PATINA PROBLEM IN ETC MOTORS 1

[uppili11]

THERE HAS BEEN A SERIOUS PROBLEM IN DC MOTOR IF IT IS NOT OPERATED IN FULL ROTATION ESPECIALLY IN APPLICATIONS LIKE THROTTLE VALVE CONTROL.DO THE COMMUTATOR TURNING CHARACTERISTICS REALLY MATTER IN AVOIDING THE OXIDATION ISSUE WHICH IS TECHNICALLY CALLED PATINA FORMATION?IF SO WHAT SHOULD BE THE IDEAL SPEC AND HOW TO EVELUATE IN THE LABORATORY RELIABLY?

 

[UKpete]

An oxide layer will form whenever the copper surface is undisturbed for a period of time (minutes). A patina usually refers to the much longer term surface film that has a greenish tinge to it.

With a standard DC commutator motor, or a slip ring machine, the copper wear rate per pass of the brush is extremely low. On copper/carbon current collection systems where the copper surface is only occasionally wiped, such as on the motor you describe or an overhead wire on an electrified railway, the copper wear rate per pass of the carbon is much higher (say 100x) because in the intervening period between brush passes, an oxide layer forms on the copper surface which is eroded by the passage of the carbon.
I have some typical wear rate figures (though not with me) if you want them.

I'm not sure how close I have come to your question. To simulate this type of problem you need a test rig where the brush lifts and lowers, I once operated such a rig.

 

[uppili11]

thanks for the response.we have a test rig to simulate this condition.but the issue is we ned to have a complicated commutator turning spec in terms of PPI,Rz,VERY TIGHT BAR JUMP(LESS THAN 2 MICRONS)which reduces the production efficiency.Tht is why my question is ,is there any method by which we can ovrcome this situation as a motor.
uppili11

 

[UKpete]

uppili11 - I'm don't know what the terms PPI and Rz are. The bar jump, I assume this is the maximum allowed step between adjacent commutator bars. In the past, I have seen specs of max 5microns and 12.7microns max (and anywhere between 0.04mm and 0.18mm max TIR for the whole commutator, depending on the machine size and application).

So your spec of max 2microns is pretty fine. What is the rating of the motor?

 

[uppili11]

PPI IS PEAKS PER INCH AND Rz IS ROUGHNESS SPECIFICATION.THE
SIZE OF THE COMMUTATOR IS 14MM OUTER DIAMETER.THE MOTOR SIZE ITSELF IS PRETTY SMALL(36MM DIAMETER) AND IS NOW WIDLEY USED IN DRIVE BY WIRE SYSTEM OF ADVANCED CARS.SO IS THE CRITICALITY FOR FAILURE-EVEN ONE MOTOR FAILURE WILL RESULT IN BIG CLAIM.WE ARE NOW CONTROLLING THE TURNING SPECIFICATIONS TO OVERCOME THIS PROBLEM.BUT WE ARE NOT SURE.BUT THE LABORATORY TESTS ARE OK

 

[UKpete]

ok, I didn't realize they were that small. The commutators I was referring to were much larger (say 90mm diameter), built up from copper segments, micanite separators and vee-rings rather than moulded.

I don't have experience of commutator operation in limited motion applications. I guess the reason you don't go to a brushless system is cost. One of the problems with larger commutators is that no matter how well you finish the surface (diamond turning, followed by a slight stoning of the surface in some peoples opinion) the profile can soon deteriorate if the conditions aren't right.

 

[jbartos]

Suggestion: Visit

http://www.morgancarbon.com.au/long_life.htm

http://www.sglcarbon.com/sg/pdf/kbuerste_e.pdf

for: patina details
etc. for more info

 

[uppili11]

thnak you so much.the solution comes from brush maker