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Old 01-14-2009, 08:57 PM   #1
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Default megging a motor wired up

Ok....i have been told by some that you can not megg a cable that is wired up to a motor.....and some have said that you can only meg from conductor to ground only. well, i wish some one that is 100 percent sure of what they are talking about to answer this.......i recently installed a MCC with both VFD and regular wiring from a bucket.....some of the motors are wired and some are not......i had a couple cables that someone else installed and was bad......well anyway, the question is...... i wanna know some details of how and what it is that i can do with a megger on both situations i have.......i cant say that i am very knowledgable on the use of meggers and would like to be educated about them and what i need to know to be able to use a megger in the future without any doubts. thanks

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Old 01-14-2009, 09:27 PM   #2
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success, the megger is used to measure the integrity of insulation on mostly motor windings. we normally megged A-B, B-C, C-A, first open the motor disconnect. if your megging a few branch circuit or MCC feeders, a standard ohmeter should be fine.

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Old 01-14-2009, 09:37 PM   #3
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I am no expert but, be careful around any electronics such as VFD's. I usually disconnect them.

I do not test any cable active to neutral if there are appliances attached.

As for motors, if it has a neutral connection such as in a single phase or star connected 3 phase, and the neutral is grounded then obviously you will get a low reading.
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Old 01-14-2009, 10:08 PM   #4
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Default megger testing

hello....thanks for the reply.......i dont think i got the question completely answered......im kinda looking for a "teacher" type response......i only understand about 50% of what meggers are used for and what they CAN do......i know its to test to see if the wires are bad.......just wanna know the DIFFERENT ways you can use a megger......for years i thought you couldnt check a cable if it was wired up........two days ago. a guy that i know is a good electrician megged some wires which was wired up in the pecker heads and i always thought you couldnt do that......i wanna know the ends and outs of how this can be done.
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Old 01-14-2009, 11:05 PM   #5
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A megger is used to test the integrity of insulation. If you are testing a motor circuit wired to a MCC bucket or a VFD;
  • Open the breaker in the bucket or isolated the VFD.
  • Always test your Megger first with leads together and leads apart.
  • Clamp onto the equipment ground with one lead and one of the T-leads with the other. This test should give you a very high resistance reading (should be 1K ohm per volt to motor).
  • Repeat for each lead.
  • If you get a low reading to ground on one or more lead, you have a problem...but is it in the motor or the conductors going to the motor?
  • Next repeat this process on the motor leads, disconnected in the peckerhead. Be sure to follow all LO/TO rules!
  • If this test gives you a low resistance you have a grounded motor...if not repeat test on the T-leads going back to the MCC.
Hope that helps!
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Old 01-15-2009, 12:49 AM   #6
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we normally megged A-B, B-C, C-A, first open the motor disconnect.
That will tell you absolutely nothing, and you stand a pretty good chance of messing up the motor if you do it that way.

You need to check between each phase conductor and the equipment ground when you're doing an insulation resistance check with a megger.
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Old 01-15-2009, 09:57 AM   #7
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That will tell you absolutely nothing, and you stand a pretty good chance of messing up the motor if you do it that way.

You need to check between each phase conductor and the equipment ground when you're doing an insulation resistance check with a megger.

Meggering between the phases will tell you absolutely nothing if you are trying to identify a ground fault...but it will most definitely identify an open winding.

I am not sure how it can "mess up the motor" though.
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Old 01-15-2009, 06:04 PM   #8
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I always check for an open winding by megging phase to phase. I've checked literally hundreds and hundreds of motors this way, and never hurt one.
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Old 01-15-2009, 07:02 PM   #9
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1. Personally I never use a megger to take readings from Phase to Phase on a motor.

(I use a ohm meter such as a simpson 260.)

I don't thinkl you will hurt a winding if you chose to use a megger between windings, because the winding should be able to handle the megger I do see electricians doing it but personally I see it as a waste of time.

2. I use a megger to take readings from phase to ground. This checks for breakdown of insulation of motor windings and feeds. The rule of thumb is 1000ohms per volt.

I always take my first megger reading after I lift the T leads from the VFD and with feeds still connected to the motor. This will Check the feeds and the motor windings. If one reads grounded I break open the pecker head Isolate the motor from feeds and then take the reading again.

(For the test I use a Amprobe Analog Megger.)

If you are meggering your motor feeds and they are disconnected at the motor peckerhead, make sure they are also disconnected from the VFD. If not you can back feed the VFD and blow it up.

3. You can always megger out your conductors seperate from the motor. Disconnected from both ends making sure they are not grounded or shorted from phase to phase. When conducting this test I tend to also sometimes tie all three phases together and take a ohm reading at the other end and look for continuity between all three phases.

4. Remember a motor can megger out to be good read balanced and still be bad.

Last edited by 5volts; 01-15-2009 at 09:52 PM.
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Old 01-21-2009, 10:40 PM   #10
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Using a megger to check a motor winding relative to ground A TO GR. B to gr. etc pretty basic ,motor failures are not always electrical ,bearings fail also lack of p.maintenance...we check loops that are placed in the ground for detection on traffic signals , loops on freeways ,on-ramps used for counting cars i've used meggers for years hate the digital ,love the old crank style
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Old 02-10-2009, 01:50 PM   #11
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Just to add to the discussion. Never meg a circuit with a VFD connected to it. Never meg any electronic devices that are connected to the circuit as a whole.

Megging phase to phase can expose a short that may not be grounded. I meg phase to phase and then phase to ground. Cover all the bases.
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Old 02-10-2009, 05:30 PM   #12
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I wish I could put my 2 cents in but I'm actually trying to learn here. There are too many conflicting answers. Some of you are posting misinformation. There's no reason everyone shouldn't be on the same page.
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Old 02-10-2009, 09:14 PM   #13
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Came across a problem today where a 480 volt 30 HP motor was kicking out on a over current Fault. Controlling it was A PowerFlex 700 VFD.

This strange thing was at Idle speed which was 17 Hertz the drive and motor would run fine. As soon as the drive ramped up to 60 Hertz The drive would Fault out immediately. After Verifying I had 480volts supply to the VFD, I lifted the T leads off a contactor that is wired on the load side of the VFD. With All three phases still attached to the 30 HP motor I meggered the T leads to ground And I read a direct short.

Then I proceeded to the Motor peckerhead and broke loose the connections. I then took A megger reading on all the feeds making sure they were not grounded and verifying they were not shorted from phase to phase. I then read the motor and it read a direct short. I even read it with a Analog simpson meter at the Rx10,000 scale From phase to ground and the motor read shorted.

Before meggering When the motor was running fine At Idle speed I took A voltage reading at the load side of the contactor and I read about 127 volts from phase to phase.

To conclude the insulation on the motor winding insulation was starting to break down and at 17 hertz which was 127 volts, not enough to "jump" or cause a overload, or the VFD to Fault. The megger 500 scale test found the faulty motor.

A couple of guys before I got to the call were about to change the VFD out they assumed the drive was the problem because the motor seemed to be running fine only a problem when the drive rammped up.
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Old 02-12-2009, 07:07 PM   #14
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Quote:
Originally Posted by p_logix View Post
A couple of guys before I got to the call were about to change the VFD out they assumed the drive was the problem because the motor seemed to be running fine only a problem when the drive rammped up.
Unfortunately, that seems to be the norm these days. Most of the guys I work with just swap parts until the machine works. They don't seem to care about understanding the whole process of the machine.

Good catch
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Old 02-13-2009, 12:28 AM   #15
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Quote:
A couple of guys before I got to the call were about to change the VFD out they assumed the drive was the problem because the motor seemed to be running fine only a problem when the drive rammped up.
Quote:
Originally Posted by InControl View Post
Unfortunately, that seems to be the norm these days. Most of the guys I work with just swap parts until the machine works. They don't seem to care about understanding the whole process of the machine.

Good catch

That kind of stuff drives me nuts...the "shotgun troubleshooting" of just replacing parts until the thing starts working

I prefer to spend a little extra time up front to TROUBLESHOOT and find the actual problem, then you can repair/replace only what actually needs it.

The "shotgunner" may get lucky once in a while, but the true troubleshooter will be right, and will get you up and running quicker, more consistently.
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Old 02-24-2009, 03:17 PM   #16
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Default inductive kick using megger

yes true you can use the megger to test motor winding endsand outs and we call this test (the inductive kick) where in if you have athree phase motor with six leads out of the winding and you do not know thethe ends and outs of the winding you can use the inductive kick method instead of opening the motor and following up the winding
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Old 04-03-2009, 05:44 PM   #17
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i downloaded from meggers website a publication called "a stich in time" a very helpfull (and long) read on meggers it also shows going from a conductor with one lead to the insulation on the same conductor with the other lead to test insulation
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Old 04-04-2009, 11:33 PM   #18
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Could you post a link for me please Ampman?

Thanks
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Old 04-05-2009, 01:28 AM   #19
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Quote:
Originally Posted by successisforme View Post
Ok....i have been told by some that you can not megg a cable that is wired up to a motor.....and some have said that you can only meg from conductor to ground only. well, i wish some one that is 100 percent sure of what they are talking about to answer this.......i recently installed a MCC with both VFD and regular wiring from a bucket.....some of the motors are wired and some are not......i had a couple cables that someone else installed and was bad......well anyway, the question is...... i wanna know some details of how and what it is that i can do with a megger on both situations i have.......i cant say that i am very knowledgable on the use of meggers and would like to be educated about them and what i need to know to be able to use a megger in the future without any doubts. thanks

Hi Success,

Ok, disconnect the motor cables from VFDS's, and from motor terminals.
Now carry out megger test of the cables, phase to earth and phase to phase - should have open circuit on all. If not trace fault on cable for defects.
Then proceed to megger test motor, sometimes our senses makes work more easier. As you approach the motor terminal box if there is any abnormalities with the motor it will show up such as burnt smell of windings, any cables black/burnt, terminals loose, broken shattered pieces etc.If not then megger is your best friend.
Usually motors are Delta connected or even star, just make a note and label the winding leads connected to the teminals. Now disconnect and separate all the leads and carry out megger test.Properly secure your earth lead to the body and test megger first. Then carry out insulation test with leads to earth. Reading should be as high as possible - open circuit. If not the tested lead is shorting to earth.
Then proceed to carry out phase to phase insulation test. If there are 6 leads, 3 sets of test will show short circuit or continuity thus a set of windings. Other 3 should result in open circuit if not then phase to phase short. Delta & star configurations should be understood when phase to phase test is performed.

DO NOT MEGGER ANY CABLES CONNECTED TO ANY ELECTRONICS, STATIC, UNDER-VOLTAGE RATED COMPONENTS.
Meggers produce high level of energy as specified in selection panel which can cause equipment failure, component damage, and even at time injuries & electric shock.

Also note the prime purpose of meggers is to determine the cable, winding installed insulation resistance or in otherwords integrity.It does not been that after megger test results found ok, the motor or installation will perform to the best.There are many other factors involved to down grade the performance as one says in motors, the bearing conditions, lubrication performance, rotor alignment, base mount and alignment, coupling to drive, envrionment, type of load applied, cable size, protection used and many more.

Hope this will show some light.
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Old 04-06-2009, 02:02 PM   #20
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kelem ,you must go to meggers website and log in then go to publications

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