megger that motor

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jonni

Member
I have a 460volt 3ph 1hp motor that I need to megger. Traditionaly I place the red lead of the megger to each lead one at a time and the black to motor ground. I check for insulation value of 2megs or better to pass. Can you combine the three motor leads to the red megger lead and place the black to ground to test the motor for shorts?

Are there any other test that I can perform with the megger to evaluate the running condition of the motor.

Jonni
 
zog

zog

Senior Member
Location
Charlotte, NC
Your minimun IR value (temerature corrected of course) will depend on the type and vintage of the motor. you should also do a DAR and PI test along with a winding resistance test.

As far as you question about the leads, think about how a 3 phase motor is wound. That should answer your question.
 
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bth0mas20

Senior Member
Location
Maryland
megger

megger

When you megger a motor you only need to put the positive leed on one of the motor leads. This is because as long as all the windings are still tied together inside the motor your will be meggering all three leads from the motor. if you disconnect the motor at the pecker head and wish to megger the feeder cables you will need to do each individually to ground and each phase to phase. Im not sure what the problem is that you may be having with the motor but it would be a good idea to check the resistance of each winding by reading a resistance from T1 to T2 then T1 to T3 and T2 to T3 to verify that the windings are balanced. if they are not balanced you may have a problem internal with your windings. If you want to do a PI (polarization Index) test you need to megger the motor for ten miniutes. write down the readings for 1 min and 10 min. then divide the ten min reading with the one min reading.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
If you megger each lead and they are not the EXACTLY the same, you have real issues...As stated one lead is as good as another if the motor leads are connected for use.
 
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bth0mas20

Senior Member
Location
Maryland
megger

megger

if your megger readings are different when you connect to different motor leads you may have a problem. chances are the resistance of the windings are not balanced. this could mean a short internal to the windings. usually moisture or the winding insulation breaks down and causes the windings to short.

what are your winding resistances and megger readings and PI?
 
J

jonni

Member
megger motor

megger motor

bth0mas20 said:
if your megger readings are different when you connect to different motor leads you may have a problem. chances are the resistance of the windings are not balanced. this could mean a short internal to the windings. usually moisture or the winding insulation breaks down and causes the windings to short.

what are your winding resistances and megger readings and PI?
Click to expand...

Thanks for the feedback. Had 2meg reading at 30sec and 2meg reading at 1minute. According to the megger manual that is poor insulation values.
 
C

cbranter

Member
Location
Provost, Alberta, CANADA
In addition to a megger testing;
I also take a resistance reading between The three motor leads ( three winding wire groupings) these values should all be very close 0.1 to 1.5 typically depending on the size of the motor.
 
tryinghard

tryinghard

Senior Member
Location
California
bth0mas20 said:
...If you want to do a PI (polarization Index) test you need to megger the motor for ten miniutes. write down the readings for 1 min and 10 min. then divide the ten min reading with the one min reading.
Click to expand...

So the unit of measure for the PI is a percentage (10 min divided by 1 min)?
 
W

wireman71

Senior Member
And are you recording your readings? Should keep a log for that motor. Can see if it starts to degrade and order a spare.
 
Krim

Krim

Senior Member
Location
Maryland's Eastern Shore
wireman71 said:
And are you recording your readings? Should keep a log for that motor. Can see if it starts to degrade and order a spare.
Click to expand...

wireman71,
OK, Since I've found this post to pick up an issue of relevance ( determining the degradation rate of a motor ) I'd like to ask for your opinion on the subject ...

Is it still considered,as I was taught,that in order to determine the condition of a motor to:
1) amp the motor at start up & record the inrush,
2) amp the motor after 10 to 15 minutes & record ramp up amps,
3) amp the motor after 30 minutes & record leveled out constant amps , 4) than compare the average with the listed continuous duty amperage on the motors' nameplate.
To determine the health of the motor in order to establish a potential run time until it would be wise (from a preventative maintenance standpoint) to remove there said motor for rebuild or replacement ?
Please let me know if this practice is still somewhat common practice or if there's an easier, more reliable way of determining this.
Carl :confused:
 
W

wireman71

Senior Member
After you spend all that time testing the motor you could have afforded a spare : ) I'd only do that on big expensive motors. I'll let others chime in on maintenance. I used to test motors a long time ago and we just meggered and recorded. I think a running amp draw wouldn't be bad either but spending excessive amounts of time doesn't make financial sense for a 1hp motor.

One thing to think about is making a inventory list of your motors then finding replacements. If they are all Dayton say and the local Grainger, if your lucky enough to have one, has your replacements in stock then you don't need to keep one on hand. But lets say you have a line that will lose X amount of dollars if this motor goes down per a day and the replacement will take a week to get. Then management can decided if they want to stock spares.
 
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U

union91

Member
to megger just do it like this phase to phase and phase to ground, in phase to phase megger show a short in phase to ground megger will give the G
 
G

Gary Shumaker

Member
Hi Pot

Hi Pot

Another method of testing the insulation (it will actually overstress the insulation by a bit) is to put a Hi Pot tester on the leads and measure to ground

Regards,
 
Krim

Krim

Senior Member
Location
Maryland's Eastern Shore
Speaking of Meggers'...
What type do you use ? Is it a 2 or 3 lead type ? Crank or battery powered ? How long have you had this particular model and would you buy the same or most similar to model again ? If not what features would you prefer in a replacement megger ?

Carl
 
electricalperson

electricalperson

Senior Member
Location
massachusetts
i use the MJ159. its a hand crank with 3 terminals + - and guard. the major megger series is a high quality megger and should be all you need. they make them in handcrank, line, battery and afew combos. cost about 1000 dollars. the fluke insulation tester is pretty nice too
 
zog

zog

Senior Member
Location
Charlotte, NC
Krim,

That all depends on what your intended use is, I own 4 Meggers that I use for different applications, OK I only use 3 of them one I just keep around for sentimential reasons.
 
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