DC motor brush question

[Clayton79]

what would be the consequences of a Carbon brush sticking in its holder in a 150hp motor??

What I think would happen is that the brush would wear, and then start to spark with and air gap between itself and the commutator leading to rapid and destructive failure of the motor/commutator.

does that sound right?
thanks

 

[kwired]

what would be the consequences of a Carbon brush sticking in its holder in a 150hp motor??

What I think would happen is that the brush would wear, and then start to spark with and air gap between itself and the commutator leading to rapid and destructive failure of the motor/commutator.

does that sound right?
thanks

That is probably worst case scenario, best case scenario is it wears, sparks because of air gap but does not do so for very long before shutting off. Next time it attempts to start there is no contact because of air gap and it fails to start with minimal damage. I would think the worst case scenario has a much higher probability though.

 

[Besoeker]

That is probably worst case scenario, best case scenario is it wears, sparks because of air gap but does not do so for very long before shutting off. Next time it attempts to start there is no contact because of air gap and it fails to start with minimal damage. I would think the worst case scenario has a much higher probability though.

As you are probably aware, my background is in variable speed drives and I've been doing it since before the Dead Sea reported sick.
Initially, it was on DC drives mostly with shunt or compound wound motors.
I've seen one or two commutation problems, sometimes on very large machines. It can be quite spectacular and may a bit frightening when the arc goes all the way round the com.
That said, I don't recall one case where the commutator or motor was destroyed. The commutator can usually be dressed and brushes/brush holders replaced. DC motors are usually pretty robust.

A point to add.
Brushes can and do wear and stick. In most of the places where we have DC drives, typically paper mills, there is a preventive maintenance regime with regular periodic inspections of motor brushes.

 

[YoungElectricalEng]

A point to add.
Brushes can and do wear and stick. In most of the places where we have DC drives, typically paper mills, there is a preventive maintenance regime with regular periodic inspections of motor brushes.

As someone who developed the 6 week DC motor brush pm's for the E&I guys at a paper mill, we regularly had brushes stick in the holders, spark like the 4th of July, fall out... Besoeker is right. These things are robust. In three years the only motors we had to changeout for com damage were because of a stall. That's not to say we changed all of the brushes every six weeks, but checked/documented brush length and changed out as required.

 

[Clayton79]

I work at a rubber mill, large 150hp dc 500volt 245 amp G.E. Motor, installed around 1967. I was born in 79.
so my regular pm, checking brushes and the like, i notice the commutator is shaped like a horse shoe, the brushes had finally eaten down about 1/8 inch? or so. So I called in a Motor shop Tech, cause i want a real experienced opinion. He's like yep needs worked on. no problem, we shut down the line, send it out, get it back, and 45min into running it starts shooting green (brass/copper) flames out of the vents. the operators obviously shut it down. we tear it out sent it backout, today it comes back cleaned up again. exact same senario except im standing next to the disconnect this time ( i was away the first time) about 40-45 min into running the motor I hear it wind up like a turbo motor and as im pulling the disconnect i see the aforementioned green spark.

I have tach feedback checked and doublechecked:?, any ideas? the drive is a 590 series eurotherm, absolutley no alarms. it has field loss enabled, feedback loss enabled, overspeed enabled. the only thing i regret is I didnt have the presence of mind to look at the armature amps as i was pulling the disconnect.

any ideas, conjectures, theorums, things to check would be greatly appreciated, the main motor controls guy from the motor shop is coming out tomorrow. hopefully he can diagnose this.:happysad:

 

[Besoeker]

I work at a rubber mill, large 150hp dc 500volt 245 amp G.E. Motor, installed around 1967. I was born in 79.
so my regular pm, checking brushes and the like, i notice the commutator is shaped like a horse shoe, the brushes had finally eaten down about 1/8 inch? or so. So I called in a Motor shop Tech, cause i want a real experienced opinion. He's like yep needs worked on. no problem, we shut down the line, send it out, get it back, and 45min into running it starts shooting green (brass/copper) flames out of the vents. the operators obviously shut it down. we tear it out sent it backout, today it comes back cleaned up again. exact same senario except im standing next to the disconnect this time ( i was away the first time) about 40-45 min into running the motor I hear it wind up like a turbo motor and as im pulling the disconnect i see the aforementioned green spark.

I have tach feedback checked and doublechecked:?, any ideas? the drive is a 590 series eurotherm, absolutley no alarms. it has field loss enabled, feedback loss enabled, overspeed enabled. the only thing i regret is I didnt have the presence of mind to look at the armature amps as i was pulling the disconnect.

any ideas, conjectures, theorums, things to check would be greatly appreciated, the main motor controls guy from the motor shop is coming out tomorrow. hopefully he can diagnose this.:happysad:

You say the motor winds up like a turbo. Do you mean it actually gets faster? Given the age of the motor, I'm guessing that the speed feedback is from an analogue tachogenerator. If its coupling was slipping the motor would speed up and may be going into overspeed without the overspeed alarm being triggered. That would explain the "winds up like a turbo" thing.
I'm not so sure it would explain the nasty sparking though.

A few other thoughts come to mind in no particular order.
When the motor was re-assembled after refurbishment, were the internal connections correct? Wrong interpoles would not be good.
It used to quite common for motors, particularly non-reversing mill motors to be slightly compounded. That is they had a field in series with the armature as well as the shunt field.
Has the drive been looked at? I'm assuming it is a six-pulse unit (six SCRs). I've known missing pulses to cause poor to very poor commutation.

But, given the sequence of events, I'm inclined think the problem is in the motor. Can you check both armature voltage and current if the same problem is observed again after the motor guy has looked at it?

 

[Clayton79]

:thumbsup: Besoeker,

Wow, with the little info I gave you you did hit it. The resolution of the issue came about today. this is a stabilized shunt wound motor. However, the series field is not used, making it more of a straight shunt motor, but as you mentioned it has interpoles. the interpole wires were put back on reversed. so as a result, the neutral plane of the commutator shifted(When under heavy load, not under light load), causing great sparking and over heating of the comm and brushes and brush holders.

i also believe but would like your opinion as the interpoles are wired in series with the armature and the series field is not being used its kind of like a small series winding and if it were reversed and under load it could act like a differentially wired compound motor and actually oppose the shunt field winding causing field weakening and therefore explain the runaway situation.

Thanks again for your replies and knowledge

 

[Besoeker]

:thumbsup: Besoeker,

Wow, with the little info I gave you you did hit it. The resolution of the issue came about today. this is a stabilized shunt wound motor. However, the series field is not used, making it more of a straight shunt motor, but as you mentioned it has interpoles. the interpole wires were put back on reversed. so as a result, the neutral plane of the commutator shifted(When under heavy load, not under light load), causing great sparking and over heating of the comm and brushes and brush holders.

i also believe but would like your opinion as the interpoles are wired in series with the armature and the series field is not being used its kind of like a small series winding and if it were reversed and under load it could act like a differentially wired compound motor and actually oppose the shunt field winding causing field weakening and therefore explain the runaway situation.

Thanks again for your replies and knowledge

Thank you for your kind words, Clayton.
I'm pleased that you got a resolution.

To your question.....
I don't think that the incorrectly connected interpoles would have resulted in a runaway condition. If the DC has speed feedback that ought to prevent overspeed.

 

[kwired]

:thumbsup: Besoeker,

Wow, with the little info I gave you you did hit it. The resolution of the issue came about today. this is a stabilized shunt wound motor. However, the series field is not used, making it more of a straight shunt motor, but as you mentioned it has interpoles. the interpole wires were put back on reversed. so as a result, the neutral plane of the commutator shifted(When under heavy load, not under light load), causing great sparking and over heating of the comm and brushes and brush holders.

i also believe but would like your opinion as the interpoles are wired in series with the armature and the series field is not being used its kind of like a small series winding and if it were reversed and under load it could act like a differentially wired compound motor and actually oppose the shunt field winding causing field weakening and therefore explain the runaway situation.

Thanks again for your replies and knowledge

Thank you for your kind words, Clayton.
I'm pleased that you got a resolution.

To your question.....
I don't think that the incorrectly connected interpoles would have resulted in a runaway condition. If the DC has speed feedback that ought to prevent overspeed.

Don't forget to send him a bill now

 

[Besoeker]

Don't forget to send him a bill now

It's reward enough to know that I may have helped even in a small way.

Slightly off the original topic....
We have kit installed in many places in the world and we, and mostly I, provide support at distance for many of the systems by phone or email.
One such example was for some high current, low ripple, rectifiers for a telecoms application - the electronic control for a controlled rectifier is pretty much the same as for a DC drive. Tenuous link, I know.
Anyway, these particular rectifiers, a set of three 1200A units connected in parallel, went to Bermuda. Quite a long time ago. I don't exactly remember when but I know it was prior to 1990. Pre-email of course.

About two or three years ago, I got an email from the plant engineer about a problem with one of them.
There followed an exchange of numerous emails over a period of a couple of weeks and the problem got resolved. At no charge to the customer.
Shame in a way. I would have pulled rank to have made a trip there.

On the other hand, I'm happy to leave Arkhangelsk and Mumbai to the service guys.