Exciter Winding Resistance Unbalance

Well, it's rather difficult to see the <...our generator...> from here.

What one would need, ideally, is the original equipment manufacturer's telephone number off it and a call to that body's Technical Helpline to answer that question.

It depends upon the type of exciter, but generally speaking as long as the DC exciter's field current keeps up with the demand of the operating point the generator won't care.

Depending upon the exact cause and degree of imbalance, the exciter may overheat, become magnetically/mechanically unbalanced, and trip causing the loss of the main generator. Turn to turn shorts will cause a slow degradation, turn to ground will instantaneously become severe when the second fault occurs.

Everything depends on what you mean by "imbalance". A 0.01% difference in the series resistance for only one rotor pole will not make much of a difference. Having two of the three exciter rotor poles open circuit will make one hell of a difference. (I'm not sure why anyone would make separate rotor windings but it can be done.)

after communication with vendor, at early commissioning we got balance resistance 7 mOhm, then when we perform resistance test (preventive) we got one winding imbalance (11 mOhm), we've been measure more than 4 times, and using more than one metering and still get this result.

Assuming your tiny resistance numbers are accurate, I suspect the contact area between brushes and that rotor winding are out of round. A film maybe on the contact, too.

thank's for your comment refred, and what will happen if we still running Generator???

That was answered in post #2, and until you provide much more information regarding the generator and exciter, no further analysis is possible.

imbalance it's about 25% , below is data that we measure

using DLRO

So what did the original equipment manufacturer say during the telephone calls made to its Technical Helpline?

this is the reply from manufacture

"The higher resistance on the one phase of the exciter armature indicates that something has changed since originally manufactured. What has caused the higher resistance is unknown.

If you decide to use the exciter armature as is *a** would recommend doing so only as a temporary back-up basis until a new exciter armature can be ordered and this exciter armature can be rewound. Higher excitation will most likely be required to maintain rated voltage."

So the answer to Anonymous Poster #1's question, <...what happen in...generator if...exciter winding resistance is unbalance?...> is that the original equipment manufacturer's statement <...would recommend doing so only as a temporary back-up basis until a new exciter armature can be ordered...> would suggest that the happening "some replacement parts need to be ordered and fitted as a matter of urgency" will take place, otherwise another happening, a "significant malfunction might ensue that might affect the ability of Anonymous Poster #1's facility to stay in business" would be the reasoned alternative.

If these measurements were taken with the windings separated and at the winding terminations, then it appears that either one of the coils was wired in series with its group, or a coil in a parallel group has a poor joint and/or has failed open.

My best interpretation of your measurement is that you have two windings per phase, Δ connected & one is open circuit - running the machine would overheat & damage the remaining winding.

Please excuse graffitti - for free advice you have to accept whatever piece of scrap paper was to hand! Figures "OK" are no-fault resistances, 2R each winding, while "1o" indicates with 1 winding Open. A "star" connection is unlikely, one half-winding would have to be shorted to match your measurements [3 ~ 5.7/2].

The 7mohm is your OK previous measurement [1R in parallel with 2 x 1R in series - 2 x 2R parallel = 1 R :all windings equal resistance 2R], while 8.77 mohm is average of your measure of two phases (third, 11.35 mOhm).

Your actual measurements high/(average of two others) gives 1.294, compared to 1.33 theoretical with 1 winding "open".

Thank you for your calculation, but I still confused with the calculation if you assume it as a delta connection. I agree if it's star connection with one winding turn to turn short as describe in the picture.

Dear Anonymous,

I assume you have 3 sets of phase windings on the rotor, which pass a DC field from windings fixed in the stator. These phase windings will each have two parts, diametrically opposite on the rotor. The two parts could be wired in series or parallel by design and then connected to make star or delta to connect to the rotating diodes. Since you do not tell us how the windings are arranged, one has to guess!

With a series connection, it is unlikely you could get a low resistance short directly across one of the windings with a low voltage test - it would take a metal-metal weld.

With parallel windings, it is possible a crack has spread across one winding conductor or a weld has broken at junction point - open circuiting one of the two windings which are supposed to be paralleled.

My calculation found the resistance between any two corners of the Δ with all windings connected [OK] and then with one 2R winding open circuit [1o]. With 1o, two measurements are the same & third is high. I then calculated the ideal ratio "same" to "third" as 4/3 = 1.33. This is close to the ratio of your "fault" 1o measurements 11.35/8.77 = 1.294 [getting 8.77 as mean of your measurement (8.9 + 8.64)/2 ]. Finally, I took your OK measurement (at commissioning?) of 7 mohm & equated it to the OK value of 2R/3 by the diagram to get 10.5 mohm, not far from 11.35 you measured "with fault".

It does not take long to calculate that if you have two windings 2R ohms in // in each leg of star, then, if one winding is open, you will get two readings of 3R ohms and one of 2R ohms - ratio of 3/2 = 1.5 - compared to measuring 2R three times if OK. N.B. that is two high & equal, one low, ratio 1.5 with star - it was two low & equal, one high, ratio 1.33 for Δ.

So my best shot is that you have two windings in parallel per phase with Δ connection by design, but one winding is open circuit.

Also, if you measured 7 mohm 3 times when OK, then, if you had a short in a winding, you would measure less than 7 ohms. Since your measurements are now all > 7 mohm, that is further reason to suspect a disconnection, not a short.

If you are still confused, please indicate what is confusing.

67model

67model

sorry for late reply , i still confusing with your calculation, can you give me reference for me to read that , and then finally understand your calculation,

understand your calculation is very important , cause in the future when we meet with same problem i can manually calculate and found where the problem is..

thank you

OK Now i got it

thank you for your detail explanation

thank you! I got your point. The measurement ~ the calculation result as you assume 1 one winding open.

Correct, mantij. And by your own method.

67model

I would suggest that most motors would have a phase current imbalance of at least 3%. This can be caused by many things other than a phase resistive imbalance. I would also say that a 5% difference in resistance would not be that unusual either.

Most of the impedance in an electric motor gen is inductance so whilst some resistance imbalance might cause some phase imbalance , possibly a little more current, some extra heating, slight change in power factor and in theory a slight distortion in the rotating magnetic field i would not be concerned.

Hi there,

first sry for my bad english.

try to say it in a few simple words.

first for understanding: you have a rotating 3 phase exciter, right?

and you have measured resistance 3 times, so if i understand it right, you have lower resistance in phase 1, (1-2 is lower and 1-3 is lower), other two phases have higher value (2-3)

so it couldnt be a poor contact in a phase. (so you should measure 2 higher values for that problem)

i think it could be a shorten turn problem in phase 1.

Did you have a surge voltage or rso test equipment at site ?

best regards

Alex