motor repeatedly shorting 1

[ka043710]

hi,

can somebody help me here? my 5hp 3600rpm weg brand HCL pump motor repeatedly shorts to ground. i have had to replace the motor 3 times within the past 5 months. i have been using weg brand motors elsewhere in the plant and they have proven to be reliable.

besides the motor running under water, what else could explain the frequent shorting to ground?

also, even if water were to get into the motor would the motor not continue to run as long as it was not shut off?

thanks for your anticipated help.

kwame

 

[buzzp]

I would verify the installation is wired and grounded properly. If this is true, then perhaps there is some voltage spikes, transients, sustained, or whatever causing an insulation breakdown. Might be worthwhile to monitor this with a high quality (fast) voltage logger, monitor or something similar. Has the motor been torn down by the OEM and analysed?
If water gets in your motor, your done. Most of submersible motors are filled with oil and any water present will out the motor. There are probably some important installation notes to follow when installing these. I have installed some larger 3 phase motors and you always have to top them off with oil when installing. Yours may not be the same. Perhaps water is getting in during the installation some how (assembling pump and motor during installation).

 

[sreid]

pragmatically

1) weg has changed to a different offshore supplier.

2) Change to a different motor supplier; easy if the motor is a standard NEMA design.

 

[jraef]

Also, how are you controlling this submersible? Do you have a VFD on it? That can contribute to winding failure if not done properly.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ka043710]

thanks, guys for your help so far.

sreid, it is too early for me to accept that weg is putting out bad product because during this same period, i have installed several weg motors elsewhere in the plant and not had any problems at all.

jraef, my motor is not used in a submersible application, except that water levels in the dike where the pump is located sometimes tend to be too high. but even that other weg motors at similar heights in the same dike don't seem to budge.

 

[DickDV]

You may be experiencing high phase to ground voltages if your AC source is an ungrounded floating delta.

These systems have no enforced voltage to ground so whatever touches them is transferred to the load.

On the other hand, if your source is grounded center wye, the voltage is fixed by the transformer windings and will not change without releasing a fuse or circuit breaker.

 

[jraef]

So it is not a submersible motor, but occassionally it is submerged while running, and you can't understand why it fails because some others don't under the same circumstances? I'd call it luck with the ther motors!

Even if we assume that water is not getting into the connection box, non-submersible motors use cheaper winding insulation that is, to varying degrees, hygroscopic, meaning it absorbes moisture. When a motor gets that wet it is very important to thoroghly dry it out before energizing it because any remaining moisture can vaporize behind the winding insulation and cause it to bubble up, eventually failing. The fact that your other motors have not done this can be attributable to luck, or perhapse there is some difference in the operating sequence so they have more time to dry out before being run again next time, compared to this one.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[stardelta]

To stand any chance of finding and overcoming the root cause you need to have the failed motor carefully examined by someone experienced in this field. Have you considered sending the motor back to the suppliers, claiming warranty by saying it has failed within a month and asking for their comments? If its not practical send it to a good repair shop and ask them to examine and report. If water has gone in and caused the problem the symptoms will be impossible to miss. Another possibility is premature bearing failure. This could allow the rotor to run against the stator and cause a core rub which would cause overheating and/or mechanical damage. Either could lead to an insulation failure and a earth fault. Bearing failure in this application could be caused by a damaged coupling between the motor and pump, a bent pump shaft, excessive pump shaft float generating preload on the motor bearings or poor shaft alignment between the motor and pump. You say you have replaced the motor several times, are sure you using a replacement motor thats suitable for the appliction? Its not unknown for makers to use a angular-contact bearing in the drive end of certain pump motors to absorb the axial forces generated by the screw effect of the pump impeller, particularly if its verticaly mounted.

 

[buzzp]

I thought these were submersibles the way you were talking about "besides the motor running under water, what else could explain the frequent shorting to ground?"
P
ick a motor thats correct for the application or come up with some way to protect them from moisture.
So is the water submerging the motor partially or is it just splashing water? Either are bad for standard motors.
Expect to have more failures until you pick one for the application.

 

[ka043710]

thanks, guys for all your thoughts! have a wonderful week.