How To Tell Your Induction Motor Is Good Using Digital Megger?

We use a UNI-T 5URG2 digital megger to test induction motors, but I don't understand the results. My boss said if the scale when testing is at least 1gig that the induction motor is good, anything less indicates that the induction motor is bad. Also, when testing he said go up to 1000V, but why go so high when the induction motor max voltage is 480V? 500V is an option so I test at 500V because I see no need to test at 1000V. In the photo you can see that the scale is at 5gig, but the display is reading O.L, why? O.L means open circuit which indicated that there is a winding that is defected or am I wrong?

My question is how to read if a induction motor is good or bad by using a digital megger? Is there a link/site/chart that I can use as a reference? Is there truely a need to test at 1000V? I have a photo to this post.

See this paper, there is a standard that is used to test in this situation, it is mentioned in the paper as well,
cheers,
Richard

Oops, sorry, didn't paste the link in,
http://www.netaworld.org/sites/default/files/public/neta-journals/NWsp08 - Bishop.pdf
https://www.instrumart.com/assets/Megger-insulationtester.pdfCheers,
Richard

I have not experienced using a digital insulation resistance (IR) tester, but I think it works similarly as to a dial/analog IR tester. An indication of an induction motor with a good insulation resistance is when your reading is at 1000 Mega-Ohms (1gig), but when you obtain a reading of 100 Ohms or below, the motor's insulation resistance is in poor condition, maintenance works for the motor should be carried out (cleaning, replacing of ball bearings, applying insulation varnish to the stator windings).
 Place the IR tester's black probe to the motor's casing (acting as ground), and then alternately place the red probe to each of the motor's winding terminals. The readings obtained will indicate the insulation resistance of that winding with respect to ground. A safety precaution: never touch the probes during testing or when the IR tester is still powered on.
Regarding the usage of test voltages of 500Vdc or 1000Vdc of an IR tester, actually we can use either of the two for a motor rated at 440V. The only difference is that, if we use 500Vdc, we need to take the IR reading for at least one minute to get an accurate reading. If we use 1000Vdc, we should only do a spot test, meaning testing each winding insulation resistance for a maximum of 10 seconds, because it may wear the windings if we test it for a longer period. The standard IR maximum test voltage to be used for induction motors is motor rated voltage x 2. So if we have a 440V motor, the maximum test voltage should only be 440x2 = more or less 880Vdc. Hope this will be helpful.

Ok so if I have a dual voltage induction motor that is capable of being wired at 208VAC or 480VAC then what voltage setting should I put the IR at? Let's say if I wired that dual voltage induction motor for 208VAC do I put the testing voltage at 500 or 1,000 (I say 1000 because 480VAC is a wiring option)? You mentioned VDC when talking about voltage setting to the digital megger. Do all megger give off DC voltage when testing insulation resistance?

I would say 500Vdc test voltage since it is the one closer to your 208-V wired motor. Most IR test instruments give off DC voltages as test voltage because we are after the motor's insulation resistance condition. If we use AC voltage then we will be getting impedance.
Having IR test is analogous to finding a leak in a water hose. Assuming that we do not know where the leak or hole is along the hose, we will apply water pressure to the hose (equivalent to the DC test voltage in IR test), and we will be able to point out where the leak is, obviously where the water squirts along the hose. The higher water pressure we apply (higher test voltage), the more we can find out the leak.

All of the questions you have asked are covered under the standard for testing motors, you need to get a copy and read it,cheers,Richard