using megger to dry out motor ?!?
using megger to dry out motor ?!?
(OP)
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.
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.





RE: using megger to dry out motor ?!?
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. }
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
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?
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
Apologies. "...hell of a lot of cranking, sir. ;-) "
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
RE: using megger to dry out motor ?!?
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.
RE: using megger to dry out motor ?!?
The first approach can lead to the water boiling point, i.e. 212°F, which could potentially damage conductor insulation. This may be compared to having the certain food warming up in a microwave oven. The heat generated from within causes small explosion and holes in the food.
The second approach would be somewhat slower; however, it will be safer, since the temperature will be highest at the surface and gradually decrease in the inward direction.