megger readings? 6

[gwalk]

I have a 10hp 3ph 460volt pump motor that has been 1/2 under water for about two weeks, now that the water has gone down I've taken megger readings at the t-leads disconnected from the starter. On the 500volt scale the (digital) megohmmeter ramps up to 80 volts and reads 0.00 ohms and holds. Am I right in assuming this is the voltage at which the windings go to ground.(access to the motor to split the winding connections, is still under water)

Any information or proper procedures for using a digital
meggohmmeter for checking grounds would be appreicated


Thanks!
gary

 

[edison123]

Dry out the motor at 80 to 90 deg C and re-check the megger value.


* Basically, I would like a full-time job on part-time basis *

 

[waross]

This doesn't sound good. Is the water contaminated?
I would do some checks with a multimeter.
I had to deal with a 350 kw generator that had been contaminated with brackish water. We were reading zero resistance with a megger and very low resistance with a multi meter. The resistance reading on the multi meter changed when we reversed the leads. We could also read a small DC voltage to ground with the multi meter. I took this as an indication that the copper windings and the iron stator were acting as a battery and I was reading the galvanic voltage.
We dis-assembled the generator and washed it for about two hours with a parts washer and fresh rain water. Then we reasembled the generator and spent about 2 days drying it. We were able to get the megger readings up in the megohm range.
We then put the generator to work.
However, based on the experience with this set, I recommended that the other 4 sets be sent by ship to the US for cleaning, dipping and baking in a motor shop. I further advised that the set we dried would probably run indefinitely, BUT if it was ever stopped and cooled down, it should not be restarted before being sent to the shop in the US for a re-dip and bake.
This set ran for 4 or 5 months. Some of the other sets returned from the US and were put in service.
The original set was shut down for awhile. One day my recomendations were ignored and it was restarted. It failed catastrophically. The difference in cost between a re-dip and bake, and a rewind are such that it never was repaired.
I would check your motor for any sign of a DC voltage to ground, indicating that the insulation has failed to the point that the copper and the iron are acting as a battery.
If so I recomend sending the motor in for a cleaning and a dip and bake. You may save some money by cleaning the motor windings with clean water and drying the motor before sending the motor to the shop.
If you get a good megger reading after drying you can probably use the motor, but when it is stopped and allowed to cool, I recommend rechecking the insulation resistance before restarting the motor.
You can dry the motor quite effectively by running low voltage dc through the windings.
Warm the motor externally until you get some improvement in insulation readings before putting any voltage on the windings. If it's a dual voltage motor connect all the windings in parallel to minimise voltage stress on the insulation.
yours

 

[54494060]

gwalk,
If you or your company (which ever owns the motor) is financially able I would pursue sending the motor out to a motor shop for rebuild or purchasing another 10hp motor. 2 weeks is a long time for a motor to be "1/2" under water. The rotor needs to be pulled and cleaned, the stator needs baked, and a bearing replacement is a (must) good idea even if they are double sealed. A motor shop can perform this in a very short time. One other thing. You said that you obtained your insulation values (megger) form the motor starter. Were the supply conductors at the motor also under water. Something else to be aware of because they "could" also affect your megger readings, even after you correct the motor deficiencies.

12fish
As Iron sharpens Iron so does a man sharpen the countenance of his friends.

 

[jraef]

Check out faq237-1157

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

 

[dwftx34]

I agree for the cost of the motor or the motor repairs it it would be best to remove the motor from service. here is a link to a Megger article about salvaging water damaged equipment.

http://www.megger.com/us/publications/etarticle.php?aid=21&pid=18&Description=