compare to on-premise solutions. Download Now
|
RE: Source of 2*Line Frequency Vibration in motor/transformers
From "Transformers for the Electric Power Industry" by Westinghouse Electric:
"Magnetostriction:
When a strip of iron is magnetized, it contracts ever so slightly. This very small change in dimension, occurring 120 times per second is a vibration which creates an audible noise."
It goes on to say that this magnetostriction is not directly proportional to flux (there is a curve), so harmonics of 120 Hz are generated as well. There is also a relationship between strain in the steel and magnetostriction and that bending the laminations increase the noise level and should be avoided.
dpc
RE: Source of 2*Line Frequency Vibration in motor/transformers
I didn't think of that myself... read it in a 1932 IEEE article by some GE old-timer.
RE: Source of 2*Line Frequency Vibration in motor/transformers
Of course, the motor is making a lot of other noise, so maybe the iron noise gets swamped out.
I've noticed that motors on adjustable frequency drives get extremely noisy at low frequency. I don't know if that is from the core or torsional oscillations.
RE: Source of 2*Line Frequency Vibration in motor/transformers
RE: Source of 2*Line Frequency Vibration in motor/transformers
RE: Source of 2*Line Frequency Vibration in motor/transformers
I'm replying from your previous thread to tell you that I'll have to lookup some info at my workplace to provide my best answer which may not be till early next week. For now, I'm inclined to agree with you that magnetostriction is not a big contributer to 2E in induction motors ; however, current unbalance, introduced by Mr. Williams above, may be the biggest player of all. Other possibilities have been raised though by some knowledgeable EE associates who have been doing some heavy duty electromagnetic analysis and testing in recent years. I need to refresh on their most recent findings. Incidentially, you apparently haven't seen the thread I started in the near-dormant Electrical Acoustics forum which is aimed at all combinations of line frequency peaks we have seen in our pumpmotor spectra. It was an effort to stir up some interest over there and I hoped to find someone to confirm our present speculative understanding of what we are seeing. So far a forlorn hope!
Regarding my comments about 2E sidebands around Rotor Slot Passing, that is definitely linked to rotor free-floating and heavily loaded eccentricity which can vary continuously with fluid film radial bearings particularly the pivoted-pad kind that allow rotor eccentricities up to 200% of nominal bearing clearance and let the rotor precess slowly between pad gaps. Whenever we see really high level 2E sidebands around RSP harmonics we worry about what the rotor was doing to our poor, defenseless bearings at the time the data were acquired. More to come. Regards, vanstoja
RE: Source of 2*Line Frequency Vibration in motor/transformers
RE: Source of 2*Line Frequency Vibration in motor/transformers
I'm a bit late with this but our UMP engagement intervened. Here's a direct quote from an "authoritative source" from 1994;
"2E noise is generated by many sources within an electric motor. The primary sources are:
1. The fundamental magnetic field which develops the steady state torque to turn the rotor. This source of 2E is primarily a radial bending of the stator core and is best minimized by increasing the backiron thickness of the motor.
2. Negative sequence fundamental voltage/current which is caused by the unbalanced line voltage. This source of 2E generates an oscillating torque and , according to analysis, could be reduced by [active control measures]. However, an alternate way to reduce this source would be to present a better line voltage balance to the motor.
3. Rotor/stator offset (rotor not centered in stator bore) or rotor and/or stator out-of-round (elliptical shaped rotor or stator bore). These sources of 2E are typically radial in magnetic force direction and would not likely be good candidates for application of [active control measures].
4. Magnetostriction, or contraction and/or expansion of the steel punchings caused by conduction of magnetic flux, generates a 2E signal in electric motors. While this source is not thought to be a strong effect in our motors, no quantitative analysis of this source has yet been done."
Some testing reportedly found that:
2E is reduced with less flux density since radial magnetic forces are proportional to air gap flux squared.
2E increases as voltage imbalance increases. The knee of the curves were 0.5% voltage imbalance for a 4-pole machine and 0.25% for an 8-pole machine.
2E is affected by changes in pressure at the stator bore. It remains to determine if this is a source change or a transmission path effect.
A 1967 report by a major motor manufacturer on motor noise implicated space harmonics, saturation harmonics and combinations of both in the generation of n2E sidebands around Rotor Slot Passing frequency.
RE: Source of 2*Line Frequency Vibration in motor/transformers
Levels of vibration observed have actually decreased, machine may be somewhat warmer...
RE: Source of 2*Line Frequency Vibration in motor/transformers
One thing I will throw out for rwilliams' benefit since it sounds like he is troubleshooting a twice LF problem in the field:
Try loosening one foot at a time and checking vibration levels (only one foot loose at a time). Soft foot is known to cause twice line frequency. Also mounting resonance or frame resonance may be involved.
Also, if unbalanced / negative sequence plays a role in the twice line frequency vibration, I think it will be an effect on the stator, not the rotor. Negative sequence also has a heating effect which is severe on the rotor... but that's an issue unrelated to the twice line frequency vibration.
RE: Source of 2*Line Frequency Vibration in motor/transformers
Thanks much!