Synchronous Generator Keeps Stopping

My level of education re generators is limited to DIY home brew types. I was going to point the finger at the stator, but you checked that out.

The generator you wish to trouble shoot, you describe it as braking and tripping a temperature fault? Assuming that is the case (and assuming you are not using permanent magnets-if you were, got any spare ones, lol), I would examine the frequency and voltage source fed to the synchronous generator used for the sole purpose of excitation of the salient rotor. I assume here, your field winding is on the rotor and the armature winding is on the stator.

It is possible (my uneducated answer) a frequency or voltage shift may have occured at your source fed to your generator, causing the heat and braking effect you describe.

If it was still electrically "loaded" in some way, this would work as a brake......even a (partial) short on the output would have the same effect.

A short would maybe explain the over temp as well......

Did you check for earth/ground faults?

Just my 2 (Euro) Cents worth!!!

Sounds like you have checked all the obvious, In order to stop that much rotational energy in that short a period, there must have been a lot of energy dissipated somewhere, either electrical or hydraulic. (no brakes or damage just about eliminates the mechanical.)

The temperature fault might be a clue. I assume it's from a number of imbedded temperature sensors on a common alarm/trip. If there was field excitation present during the coast down and a possible shorted turn condition in the stator windings, it could cause a hot spot and the shorted turns would absorb energy and create a braking action,

Shorts of this type do not generally heal unless the short point contact is still present and the shorted turn has burned open during the coast down. I'm afraid your problem may come back to haunt you again, (just some thoughts) .

Your horizontal shaft configuration and machine size indicates to me that this may be a low head "Peltton" runner, is that true.

Snakers