did this problem just start occuring while in service, or was the generator removed and checked out then reinstalled? If you have had the generator rewound, or removed, it is imperative that the generator field and the series field are hooked up with the correct polarity. For example, if you were to reverse the generator field wiring on a good unit, and turned on the generator, the loop circuit would immediately begin to build until it pulled through the brake or popped the overload. this is due to the fact that the suicide circuit is designed to put residual loop current through the generator field to negate the magnetic field produced by the series field. If either field is not in the correct polarity, then the suicide circuit will not decrease the magnetic field, but will increase it, causing the generator field to increase its output and cause the above problem. I have been on many jobs where the fields were not hooked up correctly, causing the car to run in the opposite direction commanded. to fix this the mechanics will sometimes reverse the motor field to get the car to run in the right direction. the problem is that after the car stops, the loop circuit will continue to build up either slow or fast, depending on the strength of the series field. If this problem just started out of the blue, then you must check the suicide circuit to make sure it is intact as Vic stated. usually a grounded loop circuit will cause compounding problems, not the problem you are encountering. Also, why are you measuring the generator output to ground? you should be measuring across the brush riggings of the motor or generator, as this will give you the correct output voltage.
