DC MOTOR CARBON DUST 5

[rloj]

Our crane drives are equipped with GE, 75 HP, 480 volt DC, TENV motors. The cranes are in a covered outdoor environment. The motors are meggered annually. When ever a low megger reading is found it is almost always due to carbon dust build up in the housing. A through cleaning of the interior of the motor will bring the megger readings back to an acceptable value. We have experienced no other serious problems with these motors. Due to low usage a set of brushes will last many years. Some of these motors have lasted 10+ years before they are over hauled. Our maintenance crew is telling me the dust build up is normal for a TENV motor. An electrical engineer is telling me this is not normal and it is an indication of a serious flashover condition. He is proposing an expensive modification to the motor control circuits. I am an ME and I am getting over my head with this issue. My management is expecting a recommendation from me. I have several questions. Is the carbon dust build up normal for a TENV DC motor? Are there any other conditions to look for that would indicate a flashover problem? Would a serious flashover problem soon destroy the motors?

 

[Skogsgurra]

The brushes wear when used. The dust is a result of the wear. Since the motors are TENV, there is no way out for the dust. So it accumulates in the motor. Carbon dust lowers insulation resistance.

Cleaning is part of the maintenance process and if that is all that is needed to bring back insulation to an acceptable level then there is nothing more to it.

Why is your EE saying that flash-overs happen? Does he see traces of flash-overs on the commutator surface? Is the brush wear extraordinarily high? Does the crane trip and/or blow fuses repeatedly? Are the brush edges rough or broken?

If none of these apply then you shouldn't worry.

Gunnar Englund

http://www.gke.org

 

[powerjunx]

rloj,

"An electrical engineer is telling me this is not normal and it is an indication of a serious flashover condition. He is proposing an expensive modification to the motor control circuits" - The need for "modification of motor control" is NOT an issue when you consider the carbon dust accumulation that lowers the winding insulation. It's just a simple preventive maintenance activities to ease dust build up. Even, the motor is TENV or ODP, or any enclosure - a maintenance is still recommended, to get rid of this dust.

Your EE pointed out "FlashOver," I think - if theres no tripping or interuption of the Control protection - there' s no way to worry about it.

The findings that your EE have is ,probably, a commutator discoloration. Since the machine is intended for its purpose(crane), there is a frequent starting while carbon brushes and commutator induces heavy sparking. Sparking sometimes "misinterpreted as flashover"(i myself before). If, however, after starting a severe sparking occurs then you have a problem on your machine. A simple trick, by just visual inspection to the commutator you can tell and point out the roots!, Tell it to your EE...

 

[MikeHalloran]

Tell management they should spend a little more money and get an older EE.



Mike Halloran
Pembroke Pines, FL, USA

 

[stardelta]

Carbon dust in DC machines presnts problems other than lowering the insulation resistance of wound parts. A heavy build up can cause Tracking and Earth-paths on non-conductive parts such as brushgear supports and terminal blocks. Maybe this is what is concerning the EE.

 

[powerjunx]

rloj,

stardelta, EE concern:
"An electrical engineer is telling me this is not normal and it is an indication of a serious flashover condition."

Take note, EE pointed out INDICATION in his statement. Dust build up is ruled out. Thus, a flashover had occur.

Im pretty sure of what EE refering to, "..indication of a serious flashover condition.", is the traces of SPARKING that is a discoloration of commutator surface due to frequent starting of the machine.

If you try to analize assuming no abnormalities on the control, carbon brush and commutator contacts were the only source of sparking often misinterpreted as FLASHOVER.

Furthermore part of field, interpole, compensating, and series winding's overhang on commutator side will also depicts discoloration since this winding overhang were close proximity to the brush-commutator. This might EE also refering to. And no way to worry.

By the way, rloj, why EE is proposing an expensive modification to the motor control circuits??

 

[waross]

Gentlemen, we seem to be loosing sight of the original post.

The motors are meggered annually. When ever a low megger reading is found it is almost always due to carbon dust build up in the housing. A through cleaning of the interior of the motor will bring the megger readings back to an acceptable value. We have experienced no other serious problems with these motors.

I agree with MikeHalloran. The motors are fine. Overhaul the EE.

 

[rloj]

I wish to thank everybody for their responses. They have been very helpful. This is truly a wonderful web site. From what I have heard from all of you and been able to find out on my own on Google, it appears what we may be experiencing is sparking vice flashover. If our motors are experiencing flashover it would be evident by damaged brushes (only one case in 25+ years), pitting on the commutator (none), and accelerated brush wear (none). We rarely blow fuses or trip breakers; in fact, I cannot remember the last time either has happened.

fcybil – To eliminate the flashover. What he is proposing is design and installation of an anti-flashover circuit and replacement of our Emergency Dynamic Lower resistors.

 

[waross]

The comutator turns. The brush rubs the comutator. Dust is produced. You clean the dust as needed, and you do not have flashovers. The EE is going to design a circuit to prevent nonexistent flashovers. This is not the person I would trust to change someting as important as the Emergency Dynamic Lower Resistors.
Try to find a power factor problem or a harmonic problem to divert him.
respectfully

 

[Skogsgurra]

Bill,

Hehe!

Gunnar Englund

http://www.gke.org

 

[fflinders]

Normally, if a flashover occurs there is a continous arc between brush holders. This will cause melting of the brush holder metal and damage the brushes/pigtales ect (pigtales may be severed). The commutator riser and vee ring creepage bands are also often burnt. Once a flashover occurs very often further flashovers will occur due to tracking and electrical breakdown across the commutator segments very often in the riser area. A flashover will almost certainly initiate a protection operation since essentially it is a short circuit on the power supply. In many cases it will also result in an earth fault.

It sounds like you have simply got carnon contamination from the carbon brushes. If the frequency of cleaning the carbon dsut out is an issue you could try experimenting with different grade brushes. However I caution that changing one parameter of a brush such as hardness to improve one aspect often casues deterioration of performance in other respects. Never the less it may still be worth getting some advice from your carbon brush supplier.

 

[powerjunx]

fflinders, you went quite far from the post. Consider this statement from rloj;
"...Due to low usage a set of brushes will last many years. Some of these motors have lasted 10+ years before they are over hauled."
When overhauling a motor beyond its specific maintenance routine/or schedule, surely a dust build up will occupy to machine's internal parts. Thus, Carbon Brush grade is not an issue here. Besides, i had no negative comment to GE DC motor carbon brush grade, it's durable and good quality unless replaced with other brand/or manufacturer.

rloj,im quite busy last few days.Nice to be back here!

"...To eliminate the flashover. What he is proposing is design and installation of an anti-flashover circuit and replacement of our Emergency Dynamic Lower resistors."

Flashover is nonexistent phenomenon,your EE misinterpreted SPARKING as Flashover. Since your EE is proposing a anti-flashover circuit in lieu to Emergency dynamic lowering resistor. For me,the proposal is not feasible since sparking is normal while the machine is on starting duration and it will lessen to allowable minimum spark upon it reaches to running condition. The steps of resistors will cater the starting and running sequence. Hence, a sparking is reduce and starting current as well. Further sparking while on running condition, definitely it will direct you to check the machine for the abnormalities.

For the anti-flashover circuit,i think your EE is ideally experimental person who likes an adventure with ineffective cost proposal. Anyway, i like adventure! Your EE is aggresive.

rloj,i pressume that if the machine runs on heavy or severe sparking on commutator that is why your EE had proposed to prevent it.In order to divert your EE, Propose to him to investigate the machines' parameters like incorrect NEUTRAL POINT setting of carbon brushes, unmatched brush grade, wrong polarity of poles (if disassembled) because this factors has great factors to cause sparking.

Furthermore,tell him that interpole winding of the machine itself will prevent sparking since this windings are made intended to. Provided that other parameters are all good condition. No need to propose other circuit, as proposed.


Could you tell us how's the machine condition at this time?