# megger reading question?



## ashveal

what i have is a 300 hp baldor 1.15 sf motor 460 v 3 phase motor .
Motor is suspected to be bad.Motor has 6 leads that are labeled 1,1 2,2 3,3
essentially a "3 lead dummy proof" motor pecker head cover reads all #1-L1
all #2-L2 ......etc. used a megger to test between all leads phase to phase and the reading is 0.00 same as touching test leads together. 
Is the motor bad?


----------



## GEORGE D

Try each lead to motor casing.


----------



## varmit

A megger is normally used to test insulation leakage to GROUND. If testing "T" to "T" lead, the resistance should be near zero as all of the windings are connected together internal to the motor, but will vary depending on the motor size and the ambient conditions. Most places spec a go-no go value of 50 meg ohm T lead to ground. On a new motor this value should be much higher, usually 1000 meg ohm or better.


----------



## McClary’s Electrical

Connect your pairs together in three sets. Meg between each set and ground. If not grounded move onto checking resistance between phases. If none are open, and alll three are equal, throw the coal on it and stand back.:whistling2:


----------



## ashveal

Ive meg leads to ground readings are good . ive ohm between leads readings are .2 to .4 ohms . Continuity readings are ringing the buzzer on my meter.
so why should'nt I meg leads, Would,nt that be where I would find phase to phase?


----------



## Zog

varmit said:


> A megger is normally used to test insulation leakage to GROUND. If testing "T" to "T" lead, the resistance should be near zero as all of the windings are connected together internal to the motor, but will vary depending on the motor size and the ambient conditions. Most places spec a go-no go value of 50 meg ohm T lead to ground. On a new motor this value should be much higher, usually 1000 meg ohm or better.


Here is the actual IEEE/ANSI/NETA spec

The dielectric absorption ratio or polarization shall be compared to previously obtained results ​and should not be less than 1.0. The recommended minimum insulation resistance (IR 1 min) test results in megohms should be corrected to 40° C and read as follows:

1. IR 1 min = kV + 1 for most windings made before 1970, all field windings, and others not described in 2.2 and 2.3. (kV is the rated machine terminal-to-terminal voltage in rms kV)
2. IR 1 min = 100 megohms for most dc armature and ac windings built after 1970 (formwound coils).
3. IR 1 min = 5 megohms for most machines and random-wound stator coils and formwound coils rated below 1 kV.​


----------



## Zog

ashveal said:


> Ive meg leads to ground readings are good . ive ohm between leads readings are .2 to .4 ohms . Continuity readings are ringing the buzzer on my meter.


A normal ohmeter won't give you accurate winding resistance values, not enough oomph to charge the windings enough, so need a winding resistance test set for a true reading. 



ashveal said:


> so why should'nt I meg leads, Would,nt that be where I would find phase to phase?


Think about it, each phase is connected in the motor. Draw a wye winding and it will be clear to you (I hope)


----------



## ashveal

After I posted I started thinking and I get it now. So unless its direct short I really can't test with regular digital meter


----------



## Zog

ashveal said:


> After I posted I started thinking and I get it now. So unless its direct short I really can't test with regular digital meter


You can, the readings just will nto be accurate, but as long as they are similar that is good. If you have an open winding you won't see the open but rather double (Which sounds liek the case here)the resistance of the other windings because you will read path through 2 phases. Again, a sketch makes this all clear.

Before you go replace a 300 HP motor I would get a testing company with the proper equipment to check it out for you.


----------



## ashveal

True thanks for your time.


----------



## mrmike

Zog said:


> You can, the readings just will nto be accurate, but as long as they are similar that is good. If you have an open winding you won't see the open but rather double (Which sounds liek the case here)the resistance of the other windings because you will read path through 2 phases. Again, a sketch makes this all clear.
> 
> Before you go replace a 300 HP motor I would get a testing company with the proper equipment to check it out for you.


 
First of all, If you take a continuity check between the phases, example 480v motor -lead 1-2, 2-3, 1-3 If there is an open your meter will show it ! On a ohmeter you won't see double the reading as the reading will be zero between the phases !
And we never had to get a testing co to do our work-if in doubt with the ohm readings to ground-this is when the meggar or Hi pot tester came out...........................


----------



## Zog

mrmike said:


> First of all, If you take a continuity check between the phases, example 480v motor -lead 1-2, 2-3, 1-3 If there is an open your meter will show it !


It depends if it is a delta or wye winding. I assumed delta because he stated 480V, not 277V. Say each phase winding had a resistance of 3 ohms. If phase A was open you would see 6 ohms because the meter would read phase B and C in series. 












mrmike said:


> On a ohmeter you won't see double the reading as the reading will be zero between the phases !


 There is no such thing as zero ohms, you are just using the wrong tool. 



mrmike said:


> And we never had to get a testing co to do our work-if in doubt with the ohm readings to ground-this is when the meggar or Hi pot tester came out...........................


??


----------



## mrmike

Zog said:


> It depends if it is a delta or wye winding. I assumed delta because he stated 480V, not 277V. Say each phase winding had a resistance of 3 ohms. If phase A was open you would see 6 ohms because the meter would read phase B and C in series.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> There is no such thing as zero ohms, you are just using the wrong tool.
> 
> no there is no such thing but this is what we looked for, in checking for an open between phases in motors with our ohmmeter in our plant. We had so many it- that was the threshold to look for.
> 
> ??


 What is there to understand ??? Just What I stated- you Use your multimeter first- then when you see a borderline reading between a phase and ground you bring out the meggar to verify it.....................
You don't carry a meggar around in your tool pouch...................

I meant to say you wouldn't see any ohms- it would be infinity-no reading between the phase if it is open.


----------



## Zog

mrmike said:


> What is there to understand ??? Just What I stated- you Use your multimeter first- then when you see a borderline reading between a phase and ground you bring out the meggar to verify it.....................
> You don't carry a meggar around in your tool pouch...................
> 
> I meant to say you wouldn't see any ohms- it would be infinity-no reading between the phase if it is open.


Not in a delta winding. It would be double the resistance of one phase. You seem to be missing the whole point.


----------



## CheapCharlie

Correct me if I'm wrong, but if it's a six lead motor, it can be wired wye or delta (in the case of a wye start, delta run setup). If you tie 1-1, 2-2, 3-3 then what zog mentions about double the reading is right. Any other mixture of leads can get varying results.


----------



## Tsmil

CheapCharlie said:


> Correct me if I'm wrong, but if it's a six lead motor, it can be wired wye or delta (in the case of a wye start, delta run setup). If you tie 1-1, 2-2, 3-3 then what zog mentions about double the reading is right. Any other mixture of leads can get varying results.


This has been interesting and funny. First, lets start with the math of resistance in a delta motor. Assume 3 ohms per winding. If you measure between phase 1 and 2 you will get a reading of 2 ohms. That is the 3 ohms in parallel with the 6 ohms of the other 2 windings. If the winding between 1 and 2 is open then you will get a reading of 6 ohms. Triple of what a good reading should be. 

If this was a 6 lead motor that was capable of delta wye, it would not be labeled in this manner. What I suspect you have is a motor that has 2 sets of parallel windings. To check resistance the first step would be to group the 1s 2s and 3s together and take your readings from 1 to 2, 2 to 3, and 1 to 3. You should get readings that are very close. On a motor of this size the values may be so low that these results may be worthless. Now you will have to separate the 1s, 2s, and 3s and using your ohm meter to find the groups that belong together. Now take your resistance readings from phase to phase for each set of windings. If you have an open or shorted winding, you should see some varying results. If all readings the same then time to do the insulation test by Kettering from phase to ground. 

When dealing with motors of this size we call in the motor specialists who will be able perform the right tests with the expertise to evaluate the motors condition. Ohm meters and meggers may detect serious motor problems but may not detect all.


----------

