uneven brushwear in permanent magnet dc motors

Don't know if I'm right, as I'm not much of a motor expert. (I'm a believer in Lenz's law: "If it rotates it sucks" ....that would be my classmate Dave Lenz rather than the other more famous one!)

My guess would be that the effect is electro-chemical. Are you familiar with those metal cutting machines that use an arc between a moving wire and a chunk of metal to do machining? What in hell are they called? Electro-something I believe. Anyway those machines can cut right through very hard metals. They are used to make dies and the like.

Point is that this electro-etching process is polarity sensitive. So I presume the same thing is going on with your brushes. The sparking occurring there is etching the brush material and the process works better with one polarity than the other. Hence one brush wears faster.

Benj

The wear on the brush is related to the current that goes through it. The current is directly proportional to the mechanical load on the shaft of the motor. The best way to set the brushes is:

1: Load the motor on a stand, using a dynamometer (or similar) and reading the amps the motor pulls . . .

1. simultaneously, rotate the brush holder around the commutator, such as to get the minimum arching in the brushes . . . I assume because of the use of high acceleration, etc. . . . it will be good to pull a good 120-150% of the rated current.

Needless to say that this test should be done quickly and properly to not damage the motor!!

P.S. I don't know if you can adjust the brushes around the commutator!!!! if you can not, then there is noting you can do. Unless you can reduce the acceleration rate in the motor control.

Thanks Benj..I believe the term you were looking for is "EDM". The arc melts the metal as the wire moves by. I agree that the effect has both mechanical wear characteristics as well as some polarity sensitive component, hence the faster wear rate on one side.

Thanks for the reply Al. I can't rotate the brushes in these motors and timing is something we want to preserve. We do measure and equalize the spring pressure so methinks that while pressure contributes to wear, the wear imbalance I am seeing is greater than the spring imbalance and always on the negative side of the motor. The imbalance does not seem to be a function of brush compound either.

Ya, I was suspecting that the brush holder is fixed on your motors. Manufacturers will provide that for large expensife DC motors, etc. Your brush position vis-a-vis the commutator is probably selected for 50-80% load current. But, during acceletion, your motor pulls

150-200% rated current. . . . which ware the brushes faster, etc.

You could machine the commutator to a perfect smooth surface, like a mirror!! and use a very fine sand paper to have the brushes perfectly embrace the commutator arc surface; and again, adjust the brush pressure to obtain the minimum sparks . . . . .

It sounds you got a nice toy.!!

>

Hi Al..yep..got the tools for the toys...diamond bit comm lathe and brush seating tools...everything super true, round and smooth. Need to be for 80K plus RPM. Thanks for all the ideas.

Generally polarity is not a factor at a brush contact. With proper brush design and positioning- there should be no arcing-partly as affected by brush design, material pressure and positioning. When arcing occurs, then there will be problems and extra wear. In heavily loaded motor, the demagnetising effect of armature reactance can shift the neutral axis* causing arcing. In large motors there is compensation in the form of interpole windings but in small motors these are too expensive.

*The brush shorts a coil in the armature- ideally only while the current in that coil changes polarity during commutation. Arcing usually indicates that the brush is not properly positioned so that the field induced voltage is not 0 during this period. Arcing can also be load dependent in that loads can shift the "neutral axis" due to armature reaction which distorts the field. This isn't restricted to the negative brush. If this is a factor with all motors produced by a given manufacturer- it seems that this should be a problem that the manufacturer should address. Possibly a slightly wider brush may be all that is needed. --

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

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Hi Don..thanks for shedding some more information. I do have another question with regards to your explaination. Does the fact that the motors are not "neutral" (zero) timed contribrute to the arcing? They are timed anywhere from 20 to 45 degrees. We do take care to center and size the brush hoods so that the centerline of the hoods are perdendicular to the armature shaft and pass through the center of the armature. Thanks again...

---------- The zero position refers to the position of the coil which is undergoing commutation- It should be on the neutral axis- half way between poles but shifted somewhat by armature reaction. If the coil under commutation is not at this neutral point, there will be a greater tendency to arc. Commutation involves shorting the coil at the time that there is no speed voltage induced in the coil-ideally half way between field poles. during commutation, the brush shorts the coil (not the winding but a single coil) while the current in that coil reverses. If done correctly, the reversal is complete when the brush leaves the coil. If not, there will be arcing. The resistance of the brush is of importance as this resistance controls the rate at which the current switches. Under load, unless there is compensation, unlikely in small motors, there can be a shifthing of the neutral axis due to armature reaction distorting the field flux and this can lead to arcing- sometimes very serious. Do you have a lot of arcing under load? Can you shift the brushes position at all? Unfortunately the position of the commutator bars related to this coil and the brushes may not be physically in line with this neutral axis so that is no indication. You do want the brushes to be in smooth and full contact with the commutator but wear tends to do this naturally. What you are doing with the brush hoods etc seems to be correct.

Hi Don...good stuff, thanks. The motors I'm dealing with are three coil, three pole so I am thinking if there is any advance to the timing then the neutral will be off? I do see minor arcing and it does vary from motor to motor. There are some motor style that allow the endbell to be rotated to increase the timing for more rpm, but not something I generally do. I appreciate the explainations..thanks again.