You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old December 3rd, 2018, 05:20 AM   #1
James42005
Member
United States

James42005 is offline
 
Join Date: Dec 2018
Location: Waterloo, IA
Posts: 2
Motor Insulation Test

Megging a 3ph 30hp motor. Run by a PF753 showing F13 ground fault. Getting 15K Ohms @ 1000v with megger. Yes Komhs. Bad motor correct? Should be like pry .5Mohm or higher if I remember correctly, been awhile. Motor require crane for replacement, so trying to be sure.

Thanks.
  Reply With Quote
Old December 3rd, 2018, 07:00 AM   #2
saultgeorge
Lifetime Supporting Member
United States

saultgeorge is offline
 
Join Date: Jul 2015
Location: Detroit
Posts: 152
Hi, James and welcome to the forum.
Most of the time in motors I was taught to use a rule of thumb of "1 megohm per volt of line voltage". So, 500 Meg ohms per phase on a 480 V motor. As long as I've had that or above, then I will leave the motor in service. Hope this helps.
__________________
Detroit City Limit: An Automation Alley Community
  Reply With Quote
Old December 3rd, 2018, 07:11 AM   #3
milldrone
Member
United States

milldrone is offline
 
milldrone's Avatar
 
Join Date: Mar 2005
Location: in the dog house
Posts: 1,294
Quote:
Originally Posted by James42005 View Post
. Motor require crane for replacement, so trying to be sure.

Thanks.
Is this measurement at the motor termination box? If not, do the test again (at the motor termination box) and measure the cables also (after disconnecting the VFD)

P.S. yes that is a bad reading
__________________
Vaughn

If you can read this, thank a teacher.
  Reply With Quote
Old December 3rd, 2018, 11:15 AM   #4
GaryS
Member
United States

GaryS is offline
 
GaryS's Avatar
 
Join Date: Aug 2003
Location: Lancaster Pa.
Posts: 811
First never ever meg a motor connected to a vfd or any other drive, you can damage the drive they are not designed for that level of voltage any number of things could be damaged.
if the reading you got are correct I agree the motor I bad needs repaired or replaced.
do the absorption test it the best way to check faulted motor.
if as you say the motor is located in a bad location then I would separated the motor from the wiring at the motor box and meg both the motor and the wiring separately just to be sure you don't have bad feed wire .
Also I think you 1000V test is to low I was told the rule of thumb was 2 x the rated voltage + 1000V the next level up, that would make the megger test voltage 2500V I would opt for 5000V with a 480v motor but that's just me.
in your case it may be cheaper just to replace it with a new motor over repairing it. The cost of the crane twice verses once.
  Reply With Quote
Old December 3rd, 2018, 04:01 PM   #5
lesmar96
Lifetime Supporting Member
United States

lesmar96 is offline
 
Join Date: May 2017
Location: PA
Posts: 235
Our motor shop standard is 2000V on the meg test on 480V motors and down.

I once had a motor that was tripping the vfd out of ground fault. The electrical contractor was testing it with some megger and couldn't get it to fail and didn't understand why the drive was tripping out. Called us in and I put our high voltage computerized winding tester on. The motor failed the meg test at around 1400V. The contractor had only been doing a 1000V test which he thought was good enough for a 480V motor. Drives seem to be sensitive and can pick that up quick.
  Reply With Quote
Old December 3rd, 2018, 05:05 PM   #6
James42005
Member
United States

James42005 is offline
 
Join Date: Dec 2018
Location: Waterloo, IA
Posts: 2
The motor was megged with phases disconnected from the vfd. I megged from the output of vfd to motor and also at the motor peckerhead. I knew i was right, but was second guessing myself with the severity of the situation due to it's location. I've always assumed that our 1000v megger was good enough, but looks like I'll have to find a new one. New motor and a crane on the way. Thanks guys, love these forums!
  Reply With Quote
Old December 3rd, 2018, 05:30 PM   #7
GaryS
Member
United States

GaryS is offline
 
GaryS's Avatar
 
Join Date: Aug 2003
Location: Lancaster Pa.
Posts: 811
if you are looking for a megger have a look at AEMC and look at the 5000V units
I think that 2000 / 2500V is to light to check a 480V motor
they may not be cheep but they are the best.
  Reply With Quote
Old December 4th, 2018, 01:02 PM   #8
DickDV
Member
United States

DickDV is offline
 
Join Date: May 2002
Location: Kalamazoo MI
Posts: 2,319
Gentlemen, please consider that both ANSI and NEMA spec 1000V meggars as proper for field testing of used induction motors. Also, consider that a brand new non-MG1 Motor is tested with 1200VAC at the factory and an MG1 motor is tested at 1800v DC pulse power.

Testing a motor with an insulation system designed for 480VAC at levels above 2000V can be expected to damage perfectly good insulation and convert a good motor to bad simply by testing it.

Finally, expecting to see 1000000 times the nameplate voltage as the lowest acceptable meggar test resistance is way off. It isn't unheard of for a new, bone-dry motor to test less than that.

The general rule I've seen most commonly at 480VAC is at least 1000000 ohms to ground and, when possible to test, phase to phase. Less than that indicates a motor recondition is needed and maybe even a rewind. It depends how high the resistance goes after a good cleaning and bake-out.
  Reply With Quote
Old December 4th, 2018, 02:35 PM   #9
lesmar96
Lifetime Supporting Member
United States

lesmar96 is offline
 
Join Date: May 2017
Location: PA
Posts: 235
Quote:
Originally Posted by DickDV View Post
Gentlemen, please consider that both ANSI and NEMA spec 1000V meggars as proper for field testing of used induction motors. Also, consider that a brand new non-MG1 Motor is tested with 1200VAC at the factory and an MG1 motor is tested at 1800v DC pulse power.

Testing a motor with an insulation system designed for 480VAC at levels above 2000V can be expected to damage perfectly good insulation and convert a good motor to bad simply by testing it.

Finally, expecting to see 1000000 times the nameplate voltage as the lowest acceptable meggar test resistance is way off. It isn't unheard of for a new, bone-dry motor to test less than that.

The general rule I've seen most commonly at 480VAC is at least 1000000 ohms to ground and, when possible to test, phase to phase. Less than that indicates a motor recondition is needed and maybe even a rewind. It depends how high the resistance goes after a good cleaning and bake-out.

DickDV, 1,000,000 ohms phase-to-phase?or do I misunderstand you?

In my opinion, 1000V may be good for most circumstances but it may not catch everything. But I would agree that 5000V seems very excessive.
  Reply With Quote
Old December 4th, 2018, 02:41 PM   #10
Bob O
Member
United States

Bob O is offline
 
Join Date: May 2003
Location:
Posts: 1,584
I don’t know about meggering but I think I was told this was the “Bible”.
Take it for what it’s worth.
http://www.biddlemegger.com/biddle/Stitch-new.pdf
  Reply With Quote
Old December 4th, 2018, 02:45 PM   #11
lesmar96
Lifetime Supporting Member
United States

lesmar96 is offline
 
Join Date: May 2017
Location: PA
Posts: 235
I believe that table on p.17 is the same or very similar to the charts that EASA produces
  Reply With Quote
Old December 5th, 2018, 08:28 AM   #12
DickDV
Member
United States

DickDV is offline
 
Join Date: May 2002
Location: Kalamazoo MI
Posts: 2,319
lesmar. that is one million ohms phase to phase or phase to ground. Of course, you can't test between coils that are internally connected.


These guidelines are for FIELD testing. There are other potentially higher voltage tests that are intended for SHOP use.
  Reply With Quote
Old December 6th, 2018, 09:26 PM   #13
DickDV
Member
United States

DickDV is offline
 
Join Date: May 2002
Location: Kalamazoo MI
Posts: 2,319
To clarify, when I said phase-to-phase I meant from one coil to another. Clearly, that would only be meaningful if you can isolate the coils with the leads available in the junction box. Coils tied together with connections made up inside the motor and not available in the junction box cannot be meggared since they are already shorted together. They have to be treated as one coil for testing purposes.

Sorry for the confusion on that.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Problems with Siemens CP340 RS232C communication Borte LIVE PLC Questions And Answers 10 February 17th, 2010 05:16 AM
OPC test Client cannot connec to RSLinx Proffesional Eric Le Yin LIVE PLC Questions And Answers 0 May 7th, 2009 11:04 PM
RS Test Stand lucifer LIVE PLC Questions And Answers 0 March 14th, 2006 10:18 AM
Information about TYPE TEST MDB? Platootod LIVE PLC Questions And Answers 3 December 2nd, 2004 10:23 AM
Test equipment ranjana LIVE PLC Questions And Answers 1 November 10th, 2002 06:30 AM


All times are GMT -5. The time now is 02:23 PM.


.