Hi all,
I am working as a field service engineer for the overhauling and testing of generators and exciters in power plants.
At my most recent project, I was measuring the resistance of the generator rotor winding using a Digital Low Resistance Micro Ohm Meter with the current set at 10mA. The winding resistance as per the manual is 98 m Ohm at 25 deg Celsius. But in my measurement it came to 120 m Ohm at 27 deg celsius. What does a drastic increase in the winding resistance mean! Please note that the thermal coefficient of the metal used for the winding is of the order of 10^-6 in SI units and so temperature increase has very little effect in this case.
From my experience, I understand that a lower resistance than the manufacturer's manual may mean an inter turn short in the winding. This rotor cleared the RSO check and the impedance of the winding was ok. I wish to know what this anomaly may mean.

in beginning be sure that your instrument calibration certificate is valid, and check its uncertainty of measurement.
Secondly, you can verify also your instrument a gainst working standard resistor.
however, the error about 22% which is too much, I expect this is due to lead connection or to loose connection some where.

I need to say that I agree in completely with sir Omer except I would like to recommend method for measurement resistance which is well known as "V/C method" which is based on measuring value of DC voltage between terminals of consumer (rotor's winding) by using multimeter and, on that way measured value of DC voltage, it is necessary to share with value of DC current which flows through circuit which contains DC supply source in serial with your consumer (rotor's winding).

Only what you need to do before measurement of resistance of rotor's winding is to connect terminals of DC supply source in serial with terminals of rotor's winding. After that, you need to set DC current at your DC supply source on value which is equal with 10% of rated current of your rotor's winding (see at nameplate of electrical motor). After that, you can finally measure value of DC voltage between terminals of rotor's winding and, on that way measured value of DC voltage, you need to share with value of DC current of your DC supply source and you will get appropriate value of resistance.

My opinion is that method is the simplest method and gives the most regularly results.
Otherwise, you need to include value of inside measured error of instrument, value of temperature, influence of quality of connecting between DC supply source with rotor's winding etc.

Both have covered all the scenarios.
I believe the Digital microhm meter is a bridge .
However, i hope and believe you have taken care of lead resistance.
However, there are two ways the reading could have been higher.
1 the calibration
2 loose or bad contact at one or more places.
I do not believe the problem is in the poles as the RSO was done.
please recheck for repeatability of reading under various conditions.
All the best!

The rotor resistance was measured after withdrawal and the temperature of the rotor can be safely assumed to be uniform as the rotor has been outside the stator for 8 days. The resistance was measured from the contact pins between generator and exciter rotors. The surface is sufficiently clean and the instruments calibrated. I have repeated the test with another meter and received the same result.
Also please note that the resistance of the rotor, now that it has been re-inserted into the stator is suddenly 3 times the shop and erection value of 98 m ohm. Please advise on this as well.

There are a couple of things that come to mind:

1. You didn't say which brand meter was used, but some require a warm-up period of a few minutes.

2. Depending upon the physical size of the rotor the inductance can be quite high, requiring a significant settling period before all the charging transients have died out, and the meter has reached steady state.

3. Surface preparation of the winding leads is critical to getting consistent readings.

4. Were the test leads reversed between readings? Sometimes alternating the test leads and doing multiple readings will finally provide a consistent result.

5. Once you have ruled out the meter then it's time to examine any joints in the leads between the test point and the windings themselves.

6. Worst case there is a micro-crack somewhere in the winding, usually at an end turn.

Please let us know what you find, thanks in advance.

The instrument used is DLR 10X by Megger Co. The instrument provides both forward and reverse measurements at the same time. The contact surfaces were satisfactorily clean. Interchanging the leads provided the same result. Due to shutdown schedule constraints, the generator has now been assembled.

If your dlro is not you satisfactory data, try to find a dc source with current limiting capability. Inject within the power supply capability, read off volt and amp and voila, you get R.

Problem with low current test set is they do not have the power to break down some high contact resistance boundaries. A few more amps may help.

Are you reading off the slip-rings or off the collector bars ?

Phani, Thanks for the feedback, but I cannot tell you why the resistance tripled, perhaps more detail will help solve this mystery.

You said "... The resistance was measured from the contact pins between generator and exciter rotors...". Is this a brushless excitation system that has a pilot exciter directly connected to the main rotor? If so then there are connections between the two that are suspect, also rotating diodes and fuses that rectify the output of the exciter rotor. Originally I assumed that this was a brushed rotor, please clarify exactly what is the excitation system type. Thanks, Alan

Alan, the excitation system is of brushless type with a pilot exciter rotor. However, the two rotors are in decoupled and separated condition so the fuses and diodes do not come into picture. The pins through which current flows from exciter rotor to generator rotor can be removed and fixed from the coupling face as desired in the current condition.

Phani, Thanks for the update. It's certainly puzzling why this is happening, the only thing I can add is that if I were in your position I would get another test set and repeat the tests. It's the only way to eliminate instrument error from your testing.

At this point I suspect that your generator will perform perfectly until its next inspection.

Alan

Having the same problem at the site I'm working. Winding Resistance has increased with time.

Phani, any solution?

Dear Phani,

Hope your Generator is working well now..

????