Yes it is possible to have dirty slip rings on the FEILD ARMATURE (Not Stator he has the terms reversed) ,I used too have a burnishing tool that I used too clean the commutators or slip rings on motors, when servicing machinery,
And some times after setting for long periods unused the brushes stick in the holder and do not make good contact with the slip rings , then there is no power being applied to the armature field windings, feild voltage is in the 24v to 30v range to produce 120v in the output stator windings,
To damage the stator windings takes extended overload on the generator (usually during hot weather) that heats up the generator without drawing over load current that trips the breakers on the generator, that or a sudden massive overload like a dead short, or a crossed circuit with shore power, failed stators without this kind of crisis are very rare,
Stuck brushes, dirty slip rings, or loss of armature magnetism are 2,3,4 causes after #1 failed control board,
With Onan gensets if there is no 120v output the control board will not stay in run mode,
On occasion i have run into the situation where the armature has lost its residual magnetism (needed to start the process) and the generator needs to have this restored, i did this using a corded 120v electric drill,
The procedure is simple
Turn off all circuit breakers except for one that is for the outside or compartment duplex outlets
Turn on circuit breaker for external outside or compartment duplex outlet, make sure there is nothing else plugged into that circuit , plug in the drill (there is no power, it will not run) ,
Now hold the drills on switch on , use your other hand to spin the drill manually do this for several minutes non stop
Explanation
The drill is a permanent magnet motor, it will act like a DC generator,
Turning it by hand will output a low voltage DC current back thru the power cord and into the generator windings, sometimes this is enough apply a small magnet field allowing the genset to start producing power and eliminating the need to replace the armature which is the field windings,
So after spinning the drills motor several minutes, unplug the drill set it aside , and try starting the generator,
Unless i have my memories crossed ,
Maybe it should be noted
That in an AC generator such as this the field windings are in the armature which is turning (this can sometimes corrode)
The output Stator windings are the fixed windings around the the inside of generator head/housing, " they do not corrode "
In an electric induction motor, these 2 items are reversed the feild windings used to induce the armature to turn, the feild windings are the fixed ones and power is applied, the rotating magnetic field in housing fixed windings, causes the armature to turn and chase/follow the moving feild
Good luck
Clean the slip rings, check brush movent and contact , you can check b armature winding using a multimeter on ohms setting, test at the brushes and at the slip rings if possible, and infinite high reading is a problem , a low ohms continuity reading is good,
With generator running or held in start to stay running, you can check for feild voltage at the brush holders, DC volts between 20 to 30, No feild voltage means bad control board, proper voltage but no 120v output means no brush contact, or bad armature feild windings,
Or last case least likely damaged open output "stator" windings,
Nomeclacture:
Armature the part that turns
Stator the part that dies not turn
Feild windings
In a motor that is the stator
In a generator that is the armature
The feild can be stator or armature depending on if the device is a motor or a generator,
With a motor the feild is used to induce the turning off the armature shaft,
With a generator the turning of feild armature by a mechanical force, gas engine thus rotating the magnetic field,
Is used to create voltage in the fixed stator output winding,
In the typical electric drill with brushes and a "Commutator" the armature is energized with the power needed to attract the winding segments toward the magnets, making the armature turn, it works differently than your box fan induction motor which is the inverse of your generator, a Commutator is more complicated and troublesome than your box fan induction motor, but the drill uses this design because it produces more torque than a simple induction motor, and the drill needs this torque,
I hope i have not confused the issue. But i do believe your son's friend used the wrong term when he said failed stator, I'm sure he meant failed feild armature, but from what you posted , i don't think that is the the case,
