We recently had an issue of a DNH Starter not turning the motor that was connected. I went to the site and took along a 25 horsepower motor to connect to the starter to see what was happening. When we gave the Starter a run command, the shaft of the motor viberated back and forth but did not spin. I really didn't give it too much time to do anything since I could tell that things were not good.
We brought this unit to the shop and in looking it over, we found that a wire from the firing board that goes to the snubber for that phase was burned. In disassembling the unit, we also found that the heatsinks of that phase had been very hot. Hot enough to burn the fish paper that is used as an insulator around the bottom of the heatsinks. This would indicate that the units bypass contactor is not carrying the load on this phase but rather the current is traveling across the SCR's to the motor. We have not at this time checked the contactor but will do so in the next few days as that part of the starter is still on site.
I will get back to this and give you the end result when we get some more information and draw our final conclusion regarding this matter.
I felt that this is worth writing about because in almost 30 years, we have never seen this before.