The relevant formulas are
R = E/I = P/I-squared = E-squared/P, where R = ohms, I = amps, E = volts, and P = watts.
I think this needs the input of a 'design' electrical engineer....
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I am not an EE, but I have determined the resistance of many power resistors. I assume you have a dynamic brake grid unit and you want the resistance? The way I do this is to connect it to a DC power supply, like a arc welder, and measure the voltage drop across the unit and the current. The resistance will change with temperature, if you need a very accurate measurement, enough current will have to be drawn to get the grids up to the operating temperature. The resistance will not be much, maybe a ohm or less. E=IR, so you need R=E/I, where E is the voltage drop across the resistor and I is the current any where in the circuit.
If you are not trying to measure the resistance, but calculate the resistance for an existing unit, then you will need the resistance of the wire or ribbon, usually given in ohms per foot or ohms per inch. This times the length of the wire or ribbon will be the resistance. If you do not have the resistance of the wire or ribbon, cut off a small sample and have it analyzed for the elements it is made of. You will also need to measure the wire or ribbon. Then look around the internet and see if you can find the resistance for wire or ribbon of the alloy and size you have. But again, it will be given at a particular temperature.
A very complex subject and it appears to be under constant review by engineers. I found several papers on the analysis which included many factors (forces, friction of wheels, spacing in couplers to name a couple) The elements of the motor factor in as one cannot overload the motor's electrical characteristics. Mosebach is one US mfg. Here are some of the basic formulas current of dynamic braking motor.
I also found and interesting approach to designing the resistors by thermal analysis. Dissipating the heat generated seems to be the key to a dynamic brake system's effectiveness. (paper attached)
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Dave: I want to know how the dynamic grid resistor is designed...Not how to measure it. The design engineer will, of course, need to know some detailed specs of the traction-motor-turned-generator.......Ranges of amps, volts, and watts.
Moonman: I know very well that it's a 'heat exchanger', too, and so thermal analysis is definitely involved....AFTER the EE tells me the value of R [and, of course, the max
power to be dissipated thru that resistor. [ P = I-squared R = E-squared/R]. Then ,the size [area] can be computed, given its max allowed temperature and the most difficult part: the overall heat transfer coefficient.
General: I still think we need just the 'righ't EE to provide what I seek....
