ECN Forum Forums Technical Discussion Electrical Theory and Applications Motor Failure

Does anyone know where I can find information
about the heat generated in the windings of a three phase motor with unbalanced current.
Also the effect of peak overvoltage, waveform faults, voltage fluctuations & unbalanced voltage.

I use the "Electrical Engineering Pocket Handbook" from the Electrical Apparatus Service Association (EASA). Sorry I don't have a web site, but most motor re-wind shops are members. EC&M magazine usually has an ad from Sunbelt Transformers which offers this booklet for free.

The key to understanding temperature effects on a motor is understanding insulation.

Information on the subject can be found at motor mfg websites, via search engines, or your local rewinder. (The handbook has the insulaltion class charts I believe.)

If the importance of this motor is great, then I would have it evaluated to see exactly what happened to it. The remidiation could be a higher insulation class that can handle a greater temperature rise (check out article 430 NEC.)

[QUOTE]Originally posted by ctolbert:
[B]The key to understanding temperature effects on a motor is understanding insulation.

Thanks for the info. The information I'm looking for would be a chart of the temperature rise and long term effects on the motor insulation of a motor running with a voltage imbalance of .9 to 1.1 percent and current of about 5 percent. I know that the motor should be derated maybe 1 or 2 percent because of the voltage. I have a customer that has had several motor failures in the past two years (same motor). The motor repair shop reports that I have read say the problem is voltage spikes, transients, lightening, etc. I have run several surveys with a power quality analyzer and have not seen any RMS spikes, etc. even during lightening storms, (motor has lightening arrestors and surge capacitors). I have seen the voltage imbalance of 1% and current of 5%, plus waveform faults with peak voltages of 580 volts phase to neutral (480Y/277 volt system), but they only last one cycle, sometimes 2 or 3 of them within a 15 cycle time frame. I think it may be capacitors switching on the primary. There is also a system voltage drop of approximately 5% every 30 to 40 minutes as the result of another motor starting which causes a 20% drop in current on the motor in question followed sometimes by a current increase. I have also caught a few partial phase losses which would single phase the motor for a short time. The question I have is could all of these things combined over time deteriorate the insulation to the point where a small voltage spike destroys the motor.

jmcs What horsepower motor is this. I have some motors on motorsavers that I have to set the unbalance trip at +10% to avoid nuisance trips. I did have a similar problem loosing motors but it has improved since I switched rewind shops.

[This message has been edited by dtl66 (edited 08-14-2001).]

dtl66 The motor is a 100 hp. connected to a deep well pump. There has been two motor rewind shops involved that I know of. I Know that if there is core damage to the motor that is not corrected it can create a hot spot in the windings near the core damage that could result in premature failures. Do you think this is a possibility? Is your motor saver looking at unbalanced current, voltage or both? The motor that I am dealing with has a motor saver on it but they are replacing it every month or two because it is damaged in some way but I have not been able to determine the cause.

The big issue I have is with the rewind shops. I know from personal experience that some rewind shops are lame.

I demand at least a one year warranty, test results of QA, and if it fails...I better have a rep kissing my butt and telling me why it failed and explaining what to do to fix it.

Rewinding is very cut-throat, so if they don't mind losing your business...fine. There are several that want your business even if they have to drive a little.

Anyway, the location of the failure is a prime indication of the possible cause(s). And core damage (depending on severity) will lead to replacement of the rotor or at worst the entire motor.

The motor savers and that genre are important for single phasing and some types of voltage anaomolies, but I'm more concerened about transients, type of lead wire used and the environment.

I would also check with the TCI www.transcoil.com they make line reactors and filters.

Hope this helps some.

Carl



[This message has been edited by ctolbert (edited 08-15-2001).]

The motor savers I am referring to in this instance monitor current. I use some that monitor line voltage for single phase and reversal protection only on motors up to 100hp but for some reason they will not detect a single phase on larger motors while the motor is running. The core damage is probably the culprit for frequent failures. I recently sent one to the rewind shop I switched to because it ran extremely hot compared to others in the same application. The informed me that the motor had damage to the irons. I had them redip and bake and put the motor in storage as emergency spare only. The previous shop that had been rewinding the motor could not or did not test for this or didn't bother to mention it.(small shop - plant manager trying to save a $). I know it is expensive but I would look at replacing the motor.

[This message has been edited by dtl66 (edited 08-15-2001).]