meggers

At work they just recently "allowed" the plant emt's to use meggers. Unfortunatly most of the plant emt's don't have much experience using them so we end up second guessing ourselves. Usually end up calling "central maintance" to double check the larger motors.

The megger they got us is a fluke brandand they had a rep give a class on the usage but didn't go into the low ohms in detail. Does anyone have any guidelines as far as the readings we should get on various motors, namely using the "low ohms" for phase to phase. The motors range from fractional to 150hp plus, 3phase,230v to 480v.

Any input is greatly appreciated.

Drewcrew6

In my neighborhood, emt's ride around in those square-looking vans with the flashing lights. They seem to show-up a lot when there is an accident.

I would hate to think that they are running around using the older hand-crank type megger... "CLEAR!... Whirrrrrr!

Regarding your question,
You aren't likely to find a listing anywhere (none that I know of) that would identify the expected resistance between phases. Too many motors, too many variables.

For Phase-to-Phase, I usually use my multi-meter to verify that the resistance between phase windings are close - very close - within a few percent.

If you aren't familiar or comfortable with using the low voltage scale on the megger for this purpose, read with the megger anyway, then read again with a multi-meter to verify what you read. Use the lowest possible range in both cases.

When the motor is running, read the current on all legs. They too should be very close - within a very small percentage. That will give you an indiation of how well matched the resistances are.

Between the resistance readings and the current readings you should get a very good picture of the real condition of the phase coils.

In my factory we also use multimeters to check motors.Check with the multimeter between phases to make sure they are balanced,then check from phase to earth with the meggar ( 400v setting )all phases should be balanced and not short to earth.Then check the current they should be balanced also,you could also take temperature reading and have you mechanical dept to do vibration tests on the motors.

hope this helps

I very much agree with Terry Woods recommendations with one addition. Before checking for current balance on the leads to a running motor, first check the incoming three phase power for voltage balance. You should have the same voltage across all three phase pairs within about 10 volts. If you don't have voltage balance, you surely won't have a meaningful current balance measurement.

While you are checking the incoming AC voltage balance, I always recommend checking the phase to ground balance as well. On a wye-derived grounded center source you should only see a few volts deviation to ground. On an ungrounded delta derived source you should also only see a few volts deviation but often its much worse even possibly running with one leg grounded. This is not a desireable situation except in a few specialized applications like hospitals. In fact, running a delta system with one leg grounded is highly hazardous to personnel and needlessly stressful to the phase to ground insulation systems in every connected piece of equipment.

If you find a leg grounded, I always recommend that the ground, whether accidental or deliberate, be found and removed unless it has a definite good purpose.

There are too many variables involved with motor construction to know what the phase to phase readings will be for an AC motor, a DC motor is different. AS what you call an EMT (electrical maintenance tech) I use a megger to check for shorts to ground mostly, as the others have stated a plain multimeter can give a phase to phase reading.

This may be of help if you want to print it out for your techs to read.
http://www.usbr.gov/power/data/fist/fist3~1/3~1_cont.htm

I have found that any single megger reading is all but meaningless. As long as you are not shorted to ground or phase to phase, there are too many variables to consider. However, plotting the results of regular megger readings can give you an indication of insulation degradation. This comparison of one value to another is extremely effective if the test and motor enviromental conditions are the same. Actually, recording the megger readings before the motor (and while you're at it, the rest of the wiring) is put into service provides valuable history for future evaluation.

Any ohm meter can check the windings for a short to ground. A megger measures how many mega-ohms or lack of, i.e. short to ground while a voltage is applied. Which could be caused by a break-down of the insulation. Just in case someone dosen't know what a megger does.

And of course there is the third and often definitive piece of test equipment; your nose. The acrid rank niff of a burnt out winding often greets you before the test leads are even connected.

Maybe this will help explain what I'm looking for. We had a 60hp 460v motor that started popping fuses immediatly at startup. When we read the ohms of phase to phase the reading was what would've been in my opinion a short phase to phase but all three phases were the same so we found another motor of same to compare. It also appeared that all three phases were shorted to ground. It turned out that a "bug" at the motor was the culprit, wore a hole and shorted to ground. But we almost changed that motor out with another for what would've been no reason.

What i'm looking for is just a rough idea of:
1:1hp is XX ohms phase to phase
2:5hp is XX ohms phase to phase
3:20hp is XX ohms phase to phase

up to around 150hp and not necesarrily those hp rating


I understand the purpose of the actual megger (insulation breakdown with a 2x rated voltage applied) part just the low ohms and a basis for what I should see.


Thanks
Drewcrew6

Any ohm meter can check the windings for a short to ground. A megger measures how many mega-ohms or lack of, i.e. short to ground while a voltage is applied. Which could be caused by a break-down of the insulation. Just in case someone dosen't know what a megger does.

Click to expand...

I reckon I gotta disagree with that statement, an ohm meter can NOT always show a short to ground because of insulation breakdown or other factors.

As Jim said using a megger is more appropriate when taking measurements over a time period as a preventative measure.

To quote the book on electrical testing that I posted the link for:
The megger test method for determining the condition of electrical insulation has been widely used for many years as a general nondestructive test method. A serious limitation of the megger test is that its operating voltage of 500 to 1,000 volts will not always detect insulation punctures, whereas the higher voltages used by the high-voltage, dc testers will detect these punctures.

The megger test will show (a) the relative amount of moisture in the insulation, (b) the leakage current over dirty or moist surfaces of the insulation, and (c) winding deterioration or faults by means of insulation resistance versus time curves.

All in all a megger is a tool and its proper use can be informative. Again I offer this site for info on this subject.
http://www.usbr.gov/power/data/fist/fist3~1/3~1_cont.htm

There is no responsible answer for the question you asked, Drew. Motor winding resistance is in the range of milliohms and will vary from mfgr to mfgr, from cold to hot, and from standard efficient to premium efficient. All of that in the same HP-VOLTAGE-BASESPEED motor.

The best advice is that posted above where the absolute numbers are not so important as trending over time.

And phase-to-phase can easily look like a short if the motor is internally connected (as in a three lead motor). In a single voltage three phase motor, you only get access to both ends of each winding with six leads. On dual voltage (ie. 230/460) motors, you only get both ends of each winding if you have 12 leads.

drew,

If I'm correct you are just wanting to know what the phase-pahse resisitance is according to the size of Motor.

Unfortunately there is no set standard due to differences in construction. You will need to contact the manufacturer to get that information.

Also, to accurately measure that you would need a very sensitive ohmmeter like a Valhalla, that measures in the range of 10000th of Ohms.

Hope this helps.

Phase to Phase Resistance

I Have found over the years that the smaller the motor the higher the resistance will be. Less copper in parrallel I suppose.

0,55KW and the like any where from 12 to 20 ohms
7.5KW and similar 1 to 2 ohms
22KW and over less than 1 ohm

These really are very rough estimates but the important thing is a balanced reading across all the windings. I have found the biggest factor affecting your reading will be the the connection you make with the clips on your meter and the stator connections on the motor.

when I was an journeyman at ICI in the Uk the very same Question was asked of an engineer. He came back a couple of weeks later with a list of KW/frame size and expected phase to phase readings. That was about 1982 and I just dont have the list now.

Hope this helps and I suppose I'm only here to be shot down .

ian

Three Simple Words

American Electrician's Handbook

I just had to make an edit to my previous post...

I would hate to think that they are running around using the older hand-crank type megger... "CLEAR!... Whirrrrrr!

It's ALIVE !!! It's ALIVE !!!

I also have to say, DICKDV is right...
Measure your Phase to Ground Voltage for Voltage imbalance.

I also have to say, Ron is right about what he said.

There were many other things said that were inaccurrate or just plain wrong. Read carefully and think.