Megger Testing

[Davebones]

I have a Fluke 1507 insulation tester . We have a 150 HP motor ( 480v) that the insulation has been showing signs of breaking down . Its reading 130K ohms at 500 vdc now. My question is when I first started the test the megger only showed it was putting out around 100 volts when the test was first started . Is this to prevent any more damage to the insulation since its going bad ? The voltage readings started going up as the insulation readings got a little better . it stopped around 250v at 130K . Never noticed this before just curious ............

 

[zog]

I have a Fluke 1507 insulation tester . We have a 150 HP motor ( 480v) that the insulation has been showing signs of breaking down . Its reading 130K ohms at 500 vdc now. My question is when I first started the test the megger only showed it was putting out around 100 volts when the test was first started . Is this to prevent any more damage to the insulation since its going bad ? The voltage readings started going up as the insulation readings got a little better . it stopped around 250v at 130K . Never noticed this before just curious ............

That is due to the polarization and absorbtion currents decaying, as they should, your 1507 does not have enough power to maintain those currents until they decay. You got 130k at what time? was that your 30 sec, 60 sec, or 10 minute reading? What was the winding tempature at the time of the test? Is your 130k a tempature corrected reading?

 

[Davebones]

The reading I got was after about 10 minutes . I just put the megger on and left it for ten minutes . It started out at around 30 k ohms and worked itself up to around 130k after 10 minutes . Didn't really worry about winding or ambient temperatures when we did this .

 

[zog]

View attachment Insulation Resistance Conversion Factors 40C.doc

The reading I got was after about 10 minutes . I just put the megger on and left it for ten minutes . It started out at around 30 k ohms and worked itself up to around 130k after 10 minutes . Didn't really worry about winding or ambient temperatures when we did this .

Without a winding tempature to correct your reading to they are pretty much worthless. It makes a huge difference.

 

[Davebones]

The motor hadn't been in use all weekend . The ambient was around 87 deg . Can I use this to figure the temperature at the windings ? Its a class f motor .

 

[zog]

The motor hadn't been in use all weekend . The ambient was around 87 deg . Can I use this to figure the temperature at the windings ? Its a class f motor .

Did you look at the table I attached? Assuming you meant 87F not 87C your actual insulation resistance is 130k x 0.63 = 82kOhms, even worse than you thought.

 

[Davebones]

Thanks I see the table . I have one other question . If a motor has been running and you shut it down to megger . What is the correct way to verify the temperature of the windings ?

 

[zog]

Thanks I see the table . I have one other question . If a motor has been running and you shut it down to megger . What is the correct way to verify the temperature of the windings ?

Larger motors may have RTD's, or you can use a non contact thermometer. They will usually be around 40C when running, hence the basis for the 40C correction. Most other equipment is corrected to 20C.

 

[Davebones]

If we take a reading at 15 sec,30 sec, 1 minute ,then every minute up to 10 minutes would this not be considered the "Time -Resistance Method " ? Would this be fairly accurate for checking out motors ? Does a temperature correction need to be done with this method ?

 

[zog]

If we take a reading at 15 sec,30 sec, 1 minute ,then every minute up to 10 minutes would this not be considered the "Time -Resistance Method " ? Would this be fairly accurate for checking out motors ? Does a temperature correction need to be done with this method ?

Readings should be recorded at 30 sec, 60 sec, and 10 minutes. Temp correction applies to all readings. DAR and PI should be calculated and used to evaluate insulation condition. If you can't get training from your employer I suggest you download a read "A stitch in time" from the megger website that explains the baisic procinpiles of insulation resistance testing.

 

[Davebones]

Actually I have a copy of " A Stitch In Time" . The first sentence on the time resistance method says this is independent of temperature .

 

[zog]

Actually I have a copy of " A Stitch In Time" . The first sentence on the time resistance method says this is independent of temperature .

That means the ratios for DAR and PI are independent of tempature, because they are ratios. You still need a insulation resistance value to tell if it is above whatever your spec is, and that is effected by tempature.