Delta tan Delta test versus Megger Test
Delta tan Delta test versus Megger Test
(OP)
What are the differences between a delta tan delta test versus a traditional megger test insulation integrity for induction motors? I have read that induction motors over about 4kv should have the insulation tested using the delta tan delta method....





RE: Delta tan Delta test versus Megger Test
There is some discussion of power factor test here (for all practical purposes the same test as tan delta test);
http
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RE: Delta tan Delta test versus Megger Test
RE: Delta tan Delta test versus Megger Test
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RE: Delta tan Delta test versus Megger Test
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RE: Delta tan Delta test versus Megger Test
The megger was always the first test for the "it was running and then tripped/stopped" scenario. If the megger showed it boad/grounded/shorted, subsequent tests weren't likely to change anything. If the megger test was good, there were still many possibilities that other tests could find failures.
For the "take it down for periodic testing/maintenance" the megger was often one of the tests, but not necessarily the first.
old field guy
RE: Delta tan Delta test versus Megger Test
RE: Delta tan Delta test versus Megger Test
See for example IEEE286, section 10.2 which says what I just said - test voltage shall be limited to line to neutral voltage or below.
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RE: Delta tan Delta test versus Megger Test
Megger and tan delta are two different tests of the dielectric measuring different quantities. The only thing common to both of them is that they are trending tests.
Muthu
www.edison.co.in
RE: Delta tan Delta test versus Megger Test
RE: Delta tan Delta test versus Megger Test
I think it should be important to separate standard requirements from opinions – this applies to everyone including myself. Not because standard is better than opinion or vice versa, but just so that people can judge for themselves.
Here are my observations regarding the standards:
1 – The IEEE standards identify ac hi-pot and dc hi-pot as potentially-destructive tests.
2 – The IEEE standards do not identify power factor (tan delta) as a potentially-destructive tests.
3 – The IEEE standards require dc insulation resistance test at IEEE43 levels prior to ac hi-pot or dc hi-pot.
4 – The IEEE standards do not require dc insulation resistance test at IEEE43 levels prior to power factor (tan delta) test.
5 – IEEE standards define overpotential test as one which exceeds rated voltage.
So based on the above, I hope everyone would agree:
Item A – Referring to tan delta as "overpotential test" (as was done 18 Feb 10 8:50) is incorrect. (One could argue that the neutral end sees higher voltage during tan delta than during operation, but the same argument applies to insulation resistance at IEEE43 levels).
Item B– Using item A to imply that the standards require dc insulation resistance test prior to tan delta (as was done 19 Feb 10 12:23) is incorrect.
Now, as far as my personal opinion of what should be done. I am certainly in agreement that IEEE43 winding insulation resistance test is so easy, fast and routine that we should do it every time. That was what I was trying to say in my very first comment of the thread: "I don't think it's a question of one or the other. Megger test (insulation resistance test) is a very simple easy test. " [implied but not stated: you might as well do the megger every time, regardless of whether you decide to do power factor test.].
BigMotorGuy – I can tell you among nuke plants that attend the EPRI Large Electric Motor User's group, there is one large nuke that does power factor testing routinely on large motors. The remaining 15-20 in attendance do not do it routinely. At our plant, we have the power factor test equipment available (for transformer testing), and we have people trained to use it, but we don't go through the trouble to do it on large motors for a routine test. I reserve it for a special troubleshooting test. I also request the shop to do it when in the shop so we can have a baseline to work off of. Note we do periodic insulation resistance test, dc step voltage on selected motors, and we have on-line partial discharge monitoring installed.
It is much more sensitive and provides more diagnostic info than simple megger. It has the benefit over hi-pot that IN GENERAL power factor testing is not considered potentially destructive. But there is a lot of effort to do the test and it is a judgement call whether that effort is needed given the other info available.
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RE: Delta tan Delta test versus Megger Test
Richard Nailen on effectiveness of tan delta as a maintenance tool.
http:
Shorter Nailen - While a bad tan delta value indicates a bad winding, a good tan delta not necessarily indicates a good winding.
Muthu
www.edison.co.in
RE: Delta tan Delta test versus Megger Test
Read NETA/ANSI test procedures, they require it.
But you are right about my terminology, I and others tend to lump several tests together when using the term overpotential, tan delta should not be lumped in there. I suppose that is because it is used in place of VLF or hipot tests.
RE: Delta tan Delta test versus Megger Test
Based on the secitons I quoted above, I think it is pretty clear that IEEE does not support what you are saying. i.e. they require insulation resisstance testing before hi-pot testing, but not before power factor testing. So I would be very surprised if there was some industry standard requirement (insulation test before power factor), that IEEE somehow "forgot"... even in a standard that specifically addresses power factor testing (IEEE286).
Now you mentioned NETA standards. I looked at NETA MTS-1 (2001). There is nothing that specifically addresses what we have been talking about. For ac rotating machines, what there is is a list of tests and the first one is indeeed IEEE43 is in the list before all others tests. Is that what you referring to? In that case I would suggest you are reading too much into it to infer this constitutes a new requirement which was somewhow overlooked in multiple IEEE documents. We presume that is the recommended order of tests, but that doesn't mean that there are not items within the list whose order can be exchanged provided other more specific industry standard requirements are not violated. For example winding resistance test occurs after IEEE43 insulation resistance test. Are you saying there is a technical reason that the we cannot perform winding resistance test before the IEEE43 insulation resistance test? I hope not.
Can you please provide a specific standard reference, paragraph, and quote to support your contention?
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RE: Delta tan Delta test versus Megger Test
However, you said yourself the megger test should be done before a TD, I assume Edison123 would agree based on his experience, and so do I. So if IEEE and ANSI/NETA disagree on it being required who is in error?
RE: Delta tan Delta test versus Megger Test
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RE: Delta tan Delta test versus Megger Test
Muthu
www.edison.co.in
RE: Delta tan Delta test versus Megger Test
RE: Delta tan Delta test versus Megger Test
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