PDA

View Full Version : questions on meggers for motor winding testing



andy_b
10-24-2009, 11:37 AM
has anyone on here tried one of those $80 electronic chinese meggers (winding resistance meters) that are fleabay all the time? this would just be for very occasional use and i have no desire to purchase a $500 Mercedes when a $80 Yugo will do. :)
how about the old hand-crank ones. does much ever go bad in them? there seems to always be a good assortment on the 'bay, but i have no idea what to look for in a used megger.

andy b.

J Tiers
10-24-2009, 11:57 AM
I wouldn't bother.

The "megger" is standard, but un-necessary, and can even damage windings if used wrong, since it is often capable of rather high voltage.

To check one or two motors, I just connect them to line volts with all wires together. I connect a 1500 ohm 10W resistor to ground, with a 0.1 uF capacitor across it, and ground the case through the resistor (case must be insulated), and measure the AC volts across the resistor.

more than a volt or two is an indication of excess leakage current. If you want to be fancy, you can use the "official" UL measurement circuit. which I can show you if you are interested.

The system can also be reversed, with the resistor in the AC supply line.... in which case it needs to be guarded since it is "hot" with line volts.

again, all wires together, and measure the voltage. Either way you are measuring the sum of capacitive and resistive leakage.

Most "megger" units use DC and measure only the resistive component. That is the basic difference. The AC resistor test is a lot cheaper, and shows a "real world" result that can distinguish between a good and bad motor.

andy_b
10-24-2009, 02:47 PM
JT,

can you post a diagram of what you are saying, or PM me or email me?

thanks!

andy b.

ptjw7uk
10-24-2009, 03:50 PM
A megger is used to test the insulation resistance and is usually twice the working voltage but using DC. This is used in high voltage cables or overhead wires and should be carried out for one minute. I once used a 2500 volt megger to test 1100volt power cables but it took 2 of us to use it as you couldnt turn the handle for a minute and still look at the meter was very glad when battery powered units became the vogue.

In my view if (in Uk) the motor windings wont take 500 volt dc for a minute I'd bin it!

my 2p worth.

Peter

dp
10-24-2009, 04:01 PM
Meggers intentionally stress the subject being tested. It's a feature and an important one. There's no reason a Chinese megger can't work well. They need high DC voltage and an ohmmeter. Neither is particularly difficult to come by and there's no significant design challenge involved. You can do the same thing with a gang of 50 9v batteries in series and a Radio Shack VOM meter. The Chinese megger would be more convenient and cheaper.

Mark McGrath
10-24-2009, 05:01 PM
In my view if (in Uk) the motor windings wont take 500 volt dc for a minute I'd bin it!
Peter

Totally agree with you Peter.I use a megger with 250,500 and 1000 volts and low ohms.I test all motors I work on with it and never damaged one yet.
Low ohms for continuity and high volts for resistance to earth.
Started of many years ago with a hand wind Avo but never had enough hands to use it easily,although it was handy for giving an apprentice an introduction to dc.
Jerry`s method will suffice no argument about it but a pain to use.
Mark.

quasi
10-24-2009, 05:13 PM
Mark wrote

although it was handy for giving an apprentice an introduction to dc.

Yes, a megger can be a lot of fun.

Dawai
10-24-2009, 06:31 PM
High pot something, a pulsed high voltage can blow holes in the winding insulation. I have never seen them used outside a power plant. People being over zealous can ruin a perfectly good generator.

Meggers do no damage I am aware of, 1000 volt generated into normally 600 volt insulation to test it for spike resistance.

J Tiers
10-24-2009, 06:54 PM
There is a very specific test for spike resistance. having designed/built some of the equipment to do it, I can assure you that there are good ones and bad ones, although I can't tell you any more details because they are secret.

However, bad spike testing will short the winding. Seen it happen. The good type will not.

I'll see about the diagram.... much cheaper, UL recognized (it's their test), and pretty easy.

JoeFin
10-24-2009, 07:38 PM
I thought it was generaly accepted "You Do Not Hi-Pot motor windings" hence the generaly accepted practice to Megger @ name plate voltage or less for motors, 2X Insulation value for conductors.

dp
10-24-2009, 07:51 PM
It's probably always going to be interesting to pulse large inductors with high-voltage. Particularly multiphase inductors. I'd consider it a "hold m' beer and watch this" moment.

J Tiers
10-24-2009, 10:20 PM
A number of the motor manufacturers test their motors. The ones with higher voltage withstand become "inverter rated" or whatever the brand's designation is.

often that is the only real difference in windings. The same cores with same wire and same construction go in different grades of motor, but the insulation placement happens to be better on an "inverter duty" motor.

The test amounts to a hi-pot, but with certain key elements handled specially.

There CAN be other differences, often are, and there is a definition covering the "inverter duty" type.

But at the end of the day, there is a certain relation between test voltage withstood successfully, and the chance of failure in the field when used with an inverter.

naturally, some useage is easy pie, but a long line from inverter to motor can make for "issues".

Some manufacturers don't test, and they have more troubles.

Dawai
10-24-2009, 11:20 PM
Ernie was driving us into Oklahoma, I had been drinking beer for about ten hours straight.

I had my old crank megger in my lap, a 1000 volt capacitor.. I was cranking it to beat the band.. going to throw it over into his lap to get even for him teasing a buffalo in a barbed wire fence.. (*scared the bejesus out of me)

Instead, I held it up to the window crank, it blew a chunk as big as a pencil eraser out of the pot metal crank.

NOW.. that was a EDM.. no clue where to find them big old dc high voltage capacitors anymore..

steve45
10-25-2009, 12:05 AM
A company I used to work for sold these: http://www.megalert.com/

Not what you're looking for because they cost thousands, but it's basically an automatic megger that comes on every time a motor/generator is de-energized. They are also current limited so they won't hurt you if you put your fingers in the wrong place.

kc5ezc
10-25-2009, 12:28 AM
DAWAI: I have a .25mfd, 12,500vdc cap that is about 8x6x2 with large glass insulators coming of the top. Always have wanted to charge it and see the results.

andy_b
10-25-2009, 12:46 AM
DAWAI: I have a .25mfd, 12,500vdc cap that is about 8x6x2 with large glass insulators coming of the top. Always have wanted to charge it and see the results.

the "results" will be in the discharging, not the charging. :)

back to my megger. i like that 50 9v battery idea. just kidding.

so i thought about JT's method. if i understand this, i connect all of the motor winding wires together (basically all of the wires in the peckerhead except the green ground wire), then hook them up to line voltage (making sure the motor is sitting on an insulator of some sort and not wet dirt), then take the resistor/capacitor network and hook one end to the motor frame and the other to ground. i then measure the voltage across the resistor. is that correct?

this is for a 3-phase 220V motor (which will run on a RPC not real 3-ph). do i use 220V or 110V for this test?

andy b.

rdfeil
10-25-2009, 01:16 AM
Andy,

You have the test JT mentioned right. It doesn't matter what the motor ratings are. Use 110 volts for the test and make sure to use the neutral side for the connection to the resistor network to the frame for a little more safety. Also, testing motors with a megger is a much more informative test. I have seen motors in the process of failure which will test virtually clean at lower voltage (less than 250) and show obvious insulation failure at 1000 volts. Motors are very much like wires as far as testing. The insulation MUST survive at least 2 times the highest voltage rating. I routinely test 480 volt motors at 1500 volts to ground and if they fail a 1 minute test then rewind or replace, period. Failures cost much more than preventative maintenance in an industrial setting. In a home shop you can obviously be less stringent.

Just my opinion and shop rules.

Robin

J Tiers
10-25-2009, 12:53 PM
For a higher volt motor, I might use a transformer to get the higher voltage, but I have them laying around.

120V will get the job done in the US for a 240V motor because 240V is essentially 120V to ground (actually138V). You are checking in this case for leakage to ground, not from phase-to-phase. For phase-to-phase you need to get more complex and I don't feel comfortable trying to describe it.

Here is the UL test network, the fancier one. The reading in volts should not be larger than about 1.5.

http://img.photobucket.com/albums/0803/jstanley/testnetwork.jpg

http://img.photobucket.com/albums/0803/jstanley/testnetwork.jpg

(I have posted this for educational purposes under the fair use clause, and I will probably take it down shortly.)

Dawai
10-25-2009, 05:23 PM
John: that is a "tesla coil capacitor", worth big money on fleabay. Sell it, buy more machine shop tools.

jdunmyer
10-25-2009, 09:20 PM
This isn't actually testing a motor, but it fits here:

A motor that has sat around for a long time, especially in a damp/unheated environment, will often develop some leakage in the insulation. IF you test it, you'll find the leakage, if you don't, the motor will sometimes simply short out. A simple procedure will usually eliminate the leakage and prevent failure: connect a battery charger or other low-voltage DC power supply to the motor leads. You want a couple or 3 amps going through it, but always less than nameplate. The motor will appear to be "locked up" if you try to turn the shaft. Leave it sit like this for at least 12 hours; a small motor will get a bit warm to the touch.

Not having a megger, I usually check a motor with my trusty ol' Radio Shack DVM. It gets the above treatment if it shows ANY leakage to ground, and my experience has been that such leakage is gone afterwards. I'll often subject the motor to that treatment even if I can't measure any leakage, "just in case". I've had motors fail that didn't get the treatment, and have never had one fail that did.

FWIW:

deltap
10-25-2009, 09:26 PM
has anyone on here tried one of those $80 electronic chinese meggers (winding resistance meters) that are fleabay all the time? this would just be for very occasional use and i have no desire to purchase a $500 Mercedes when a $80 Yugo will do. :)
how about the old hand-crank ones. does much ever go bad in them? there seems to always be a good assortment on the 'bay, but i have no idea what to look for in a used megger.

andy b.

I routinely use a megger for 480 volt hvac work. My favorite is an inexpensive Supco battery powered unit with lights to indicate megs. It has been in my tool box for at least 20 yrs. and still works fine. In my work low megs are an indication of moisture in the motor windings. A new motor will test nearly at infinity. VFDs measure current to ground and trip a diagnostic when leakage is too high. This condition occurs in the cold wet environment of an air handler. The motor will run fine on bypass(across the line) but trip on VFD. The real value of a megger is for trending to see if resistance changes from inspection to inspection so the motor can be replaced before it goes from a small problem to a large expensive problem.