Vehicle Alternators

Alternator... isn't the clue in the name?
Doesn't it produce AC, which is the rectified through a diode bridge?
Del

That's right, but how does it answer my questions?

Sheesh...
If it's AC the electron flow is alternating, so it is flowing in both directions. thus it is exactly the same for both brushes...
Del

Never mind sheesh, you clearly don't understand how alternators work. The question is about the sliprings and brushes. These supply the rotating field, which uses DC. The stator outputs AC, which is rectified to supply the battery, and also the field via the voltage regulator.

Perhaps you missed the question marks in my original response?
Maybe you should look up how polarity in DC arc welding effects the weld.
I'm done here.
Del

No I didn't, I answered it in #2. You then used impoliteness (sheesh) in #4, and what you said there is wrong, as I explained in #5.

Arc welding might be relevant to my original question, but I doubt it.

Indeed you are right and I was wrong.
But in my defence I'd say that it wouldn't have been hard from my initial post for you to deduce that I thought AC was going through the brushes, and that rendered you response rather unhelpful.
Del

Thanks for that, no hard feelings I like to think I'm also able to admit it when I'm wrong.

In #1 you asked "Doesn't it produce AC, which is then rectified through a diode bridge?". In my #2 I meant to just confirm that it does, but perhaps I should have added at that point that the AC is rectified before it gets to the brushes and field coil.

My observation on the brush wear is that I have noticed the same effect but deduced that the wear was on the brush furthermost from the bearing and therefore less protected from the dust ingested through the alternator acting as fine sand paper which is what is used to bed brushes in.

Not quite.

The rotor in a vehicle alternator is the electromagnet that then induces a voltage in the stator field, the stator produces the AC. The brushes carry only DC to energise the electromagnet

Alternator brushes generally last considerably longer than generator ones due to lower currents and an unsegmented surface to run on.

Most alternators these days are three or even more phases achieved simply by adding more stator poles while still using only two brushes.

I'm thinking you feed DC to the rotor through the slip rings and get AC off the stator.

https://www.picoauto.com/library/training/the-alternator

Thank you, this answers the implied questions in my post, in an both informative and polite manner.
Del

There's also the voltage regulator in the rotor circuit, as below. These are Lucas, where the regulation is in the earth side of the rotor, but I believe some makes do it in the +ve side.

Interesting question. I don't know the answer and Del has a very valid point.

I suspect that uneven spring tension and orientation of the grain pattern in the brush affect wear rates.

Possibly some devices might have wear rates related to polarity, some devices related to spring tension and grain pattern and many devices have stories that get mixed together. Truth is, I don't know.

I never heard of this...It could be a broken spring though, or somebody might have replaced one of the brushes with a shorter or longer brush than the original...it happens

Yes, you wouldn't expect them to wear at exactly the same rate. The claim is there's a consistent difference due to polarity.

This could maybe happen if the regulator went bad....If this was a problem wouldn't the manufacturer make one brush longer than the other to compensate for the wear?

It would be nice to think so, but see my #15. Early Lucas ones had one brush more prone to wear from its position and the polarity effect, if I've got it right.

Of course alternators nowadays AFAIK all have cylindrical sliprings, and life is pretty good. I've done 205000 miles in my car and no alternator problem yet. Not bad for bearing life, apart from brushes and controls. Maybe helped by being controlled via the engine management system.

Hi Codemaster but just to add to the confusion I have a number of alternators salvaged from Japanese cars. Some have cylindrical sliprings and some have concentric slip rings which one would think would cause the outer brush to wear more due to the higher peripheral speed.

As for regulators, some require the battery connection through the lamp to establish initial excitation and unless this connection is made it is game over. Alternators come in all manner of connections, 2 wire, 3 wire 4 wire. I just installed an alternator onto an old BTD8 inter dozer and it needed the lamp circuit to establish initial excitation.

Regulators, gone are the days of discrete components in the box, no now it is all potted and a throw away item but replacements can be bought at a reasonable price or if too expensive then an alternator change becomes cheaper.

The only ones I've seen with concentric sliprings are the early Lucas ones, but it seems there are others. The outer one does wear quicker, I've seen it many times.

Unlike a generator (dynamo) an alternator needs the lamp for initial excitation. When I first read about that I was sceptical, thinking there should be enough residual magnetism to get it going, but when I tried it without, the book was right!

A friend of my brother is doing up a boat with inboard Ford engine, fitted recon alternator but it wouldn't charge, though it had a lamp, presumably typical dashlight 3 watt. I suggested he try a more powerful bulb, eg 21 watt indicator, he did and then it worked. Interesting, haven't come across that before. Told him if he wants the original bulb he could wire a resistor in parallel.

Bro is big into old tractors, earthmovers etc, he has at least 10 of them, International Harvesters, Drotts, Fergy TE20, etc some tracked, some on rubbers. Diesel, petrol/paraffin, petrol/diesel. Most have dynamos, as original spec.

I retrofitted an old tractor with a one wire alternator.For exciter voltage I used an oil switch that is closed on low pressure,open on normal pressure. This provides the starting excitation and then turns off when pressure is up.Works like a charm.`Some one wire alternators require revving the engine up a little high after startup to generate enough excitation voltage.After that,they can be returned to normal speed.

Clever! I suppose that would be a 2-wire oil pressure switch. The ones I've seen have 1 wire, ground via the engine block when oil pressure is low.

Yup! Got to feed + for just a few seconds to energize field.

An even more archaic analogy: Remember which ignition breaker-point contact would pit (other contact would "grow") caused by incorrect capacitor value?

Here's the "Old Mechanics's" Jingle: "...MINUS on the MINUS is a MINUS" meaning a pit on the 'ground' contact meant too little capacitance.

Dude! you are talking about my Shelby's old Ford 12127 dual points distributor.

Shelby used different weight springs to help with that but I found that replacing the points frequently was the best solution until I pulled the entire ignition system and went Mallory electronic.

Electronic ignition! That was a huge improvement in both less maintenance and performance.

Thanks for that, interesting.

The early-ish Lucas alternators (15/16/17 ACR) had face-type sliprings on the end of the shaft, one nearly concentric with the shaft, the other at a radius ~ 12mm. Brushes carbon of course. Unsurprisingly, the outer brush wore quicker, lasted about 80000 miles, at which point you could swap them as a stop-gap.

But the inner brush is the +ve, so from your link the outer gets it from both effects. As far as I can tell, at design stage the polarity could easily have been reversed. Perhaps Lucas weren't aware of the polarity effect at the time - early 1970s.

So you need one type on the positive side and another type on the negative side....case closed...haha

It could be due to microscopic roughness ie stalactites forming on one slip ring, and erosion on the other slip ring, the relative hardness of the brush determining wear. There was a huge study on relay contacts in 1958 by Western Electric since switchboards were all mechanical, very interesting how various metals were combined based on current density and voltage.

Is this really a problem?

I put 220,000 miles on my Ford PU alternator with no problems.

Everything still worked on it when I sold it to a friend.

This brings to mind a problem I have with an old conversion van. Will start a new thread.

I had a client with 306K on her F150 with original alternator. I believe they're made by Visteon/Motorcraft.

I hear very little alternator problems in modern cars. Older cars - yeah. When I was in high school, I had a 1976 Ford Thunderbird with a big Motorcraft alternator - went bad before 100K miles and 5 years.

A little useless information. New cars have computers control the field current, which controls alternator output. I wonder if this is what helps lengthen the life ... or it could be better engineering or build quality.

The one item not touched on is the voltage sensing. Battery sensing allows the regulator to directly measure the critical variable, cell voltage. If measured at the alternator, you have to account for the voltage drop of the wiring, which varies directly with the charging current.

Modern regulators adjust charging according to battery cell temperature, charge state and load. I assume that is why my 2008 Saturn Outlook 195,000 miles still has OEM battery, behind passenger front seat in fully instrumented compartment, current shunt, voltage sensor leads, and temp sensor. (Admittedly North Carolina weather for 90% of life, not much upstate NY or New England in winter )

I noticed that but didn't want to press the point!

I still can't see any obvious reason machine vs battery sensing would affect brush life, apart from what I suggested in original post.

Yes, no effect on brush life, just battery life. Though your theory about rapid changes in excitation increasing brush wear is borne out in industrial environments, dc motor armature brushes spark more, wear more. The voltage regulator could be fast enough to respond to the ripple near the bridge.

You have an incredible battery! I haven't seen a car battery last 11 years and 195K miles. Depending on where in VA you live, it could be from the cooler summers. We have a belief that cold temperatures kill a car battery, but the opposite it what kills it - intense heat. Cold weather will make starting a car harder, but it doesn't kill the battery. If a car has a weak battery, cold weather will bring out the weakness.

Here in CA, we have hot summers. Many cars have a dead battery within the 3 year 36K mile warranty. Even those heavy duty Bosch batteries die before the 4 year 50K mile warranty - not always, but some do.

Battery is not under the hood, in it’s own little box behind passenger seat. Car always starts with maybe less than 1 revolution, original starter motor. Actually closer to SC than VA, a year in KY and a year in Lynchburg VA failed to kill it, nervous as I was....

My E34 BMW has the battery under the rear seat. A big Bosch battery lasts about 4 years.

My E36 and E90 BMW's have their battery in the right rear fender well - behind the wheel well. Let's see, my E90 is a 2009 and the battery was changed 4 years ago. So the original lasted six years. I had a 2006 E90 - I changed the battery, which was an aftermarket Interstate about 3 years ago, so the first and second battery had a ten year combined life.

My 2007 GMC Sierra is on it's second battery. I bought the truck new in Oct 2008 and 5 years ago, I changed the battery, but the truck only had 32K miles.

Thanks to everybody who responded. Seem to have confirmed the effect is real, even if the explanation is not known.

As in welding, the heat and wear comes from electron impact. You drop an electron across a gap and it deposits heat where it hits. The positive side carbon should wear quicker.

That's opposite to what the link in Rixter's #10 says.

I need to correct my misstatement. The brush supplying electrons to the wiring system will wear quickest. The commutator supplies electrons to the negative brush. The negative brush has a positive potential relative to the commutator as the commutator is delivering electrons to the negative brush. The negative brush supplies electrons to the rest of the electrical system.

It is the drop of the electrons from the commutator to the brush that delivers the heat.

You've lost me there!

First, to be a little pedantic, it's not a commutator, it's a pair of sliprings.

The brush supplying electrons to the wiring system will wear quickest. What do you mean by the wiring system, the rotor field coil (between the sliprings) or the circuit that supplies it (stator coils and rectifier)?

The commutator slipring supplies electrons to the negative brush. OK - electron flow from +ve to -ve.

The negative brush has a positive potential relative to the commutator slipring as the commutator slipring is delivering electrons to the negative brush. Not with you. Did you mean The negative brush has a positive negative potential relative to the commutator slipring as the commutator is delivering electrons to the negative brush. ?

The negative brush supplies electrons to the rest of the electrical system. OK if by "the rest of the electrical system" you mean the wire from -ve brush to ground, incorporating the main transistor of the voltage regulator.

I remember some Ford motor starters had two different type brushes.One was carbon compound,and the other one was a copper/carbon compound,and the difference was easily seen. I do not know why they were different,but I will ask my son when I see him.He worked in the lab for a company that designed and manufactured brushes for automotive and other industries. I will get back to you when I have more info.

That will be useful.

One obvious reason, following Rixter's #10, is that one of each gives best of both worlds (assuming starter commutator behaves the same as alternator sliprings). Suggested by SolarEagle in #19.

My son said there were many factors involved in the cause of uneven brush wear,even when brush spring pressure,brush material ,and contact area were all equal.The electron flow direction,capacitance discharge current between the fields and ground.The subject gets very deep,and there is not an agreed upon consensus among experts on all of the causes.Mostly,they accept it,and use different brush compounds to partially compensate.Sorry I could not provide a more precise answer.

I do remember that old cars used a positive ground instead of negative ground to reduce rust of the body. Rust is just a slow oxidation process,so maybe one of the factors in the wear is the oxidation of the brush material. A negative brush would tend to oxidize more readily than a positive one. As I stated previously,there is no single cause of this effect.

From memory, and not having (tried to) go into the electrochemistry of it, I thought the change to -ve ground was to give lower corrosion. But I could easily be wrong.

Exactly MHO too!

Which implies to me that the positive will wear quickest, so stopping the rust on cars with previously positive ground, by switching to a negative ground!

Electroplating also came to mind, where the positive anode gives up the material to be deposited on the negative anode!!

It all starts to agree (IMHO) that with all other points being equal, the positive will wear fastest!

Sorry to be pedantic but I have never heard of a negative anode. The positive is anode and negative is cathode. I agree with what you say but it bothered me to see negative so close to anode.

I loved your comment:-

I do remember that old cars used a positive ground instead of negative ground to reduce rust of the body. Rust is just a slow oxidation process,so maybe one of the factors in the wear is the oxidation of the brush material.

I was going to mention it as well, but you got there first! Though I am personally sure that polarity does play a role, as in electroplating for example! Where metal is taken from the positive side and deposited on the negative side, all other factors being equal of course..

Here is a picture of copper being plated onto a key:-

So IMHO, the positive side will wear quickest.

Regarding positive ground cars having less rust, I believe that's only true if the car body is positive compared to earth and there's a path for the material oxidizing the car from the body to the ground - complete the circuit.

If you're looking at the car body in regard to the car battery, then a positive ground will pull material from the body to the wire or connector that leads to the battery. That amount of material is so miniscule and it's not going to make the body rust.

If the circuit between the car body and the earth ground is completed and the car body is positive, then material will pull from the car body to the earth.

The true reason car bodies rust is a foreign substance that oxidizes the steel. Standing water will do damage, acid rain even more damage and driving through snow that has been salted, then letting it melt in your garage and leave salt water on unprotected sheet metal. That's why those British cars rusted. 1960's Lotus, MG, Jaguar, Triumph, etc. Positive ground and rust buckets!

I found this that may help from Quora:-

Why did cars and trucks in the 1950's switch from positive ground electrical systems to negative ground systems, which we use to this day?

Good question. As best as I can tell, it has to do with galvanic corrosion.

When two metals (e.g. copper wire and a steel automobile body) come in contact in the presence of an electrolyte (e.g. rain water), corrosion will occur, dissolving the anode and depositing it on the cathode. Without the presence of an applied DC voltage between the two metals, the more noble metal will be the cathode, and the other the anode (see galvanic series).

However, when a DC voltage is applied between them, the negatively-charged metal becomes the cathode, and the positively-charged metal the anode. In many applications a DC voltage is applied purely for this purpose (see cathodic protection).

So, in a vehicle where there is a voltage between the wires and the chassis, you have to pick one to be the “sacrificial” anode (i.e. the one to be positively charged).

Up until the 50’s, most wires were covered with natural rubber and cloth insulation, which you may guess did not do well to protect the wires against moisture. So, choosing between frequent electrical faults versus slightly diminished chassis life, many manufacturers opted to protect the wiring. This meant a positive ground vehicle.

Replacing rubber/cloth insulation was PVC. It was developed in the 30’s but didn't start gaining popularity until after WWII. PVC offered a dramatic improvement in the resistance of the wires to moisture, and therefore allowed auto makers to switch to a negative ground, gaining back a little extra longevity on the chassis before it succumbed to the elements.

I've read some competing theories as well, but this one seems the most logical to me. Take it for what it's worth.

Interesting. So a positive ground will make the auto body oxidize - it's the sacrificial anode when DC current is applied.

So a negative ground vehicle is suppose to be better for a car - in theory.

What I've known from growing up in Chicago, is that salted streets are great for melting snow and ice, but horrible for car bodies. The British and Italian cars did horribly in regard to rust. Ditto for the thin sheet metal Honda's, Toyota's and Datsun's of the 70's to late 80's. The domestics had thicker sheet metal, so it would take 3-5 years for the first rust hole to show, then it was downhill fast.

Here's an interesting thing I was told. The best thing to do after driving your car in salted snow/ice was to leave it in the driveway and let it remain cold. The rust wouldn't form as long as the snow/ice didn't melt and become a briny solution. People would park their cars in a "warm" garage and the snow/ice would melt and the briny solution would creep into areas and start rusting the sheet metal. Ziebart, Rusty Jones, etc were companies that would spray rubberized rust protection on the back side of the sheet metal - many times it helped, but sometimes it would allow the briny solution to sit in an unprotected area and then the rust would form very quickly.

Now cars have much better rust protection and rust holes take much longer to form and cars look better for longer. Though the cost of vehicles has gone up tremendously.

Yes, I've heard the theory that it's better from a corrosion standpoint to leave a car outside than in a garage.

I cannot paste or import picture to the forums.What has changed?All of my Firfox add on's are disabled,no add blockers,pop up blockers,etc. It used to allow pictures.What happened?

Maybe not related at all and I Am Not An Electrician, but, here's my 2 cents.

In EDM (Electrical Discharge Machining) there is a big difference in how you set it up. You can actually control where your wear will be (ram work-piece or table work-piece) by changing the direction of DC flow.

Just my thoughts on wear vs DC flow.