Looking for megger procedure for 1,000 hp motor

[shockking]

We're helping some folks troubleshoot a 1,000 hp motor (GE 752). They were advised by a consultant that the unit could remain in service as long as the megger results are ok. We have a megger MIT515 but we're typically testing cable, we've never tested a motor like this. They're also considering adding this measurement to their regular maintenance routine. Is there a good IEEE or GE instruction for how to perform this testing? And do they need something like the MIT515 or would they get what they need with something cheaper? Any advice appreciated.

 

[petersonra]

Perhaps the motor manufacturer would be a good place to start.

It is is an old motor and is suspect, it might be more cost effective to just send it in for rebuild. It will likely need it eventually anyway.

 

[garbo]

Not familiar with a MIT515 megger but started with a Biddle hand crank 500 volt megger back in the 1970s then to a great Fluke combination VOM/ Megger that I would use the 1,000 test volts for testing 480 volt motors. I have my own Simpson megger that the highest test voltage is 5,000 volts and have picked up low megger readings while testing 4,160 volt motors. Rule of thumb is test voltage should be double the motor nameplate volts plus 100 volts. Great thing with battery operated meggers is I would apply the voltage for at least a minute and write down start, middle & ending time megger readings. You might want to have a mechanical contractor or other service company use test equipment to check condition of bearings. Suggest that you check on Biddle & Fluke web sites about thier great megger testing instructions.

 

[shockking]

Thanks for the feedback. It looks like IEEE 43 is a good reference. It advises testing at 500V for anything up to 1kV, which this motor operates at 750 V-dc so that should be fine. The MIT515 goes up to 5 kV so it looks to be overkill for their needs. They will rebuild the motor soon but they're looking to add this to their routine inspection procedure, which seems like a good idea. The manual just says to do megger testing, no more detail than that. I see a lot of info for megger testing smaller motors and I wanted to check for differences for larger motors. It looks like it's basically the same though.

43-2013 - IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery - Redline

The dc voltage tests procedures for the measurement of the insulation resistance and polarization index of insulated stator, and rotor windings and how to interpret the results are described in this recommended practice.

ieeexplore.ieee.org

 

[garbo]

Thanks for the feedback. It looks like IEEE 43 is a good reference. It advises testing at 500V for anything up to 1kV, which this motor operates at 750 V-dc so that should be fine. The MIT515 goes up to 5 kV so it looks to be overkill for their needs. They will rebuild the motor soon but they're looking to add this to their routine inspection procedure, which seems like a good idea. The manual just says to do megger testing, no more detail than that. I see a lot of info for megger testing smaller motors and I wanted to check for differences for larger motors. It looks like it's basically the same though.

43-2013 - IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery - Redline

The dc voltage tests procedures for the measurement of the insulation resistance and polarization index of insulated stator, and rotor windings and how to interpret the results are described in this recommended practice.

ieeexplore.ieee.org

Forgot to state make sure motor is at a dead stop. Had electricians thinking that motors were bad because of readings jumping all over the place. Return fans and pumps without check valves often would have thier motor rotating. Usually measured resistance from A to B motor winding, B to C then A to C. If all were within 5% of one another during routine PM'S would attach one megger lead to a good clean ground and the other lead to all three windings. For VFD'S liked to see at least 10 megaohms to ground. Do not touch motor leads after meggering especially if using 1,000 volts or more. Motor windings hold a charge and have received a nice shock while rushing to get equipment back on line. Many years ago while on a one man tour of a large Ma Bell building I saw one of thier facilities guys writing down megger readings. He told me they compare readings from previous years and if readings changed a lot and was important they would replace the motor and send it out to motor shop.
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