Help with megger

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dmanda24

dmanda24

Senior Member
Here is the scenario
a VFD controlling a 3PH 480 V motor, the vfd stops working, is replaced with a new one, a few minutes after being energized it fails also, the boss orders the vfd to be bypassed, wires are checked for voltage and connected without the VFD, motor runs perfectly. Controls company installing the vfd says the wires and motor need to be meeged because the vfd failure is due to a leakage current. The boss bought a megger and has a schedule shutdown to do the meassuraments.
What kind of values are acceptable for the wires as well as for the motor, I do not know the horepower for the motor since it is not easily accessible, the motor is wired with #6 wire and fused at 100 A.

Also on the VFD one of the contactors welded shut what else could be the reason why the brand new vfd failed, and if there is leakage current why isn't it tripping the breaker without the vfd.

Thanks
 
dmanda24 said:
Here is the scenario
a VFD controlling a 3PH 480 V motor, the vfd stops working, is replaced with a new one, a few minutes after being energized it fails also, the boss orders the vfd to be bypassed, wires are checked for voltage and connected without the VFD, motor runs perfectly. Controls company installing the vfd says the wires and motor need to be meeged because the vfd failure is due to a leakage current. The boss bought a megger and has a schedule shutdown to do the meassuraments.
What kind of values are acceptable for the wires as well as for the motor, I do not know the horepower for the motor since it is not easily accessible, the motor is wired with #6 wire and fused at 100 A.

Also on the VFD one of the contactors welded shut what else could be the reason why the brand new vfd failed, and if there is leakage current why isn't it tripping the breaker without the vfd.

Thanks
Click to expand...

600V rated cable should be >100M @1000VDC, motor depends on size, design, and vintage, different specs for each. Post more motor info when you get it but >5M should be good for most types when corrected to 40 degrees C after 1 minute, continue test for 10 minutes and calulate PI, should be at least 2.0
 
zog said:
600V rated cable should be >100M @1000VDC, motor depends on size, design, and vintage, different specs for each. Post more motor info when you get it but >5M should be good for most types when corrected to 40 degrees C after 1 minute, continue test for 10 minutes and calulate PI, should be at least 2.0
Click to expand...

excuse the ignorance but how do i calculate the PI
 
dmanda24 said:
Here is the scenario
a VFD controlling a 3PH 480 V motor, the vfd stops working, is replaced with a new one, a few minutes after being energized it fails also, the boss orders the vfd to be bypassed, wires are checked for voltage and connected without the VFD, motor runs perfectly. Controls company installing the vfd says the wires and motor need to be meeged because the vfd failure is due to a leakage current. The boss bought a megger and has a schedule shutdown to do the meassuraments.
What kind of values are acceptable for the wires as well as for the motor, I do not know the horepower for the motor since it is not easily accessible, the motor is wired with #6 wire and fused at 100 A.

Also on the VFD one of the contactors welded shut what else could be the reason why the brand new vfd failed, and if there is leakage current why isn't it tripping the breaker without the vfd.

Thanks
Click to expand...
sounds like an expensive job
 
Looking for something simple?

My quick and dirty way of checking a motor is to first "ohm" out the motor leads, at the vfd/contactor. Checking for balance. I also check them to ground, before digging out the megger. They should be balanced, but if they are not exact it is NOT a guarantee of a fault. If the reading is way out of balance you may have a shorted winding. It's quick and it's easy, and if there is an open you don't need to dig out the megger. Next I connect one lead of the megger to a known ground and the check it against each motor lead. ( This is assuming the motor is grounded )

If there is a brake controller at the motor this can/will muck the readings up, so if you don't know start at the motor.

If I need to proceed from there I'll take things apart at the motor and check the motor, and the feed wires. Make sure when megging wiring that it is NOT connected to a vfd.


This process is not fool proof, but is a great first step. Very rarely do I need to go beyond this process in finding a problem with a motor/feed wires.


Good Luck,
Doug S.
 
I dissagree with Zog. The rule of thumb we always use is 1 megohm per 1000V(insulation class.) I.E. a 480V motor should meg 1 megohm or better (@1000V) to safely energize. A 4160V motor, 4 megohms, etc.
The fact that the motor runs fine when bypassed already eliminates the problem being in the motor and no need for the megger test.
Besoeker asked the right question. What do you mean the VFD stops working?
Do fuses in the VFD blow? Do you get an error message on the display?
What does the diagnostic menu indicate when the VFD shuts down?
Was the VFD wired correctly. Phase rotation correct, etc.
We need more info.
 
If you don't even know the motor HP due to it being inaccessible, how are you setting the O/L's correctly? How do you know the VFD is sized correctly?
 
One other thing. Is this a new installation? Some older motors when applied to VFDs can cause problems. A VFD will not be compatible with just any type of motor (motor has to be designed to run off a VFD.)Consult the motor manufacturer for the answer to that question.
 
Cow said:
If you don't even know the motor HP due to it being inaccessible, how are you setting the O/L's correctly? How do you know the VFD is sized correctly?
Click to expand...
I trust the engineers that designed it and the fact that it was working properly for years, the motor has to be shut down before it can be accessed and the down time has to be minimized.
 
tonytonon said:
I dissagree with Zog. The rule of thumb we always use is 1 megohm per 1000V(insulation class.) I.E. a 480V motor should meg 1 megohm or better (@1000V) to safely energize. A 4160V motor, 4 megohms, etc.
Click to expand...

Zog follows nationally recognized standards such as ANSI, NETA, and IEEE. As I said the actual spec depends on the type, vintage, and size of the motor, your rule of thumb hasnt been valid for 30 years, it was removed from all standards in 1979. So disagree all you want.
 
dmanda24 said:
Is there any reference document that shows this so that I can have a reference when presenting results to the boss
Click to expand...

ANSI/NETA MTS 2007 edition covers both, the cable and motor specs can be found in a few different IEEE standards but all of it is contained in the NETA spec.

I fyou are looking for a free refernce use the link you got earlier for "A Stich in time".
 
zog said:
ANSI/NETA MTS 2007 edition covers both, the cable and motor specs can be found in a few different IEEE standards but all of it is contained in the NETA spec.

I fyou are looking for a free refernce use the link you got earlier for "A Stich in time".
Click to expand...
The free reference that you just suggested using mentions the 1M/1000V 30 year old standard.
 
If the contactor that welded shut is on the output of the VFD I would start looking there. If a bypass contactor(or any contactor) on the output of a VFD opens or closes after the VFD output is started the results on some VFD's can cause complete failure of the VFD. Any contactor on the output of a VFD should close first then the drive should start, and that contactor should not open unil the drive is stopped. So if a loose wire or faulty auxiliary contact going to the contactor is causing your problem you could go through a lot of drives.
 
tonytonon said:
Regardless, I have used it for 39 years and have not blown up anything:(yet)grin:
Click to expand...

One up-man-ship....I used it for a bit more than 40 years and never had a problem. I've seen new cable megger less than 100 megs, and nothing was wrong with it (no damage). Zog would have you buying new motors and replacing the cable! Not necessary.
 
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