5kv dielectric test set, for insulation test 1

[jlhmaint]

at work we have a 5kv DC dielectric test set from Biddle. i have never used one of these it has a 2.5 kv and 5 kv setting with an analog scale and a DC amp analog scale. You have two wires one to hook to ground and one to hook to motor lead my question is if i test a 3phase 480V motor on each phase what am i suppose to see or not see on the DC amp analog scale it testing at 500Vdc. and at what voltage should you test at any info is great or web sites

 

[electricpete]

500vdc is usually a good test voltage for 460v motors.

You should hope for a megger readings >1.5 megaohms. At 500 volts that would equate to a current of 333 microamps or less.

Also you should test polarization index which is ratio of 2 minute reading to 1 minute reading... should be >2.

I would suggest you read the instruction manual carefully. It should have some good info in there.

Any other thoughts?

 

[jlhmaint]

that is the problem no instruction manual or i would be ok, we are waiting for one by mail.

test polarization index which is ratio of 2 minute reading to 1 minute reading... should be >2.

how is this done?

also for istance we hooked up to an old motor we had on the shelf at 500vdc and the amp went up very little for about a second or two then stayed at 0 indefinetly.

i know i sound lost in the past all i ever used was the simple hand held megger that told you motor bad or good and gave an ohm reading not a current reading but from what your saying just use ohms law to figure your ohms from your analog scale if you have I=v/r then r=v/i or 500v/333micoamps = 1.5 megaohms, right

 

[electricpete]

Whoops, I goofed up... should have been:
"test polarization index which is ratio of 10 minute reading to 1 minute reading... should be >2."

What that means is you apply the voltage and wait. At 1 minute you record a current I1. At 10 minutes you record the current I10. From these you could calculate resistance R1 and R10

Polarization Index PI = R1/R10 = I10/I1 <2 is the acceptance criteria identified in IEEE standard

You are right about that the 333 microamp limit I mentioned was just from using ohm's law.

 

[jlhmaint]

Ok i found some info on your Polarization test iam at work now and iam going to go out and try a couple things turn up the multiplier on the amp scale and see what i read. thanks for the help. i will check back later.

 

[motorman]

Biddle manuals are on-line, no need to wait for snail-mail… can’t believe they didn’t tell you that.

http://www.avointl.com/documents/BM25_6172-083_UG_V08.pdf

 

[jlhmaint]

cant find the manual for the model we have, 220005 5kv dielectric test set.

tried to test motor agian and nothing same as before. set to 500vdc the microamp analog scale moves up a little then stays on 0. what am i missing here.

 

[busbar]

motorman's reference may not be an exact match, but it contains significant conceptual, application, and SAFETY information applicable to lower-range DC-insulation testing.

Another one of their long-standing manuals worth buying outright is titled, &quot;A Stitch in Time.&quot; There are also a couple of applicable titles at

http://www.avointl.com/us/publications/Index.asp

 

[UKpete]

To find out whether the meter is working you could do one or more of the following (sensible personal safety precautions assumed where required):

1. send it off for calibration, if you want to use it at higher voltages you need to know that you are actually applying the voltage that you think you are - faulty test gear is a pain

2. with the megger off, connect the leads together, switch on and see what the meter does - it should show a failure i.e. excessive current.

3. if you have a multimeter (Fluke etc.) set it to 1000V DC range and connect it up to the megger leads to check the voltage - using megger voltage settings not exceeding 1000V of course.

If you are getting a low reading when you connect up to the motor, it should mean that the motor is ok, assuming the megger is working properly. An initial surge of current can occur due to the capacitance of the winding charging up.

I have to differ with electricpete, I think that for a 480V motor, a better test would be at 1000V DC - at 500V you are not really putting enough stress on the insulation. If the motor has all six leads brought into the terminal box, you may be able to isolate all three phases and test phase to phase as well as phase to ground - this will show up all the possible insulation faults except inter-turn shorts (for that you will need surge testing, expensive!)

 

[electricpete]

Some more comment on the subject of test voltages

IEEE43-2000 Table 1 - Guidelines for voltages to be applied during the insulation resistance test - indicates 500vdc for windings rated less that 1000vac.

Higher voltages can be used for hi-pot test but should not be used repeatedly during routine insulation resistance tests.... small possibility that repeated application of voltage well in excess of rating will degrade the insulation.

That's just an opinion. Along with a standard. I'm sure practices differ and who's to say who is right.

 

[jbartos]

Suggestion: Visit

http://www.davisontheweb.com/showpage.asp?L3ID=142

(click on specs)

http://www.primoinc.com/Products.htm?CD=52&ID=356#

http://www.qualityinstrumentsinc.com/products/biddle/5kVDiel.pdf

Instruction manual Cat. No. 220005
AVTM22-5JB
etc. for more info

 

[jlhmaint]

thanks jb but i have been to those sites and they didn't give me any more info than i had. but i did order a full manual from the one site last week.

 

[jbartos]

Comment: My intention was to supplement other postings, e.g. Motorman posted somewhat related and useful link to literature. There are Application Notes addressing concepts, e.g. page 18. The typical terminal voltage characteristic at 500VDC at 2mA is included. Test results in agreement with concepts should be approximately the same for the discussed 220005 5kV test set. The insulation testers or analyzers are often calibrated and tested for their proper functioning and accuracy by simulation the &quot;would be tested insulation.&quot;

 

[utvol]

PI is a good indicator of insulation health, but you need to go a step farther. We look at the profile of the entire ten minute test. There may be insulation breakdown as indicated by dips in the charted values and still have a good ratio (2-4). The test made by the PdMA tester plots a chart, based on a value of the average of five readings one second apart, and recorded every five seconds. This is then plotted for ten minutes. It will also record capacitance to ground. This can be trended to indicate dirty or contaminated windings. look at their web site for more info.

 

[electricpete]

utvol - and what acceptance criteria do you use to judge whether a dip is satisfactory or not?

 

[utvol]

There should be no dip at all. Touching the lead insulation will cause a dip but this is a result of a change in capacitance to ground and of no concern. Barring this, I would look at anything 10% or more. The overall megohm value would play a part also. At 3,500 meg, a 10 meg drop is too bad but at 50 meg it would be more serious. Look at

http://www.pdma.com.

We have 25,000 motors in our plant with about 200 being medium voltage. The PdMA tester is really valuable to us.

 

[electricpete]

good comments utvol. Our plant has considered PDMA in the past but not yet ready to jump on that bandwagon.

I hope I am not out of order for a sidetrack question:

Can you give an example of one or perhaps two confirmed good catches with PDMA that wouldn't have been caught with a megger?

 

[jlhmaint]

thanks for everything guys. My manual came in yesterday but i have been knee deep in work hope to check it out tomorrow. had a mazak cnc to work on the past 2 days.

 

[utvol]

TO electricpete:
Drop me a note to cdhawk@eastman.com and I'll send some good examples.

 

[GSI1]

Hello;
I have always considered the min test valtage to be
2X line voltage + 1,000 volts or for 480 V system the test votlage for a megger should be at least 2,500.
A 500 Volt test on a 480 Volt syten will tell you nothing and i would not use it. The peak voltage on a 480V system is 700 Volts so a 500 volt test is not a test at all.
A DC Megger will not damage the motor an is considered a nondistructive test. on the other hand an AC Hipot test could distroy any equipment that is questionable and is considered a distructive test.