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Old 01-24-2009, 11:09 PM   #1
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I just want to make somthing clear If you do a megger test to gound on a motor less than .5 megs is no good phase to ground and phase to phase your ohms should be low but close like 10ohms 10ohms 9ohms depending on the size of motor the bigger the motor the less R value is
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Old 01-25-2009, 03:07 AM   #2
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I just want to make somthing clear If you do a megger test to gound on a motor less than .5 megs is no good phase to ground and phase to phase your ohms should be low but close like 10ohms 10ohms 9ohms depending on the size of motor the bigger the motor the less R value is
That .5 would be acceptable if your talking about a 480 volt motor.


Phase to Phase you want to be balanced.

True the resistance reading will be lower on a larger motor but the motor depends on the inductive reactance to limit the current.
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Old 01-25-2009, 09:49 AM   #3
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So .6meg would be good for 600v ac phase to ground? Is there any other way to check phase to phase on the windings than just with and ohm meter and what reading should you get? I know on a dc moter you can check phase to the brush holder.
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Old 01-25-2009, 10:38 AM   #4
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So .6meg would be good for 600v ac phase to ground? Is there any other way to check phase to phase on the windings than just with and ohm meter and what reading should you get? I know on a dc moter you can check phase to the brush holder.

The rule of thumb is 1000ohms per volt.

I use a analog meter such as a simpson 260 when reading phase to phase never a digital.


Some of these other threads have some good info as well

Testing a motor and/or drive

megging a motor wired up
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Old 01-25-2009, 11:09 AM   #5
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If you have a motor that won't get to 1 meg, I wouldn't start it. I have always went by 1 meg. for motors and 10 meg for xfmrs.
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Old 01-25-2009, 02:01 PM   #6
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I was taught, about a dozen years ago, that 1K ohm per volt is acceptable. I have followed this ever since, and have yet to have a problem...as far as phase to phase...I have to agree with p_logix again, a good analog meter is the way to go, and make sure they are balanced.
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Old 01-25-2009, 02:53 PM   #7
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I was told you should always megger phase to phase to open a bad winding then do a ohm check is this true
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Old 01-25-2009, 05:35 PM   #8
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Look as folks say for 1Mohm plus to earth from any phase and use the lKohm per volt rule for appx winding resistance - but this may be well out in some instances. It's a rule - not a a law. Test at 500min and 1Kvolts if possible for all tests. Then using an analogue tester, test ohms phase to phase (preferred instrument) and if balanced within an ohm or two then switch on.

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Old 01-30-2009, 11:32 PM   #9
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I do not know of any published phase to phase resistance values. what is impotrant is that they are all nearly the same. I use a Fuke DMM ( 1587 )to check motors the Ohmmeter portion does a fine job with phase to phase winding resistance. The megger portion does a fine job up to 480 volt motors. If I have a large 480 volt motor or a high voltage motor I use a "DUCTOR" which is a Ohmmeter designed to check low resistances.
It is not uncommon for a motor to have less than 1 Ohm winding resistance phase to phase.
As a rule of thumb horsepower and winding resistance are inversely proportional to each other.
Anouther thing is that if I have a motor that is hooked up to a drive I use a 2500 volt Megger . It is not uncommon for a 480 volt motor to see 1000 volts in operation. A drive rated motor has special insulation and it will not hurt it to megg it at 2500 volts.
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Old 04-09-2009, 01:03 PM   #10
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hi,caan you help me how to check a motor 3 phase,using a megger?
how can i no if reading is good?thx
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Old 04-09-2009, 01:05 PM   #11
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Default how to check a motor 3 phase?

[how to connect probe to motor terminals?what readin shuld gt?
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Old 04-23-2009, 01:58 PM   #12
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Connect the two leads together on the megger. Push the button. This is a dead short. Take the leads loose and push the button. This is as close to infinity as your meter can read. This is ideally what you want to see when megging the motor. You most likely will never see infinity. But you get the idea. Right?
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Old 04-23-2009, 02:13 PM   #13
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Connect the two leads together on the megger. Push the button. This is a dead short. Take the leads loose and push the button. This is as close to infinity as your meter can read. This is ideally what you want to see when megging the motor. You most likely will never see infinity. But you get the idea. Right?
I know what you are saying when you say "infinity" but be careful using that term. There is no such thing as infinite resistance, just the limits of your megger. Some only read up to 999M, I have some that read up to 500G. Anytime you max out your megger you should record this as >whatever you megger maxes out at.

Say you have a 1G megger and you record infinite, then someone with a better megger tests it 2 years later and gets 120G. What is the trend? Has the insulation gone from 500G to 120G (Could indicate the start of insulation breakdown) or was it just >1G before and that was the limit of the megger used.

Make sense?
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Old 04-23-2009, 02:16 PM   #14
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Originally Posted by ryang View Post
I just want to make somthing clear If you do a megger test to gound on a motor less than .5 megs is no good phase to ground and phase to phase your ohms should be low but close like 10ohms 10ohms 9ohms depending on the size of motor the bigger the motor the less R value is
Here is a real spec, comes from NEMA and is also recognized by ANSI and NETA as a standard. all values need to be temperature corrected to 40 degrees C (Very important, otherwise your readings are meaningless)

Minimum IR readings:
· IR 1 min = 100MW for DC armature and AC winding built after 1970
· IR 1 min = 5MW for most machines and random-wound stator coils and form wound coils rated below 1kV
· IR 1 min = kV+1 for machines made before 1970, all field windings, and others not listed above


You should also record and calulate DAR and/or PI. This will tell you more about the insulation than just a spot test IR.
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Old 05-16-2009, 02:20 PM   #15
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I know what you are saying when you say "infinity" but be careful using that term. There is no such thing as infinite resistance, just the limits of your megger. Some only read up to 999M, I have some that read up to 500G. Anytime you max out your megger you should record this as >whatever you megger maxes out at.

Say you have a 1G megger and you record infinite, then someone with a better megger tests it 2 years later and gets 120G. What is the trend? Has the insulation gone from 500G to 120G (Could indicate the start of insulation breakdown) or was it just >1G before and that was the limit of the megger used.

Make sense?
If you read my post you would have plainly seen these exact words.

"This is as close to infinity as your meter can read. This is ideally what you want to see when megging the motor"
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Old 05-17-2009, 11:04 AM   #16
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If you read my post you would have plainly seen these exact words.

"This is as close to infinity as your meter can read. This is ideally what you want to see when megging the motor"
I know what you meant but my point is that what my megger maxes out at is probally a couple hundred times more than what your megger maxes out at. So you want to record the value as >500G or whatever the max on your megger is. Some meggers only read up to 1G, so what if someone had one of those and recorded "infinity" when it was actually only 2G and his megger maxed out at 1G. Now someone else comes along next PM period and uses a better megger (Say a max of 500G) and sees it is only 2G, he would see his as a rapidly degrading insulation system and may replace the motor based on trending when if fact it is just fine.
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Old 05-17-2009, 11:20 AM   #17
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I know what you meant but my point is that what my megger maxes out at is probally a couple hundred times more than what your megger maxes out at.
Cool. You guys should have that made into a bumper sticker.

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Old 05-17-2009, 11:42 AM   #18
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Cool. You guys should have that made into a bumper sticker.


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Old 05-17-2009, 08:25 PM   #19
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Cool. You guys should have that made into a bumper sticker.

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Old 06-04-2009, 12:09 PM   #20
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I hope I am not hijacking this thread, but it seemed like the perfect thread or posting.


Lets say your first test A-B, B-C, A-C indicates (using a reg DMM), 0.2 Ohms phase to phase. Whats the point in megging each phase to ground? Wouldnt it make more sense to just meg say A to ground. If it is B that is shorted to ground wouldnt it show up while megging A since A and B are connected?
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