Diagnosis of large brushed DC motor

[Dans250cortina]

Hi everyone,

just found these forums and had a look around, there's some great info here!

I usually deal with the run-of-the-mill industrial three phase motors so this is not my strong suit so please bear with me here.

I have a large DC motor here that was brand new but failed after about 2 minutes of run time, manufacturer sent a new one down and told me to scrap the old unit, so i thought I'd take the opportunity to pop it open and see what went wrong, typical burnt out windings but this only raised further questions.

It's a 180VDC 18.5hp motor, commutated with two brushes at 180deg appart, so far pretty standard.
but the damage is what's made me scratch my head a bit. only two sections of the commutator and that portion of the windings is burnt, but the separation of those two sections is only 90 deg apart.

I'm not expecting to fix it or anything, just hoping someone may help me understand how/why

thanks in advance for any input

 

[Asymptotic]

Can you upload photos?
What is burned 90° apart - the commutator face, or the windings?

 

[Dans250cortina]

yep, comm face,
I'll take a few pics and get back to you,
thanks for the speedy reply

 

[Dans250cortina]

sorry for the bad pics,
ill take it back apart when I'm not on the clock :D

closer look, it's the winding as well, burnt clean through where the winding meet the bar, it may be a tad more than 90 deg of separation but definitely not 180.

 

[Asymptotic]

Google the carbon brush manufacturers 'Mersen' and 'Helwig'. Both of them have excellent guides for troubleshooting DC motor problems by examining commutator and brush condition.

Given it survived for only two minutes, if the motor was stalled bar burning would have been under each brush face (180°). With 90° separation, and a burned winding the only thing that comes to mind is the armature was improperly wound at the factory.

Wrote the above before seeing the pictures, but a bad armature winding is still my best guess. In photo #4, seven comm bars are badly burned, and so are the risers. I can't see the brushes well enough, but doubt there are more than 3 or 4 bars under the brush face simultaneously (if 'stall marked', burned bars would match up with brush width).

 

[Dans250cortina]

will do, thanks for the pointers, I got some homework to do tonight!
it's driving a reasonably high inertial load (this was spec'd up by the engineers and an old identical motor ran for years in this application)
the motor is a Baldor, whose a reasonably large manufacturer, I would assume (the root of most problems) this would be factory tested, but it could explain why they didn't question me too much and just sent a replacement fairly quickly with little prompting. some suppliers it's easier to get blood from a stone than lodge a successful warranty claim.