High Pot Testing Motors
High Pot Testing Motors
(OP)
Pro and Con's of High Pot testing Motors. as a PM service. Any input would be appreciated.
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RE: High Pot Testing Motors
to a Doble test. It also depends on your voltage rating. A megger should suffice for voltages less than 5 kV.
RE: High Pot Testing Motors
I lean towards a megger test in place of a hi-pot test. The hi-pot will definately verify the insulation but if it is weak at all, it will damage the equipment. In my mind, the hi-pot is just going to give you a warm fuzzy feeling if it passes, if it fails, time for a rewind. Unless its real critical, its probably better to let it fail when it wants rather than hurrying it up by performing a hi-pot. Maybe to summarize, if you have a spare motor around then hi-pot, if you don't then skip it since you might get a little more run time out of it before it smokes.
RE: High Pot Testing Motors
RE: High Pot Testing Motors
Regarding hi-pot testing: a winding that has been in service shouldn't receive a hi-pot test at the same voltage as the manufacturers test (as given in IEEE 112 etc) - 50 to 75% at most. Even the manufacturer won't (or shouldn't) apply it more than once, for fear of shortening the life of the insulation. I suspect hi-pot testing is only done on machines out of service - unlike IR testing (which at least gives some warning of deterioration), a hi-pot test can instantly push a motor over the edge.
Hi-pot testing gets discussed on this forum every so-often, you might try a search.
RE: High Pot Testing Motors
RE: High Pot Testing Motors
Before to perform this test made a Megger Readding and if you have a good reading perform the HIPOT Test.
The DC HIPOT is a practice to evaluate the insulation system integrity, I was used for many Years with a few problems, for a machines with 5 or more years old we use a Voltage test of 1500 V for 460 machines and 1000 V for 230 machines.For a new, rebuilt or age less than 5 Years use 2 Rated Voltage + 1000 V.
The DC HIPOT Test will help like a predictive maintenance tool, avoiding DOWN TIME, because you could detect earlier problems and take decisions. I was asked to the hipot manufacturer about the Voltage Test and they conclude all motors most be tested at 2 rated Voltage + 1000 because the most critical condition is the motor´s start and you could find high voltages like rebound voltages due to the contactor don´t have instantaneus close. Example: If you perform a DC HIPOT Test in a some 460 V motor at 1880 VOLTS and you find a failure at 1200 Volts, you most abort the test but the motor could be running, because only will fail when it reaches this 1200 Volts, but you have the time to find a spare motor or to make the preparations to send it a repair shop. This is different like the motor burnout in operation because you will have down time.
I think is a Good Preventive maintenance tool.
Regards
Petronila
RE: High Pot Testing Motors
John Morris
RE: High Pot Testing Motors
2x(line V rms) + 1000 V ac rms.
(if a manufacturer uses dc rather than ac for a highpot test, then the above formula is usually multiplied by 1.7).
In other words, Petronila's test is a megger test plus, rather than a true hipot. Megger tests are dc of course.
The following document comments on the differences between ac and dc tests in sections 22 and 23:
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I believe that a full hipot test (dielectric or flash test, in IEC-language) is more a proving test carried out to show to the customer that a new winding has been properly rated and correctly manufactured, rather than a predictive maintenance test. It should only be repeated at reduced voltage, and probably at the beginning of a maintenance strip-down. There are better methods of PM, using insulation resistance as specified in IEEE 43.
RE: High Pot Testing Motors
A “Controlled Overvoltage Test” per IEEE STD 95 is a very good test to evaluate the winding insulation condition. For that test the maximum DC voltage is selected around 2.1 to 2.55 times the rated AC machine voltage. The initial value is 30% of the maximum and the steps not to exceed 3% of the final value (around 7.5% of V nominal). This test is recommended for large machines since it takes long time to test and discharge every winding phase and the operator should be well trained to stop the test before destructive conditions are imposed.
RE: High Pot Testing Motors
I would like to see any test tech stop such a test before damage occurs. This is basically a go-nogo type of test in that when it breaks down, it goes quick; there is no lag time where a human could ever possibly stop the test. What I don't know is if this breaks down, how much damage is done? Will the machine continue to run? Its anyones guess and depends on the voltage level where it broke down.
RE: High Pot Testing Motors
That is not a go-nogo test. The voltage is kept for 60 seconds on each step, at that time the leakage current is evaluated aginst the test trend, it could take around 50 minutes to test one phase and 2 hrs to discharge it. Please read IEEE Std 75.
RE: High Pot Testing Motors
RE: High Pot Testing Motors
Whats interesting about what your saying about the discharge portion of this test. Do they want this charge to bleed off on its own or do you put some high impedance load on the winding to discharge it slowly? Our procedures here (again I dont agree with) allow us to ground the winding immediately after the test (then a ground strap is placed on the winding). Which, causes problems of its own.
I have never read IEEE 95 but it sounds like I better. I am not debating that standard (as I never even read it) just how the ramp tests are done here and the value of the test, as it is done now.
Say we are talking about the IEEE 95 standard, why would not the winding be damaged when you had an insulation breakdown in doing a hi-pot test?? I am not talking about damage being done below the rated voltage but damage that may occur by applying a higher voltage during the test. The equipment may not ever see this high voltage in operation, which means it would never break down until the insulation was weakened further, which would occur if the winding broke down during the test at whatever voltage.
Alright, Ill go read the standard and see where the tests we are doing came from.
RE: High Pot Testing Motors
Is this type of testing still in use, and would it be appropriate for motors?
yours
RE: High Pot Testing Motors
My findings may not be relevant in the world of motors.
I decided when you test is the key.
Some manuals required a hipot after every repair regardless of the failure.
In our situation, a unit might come in for LCD glass failure. Why stress the circuit cards?
This meant that lemon units were tested repeatedly, while good units that stayed in service for years were never tested.
Sometimes hipot tests required for device certification make their way into maintenance manuals for no good reason.
I eliminated the requirement for high pot testing at repair unless there was evidence or a repair releated to arcing or insulation degradation.
If there was a scheduled maintenence requirement to test ALL units at some interval, that would have been different.
I felt there was no added value in just testing units being repaired regardless of the failure.
RE: High Pot Testing Motors
RE: High Pot Testing Motors
RE: High Pot Testing Motors