Proper Use of Megohmmeter

Disconnect the motor leads, test the wire and motor seperately. Pay attention to the motor terminations! Hand crank,,, yikes, I would test the conductors and motor to ground at

Any chance of moisture in the motor? If so dry it out for 24 hours with hot air and re-test.

Then test the motor phase to phase, I would use a regular ohm meter on your VOM first. What kind and size of fuses are you using?

Such a megger is for measuring the insulation resistance. Disconnect all power and put one lead on any one of the three phase leads going to the motor, the other lead to a good ground connection that is also tied to the motor frame in some way. Crank until the reading stabilizes.

If its over 1 megaohm, then that's not the problem. If it is under, separate the supply cable from the motor at the motor terminal head and check the motor again. It it is good, check each of the three cable conductors separately.

From your description, it sounds like a temperature related fault. If the motor was overloaded, I'd expect the motor starter overloads to trip instead. To blow a fuse requires some sudden fault developing. Any 'acrid odors', or 'visual inspection' clues? Open the motor terminal head and take a look at the cable/motor-lead connections. This often can develop a fault if the insulating tape is rubbing against the inside of the terminal head.

If this is a temperature related fault (and it sounds like it since it starts and runs for a while before faulting), try to do the tests the moment the thing blows a fuse, before things cool off.

Good luck,

daestrom

Since only the fuses blow and the motor overloads do not trip, I would recalculate the fuse size to make sure they are correct for the application. You have not given all the information. What is the hp of the motor, the FLA on the motor nameplate, the application for the motor, what type and model of motor controller and size, and what type of fuses are you using? Also how old is the motor and how long in feet is the circuit?

Some more information on the motor etc. : the motor is 20 years old, it is 29 FLA, it is running an air handler, the fuses are RK5 40 dual element time delay, the circuit lenght is approx 40 feet. There are no signs of heat or burning, no unusual smells, I opened the "pecker head" and it looks fine. The motor starter is a Square D Nema 2. When testing the motor should I test the 3 phase leads starting at the motor starter or open the "pecker head" and test the 3 phases there? Also do I need to test phase to phase at the motor and if so what readings should I look for? Many thanks to all of you.

Testing phase-phase with anything other than a very sensitive milliohm meter is pretty much a waste of time. The difference in resistance between a good motor and one with a fault is often much less than 0.5 ohm (not megaohm, just ohm).

If you were recording the insulation value for trending/maintenance rule, I'd say to measure at the 'pecker head' with the line leads disconnected. But for troubleshooting, don't bother disconnecting all the leads, just test from the motor side of the starting contactor. Unless one of the phases was open, the megaohm reading will be the same for all three phases (plus or minus winding resistance of

You fail to mention whether the motor is delta or wye. It may be best to disconnect all windings (marking them correctly for re-assembly) and then megging winding to ground and winding to winding.

What about the bearings getting hot and causing problems? I think you said in another post it was fused at 40 amps with a FLA of 29. The US code permits fuses up to 175% of the value in Table 430.250. (Exceptions apply) Possibly the motor overload is mis-sized or malfunctioning? Dean