using megger to dry out motor ?!? 7

[electricpete]

Please don't laugh at this one. I don't believe it either.

Several electricians at our plant swear that they can make the insulation resistance reading on a random wound motor go up significantly by leaving 500vdc applied for hours.

I asked whether perhaps this is only a phenomenon of charging the capacitance and polarizing the insulation to "fool" the measurement. They assured me the motor was grounded after 500vdc applied but before megger... and further more lead polarity was swapped to eliminate effect from polarization.

Has anyone ever heard of this or seen this...a low megger reading cleared up by applying 500vdc for hours.

 

[busbar]

Yes indeed, and if your pull the endbells and see the water mark at ~50%, the motor clearly got low on cooling water. The hose jockeys are slipping again. { ;-) Sorry. }

 

Yes it will work depending on the size. The megger can warm the motor up slightly which dries it out and improves the megger reading. I have used a welding set to pump a bit of DC current through to dry out bigger motors, but be careful not to overload the windings

 

[electricpete]

John - Thanks for your comment. I am certainly not arguing with you but I don't understand how the megger has much heating effect.

Let's say I have 1000vdc across 1 megaohm insulation resistance.

That gives I = V/R = 1E3/1E6 = 1E-3 amps.

Watts = I*V = 1E3*1E-3 = 1 watts. That hardly seems enough to make any difference.

I can see where dc welding machine gives a lot more... just 10A at 0.1 ohms winding resistance gives 10 watts. 50 amps gives 250watts.

 

[busbar]

Er, mind you that the welder is applied coilend to coilend. Megger is coil to lamination. Please, please, please tell me if I'm missing something.

Besides, that is a hell of a lot of cranking.

 

[electricpete]

Yes the welder is applied end to end (T-lead to T-lead) while the megger is applied from all leads to ground.

That is why I used a different resistances in my calculation... 1 megaohm for the megger (insulation resistance), 0.1 ohm for the welder (winding resistance).

We have meggers that don't require cranking. Battery or plug-in. I'm not sure if megger is still the right term (insulation resistance tester?), but that's still what I call it.

I'll have to look into that cooling water thing ;-)

Any other thoughts?

 

[GordS]

Maybe to 1 watt of heating effect is being applied at a very small location where the insulation resistance is low? This could hardly be called "drying out a motor" but it may improve a meggar reading.

 

[busbar]

Apologies. "...hell of a lot of cranking, sir. ;-) "

 

[BJC]

Gords is right- The megar is drying out parts of the motor not the whole thing.
You can also wire up lamp holder through the motor windings and power them through the motor windings. you can start with 60 watt bulbs and go up to 150s. the bulbs may be dim but you just using them to limit current not read by.

 

[electricpete]

Gord/BJC - Have you guys actually seen this work?

 

[GordS]

Pete...personally I have not tried this but have been told this phenomenon happens by some of the electricians at my plant.

 

[BJC]

Electricpete- Yep I've seen it.
The meggar drying things out was I'm sure wet or damp leads and not any internal faults from coil to iron.
The light buld thing I know works. Wire L1 from a 120 volt circuit to X1.X3 and X5 of a motor. Wire a standard pigtale scocket ( rubber base with guard to each of X2,X4 and X6. connect the other side of the three sockets together and connect them to the neutral of the 120 volt circuit.
I should have mentioned that we usually covered the motors and put the light bulbs in the box. They were drying out from current in the winding and the heat of the lamps.

 

[sciguyjim]

I think Gord is on the right track too. In addition, at sharp points, the voltage per unit area can get very high and may be causing a slight corona effect which can expel water from the surface. I've done this myself, but with higher voltages.

 

[busbar]

sciguy—that’s an interesting point, but besides expelling water, it generates electrical stress {and ozone} that are usually a bad combination for enamel or other insulation. The method might be a last-ditch trick for ‘kotex’-grade motors, but used with any regularity, risks seem to far outweigh benefits. If it has been reported as successful, insulation is probably in dire straits and can’t be seriously expected to have much life left; being close to eminent production interruption.

The usual method of birdogging leakage current during dc insulation testing—to back off on test voltage—or discontinue the session—would not seem practical given the suggested time interval involved. Success sounds more anecdotal/accidental/not consistently reproducible.

 

[kuwait]

Friends,
MAY BE TO SURMOUNT THIS PROBLEM THEY HAVE POLARISATION INDEX IE., (INSULATION READING AFTER 15 SECONDS)/( INSULATION READING INSTANTANEOUS) AND THE PASS VALUE IS SOMEWHERE 1.5 OR SO.

 

[sciguyjim]

Another way I've used a megger in the past is to burn off some types of contamination on ceramic high voltage vacuum tight electrical feed-throughs. It could get rid of a little moisture and sometimes small amounts of carbon deposits. Just connect the megger to ground and the high voltage wire in the feed-through, bring up the voltage till you get breakdown across the feed-through. The second time you do this you may see that the ceramic can insulate against higher voltages now.

 

[UKpete]

I go with Busbar's last post - if leaving the megger on several hours does restore a decent IR reading, I wouldn't trust the motor for long anyway.

 

[asdman]

I would not use a megger to dry out any motor. You run the risk of breaking down the insulation and possibly causing tracking.

The method our electricians used at a paper mill I worked at was to pull at least one end bell and place a 60 to 100 Watt light bulb (touble light) inside the motor. This creates enough heat for any motor below 200 hp. I suspect larger motors might require a higher wattage bulb. This solution is cheap and requires no muscle power either.

 

[Aranrhod]

You might get two effects here. One would be removing surface contamination which was tending to short out the winding - this wouldn't be much - and a continued electrical stress could remove it by burning or some form of electrical decomposition which gasified it especially if it was hydrocarbon based. The other effect might be as other contributers have noted, removing conductive ions from the insulation by electromigration of some sort. But if the insulation was in such a bad way that this effect was noticeable - have the motor rewound at the earliest opportunity!

 

[electricpete]

Thanks for all the suggestions and opinions. For what it's worth, here is my general feeling after considering all this input.

I was initially predisposed to thinking the heat from the megger would be negligible. But for the numbers i used on my 9/26 post, (1000VDC megger across 10megaohms winding insulation resistance to ground generates one watt while 10A directly through 0.1ohm winding conductor resistance), the watts from the megger are 10% of welding machine at 10amps.

And more importantly as some have pointed out, the watts that are generated are localized to precisely to the weak points in the insulation where the leakage current flows. Most likely wet insulation, possibly surface contamination.

I recognize there are valid reasons presented for being cautios with this approach, but my gut feel is it really isn't going to hurt the motor. Although I don't have any sound basis for that.

Thanks. Any more comments.