I would assume that you are asking this for application on a steam type turbine/generator rotor?
To clarify, this application is on a non-sailent pole type rotor?
Yes, I can explain.
This test measures the magnitude and the direction of the magnetic field generated by the current.
The first thing you need is a search coil that can be placed on the rotor preferably near the active center of the rotor. This search coil is usually about 2 inches in diameter and has about 200 to 400 turns of fine wire wrapped around its wooden or air core. The leadsare fed to the input to an oscilloscope.
You now need a 60 Hz (or 50Hz as available) connected to the slip rings of the rotor. This is usally done through a variac, to adjust the current/voltage to the rotor circuit. Also place an AC volt meter across the scope terminals to accurately measure the search coil voltage. With AC exciting the rotor winding, start at the edge of the pole. Adjust the scope such that you have 2 full cycles on the screen. As you move the search coil around the the circumference of the rotor you will note the magnitude and direction of the search coil will change by reading the phase angle on the second zero crossing on the scope.
Do this at the first edge of the pole, center and the other edge of the pole, jump to the next pole and the direction changes, i.e. N and S.
No I do not. What i wrote here is what I used from Allis-Chalmers test procedure that was originally written about 1955.
I would further add if you are trying to determine shorted turns on your generator/synchronous motor there is an easier way that is fool-proof.
Yes. Further, do not tell me that an ac test is better than a dc test because it is not. For one very big reason, it (ac drop test)is also measuring the reactance voltage drop, which is much larger than the resistance voltage drop.
Remember that when you use ac, you are dealing with the impedance of the circuit of which the iron is part. The dc test is only the wire of the winding.
To go deeper, it is very easy to calc what the resistance drop should be. For example 120VDC and 10 poles gives 12DCvolts per pole. Typical mfg tollerance was +/- 2% as a go / no go.
