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#6706 - 01/09/02 08:27 PM megging/testing a 3 phase 480V 10HP wye motor
menegt Offline
Member

Registered: 01/09/02
Posts: 35
Loc: sacramento, ca
I'm an electrician working for a major university. I was told by our stationary engineer to meg out a motor because he was getting "ground fault" errors on his VFD.
I said okay I'll meg out the conductors from the vfd to the disconnect first then from the disconnect to the motor second. The first meg went okay i got 9.99 meg ohms (1k megohm meter). The second test (disconnect to motor) through me for a loop. I got .04 on all three legs to ground. I took out my volt/ohm meter and got a low resistance to ground reading also. I tried the same tests on a good motor nearby. Same readings! The stationary engineer says that the first motor i megged was going bad, but i megged almost a brand new motor and got the same readings. I would like to know if there is a certain reading/value i should be looking for in a motor. The stationary engineer said anything below 4k between phase ground. My real question is how do i properly check insulation of a motor and how do I check to see if the motor is still good (no matter what meter i use)

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#6707 - 01/10/02 03:06 AM Re: megging/testing a 3 phase 480V 10HP wye motor
George Corron Offline
Member

Registered: 05/16/01
Posts: 728
Loc: Lorton, Va USA
I dont't think your stationary engineer has ever megged a motor. 4k to ground is unheard of. I don't recall the right hand rule of thumb, but what you got sounds just fine. I prefer to have data from the mfg, but you can't always get that, so what you did is just what I would do, go get a new motor and compare the only other test you could do, that I don't see you've done here is a megger test between the "T" leads, but I don't think you'll find anything aberrant here given your other readings.

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#6708 - 01/10/02 04:24 AM Re: megging/testing a 3 phase 480V 10HP wye motor
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
Im not following you completely. Did you test at the motor or through the wiring from the disco. to the motor? Try isolating the motor completely. 40,000 ohms seems low. As a rule of thumb, 1 megohm is a good lower limit for equipment rated 1000 volts or less. Over 1,000 volts, 1 meg per 1KV is the rule.
Another useful standard is Rm = kV + 1.
Where Rm = minimum insulation resistance of the windings (in megohms.)
kV = rated input voltage of motor.
Keep in mind that meggering is also used to establish a trend, and periodic testing is useful in anticipating insulation failure. Thus, a slightly low reading that remains stable is preferrable to a higher reading that gradually is decreasing.
This information is available in the Biddle Instruments Manual on Insulation Testing.
I hope it helps you.

[This message has been edited by Redsy (edited 01-10-2002).]

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#6709 - 01/10/02 04:37 AM Re: megging/testing a 3 phase 480V 10HP wye motor
ctolbert Offline
Member

Registered: 08/09/01
Posts: 33
Loc: Cumming, GA USA
I'm a little confused...


The fluke manual (I happened to be looking at something this morning anyway) says that if you have a nameplate voltage of 240-600 vac then the minumum resistance should be 300 kohms.

Some drive mfgs call for a greater spec...Hitachi (the brand that I support) requires 1 Mohm per phase. The reason is the sensitivity of the invertor section of most modern VFD's.

The same motor that works with across the line starting does not mean it will work with a VFD.

Check out the way you are performing the test.

Carl

[This message has been edited by ctolbert (edited 01-10-2002).]
_________________________
Carl Lee Tolbert
Technical Support
AC Drives
Automationdirect.com
Your source for the most practical automation products at almost-free pricing, delivered by 11 a.m., just by clicking a mouse.
PH: (770)844-4200
Fax:(770)886-3199
http://www.automationdirect.com

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#6710 - 01/10/02 07:50 AM Re: megging/testing a 3 phase 480V 10HP wye motor
WARREN1 Offline
Member

Registered: 05/09/01
Posts: 184
Loc: Greenville, SC, USA
There has been much written about the cables and motors for PWM drives. If you want to indulge into some really technical stuff, see "Selection of Power Cables for PWM AC Adjustable-Speed Drives" by C. David Mercier and David A. Cooper at http://www.southwire.com/tech/library/papers/pwmpaper.htm
I read it, but it is a little over my head, although I did get some things out of it.
Another source is "Cable Alternatives for PWM AC Drive Applications" by Eric Bulington, Scott ABney and Gary L. Skibinski at http://bwcecom.belden.com/college/techpprs/capwmtp.htm
They compared several cables and seems to conclude that shielded cable is best, with the shield grounded at "both ends". If you are having a problem with AM radio interference, then shielded cable on the drive input is recommended.
Shielded cables is what we specify for our designs here at my company.

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#6711 - 01/10/02 09:29 PM Re: megging/testing a 3 phase 480V 10HP wye motor
menegt Offline
Member

Registered: 01/09/02
Posts: 35
Loc: sacramento, ca
Guys, thanks for input. (Redsy). I performed two tests the load side of the VFD to the line side of the disconnect.(first test) Then I megged the load side of disconnect (switch open of course) to the motor - no leads disconnected. I talked to another BSE guy today and he told me they have a contract w/ a guy that wound motors for them a while back and he remembers the contractor bringing out an osciloscope(sp) looking thing to test the motor. He said he tested how efficient the motor was running and could determine if the motor was on it's way out... Does anyone know what test he did? Our BSE (bldg. stationary engineer)doesn't remember what type of meter/device he used. I'd like to know. Thanks guys. (Mike Sac Sate Univ)

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#6712 - 01/11/02 04:03 AM Re: megging/testing a 3 phase 480V 10HP wye motor
ctolbert Offline
Member

Registered: 08/09/01
Posts: 33
Loc: Cumming, GA USA
All the other toys on the market for motor testing are nice...I learned to use the crank megger in the Navy and it has never done me wrong.

A quick and dirty multimeter check should read infinite from phase to ground and 10-40 ohms phase to phase (the resistance gets bigger for smaller motors).

A basic megger test (not the polarazation stuff or the like) should be greater than 300 k ohms. The phase to phase should show similar results as the multimeter does.

If these fail...the motor is suspect, and I would call the dealer.

Carl
_________________________
Carl Lee Tolbert
Technical Support
AC Drives
Automationdirect.com
Your source for the most practical automation products at almost-free pricing, delivered by 11 a.m., just by clicking a mouse.
PH: (770)844-4200
Fax:(770)886-3199
http://www.automationdirect.com

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#6713 - 01/17/02 09:00 PM Re: megging/testing a 3 phase 480V 10HP wye motor
George Rueff Offline
Member

Registered: 01/17/02
Posts: 12
Loc: Corpus Christi ,Texas
Recently graduated from a VFD school.Never meg a VFD.Use polarization index using 1000volts for 480volt motors.Divide your ten minute reading by your one minute reading with the minimum result being 2,This is detailed in IEEE standard #43.If you get an immediate infinity ,giga ohms or tetra ohms it don't get no better!!Always meg motor isolated from VFD as not to destroy electronics.

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