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Posted By: Old Town How To Meg A Motor? - 12/25/05 05:18 PM
Merry Christmas,

Can anyone here go over with me how to properly test a motor with a megohm meter(Megger)? I have two suspect single phase 2 Hp motors on my bench. How do I go about testing them?

Thanks in advance.

Ty
Posted By: RODALCO Re: How To Meg A Motor? - 12/25/05 08:39 PM
Megger from windings to frame / earth lug on motor case. start and run winding.

Megger from start to run winding.

Megger capacitor terminals to earth if it got one.

Check continuity of windings

Check operation of start , centrifugal switch if it has one.

Start winding has usually a higher resistance than run winding. It also switches out when the motor is up to speed.

Hope this helps. Ray.
Posted By: RODALCO Re: How To Meg A Motor? - 12/25/05 08:40 PM
Sorry I forgot, Readings should be at least 1 Mega Ohm or better.
Posted By: Old Town Re: How To Meg A Motor? - 12/25/05 09:03 PM
Thanks Ray.
Posted By: winnie Re: How To Meg A Motor? - 12/26/05 12:42 PM
A couple of additional points to consider:

1) Check you Megger voltage. The test voltage should be 500Vdc for motors rated 600V or less.

2) Polarization: "The resistance of a good insulation system will increase when subjected to a test voltage for a period of time." The resistance 60 seconds into a particular test should be at least 1.25 times the resistance 30 seconds into the test.

Both of the above are taken from EASA Principles of Large AC Machines, a book that I have on my shelf for other reasons. I've never had to personally run these tests or deal with the results, so absorb the above information with care. Point 1 is IMHO solid, point 2 less so.

-Jon
Posted By: RODALCO Re: How To Meg A Motor? - 12/26/05 09:52 PM
Thanks Winnie for adding that extra info.

cheers Ray.
Posted By: AllClear Re: How To Meg A Motor? - 12/26/05 10:14 PM
Winnie, I Beleive Your second Point "resistance of a good insulation system will increase when subjected to a test voltage for a period of time" Is meant For Hi-Pot testing. Megger testing is for short Durations. Just be aware that there are Two Types of tests.
Posted By: iwire Re: How To Meg A Motor? - 12/26/05 10:24 PM
Allclear timed test are applicable to Mega testing.

A one minute ten minute test is common. We just had a Mega salesman at the shop giving us some lessons. [Linked Image]

Quote
The best way to quantify the results of a time resistance test is through a dielectric absorption ratio. The dielectric absorption ratio consists of two time resistance readings. A commonly used set of intervals is a 60-second reading divided by a 30-second reading. Another frequently used set is a 10-minute reading divided by a 1-minute reading. This resulting value is referred to as the polarization index.
Posted By: winnie Re: How To Meg A Motor? - 12/27/05 12:29 AM
As I understand it, a 'Hi-Pot' test is used infrequently, and is a 'pass-fail' test. The voltages used are such that they are potentially destructive, and which probably cause insulation deterioration. A megger test, on the other hand, is done at lower voltage, generally in the range of what the insulation should be able to tolerate continuously.

Again looking at the EASA book, the High Potential tests are run with AC of 2xrated voltage + 1000, or with DC at 1.7 x the AC test voltage.

I presume that dielectric polarization occurs in both types of test, at least at DC.

-Jon
Posted By: macmikeman Re: How To Meg A Motor? - 12/27/05 04:27 AM
I keep a folder on my computer desktop that I drag and drop great threads onto. This one goes in. Thanks to all who contributed.
Posted By: Zog Re: How To Meg A Motor? - 12/27/05 02:24 PM
The minimum test voltage for 600V rated motors is 1000V, 250V rated motors test voltage is 500V.

A PI test is typically only done on motors >250 hp, a DAR test is done on smaller motors.

The minimum IR readings depend on the type and vintage of the motor and are corrected to 40 degrees celcius.

IR (1 minute)= kV+1 for most windings made prior to 1970 and all field windings.

(Example 480V motor = .48 + 1 = 1.48 Meg

IR (1 minute)= 100 Meg for most DC armature and AC windings built after 1970 (Form wound coils)

IR (1 minute) = 5 Meg for most machines and random wound stator coils and form wound coils rated below 1kV.

It sounds like the last would apply for your motor, take a reading (at 1000V) at 30 seconds and 60 seconds, your DAR should be greater than 1.0.

This info is from ANSI/IEEE standard 43 for motor testing.
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