The neutral connection is probably providing ground current detection.
What are the primary voltages?
Line to line and line to neutral?
It is possible to have equal line to neutral voltages along with unequal line to line to line voltages. This is quite common in long rural lines with mostly...
We always used Spray on Electrical Insulating Varnish on all ground connections.
The bolt corrosion appears to be atmospheric corrosion.
Try Stainless steel bolts.
The most likely cause, but not the only possibility:
Someone servicing the brushes accidentally caused a short to ground from one of the brush holders.
Yes. Common practice and a requirement of most codes.
Yes.
First at the motor. Note, there may be an alternate ground path if the motor is mounted on steel connected to structural steel.
If the tech's have a DC clamp meter, measure the current in the PE conductor.
A long shot, disconnect the...
Does that make the commutator burning a design issue?
Rule of thumb for sparking/arcing at the commutator.
Move the brushes to cover the sparks.
So when we cover those sparks may we expect sparking at every second alternate bar?
Or, do we split the difference and go with minimum sparking at each...
Yes, general causes of sparking but mostly not applicable to sparking at every second bar.
Look for something inducing an AC ripple on the generated DC.
The first thing that comes to mind is a mismatch between rotor slots and field width and or spacing.
Energize the field with 120 VAC.
The voltage at the brushes should be zero or very close to zero.
Slowly turn the rotor while monitoring the armature voltage.
Any variation of the voltage at the...
I suspect either a ground fault somewhere,
OR
Someone has grounded the neutral in one or more locations.
Look for the missing current on the protective earth conductor.
If you find the missing Amps on the PE, then try to follow the current back to either the drive or the motor, or possibly both...
One solution may be to run both bus to a common equi-distant location and from there together back to the transformer.
Old text books, 80 to 100 or more, described a wireless reactor used for transformer impedance matching and load sharing on 110/220 Volt secondaries.
Transformer life was 20...
This is more of an anecdote that you may find amusing more than useful.
The fishing boats in Central America typically have 32 Volt starting systems and 12 Volt radios and navigation.
They Use 24 volt bulbs in the engine room.
The 32 Volts is connected to the engine room lights which are not...
I have seen this many times in Central America,
The missing/blown fuse is feed the distribution circuit.
Wye/delta transformer banks were common.
A wye/delta bank will backfeed into a dead primary phase and try to pickup the upstream load on the missing phase.
The back EMF of a motor will do the...