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]

GSI1 - 500VDC for 460 volt motors comes from IEEE43-2000. It is very widely used for routine maintenance testing.

The higher voltages that you mention (2500vdc for 460v motor) are considered a high-potential test and a potentially destructive test, in the sense that a motor which has weak insulation (but is still able to withstand normal operating voltages) can fail and become unuseable during the test. Hi-potential testing is governed by IEEE95. Hi-potential testing has it's place (infrequent testing when a spare motor is available), but it should not be confused with routine insulation resistance testing which by standard is performed at 500 volts and suitable for repetitive testing, even if no spare is available (it will not damage the motor).

 

[jbartos]

Suggestion: There are some postings in this Forum which addressed low frequency AC test as superior to DC tests.
However, the search function in this Forum does not work as good as browser searches.

 

[jbartos]

Small encore: Those low frequency insulation tests might also have been applied to cable insulation tests.

 

[jlhmaint]

well i did a test on the machine i was talking about.
hook up to one leg and ground and wham trips the breaker on the tester asoon as the voltage gets to about 200V

pulled the motor out and tested the motor at its leads and the same thing

we are getting a new motor tomorrow.

 

[GSI1]

electricpete

In response to your last posting concerning the 500vdc megger test.
First I thought the reason for the megger test on a motor is to see if the motor insalation is good or bad. If its at all questionable then pull the motor out. If the purpose is to just do the test to fill paper work, then why waste the time. I have seen more motors then I can count, pass the 500 vdc megger test and fail shortly after going back in service. You may be satisfied with that but my standards are much higher. When I check a motor I want to know it good if I put my name on the test then I must be sure. If the 500vdc is the standard for IEEE43-2000 then they need to move their standard up, I can’t compromise my standards my moving them down. I have clients ask all the time why they have a motor fail after its been tested at 500 vdc. 30 years of experience and simple math tell me that 500 vdc is not enough. 500vdc is enough for a 120 vac household appliance but not 480 vac motor. And jus so you know I find a lot of their standards don’t come up to mine but that’s a topic for another time.
As for ac hipot testers are not destructive keep in mind when the equipment under test fails their will be an arc. Any arc through the insulation is destructive and I would never put it back in service. If you would then I am glade I don’t have to work with you.

 

[jlhmaint]

well i got called in early to put the motor in and everything checks out and machine is working fine agian. thanks for all the help guys

 

[jbartos]

Suggestion: There are various tests available. Visit

http://www.interfaxsystems.com/products/powertest.htm

and notice:
Electric Motor Performance Analysis and trending hardware using technology of motor current signature analysis (MCSA). Patented signal conditioning circuit.
=============================
Some tests are more thorough and informative, others test for one value, e.g. insulation resistance or impedance.

 

[electricpete]

GSI1 – you wrote:
As for ac hipot testers are not destructive keep in mind when the equipment under test fails their will be an arc. Any arc through the insulation is destructive and I would never put it back in service. If you would then I am glade I don’t have to work with you. [B/]

In my opinion that comment has no basis since I have said nothing about ac hi-pot testing (IEEE95 is the standard for dc hipot testing), and I have never said I would put a machine back in service following a failure of an ac or dc hi-pot. I am happy to discuss it further, but I hope we can discuss it and even disagree without making it personal.

Here is my view:
There are at least two dc tests available:
#1 - The insulation resistance test/polarization index (IR/PI) per IEEE43-2000 at 500vdc for 460 motors, 2500vdc for 4kv motors, and 10kv for 13.2kv motors.
#2 – The dc hi-potential test per IEEE95-1977, where voltage may be increased over time in a variety of fashions, but to levels far above peak line-to-ground voltage.

There is no disagreement among anyone I know of that #2 = dc hi-pot will identify more problems than #1 = insulation resitance test.

I think there is also no disagreement among anyone I know of that testing by # 2 = dc hi-pot will cause more motors to fail (become unuseable requiring rewind) than #1 = insulation resistance test. You yourself have said that a motor should not be used after it fails a hi-pot and I agree.

One unknown and controversial question: how bad are the motors which fail the dc hi-pot? ie long would that motor that failed the dc hi-potential test have lasted if you hadn’t tested it. or put another way: how many motors will fail that were really “good” in terms of their ability to operate for many more years.

Given the above considerations, there are some situations where the economics will clearly dictate that the dc hi-pot is required and some situations will dictate that the dc hi-pot is clearly not appropriate. If you have a critical motor whose failure will cost 1000 times as much as the cost of a motor rewind AND you have a spare motor available, the argument for performing a dc hi-pot is pretty strong. But there are many situations where dc hi-pot is not a good idea:

1 – cost of rewind is much more than cost to production. Let’s say you have 3 installed motors, any one of which can startup automatically to support production. Cost of motor failure to production is zero. Cost of rewind is large. It does not make sense to hi-pot test these motors to a point that might cause failure. Let them run to failure if necessary. But also check insulation resistance periodically when motor is down for air-filter and lubrication maintenance soo you can take corrective actions (clearning/drying etc) to avoid the cost of a failure/rewind.

2 – You have a motor which is critical to production but you have no spare. It will only be available for testing for a short time before it must be returned to service. You can confidently do an insulation resistance test without fear of jeaparding production. If you see a clear indication of a problem then you may be able to initiate corrective action justifying the purchase of a spare motor if required. If you apply a dc hi-pot at the voltages you describe you might damage that motor to the point that a rewind is required. In case of a 13.2kv motor the time to rewind can be at least a month even when you expedit the rewind shop (special coils). Productoin will be disrupted and I don’t think the plant manager will be happy to find out that a motor tester has caused it.

3 – You have a motor though underground vaults, through XLPE cables which sometimes get wet. This medium-voltage motor requires taped connections due to the tight clearances within the terminal box. Removal and resinstallation of taped connections will require several hours, and also raises questions about the integrity of the joint after reassembled. You can easily do a quick check of the motor/cable from the switchgear. If you apply a high-voltage dc test at 2*VLL+1 to the motor though an XLPE cable which has absorbed moisture, you will likely degrade the cable and possibly fail it.

To you question: why even bother doing an insulation resistance test alone (without hi-pot)?

1 – You have a motor that is wet (maybe you don’t know that). Careful analysis of the insulation resistance/polarization index may identify that without damaging the winding. Then the motor can be dried out (space heaters repaired or whatever) before energizing without damaging the motor. If you had applied a dc hi-pot to a wet motor at the voltages you describe you might damage that motor to the point that a rewind is required.

2 – You have a motor whose windings are contaminated with oil and dirt (you may not know it). Careful analysis of the insulation resistance/polarization index may identify that without damaging the winding. Then the motor can be cleaned. If you apply a dc hi-pot at the voltages you describe you might damage that motor to the point that a rewind is required.

I agree the insulation resistance test is not perfect and can sometimes miss moisture, contamination, and other winding degradation. There is a lot that can be said about methods for trending, temperature correction, consideration of humidity etc. Neither is the hi-pot test perfect in detecting insulation problems…. certainly neither of these tests will detect turn insulation problems. That will require a separate potentially-destructive test – the surge test.

 

[jbartos]

Suggestion: The "#2 – The dc hi-potential test per IEEE95-1977, where voltage may be increased over time in a variety of fashions, but to levels far above peak line-to-ground voltage." appears to be appropriate for the motor manufacturer, since this data is needed for the motor data sheet. Once the motor is installed, then this test may be circumvented by various signature tests that eventually may lead to a reasonable prediction as to when the motor will end its life-cycle.

 

[electricpete]

jbatos - it will be a matter of opinion what tests you select. BUT there is really no signature test that is as sensitive to insulation degradation as a hi-pot test. For very critical machines where the production cost of failure in service is far larger than the cost of failure during test (=cost of rewind) AND a spare is available, I think hi-pot is appropriate. I have some motors whose failure will cost about $5 million in production and we perform dc hi-pot on those.

 

[jbartos]

Comment to the previous posting: That is one way to do things. Fortunately, engineering is not entirely dependent on one way to do things. If one decides that the prediction of the motor failure is about to impact $5000000 production, one may as well have the motor replaced by the new one or overhauled.

 

[UKpete]

Yes, there are two distinct cases being discussed here:

(i) new build (and rewinds where the circuit under test contains no old insulation whatever) - hi-pot applicable

(ii) routine condition monitoring of machines in service - insulation test applicable

- there is also fault-finding on machines giving trouble (ok, three cases).

I don't think I can add anything on (ii) other than what electricpete said makes good sense.

To add to the debate: on the new build, I can remember having it drummed into me that the full hi-pot test voltage was only ever applied once in the lifetime of a machine (unless rewound); this was done with the windings cold before the load test. Perhaps I ought to add that this applied to dc traction motors with class H insulation, but as jbartos implies, I assume that the full hi-pot test is only intended for new insulation.

London Underground used to specify that a machine subsequently tested during routine maintenance should be hi-pot tested at 55% full hi-pot voltage, but never at the full voltage. Another operator used to specify 75%. These were simple pass/fail tests, I don't think there was any rigorous condition monitoring.