megging readings

[mickeyrench]

I did some megging of motors today and some of the readings I got I am not sure are correct. All are 3 phase and 480 volt.

1] 7.5 hp got 5.1 megs @ 1000 volts

2] 40 hp got > 1999.9 megs @ 1000 volts this is the highest reading on meter and i am not sure its right.

3] 20 hp got 189.9 megs @ 1000 volts sump pump motor

can these high readings be possible. The meter used is a amprobe dm-3 multi test.
Also the co. has a pm list to check- voltage, amp., and resistance. Some times motors are not running and i think a amp reading is not needed if it meggs out good. does anyone think the amp reading is needed either way? we also do temp, and vibration reading. anything to add willbe appriciated.

 

[electricalperson]

do you have any records of past readings? if those were the first reading they look ok to me. if past readings were higher it might indicate a problem. ambient tempature comes into play too. check out the book a stitch in time by megger

 

[Mule]

I did some megging of motors today and some of the readings I got I am not sure are correct. All are 3 phase and 480 volt.

1] 7.5 hp got 5.1 megs @ 1000 volts

2] 40 hp got > 1999.9 megs @ 1000 volts this is the highest reading on meter and i am not sure its right.

3] 20 hp got 189.9 megs @ 1000 volts sump pump motor

can these high readings be possible. The meter used is a amprobe dm-3 multi test.
Also the co. has a pm list to check- voltage, amp., and resistance. Some times motors are not running and i think a amp reading is not needed if it meggs out good. does anyone think the amp reading is needed either way? we also do temp, and vibration reading. anything to add willbe appriciated.

An amp reading is going to give you more of a mechanical or, loading condition. Where a megger gives you the condition of the winding varnish.

Those readings are plenty good enough to run the motors, but do reflect the condition. Find the small book called "A stitch in Time" by Biddle...Its easy to understand, and is very enlightening. You probably can find it on the net as its a small phamplet that comes with a new megger when you purchase it.

Also look at I think its IEEE#43,??If my memory is correct, its more in depth, but good stuff.

 

[ohmhead]

Megging

Megging

Well we also agree they look ok and the amp reading is aways done around here , we use a fluke and highest reading is 4000 infinity we also have a avo and its highest reading is 999 infinity . Were they warm or cold when you megged in the morning or afternoon test are different you get high readings in afternoon then in the early morning its dampness in the windings ,take care

 

[Cold Fusion]

Well we also agree

I'm minorly curiously about the "we""
Royal "we"?
You and your frog?
Imaginary friends? (Don't get me wrong - I like my imaginary friends :smile
Split personalities ganging up on us? (I like this one the best )
More than one warm body colaborating on the response?

not picking on you , just curious

cf

 

[ohmhead]

Well i guess you could say our crew or our company that i work for would that be acceptable , dont understand what your getting at explain ?

 

[zog]

I did some megging of motors today and some of the readings I got I am not sure are correct. All are 3 phase and 480 volt.

1] 7.5 hp got 5.1 megs @ 1000 volts

2] 40 hp got > 1999.9 megs @ 1000 volts this is the highest reading on meter and i am not sure its right.

3] 20 hp got 189.9 megs @ 1000 volts sump pump motor

can these high readings be possible. The meter used is a amprobe dm-3 multi test.
Also the co. has a pm list to check- voltage, amp., and resistance. Some times motors are not running and i think a amp reading is not needed if it meggs out good. does anyone think the amp reading is needed either way? we also do temp, and vibration reading. anything to add willbe appriciated.

What was the winding temp at the time of test (Major factor), your reading dont mean anything without that info. How long was the test? Your PI is more valuable that the IR values.

 

[roger]

Well i guess you could say our crew or our company that i work for would that be acceptable , dont understand what your getting at explain ?

Well, I guess some are wondering if your crew (or whoever "we" consists of) actually agrees with you.

Roger

 

[zog]

Well we also agree they look ok and the amp reading is aways done around here , we use a fluke and highest reading is 4000 infinity we also have a avo and its highest reading is 999 infinity . Were they warm or cold when you megged in the morning or afternoon test are different you get high readings in afternoon then in the early morning its dampness in the windings ,take care

Thee is no such thing as "infinity", you are just stating the limitations of your meters. Very important concept to understand. And no, the readings do not look OK.

 

[ohmhead]

Well my instructions that come with the fluke meter say 4000is infinity Zog now what do i do now? Ok whats wrong with the megg readings ? comments

 

[ohmhead]

Well, I guess some are wondering if your crew (or whoever "we" consists of) actually agrees with you.

Roger

Well if everyone agreed with everyone then there would be no one to teach. I must say we learn everday . we have megged a few conductors and motors and would like to learn or know more of what we said was not correct ? Roger best to ya

 

[zog]

Well my instructions that come with the fluke meter say 4000is infinity Zog now what do i do now?

No they dont, you are misunderstanding what it is telling you. If it was infinity then you should be able to safely put 10 billion volts across the insulation and not have any leakage, so would you touch the insulation with a billion volts on the conductor? No you wouldnt, cause no insulation could handle that, no insulation in "infinity ohms", just dosent exist.

Ok whats wrong with the megg readings ? comments

Lets wait and see what the Op says about temperature of the windings and the duration of his test , but the 1st and maybe 3rd motor are bad and need to be replaced or rewound.

 

[Mule]

Heres the phamphlet I was talking about...Im sure alot of folks have seen it...
And I agree these readings need to be temp corrected and plotted to a PI to see if the values stay positive...

http://www.biddlemegger.com/biddle/Stitch-new.pdf

 

[wptski]

In order to do a PI test you'd need a megger with a 1000G or 1Tera ohm max range. Many handhelds won't even come close to that.

 

[ohmhead]

megging basics

megging basics

Well Zog good point made we have heard in the past that one megohm for each 1000 volts applied ? True or False , next if one had a 2400 volt motor and the megg reading was 2.4 megs is it good or bad on them windings ? Also temp would be at running temp not cold better or worst case ? last why is giga megg ohms really needed on insulation testing ? We always wondered about air temp when testing at high voltage is cold or hot more conductive ? We would like to learn about megging Roger thinks iam not a good crew leader so i would like to improve and willing to listen to you you Zog as your posts are much more educational and to the point . Best to ya

 

[mickeyrench]

When I took the megger readings I did not take any temp. reading. Most of the motors were running at the time and i shut them down and megged the motors for one minute on each phase. All motors are indoors. They are the first recordings i have taken on the motors, Iam new employee and trying to start something with pm and i know I don't know about megging. The last person taking the readings was using a ohm meter and just put in a number in.
am i suppose to take the ambeint temp? Can I use the temp on the frame of motor?
Thanks for all the tips and responses.

 

[brian john]

Well my instructions that come with the fluke meter say 4000is infinity Zog now what do i do now? Ok whats wrong with the megg readings ? comments

Use one on my meters that read in the gigohms or maybe the other meter that reads in the teraohms.

 

[RHJohnson]

I did some megging of motors today and some of the readings I got I am not sure are correct. All are 3 phase and 480 volt.
1] 7.5 hp got 5.1 megs @ 1000 volts
2] 40 hp got > 1999.9 megs @ 1000 volts this is the highest reading on meter and i am not sure its right.
3] 20 hp got 189.9 megs @ 1000 volts sump pump motor
can these high readings be possible. The meter used is a amprobe dm-3 multi test.
Also the co. has a pm list to check- voltage, amp., and resistance. Some times motors are not running and i think a amp reading is not needed if it meggs out good. does anyone think the amp reading is needed either way? we also do temp, and vibration reading. anything to add willbe appriciated.

#1 - You should be meggering those 480 volt motors @ 500 volts. The 7.5 hp motor reading is low, if you megger @ 500 volts you should get a higher reading. Without a past history I'd be happy with a reading of 20 megs.
That reading of 5.1 megs does not mean you need a new motor or a rewind!
If you do have a low megger reading, take the motor apart and blow it out with air, maybe use some degreaser, and then dry it out - you may end up with a reading then of 500 megs. While the motor is apart is a good time to change the bearings too.
#2 - I'm not familar with your amprobe dm-3 mullti test. You need a real megger to get accurate readings. I would not trust a multi tester. That may be because I am not familar with one I have been able to compare readings with those from a real megger.
#3 - It is normal to include volt reading, running amps, and motor resistance readings phase to phase. And yes,....the amp reading is necessary.
#4 - Vibration and Temperature readings too? Not usually done on small motors like you mentioned above. Unless you are going by feel. You can feel excessive vibration or heat with your hand. Check a bunch of motors until you know what you expect to feel in the way of vibration or temperature.
#5 - Be sure you add to the PM schedule greasing the bearings. You better read how to do that properly so you don't force grease into the motor or rupture any seals.
#6 -You don't need to worry about ambient temperature normally, it won't make much difference if you use a megger at the correct test voltage. If you bring a motor inside after it was out in the cold condensation could "possibly" be a problem. And I have seen a 2300 VAC motor megger good (+200 megs) that had so much condensation that it threw water out the air vents when energized. That was a case of energize it now....that we would flood if we didn't.

 

[zog]

When I took the megger readings I did not take any temp. reading. Most of the motors were running at the time and i shut them down and megged the motors for one minute on each phase. All motors are indoors. They are the first recordings i have taken on the motors, Iam new employee and trying to start something with pm and i know I don't know about megging. The last person taking the readings was using a ohm meter and just put in a number in.
am i suppose to take the ambeint temp? Can I use the temp on the frame of motor?
Thanks for all the tips and responses.

Temp effects the indicated result, in order for that number to have any meaning (Comparing to a spec and/or previous readings) you need to temp correct it. For inductive equipment like motors the readings are usually corrected to 40 degrees C per ANSI/IEEE 43-2000 and ANSI/NETA MTS. If you meggered the motor right after you shut it down the windings were probally around 60-65 degrees C but that depends on the motor and how long you waited. The duration of your test is also vital, you should record your readings at 30 seconds, 1 minute, and 10 minutes to calulate DAR and PI. There are 3 different currents you are measuring with inductive equipment and they all tell you different things about the insulation of the motor. See attached

 

[zog]

#1 - You should be meggering those 480 volt motors @ 500 volts.

Based on what standard? Every standard I have read says at least 1000VDC

#
#6 -You don't need to worry about ambient temperature normally, it won't make much difference if you use a megger at the correct test voltage.

No but you do need to adjust for winding temerature, it makes a huge difference, all ANSI/IEEE standards say to correct to 40 degrees C. See attachment