DC Motor Rapid Brush wear
DC Motor Rapid Brush wear
(OP)
Hi all.
I followed with great interest the previous threads with experiences on selective brush wear on offshore drilling installations - now I've been called to something similar! But in this case, all the brushes wear out very quickly. Last incident; in a space of 5 hours the brushes went from ok to gone (literally) and the motor was lost!
Motors: BBC GMW 400 M32, 746kW, 720Vdc. 7 brushes per arm x 4 arms. Brushes were/are EG236S.
Top mounted water/air heat exchanger with closed loop internal air circuit. Purge air for keeping internal air pressure positive.
Application: mud pump. Two motors mechanically coupled with SCRs in master-slave configuration.
Floating neutral supply to the SCRs.
First incident: Routine inspection, well within the expected life expectancy of the brushes and both motors found to have brushes dangerously low just a few millimetres left. All brushes changed.
Second incident: Month later and Motor 1a all brushes worn down, motor 1b OK
Last incident: Another month gone by, Motor 1b all brushes worn down, motor 1a OK.
Period: All happened within last 3 months but actual running time much less.
Supposedly motor 1a SCR is always the master.
OK, we're going through all the classic reasons now and looking for recent changes to anything and everything but I'd like to ask you all a few questions in the meantime.
1. Anyone else had this kind of problem where all the brushes wear out 'suddenly'?
2. What grade brushes are used elsewhere for this application on these motors?
3. The maintenance guys say they've always had a problem with low insulation readings to ground with all of their mud pump dc motors. Now, the cooled air is blown down at the commutator end, through the motor, up and through the filters and then through the heat exchanger. Is this normal elsewhere? Seems like they are blowing carbon dust into the motors.
I look forward to the replies.
I followed with great interest the previous threads with experiences on selective brush wear on offshore drilling installations - now I've been called to something similar! But in this case, all the brushes wear out very quickly. Last incident; in a space of 5 hours the brushes went from ok to gone (literally) and the motor was lost!
Motors: BBC GMW 400 M32, 746kW, 720Vdc. 7 brushes per arm x 4 arms. Brushes were/are EG236S.
Top mounted water/air heat exchanger with closed loop internal air circuit. Purge air for keeping internal air pressure positive.
Application: mud pump. Two motors mechanically coupled with SCRs in master-slave configuration.
Floating neutral supply to the SCRs.
First incident: Routine inspection, well within the expected life expectancy of the brushes and both motors found to have brushes dangerously low just a few millimetres left. All brushes changed.
Second incident: Month later and Motor 1a all brushes worn down, motor 1b OK
Last incident: Another month gone by, Motor 1b all brushes worn down, motor 1a OK.
Period: All happened within last 3 months but actual running time much less.
Supposedly motor 1a SCR is always the master.
OK, we're going through all the classic reasons now and looking for recent changes to anything and everything but I'd like to ask you all a few questions in the meantime.
1. Anyone else had this kind of problem where all the brushes wear out 'suddenly'?
2. What grade brushes are used elsewhere for this application on these motors?
3. The maintenance guys say they've always had a problem with low insulation readings to ground with all of their mud pump dc motors. Now, the cooled air is blown down at the commutator end, through the motor, up and through the filters and then through the heat exchanger. Is this normal elsewhere? Seems like they are blowing carbon dust into the motors.
I look forward to the replies.





RE: DC Motor Rapid Brush wear
Bill
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"Why not the best?"
Jimmy Carter
RE: DC Motor Rapid Brush wear
Silicone fumes create very abrasive chemical products that kill brushes like emeraud paper.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: DC Motor Rapid Brush wear
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If we learn from our mistakes I'm getting a great education!
RE: DC Motor Rapid Brush wear
The motors are cooled by on top mounted water/air heat exchangers and there is a purge air line from a compressor that injects air into the terminal boxes making them and the rest of the motor positive pressure. The air purge line feeds 4 dc motors in parallel of the 2 mud pumps but mud pump 2 motors a & b have not apparently suffered any rapid brush wear.
RE: DC Motor Rapid Brush wear
RE: DC Motor Rapid Brush wear
Chris
"In this house, we obey the laws of thermodynamics." Homer Simpson
RE: DC Motor Rapid Brush wear
RE: DC Motor Rapid Brush wear
Yes moist and dirty air is a possibility but he stated two motors out of four do show this condition.
RE: DC Motor Rapid Brush wear
RE: DC Motor Rapid Brush wear
RE: DC Motor Rapid Brush wear
Chris
"In this house, we obey the laws of thermodynamics." Homer Simpson
RE: DC Motor Rapid Brush wear
Please see attached.
RE: DC Motor Rapid Brush wear
My dc motors background is in traction motors - these are similarly rated to yours (but not with so many brushes - commutators don't like running with brushes at low current density, the surface film gets worn away). Ventilation is either forced, by an external blower, or self (by an armature mounted centrifugal fan). Air in is always at the commutator end. On the self-ventilated motors the air is drawn in under the train and frequently draws water droplets, dust and leaves through but this doesn't result in abnormal commutator conditions. The usual victim is the winding insulation.
Is your commutator mechanically sound? You should run a clock around it, it shouldn't exceed about 0.2mm TIR, ideally 0.1mm, and 0.01mm bar-to-bar. Sudden steps in commutator profile is a brush killer, the sparking set up there also gives selective wear of the commutator at that point and gets progressively and irreversibly worse until the commutator is re-turned in a lathe. If you measure on and off the brush track, you can determine if you have electrical wear, or mechanical wear respectively.
I dare say that your worn brushes already show evidence of sparking on their trailing edges (soot), but if you look at their sides for wear marks from sliding in the brush boxes, it is a quick indication of whether you have brush/commutator instability.
RE: DC Motor Rapid Brush wear
A complete as possible 'shake-down' of the drives and motors was performed first. A history check also to establish recent changes. And then testing and waveforms checked with everything looking as I would expect. Of possible significance; There was a possible loose connection of the phase-back signal on the back of the regulator rack - these are analogue drives with wire-wrap connections between the cards - and it was either already disconnected or was certainly so when the wire was disturbed a part of the checks. This connection is on the output of the speed regulator; An AC capacitor - part of the snubber circuit - had a very slight sign of a leak in the past and was swapped out but it checked out OK with a meter on the bench.
What was left of the brushes is to be sent for analysis. The guys onboard correctly do not use silicon 'glue' to seal junction boxes covers etc.
My last report from the vessel is that after 5 days everything is still OK with commutators looking good.