AC Generator Brushless Excitation not working
AC Generator Brushless Excitation not working
(OP)
I have a 45MW 13.8Kv 60HZ Generator with a Brushless Excitation system on it. The Generator is a 3600 rpm two pole machine. There is a PMG (Permanent Magnet Generator) mounted on the end of the Brushless Exciter that puts out 292Vac at 240HZ to a Brush Prismic A30 Automatic Voltage Regulator (AVR). In the manual mode of the AVR we can slowly increase the DC current to the Pilot Exciter field from 0 to 5Adc. That DC current is used to excite four (4) stationary field coils around a shaft mounted 3 phase AC Generator. The AC Generator should produce 3 phsae AC voltage which is then full wave rectified by 6 diodes located on a diode wheel which in turn produces 0 to 350Vdc which is passed through to the Main Generator Field. The nameplate on the Generator indicates that approximately 2.5Adc should be enough to get to full voltage no load.
As the Generator is accelerated to 3600rpm we can see a small amount of residual voltage buildup on the Generator stator. (approximately 50Vac per phase) by watching the Generator Potential Transformer (PTs Open Delta)
As we increase the DC current from the AVR from 0 to 2Adc we see no buildup of voltage on the Generator Stator.
We have checked the diodes and fuses and verified the main Generator field (0.485 ohms) is correct. We have disassembled and reassembled the entire diode wheel and fuses. Is it possible that if the field coils were not connected in the correct polarity it could prevent the shaft mounted AC Gnerator from producing any voltage?
Does anyone have any ideas of any tests I can run to prove the various parts are working?
As the Generator is accelerated to 3600rpm we can see a small amount of residual voltage buildup on the Generator stator. (approximately 50Vac per phase) by watching the Generator Potential Transformer (PTs Open Delta)
As we increase the DC current from the AVR from 0 to 2Adc we see no buildup of voltage on the Generator Stator.
We have checked the diodes and fuses and verified the main Generator field (0.485 ohms) is correct. We have disassembled and reassembled the entire diode wheel and fuses. Is it possible that if the field coils were not connected in the correct polarity it could prevent the shaft mounted AC Gnerator from producing any voltage?
Does anyone have any ideas of any tests I can run to prove the various parts are working?





RE: AC Generator Brushless Excitation not working
With a PMG that is not an issue.
However, check the polarity of the four field coils. If they are not connected with the correct individual polarity that could be the problem.
You haven't said if this is a working machine that has failed or a machine being started up new or after repairs.
I would put a small current through the four field coils and check the polarity with a compass.
I would expect North-South-North-South.
Bill
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"Why not the best?"
Jimmy Carter
RE: AC Generator Brushless Excitation not working
Some more background info please.
rasevskii
RE: AC Generator Brushless Excitation not working
RE: AC Generator Brushless Excitation not working
That is most likely the problem.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Generator Brushless Excitation not working
I am beginning to wonder if there is something open circuiting when we are at 3600rpm. Something in the Main Field. Tomorrow we are going to try providing the field current from a simple Variac and diode bridge instead of the AVR. If that doesn't help we are going to take the exciter apart and check all the windings, diodes, and fuses again. We are going to ran an AC impedance test of the main field, and check the resistance of the main field and the AC Generator armature.
RE: AC Generator Brushless Excitation not working
If by the "fan rotor on the exciter end" you mean the small fan on the diode wheel, you would be right. If you meant the rotor cooling fan on the exciter end, then you still have two to do, the small fan on the diode wheel and the other rotor fan on the driven end. Both rotor fans have to be reverse when direction of rotation is changed. Plus, the fan on the diode wheel can't be change because it is aluminum shrunk onto aluminum and you can't heat it fast enough for it to expand away from the diode wheel.
If you haven't reversed both fans on the rotor, it may be a good thing that you can't get it to work.
Be double sure that you are right about the ability of the generator to operate 50 or 60 Hz. If it is only 50 Hz design, the rotor may not be designed with enough metal in it for 60 Hz and also may be near or at a critical.
rmw
RE: AC Generator Brushless Excitation not working
Yes the Generator is rated for 60Hz and has all the manufacturers nameplates for the 3600rpm ratings. We have all the factory tests for both 50 and 60Hz ratings. It is not a Brush generator it is a Meidan Unit from Meidansha Electric out of Japan. We are using a Brush A30 AVR. The exciter we have does not have any fans located on the diode wheel that I can see. The PMG is eight poles and is putting out 295Vac at 240Hz and doesn't seem to vary. The fans they reversed are inside the main generator housing and circulate air through the Generator stator windings.
RE: AC Generator Brushless Excitation not working
Maybe it is just that the AVR has to much pride to run a Meiden generator - HA!
Hope you get it sorted. Where in Venezuela are you?
rmw
RE: AC Generator Brushless Excitation not working
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Generator Brushless Excitation not working
RE: AC Generator Brushless Excitation not working
I have one question about the failure. During the reversing of the cooling fan the field coils were accidently swapped so that the oreintaton was NNSS instead of NSNS. What damage could this have caused to the Exciter AC Generator Stator windings?
RE: AC Generator Brushless Excitation not working
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Generator Brushless Excitation not working