Surge test on generator field coil
Surge test on generator field coil
(OP)
Hi
We have been asked to perform a surge test on some salient pole field coils at 1500V. I am familiar with voltage drops using an AC supply at 50/60hz. Also aware of tests at higher frequencies (400Hz). But should I be concerned that user a standard surge tester (Baker type) we will overstress the first turns? The coils are single layer strap type copper and the test wil be done on the rotor.
We will perform some tests at lower voltage to see the distribution, but interested if others have any experience.
Many thanks
We have been asked to perform a surge test on some salient pole field coils at 1500V. I am familiar with voltage drops using an AC supply at 50/60hz. Also aware of tests at higher frequencies (400Hz). But should I be concerned that user a standard surge tester (Baker type) we will overstress the first turns? The coils are single layer strap type copper and the test wil be done on the rotor.
We will perform some tests at lower voltage to see the distribution, but interested if others have any experience.
Many thanks





RE: Surge test on generator field coil
Surge test (IEEE522) is a potentially destructive test. Accordingly, attention should be paid to the proper selection of test voltage, considering the condition of the coil insulation (cured or uncured), stage of assembly, etc. Also (like a hi-pot) you wouldn’t test unless you had good IR/PI readings. The traditional assumption is that if the test passes, no damage has been done.
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(2B)+(2B)' ?
RE: Surge test on generator field coil
If you could test the coils when they are dis-assembled from the rotor (no ferromagnetic material around the coils), then you would have more or less linear voltage distribution over turns.
RE: Surge test on generator field coil
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(2B)+(2B)' ?
RE: Surge test on generator field coil
I guess the real question would be how much voltage can be safely applied… would we use the same type thumbrules based on machine voltage rating as used for ac stators? Beats me. I would be a little nervous about that knowing that the pulse may act a little differently. Would look for some standards and experience to guide me. I have none to share though - Sorry. In retrospect I probably haven’t said anything useful for motorspert.
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(2B)+(2B)' ?
RE: Surge test on generator field coil
All useful info. We will do them away from the rotor, so that sounds like it could work.
I share your nervousness electricpete but am also keen to experiment! So, we will take it slowly. Luckily it's bare copper so we can put a scope on pretty easily.
RE: Surge test on generator field coil
RE: Surge test on generator field coil
There is a fundamental difference between RSO and surge test in that RSO detects shorted turns and surge test detects weak turn insulation (insulation that might fail under stress in the future but is currently bad). So, surge test is a much more senstive test, and necessarily is also potentially destructive to accomlish that sensitivity.
Why do we tend to use RSO test on generator rotor (only detects shorts) but surge test for ac machine stators?
I think it ties to the consequence of turn to turn short.
On an ac stator, as soon as turn to turn short occurs, there is immediate high circulating current due to auto-transformer effect. It will melt copper and quickly escalate to a ground or phase fault causing a machine trip. There is no benefit to RSO test because if you have shorted winding your machine has most likely already tripped. But there is advantage to determining weak turn insulation because it can result in trip almost immediately if it does short in the future.
In contrast on a dc winding, turn to turn short results in practically no change in current (perhaps current increases in inverse proportion to the fraction of total turns shorted).. The machine can operate for long period of time with that short present with little effect (often only increase in vib is the most noticeable effect). We can use RSO to detect/confirm that shorted winding. There is no need for invasive / potentially destructive surge test because turn short is not as severe a fault on dc winding as it is on ac winding (we have time to respond).
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(2B)+(2B)' ?
RE: Surge test on generator field coil
should've obviously been:
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(2B)+(2B)' ?
RE: Surge test on generator field coil
It would help to understand what the OP is trying to diagnose - insulation strength I guess as a surge test was proposed initially.
RE: Surge test on generator field coil
I am familiarized with this kind of test and I.M,O the dc surge test is not destructive. Note that usually the field voltage is low and the D.C. test voltage is 1.7 (2 V + 1000 ) so if the machine is new you can test at aprox. 1700 to 1800 Volts (considering that field winding voltage don´t exceeds 110 V) if the windingb is not new 1500 volts will be high (I will test it with 1000 V), our rule of tumb is perform surge in old windings at 60% of new voltage test.
Some considertaions about the field test (salient pole rotor):
If the machine is assambled then you can perform a resistance test and compare with ratio V/A, this reading will give you an idea about the winding condition. Another consideration is if you perform the test to conected field winding unless you have severe short circuit, you are not going to find nothing significative. Is possible that the field winding pass but really you are not making a comparisson (the winding is single phase). In this type of salient poles the best way for testing is disconecting the coils by pairs and compare one with the adyacent (also you can compare the ohmic resistance of each pair of coils), to do this coil comparison follow tester manufacturing direction for single phase windings.
Regards
Carlos
RE: Surge test on generator field coil
@scottyuk et al - we are just proof testing a new set of coils, we normally test for interturn faults bt applying 220V 400Hz, this confirms the coils are ok, but the coils are only effectivelty strength tested at 0.4V/turn. This customer wamnts a more demanding test.