[TenTec] Generator field current supply capacitor. Completely off topic.

[Ken Brown]

Hi all,

There are so many knowledgeable people on this reflector, I'm sure 
someone will have the information I need.

I am repairing an old generator (alternator) set. I spliced the broken  
wire in one of the field coils, and replaced the diode bridge. I still 
need to replace the capacitor used in the DC supply from the diode 
bridge to the field coil. I don't know what value of capacitance to use. 
It is about an 8 kW 120/240 VAC alternator powered by an 1800 RPM single 
cylinder Diesel engine. The field coils (four coils in series) have a 
total series DC resistance of about 47 ohms. I was thinking that the RC 
time constant of the capacitor and the field coil resistance ought to be 
on the order of magnitude of about 1/60 of a second (to keep the field 
current flowing between cycles of the alternator output which feeds the 
diode bridge). However, if I calculate the capacitance needed to 
accomplish that, it comes out to hundreds of microfarads. With a voltage 
rating of 200 VDC this would be a sizeable capacitor, even with a modern 
capacitor. This is a 60 year old machine. The original old capacitor is 
about 1/2 inch diameter and an inch long. So my reasoning is clearly wrong.

Anyone with experience with generator sets that would know an 
appropriate capacitance value?

Thanks,

Ken N6KB