Using resistor and DC motor acting as generator as a brake

[Ry122]

how would you calculate the resistance required for a resistor which is in series with a generator (DC motor in reverse) to achieve a certain torque from the generator?
Eg, there's a torque of 30kN acting on a 100kW generator while it moves at 50RPM.
The is the same mechanism as electric diesel locomotives use to brake that employ rheostatic braking.

 

[Simon Bridge]

You have to ask yourself how much work is being done to run the generator circuit.

 

[sophiecentaur]

how would you calculate the resistance required for a resistor which is in series with a generator (DC motor in reverse) to achieve a certain torque from the generator?
Eg, there's a torque of 30kN acting on a 100kW generator while it moves at 50RPM.
The is the same mechanism as electric diesel locomotives use to brake that employ rheostatic braking.

If this is a real, practical problem then the motor would need to be a suitable one. Many motors will not work as generators, particularly not high power motors, with field windings.
Your suggested 100kW generator is quite a meaty piece of kit.

Note. If you want to calculate power (in Watts), as Simon B suggests, you will need to convert your RPM to angular velocity (and get your Torque in the right units, too - do you mean kNm?)