motor megger, the results

[matlark]

megged a 50hp 480/3/60 motor (each phase to ground) and the megger would not stabilize. It just dropped in increments: .8Mohm,.6Mohm,.4Mohm,.2Mohm then stay at 0 while continuously holding the test button. I could hold the test button until it starts dropping and then it would just indicate one of the 4 figures depending on when I released the test button. When I opened the motor electrical box there was water in there, am I looking at a fried motor. Seems the megger was indicating a short from ground to any of the phases. Sorry if this is confusing.

thanks

 

[Jeff23spl]

Let it dry and meg it again but good chance that you may have to get it rebuild to at least change seals.

it's the way a megger work, if it is bad, ohms get lower...Water is conductive

you could test your megger in the air and see what it must read on a correct motor from wiring to ground

 

[matlark]

I did test the megger on a known good motor and got .3Gohm and in air was showing OL, so I think the megger is good. I am guessing the rusty water got into the motor, there was no rubber boot where the motor wires came from the motor ( in the electrical box ). This motor is in direct rain.

 

[DickDV]

Try your luck at thoroughly drying out the motor. Then re-meg it. Often it will recover high ohms but, if you're having a bad day, of course it won't.

 

[leitmotif]

Try your luck at thoroughly drying out the motor. Then re-meg it. Often it will recover high ohms but, if you're having a bad day, of course it won't.

I would heavily consider disassembly of motor - get new bearings - wash stator as best you can in soap and water and rinse very well. Then isopropyl alcohol bath. Then dry for couple days and remegger.

You can dry by using low voltage DC say 6 VDC and maybe 10 or 15 amp across two stator leads. Watch temperature - I would not go above 125 F. Give it a couple days.

Worked well at sea with no spare parts - also works well for Fluke VOM dropped in dirty wash water - Fluke said that is exactly what they would do.

Dan Bentler

 

[maxketcham]

water in a motor connection box is a common cause of failure. Your best bet is if you have any oven around to bake it at 200 deg F for 12 hours.(pull bearings first) then before re-assembly megger the stator windings. If they are greater than 1 meg per 100 working volts it is good. (the spec actually says 1 meg per 1000 v but I prefer the latter for better results) Re-assemble and put back into service if it passes. This is what a motor shop will do before they even look at rewinding it, as usually that is all it takes to fix the problem.

 

[iant]

@ 50 HP I assume it uses a star delta starter or similar.
Disconnect all 6 of the phase wires from the terminal block
test each coil separately.
I agree with drying the motor / changing bearings etc.
But this might be water in the terminal block only...
worth a check

 

[maxketcham]

@ 50 HP I assume it uses a star delta starter or similar.
Disconnect all 6 of the phase wires from the terminal block
test each coil separately.
I agree with drying the motor / changing bearings etc.
But this might be water in the terminal block only...
worth a check

I agree, but you still ought to take the motor to 200 Deg F for 12 hours, Unless you have a hermatically sealed and potted opening to the stator in which case just drying the box and connection wires will be OK. otherwise you can count on water in the stator which can cause it to short at where the windings are connected. Might show good on megger and fail upon start when the rotor causes it to move around in stator. IF nothing else if you see no short, take it apart and use dry compressed air, then a good heat gun to dry any moisture out, be careful not to burn insulation on windings! Remember You may be applying 240/480 to the windings, but they can actually see 2-3000 volts or more depending on harmonics in the system.

 

[maxketcham]

I would heavily consider disassembly of motor - get new bearings - wash stator as best you can in soap and water and rinse very well. Then isopropyl alcohol bath. Then dry for couple days and remegger.

You can dry by using low voltage DC say 6 VDC and maybe 10 or 15 amp across two stator leads. Watch temperature - I would not go above 125 F. Give it a couple days.

Worked well at sea with no spare parts - also works well for Fluke VOM dropped in dirty wash water - Fluke said that is exactly what they would do.

Dan Bentler

I have done this also, it works, but takes a lot of time and monitoring, oven works better, though compressed air then a heat gun will do if you don't have an oven and time is critical and no spare is available. We used the same technique to keep DC motors and generator sets dry, by keeping the fields on but in a low current mode, to create just enough heat to prevent condensation inside the box.

 

[shooter]

hoping you did not burn the isolation of the stator with your megger.

 

[Jeff23spl]

squirel sometimes have space heater that could help heating up ?

 

[Jeff23spl]

hoping you did not burn the isolation of the stator with your megger.

if a megger burn motor isolation, it's time to rebuild it anyway because varnish is dry or too thin at some critical place...