In the last year I have replaced the traction motor again. This was necessary because the motor shorted out between commutator and chassis. Shorting is caused by carbon dust from the brushes being blown through the armature. High speed causes this dust to build up into a conductive layer inside the armature (you cant see it without a boroscope). In winter moisture from the air adds to the problem increasing its conductivity, eventually there is enough current to cause an arc. The arc destroys the enamel insulation and a fire starts inside the armature.
Liquid copper then shoots out and makes contact with the field winding which is NOT protected. The field winding supply is not isolated from the armature supply so damaging current can flow between the two circuits, this destroys both the motor and speed controller....
This is a post mortem video.
This view is of the flashover region.
The DVM shows low resistance to chassis.
Carbon dust coming from armature interior.
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