Synchronous motor field discharge resister

[jschwei314]

I am looking for any rules of thumb about sizing synchronous motor field discharge resister. One just has motor name plate data. I want to keep the discharge voltage around 500 volts, as I will be using a solid state field applacation panel.

Motor Data: 500 HP, 514 rpm, 4000 volt, 69 amp, .8 PF, 60 Hz field: 250 volts 32 amps

The motor will be started across the line, no load condition. The load is a two-piston compressor.

 

[DougMSOE]

The way I have approached this problem is to consider the winding as a battery with 2 separate resistances, one is the field resistance the other is thwe discharge resistor.
I would rather have the greatest voltage drop on the external resistor than on the winding resistance resistor. Since the field resistance is about 7.8 ohms, I would go slightly higher e.g. 8 to 10 ohms.
There are several other ways to approach this problem as well.