I am looking at a generator rotor. It has a rotating exciter armature, a rotating rectifier with 3 pair of diodes, and a resistor in parallel to the rotor field. I am a mechanical engineer, so this bit of electrical "voodoo" is outside my realm a bit. Can someone explain the need for the resistor? I understand general function of the rectifier, but the resistor is a finer detail that I don't understand.

 

I don't think there's a "need" for that resistor. I think it either represents some resistance associated with the generator field (rotor), or the resistance of the loads on the generator.

 

It's an actual resistor on the rotor. Embedded in a large finned aluminum heat-sink.

 

It may be there to act as a brake when the power is disconnected.

Keith

 

it may protect diodes from over voltage ?? forward over voltage . . . i donno . . . or just suppress inductor voltage and avoid excessive sparking at rotating bridge contacts
__________________________

there are however parallel resistors in 1-phase rectifying bridges . . to cancel large junction capacities in diodes -- the higher the forward current the higher the charge stored in junction

https://forum.allaboutcircuits.com/...tect-rectifier-diodes-in-power-supply.148439/

 

Here is an explanation that will probably confuse you even more:

Vivek Choubey (विवेक चौबे), B.Tech Electrical Engineering, Narula Institute of Technology (2018)

Field coils are present on the rotor (rotating part) of the synchronous generator. This field coil is excited by a DC supply and made rotated. The current through which it is excited is called field current.
There remains another set of poly-phase coil on the stator. When rotor carrying field current is made to rotate, the flux generated by rotor is cut by stator windings and hence they generate emf. When some load is connected to the stator output lines, current begins to flow and this current is called armature current.