Alternator Excitation Current

I can't see anything obviously wrong with the figures. Do you think there's a problem?

Are you concerned because the Excitation currents don't match? If so then you need to include the generator terminal voltages for both sets of readings. Remember that on a Brushless Excitation system the Excitation current is only a proxy for the actual field current in the rotor. It will vary with the terminal voltage of the machine, which in turn varies according to the system voltage (assuming you are not the only generator) and the loading on the machine. As CodeMaster said, there's nothing inherently "wrong" with those readings, they just indicate that the terminal voltage was higher for the "After" set of readings.

How did you measure the excitation current in brush less excitation system? Was the rotor excited with other excitation source with technological slip rings? Have you plotted the open circuit characteristics and compared with the previous plotted characteristics? How do they compare? Kindly furnish these data for further analysis.

thanks for the feed back

• as generated feeding power to utility so genrator terminal voltage may have diff values for both sets. I will also share this data.
• excitation/field dc current is measured with transducer(may be using hall effect sensor, not sure) and I have taken the comparison from historical trends of HMI; no sliprings we have here.

Just FYI, the term "brushless" denotes up front that there are no slip rings. Field excitation current is induced across the air gap to the rotor as AC, then there is an on-board rectifier rotating with the rotor that converts it to DC for the field excitation.

...and there is usually an ammeter in the circuit of the stator/field winding of the pilot exciter, which is what the OP has shown in his chart.

Thanks. I am aware that in brushless excitation system there are no slip rings. Therefore I have asked OP how the excitation is measured. Some manufacturers use telemetry whereas some use field current of exciter which is not so accurate and may vary specially after re=winding the rotor. In manufacturers works, technological slip-rings are used for testing the generators and excitation current is directly measured.

Why? When you rewind anything, there is no possibility that absolutely every aspect of the rewind will be exactly the same as the original. What matters is whether or not it is within tolerance of acceptable deviation, something you can ask of the original mfr, or determine yourself. Usually 1-2% is more than acceptable, you appear to be less than that..I don't see a problem here.

Perhaps the OP is concerned about the 21% increase in the excitation current which, as pointed out in an earlier post, could simply be the result of a higher terminal voltage.

On the other hand he may be suspicious that there is a poor connection(s) from the pilot rotor output through the diodes and fuses into the main field which is causing the excitation system to require more current to overcome the additional resistance.

There's also the possibility that he feels that the rewind shop "cheated" him by using fewer turns of a lighter conductor than originally specified, thereby requiring more current to pass through fewer windings to get the same Ampere-turns in the air-gap.

Hopefully OP will tell us why he posted his question.