Ones side sends electrons one side receives.

With arc lamps the anode is about 12x as massive to deal with the pounding.

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I suspect a similar situation is going on. (I love how the ± are cut into the contacts)

Anyhow it's definitely a fact of life/physics and the field winding doesn't give a crap which way the current flows. It'll make N-S instead of S-N at the field winding but when it's spinning it doesn't matter at all.


–awr–

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Doesn't it cause phase shift and lag on the alternator or is the optimal phase shift so close to 180 degrees that the change is negligible?

The windings generate A/C which gets converted to DC with the diodes so phase is meaningless.
Electrons flow from negative to positive and take material with them as they leave which is how cathodic protection (pipe lines, boat motors) works ;)

I was talking about the excitation side of things but was too much thinking it as a motor. On exciter side it doesn't matter which of the poles are north and south. This slip ring wearing is just a simple matter of DC arcing.

Alternator has no idea. The phase of the 3 AC phases is shifted but the start point is completly arbitrary and as 80s mentioned the ≈ 400hz three phase AC is rectified to DC.

I wondered for decades how on earth does the car regulate the output and can usually maintain 13.8 ± 0.2v regardless of the RPM or load.

It's actually so simple: there's a direct feedback loop that increases the field winding current if voltage is less than the set/design voltage.

Higher field current, stronger magnetic field, higher 3-phase AC voltage that's rectified.

It's fully automatic and virtually instantaneous.


–awr–

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Clearly, there is a difference in ring wear on this alternator. I don't think it's due to polarity, though. As mentioned, the current flow through each is the same. There is some excitation differences and it is true that slip rings will wear differently, but on a small generator like this, we're talking microns over the 5000 hours of operation an alternator might see. On much larger (multi-KW) generators, slip ring wear is indeed monitored and accounted for. Polarities are switched on a schedule, but the currents are much higher, they're in constant operation and service lives are measured in decades. More than likely, it's that the alternator works in the worst environment imaginable for a brushed machine. It's hot, oily, and dirty all of which will increase brush and slip ring wear on a much more meaningful and noticeable scale.

Preventative replacement of alternator?
 

There's literally only one difference between the two slip rings (polarity), and if big generators switch polarities, there's the answer. As crazy as it seems, it has to be the why.

It’s a bit crazy that 6000 hours will do it in. I've collected dozens of end-of-life arc lamps, and the anode made of over half a pound of tungsten is beat to hell, and there's never wear on the cathode.

If you compare the flux density of the current through the brushes, I suspect that will play into the equation heavily.

I don't know what the current through the field coils is typically, but when the engine is cold and running at idle, I'm sure there's a healthy chunk of the 1.5 to 2.5 kw output going back into the armature, and those brushes have maybe 1/10th square inch contact patch.

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–awr–

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The abridged version of my brush polarity swap.

I measured the difference in the wear and it is about 4x the rate of wear on the one that takes the abuse.

I'm telling you it's not. I've seen it many times in much, much, much larger generators. Increased brush wear or slip ring grooving in between monthly or biannual maintenance periods always attributed to a change in operating conditions or contamination. Flexibrasing or a quick stoning would bring it back where we'd want it and we'd replace the brushes. On DC generators it was more apparent but commutator action is MUCH more aggressive and much more affected by high temperature and contamination. The polarity swap was done on every 36.months on the multi-gigawatt AC generator and 72 months on a moderately sized few hundred-KW AC generator. The larger generator is expected to be in service for 40 years, the smaller one runs continuously and is generally overhauled every eight or nine years. Again, even on these larger generators with much larger current, we'd expect millimeters over their service lives. An alternator's life is too short, and it's just too small for this action to noticeably affect ring wear. Operating conditions have much more of an affect. There's nothing wrong with swapping the polarities, though.

I'm agreeing it's the operating conditions. But it's causing an exponential increase in the only candidate for one side wearing faster than the other.

Next time i have an alternator apart i will confirm which polarity gets the abuse. It's always one side and every alternator it's the same story.


–awr–

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Very little current is needed, a few milliamps in the case of water heaters and underground gas tanks, larger distances of pipeline gets into amps and the current needs to be impressed, alternator field is a few amps so for sure a factor.
I was told in the old days they used to bury an old car beside the pipeline, connect it and impress a bit of current to make sure it was the car giving off the electrons and being sacrificed and the pipe taking them in. ;)

In the big generators there is so much else going on that the transfer is probably not noticed as much.