I have a 92 polo and have just had to change the alternator brushes.
One brush has about 10mm left on it but the other only 3/4mm any ideas why they might be wearing unevenly??
Cheers
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Different spring pressure holding the brushes ontothe commutator
Was the commutator ok and not scored to deep ?
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The reason alternator brushes wear unevenly is due to electrical erosion when sparking occurs. When a spark occurs, the negative electrode will erode as the current flows through the ionised gas. On an alternator, this means that the slipring associated with the diode triplet (warning lamp) connection, and the brush associated with the field connection will wear more.
What you have found is entirely normal, but some machines (Valeo for example) which have very small rings will erode the positive ring through before the brushes are worn out, effectively making the machine uneconomic to repair. Bosch machines have bigger rings, and last longer.
659.
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Belt too tight can contribute too uneven wear. An older alternator will complain if the belt is too tight by making a zoop! zoop! zoop! noise on idle.
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An overtight belt merely wears prematurely the drive end bearing. This has no effect on the sliprings which are at the other end of the machine.
659.
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Thanks for the replies.
Its a bosch alternator. Someone mentioned that there may be a flat spot on one of the rings. Would this cause it to spark and so wear down one brush more than the other.
If this were the case are the rings easy to replace or best left to a garage to sort out?
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If there was a flat spot on the commutator (ring)then this would affect the wear rate of all the brushes
Many things affect carbon brush wear rate:oil injestion,atmospheric air pressure, ambient temperature,current loading,scored commutators,spring tension and commutators being out of balance.
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The reason why the brushes wear unevenly on an alternator is because each brush operates at a different radius on the slip ring, hence one covers more area per revolution.
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Bikemade3 is right the springs determine the pressure of the brush to commutator.even if the commutator is oval wear would be the same/similar on both brushes.would be inclined to look at spring pressure?
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May I point out that an alternator does not have a commutator.
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And perhaps I should point out that the Bosch machines have sliprings of equal initial radius. It was only on the less than ideal early Lucas macines that concentric face type rings were used.
659.
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Sorry for some reason I thought of starter motor.we all make mistakes.didn`t read properly.
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If as "lauriew" says that a alternator does not have a commutator( slip ring ) then there would be no need for brushes as the rotor would have to be permentantly magnetised to induce EMF in the fixed stator.
Simply put if there are brushes "that wear" they either connect to the commutator to pickoff the induced EMF in the rotor
OR
They supply voltage to the rotor in order to create a magnetic field which when turned at speed induces voltage in the fixed stator.
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bikemade3
Alternators have two slip rings at the rear of the rotor. One is at the centre, the other a short distance out. Not surprisingly, the outer one wears the brush more than the inner. Seen this several times.
Rgards
John S
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Bosch machines and most others have always had two cylindrical sliprings of equal radius. The face type sliprings of unequal radius were an old Lucas outrage.
659.
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For Clarification: A commutator is basically a rotary switching device found in motors and some generators and old style dynamos. Starter motors have them to switch the poles on the main stator. Slip rings do not switch they provide a continuos connection through a rotating device. These can be radial or annular slip rings Annular slip rings wear at a farely even rate whereas the radial slip rings wear unevenly in proportion to the circumference of the two slip rings. This is often two to one or greater, add to this the fact that the debris from the inner brush contaminates the outer brush then the outer wears even faster in addition the outer brush runs hotter due to the tip speed and this increases wear. Less and less places stock regulators these days never mind replacement brushes. I hope that clarifies the differences for you. Regards Peter
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Hi Tim,
Are these the brushes on the original alternator? How many miles has the car done?
I ask since I also have a 92 Polo, 61,0000 miles, and am wondering how long the alternator brushes will last.
Thanks!
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Change them in a polo at 92k due to flickering charge light, car now dome 120+ miles. If you have nothing better to do take them out, unsolder the braided cable connections and swop them round, car will co 120/130K with the original brushes. Lot of effort though. Regards Peter
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Thanks for your replies
At least I now know all about commutators!!
The brushes were on the original alternator and I would think they were the orignal brushes.
The car has done 133k.
The battery light came on on the way home from work one night, messed about with the battery connections thought I had sorted it out but the next day it just died!
Had a look in the Haynes manual for reasons that this may happen, nothing, did the next best thing asked my dad. He suggested looking at the brushes, and so I got a new set at the local garage for £15. Took ten mintues to fit and everthing has been alrite since (touch wood!!)
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