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  1. #1
    Look! My Spine! RubenX's Avatar
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    Want to understand car alternators...

    Permanent magnet DC I fully understand... give it power and it turns, turn it and it gives you power. I broke many toys as a kid just to play with these concepts. But I was baffled when I once broke an old fan and found no permanent magnets in it.

    I know understand the AC induction motor (regular household fan)... kinda.

    But something that's still a mystery to me is the car's alternator. There is some kind of induction there too I guess. But when you turn an ac fan you do not get any electricity out of it. So how come it works?

    I've been asking around to people that supposely know the stuff but all I get is that some of the energy produced by the alternator, is used internally to produce it's own magnetic field. Then this magnetic field is used to make electricity.

    I reply that it looks to me like the chicken/egg paradox... what comes first? You need electricity to make the magnetic field so you could generate more electricity. But if you have no electricity to begin with, how you power those coils to get the whole thing going?

    The only thing I could think of is that you use the car's battery to power the coils, create some magnetic field, then when it's turning, it generates some voltage, some of it keeps sustaining the field and the rest is your output. BUT... I remember that as a broke teenager I once used and old mazda for a full month with no battery, pushing the darn thing to get it started (I had good friends back then). So that theory should be wrong.

    Now here I stand, 38 years old (give or take a year), at 3am, googling around, trying to understand the mystery of the car's alternator.

    PS: I'm sure Danny knows this.... lets see if somebody can beat him to a decent explanation.
    "Hoy es un dia normal, pero yo voy a hacerlo intenso" ~ Juanes

  2. #2
    Look! My Spine! RubenX's Avatar
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    Buahaha.... I think I got it...

    *googles "field exitation"*
    *googles "iron core residual magnetism"*
    *goes to sleep*
    "Hoy es un dia normal, pero yo voy a hacerlo intenso" ~ Juanes

  3. #3
    Senior Member ron521's Avatar
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    You seem to have grasped the concept quite well.
    Basically, the stationary FIELD windings (the stator) replaces the permanent magnets which you found in toys.
    The only difference between an automotive GENerator and ALTernator is how the rotating part (the rotor) is arranged.
    A generator has a segmented commutator and generates D.C. A generator will work as a motor if fed D.C. from a battery. Very early Dodges used their generators as the starter motor, and some small motorcycles also have used this arrangement.
    An alternator has, instead, two slip rings, and makes A.C., which is converted to D.C. by a rectifier. An alternator will NOT work as a motor when fed D.C. from a battery.
    The advantage of an alternator is that they are capable of spinning higher rpm without coming apart, and thus, can produce more electricity from a similar sized unit.
    In both cases, the voltage regulator senses battery voltage, and when it reaches the preset limit, turns OFF the current to the Field windings. At that point, the device isn't producing electricity.
    When battery voltage falls enough, the voltage regulator turns on the current to the Field windings, and the device produces current. And the current required to energize the Field windings is much less than the device is capable of producing.
    In theory, neither one should produce current without a battery to energize the Field windings, BUT, there is very often a little bit of residual magnetism in the iron core around which the wire is wound, and this can be enough to allow charging to begin.
    Last edited by ron521; 09-01-10 at 10:26 AM.

  4. #4
    Senior Member DannoXYZ's Avatar
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    Electromagnets gives you stronger magnetic field for more efficient design. Bosch now has a super-efficient series of alternators using rare-earth magnets that gives you double the output for the same size alternator. Or smaller and lighter alternator for the same output.

    One annoying thing about all this compactness is that the most common part that fails is the same one as from decades ago, the VRM. But instead of just replacing it like in the old days, you have to practically disassemble the alternator just to get to it. Well, at least it's an all-in-one repair for most alternator designs where the replacement VRM also comes with new brushes.

  5. #5
    derailleurs are overrated bigbenaugust's Avatar
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    Should you be reading about field excitation?
    --Ben
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  6. #6
    Look! My Spine! RubenX's Avatar
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    OK... now everything makes a lot of sense.

    My old car was probably starting when pushed, due to some residual magnetism in the rotor's iron core, causing enough induction on the stator to charge up the rotor's coils and get everithing up and running from there.

    Probably the same explains why it's sometimes impossible to start up a car that's been sitting there for a while, even if pushed downhill. On those cases it seems the rotor's iron core has lost all residual magnetism and there's no induction at all. With nothing to charge the coils, it's basically the same as spinning a household fan.

    This also explains the mystery of the multiple connectors on the alternator unit. Or at least one of them, since it must be where you supply that initial voltage to fire up the coils.

    It can also be deducted that aftermarket high output alternators that have only one connector, probably use permanent magnets instead of electromagnetic coils, therefore no need for initial "excitation", therefore no excitation connectors... just one positive output to the battery and grounded trough the casing that's bolted to the chassis.

    And finally, even tho I have no idea if this is done, it's probably possible to regulate the stator power output by increasing or decreasing the voltage going to the rotor's coils, regardless of the engine's RPM.... or at least to a certain extent.

    PS: Field Excitation (as I recently learned) is the mechanic specific lingo for the initial electrical current (aka excitation) that is passed to the rotor's coils to create the magnetism (aka field). So, like many other things, a car's alternator needs to be "exited" before you can "get it going" and without that "initial excitation" of the "field" there shall be no "spark".
    "Hoy es un dia normal, pero yo voy a hacerlo intenso" ~ Juanes

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    Banned. ModoVincere's Avatar
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    excellent....now, how many phases does a standard alternator produce?

  8. #8
    You Know!? For Kids! jsharr's Avatar
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    Quote Originally Posted by colorider View Post
    Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.

  9. #9
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    Quote Originally Posted by RubenX View Post
    OK... now everything makes a lot of sense.

    My old car was probably starting when pushed, due to some residual magnetism in the rotor's iron core, causing enough induction on the stator to charge up the rotor's coils and get everithing up and running from there.

    Probably the same explains why it's sometimes impossible to start up a car that's been sitting there for a while, even if pushed downhill. On those cases it seems the rotor's iron core has lost all residual magnetism and there's no induction at all. With nothing to charge the coils, it's basically the same as spinning a household fan.

    This also explains the mystery of the multiple connectors on the alternator unit. Or at least one of them, since it must be where you supply that initial voltage to fire up the coils.

    It can also be deducted that aftermarket high output alternators that have only one connector, probably use permanent magnets instead of electromagnetic coils, therefore no need for initial "excitation", therefore no excitation connectors... just one positive output to the battery and grounded trough the casing that's bolted to the chassis.

    And finally, even tho I have no idea if this is done, it's probably possible to regulate the stator power output by increasing or decreasing the voltage going to the rotor's coils, regardless of the engine's RPM.... or at least to a certain extent.

    PS: Field Excitation (as I recently learned) is the mechanic specific lingo for the initial electrical current (aka excitation) that is passed to the rotor's coils to create the magnetism (aka field). So, like many other things, a car's alternator needs to be "exited" before you can "get it going" and without that "initial excitation" of the "field" there shall be no "spark".
    I'm not sure you're all the way there. While you can run a car on the alternator, I'm not so sure the initial spark isn't coming from the battery. My guess about push starting (assuming a manual here) is that some times you had enough juice to spark, and others you didn't. In both cases you lacked the current capacity to run the starter (orders of magnitude larger). Danno can probably confirm or refute this suspicion. Once its turned over a couple times then your alternator kicks in and keeps things going. I suppose you could test this by running a car, shutting it off, disconnecting the battery, and then trying to push start it. Also, my alternator , which was not made in the last decade only has 1 plug (which really just means they split the wires internally).

  10. #10
    Look! My Spine! RubenX's Avatar
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    Quote Originally Posted by ModoVincere View Post
    excellent....now, how many phases does a standard alternator produce?
    After all my youtube watching / wikipedia reading I can answer that the rule of thumb is 3 phases.

    However, 42 sounds more appealing.
    "Hoy es un dia normal, pero yo voy a hacerlo intenso" ~ Juanes

  11. #11
    Banned. ModoVincere's Avatar
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    Quote Originally Posted by RubenX View Post
    After all my youtube watching / wikipedia reading I can answer that the rule of thumb is 3 phases.

    However, 42 sounds more appealing.

  12. #12
    A treat for the freaks! MCODave's Avatar
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    Quote Originally Posted by RubenX View Post
    OK... now everything makes a lot of sense.

    My old car was probably starting when pushed, due to some residual magnetism in the rotor's iron core, causing enough induction on the stator to charge up the rotor's coils and get everithing up and running from there.

    Probably the same explains why it's sometimes impossible to start up a car that's been sitting there for a while, even if pushed downhill. On those cases it seems the rotor's iron core has lost all residual magnetism and there's no induction at all. With nothing to charge the coils, it's basically the same as spinning a household fan.
    Keep in mind that ignition is only part of what you need to get a car running. Some cars with electronic fuel injection can't be push-started because the fuel injectors won't squirt fuel.

  13. #13
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by jccaclimber View Post
    I'm not sure you're all the way there. While you can run a car on the alternator, I'm not so sure the initial spark isn't coming from the battery. My guess about push starting (assuming a manual here) is that some times you had enough juice to spark, and others you didn't. In both cases you lacked the current capacity to run the starter (orders of magnitude larger). Danno can probably confirm or refute this suspicion. Once its turned over a couple times then your alternator kicks in and keeps things going. I suppose you could test this by running a car, shutting it off, disconnecting the battery, and then trying to push start it. Also, my alternator , which was not made in the last decade only has 1 plug (which really just means they split the wires internally).
    I think that's right:

    Automotive electronics <------ BATTERY <------- alternator

    It's the battery that supplies all the juice to all the electronics. The alternator's only job is to re-charge the battery. Give it a test, go unplug the alternator from your car and try starting it. If your battery has juice, the car will start. However, the other way doesn't work. Induction works on field-strength and velocity. The faster you move an inductor through a field, the more current is produced. The amount of revolutions and velocity you get out of an alternator by push-starting a car simply isn't fast enough to produce any usable current.

    Push-starting with a low-battery really only works to replace the starter by spinning the tyres. This spins the engine and compresses the air-fuel mixture. However, you still need a little juice in the battery to power the ignition and fuel-injectors. With a completely dead battery, push-starting won't work at all. However, even a car that's been sitting around for a year has enough juice in the battery to power the electronics (but not enough to power the starter) With old points & condenser ignition, you can start a car with a 9v battery!

    Many racecars and motorcycles use a total-loss system with no alternator or starter to save weight. They have just large enough a battery to power the fuel-injection and ignition for the length of the race and that's it.

  14. #14
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    Lets not forget the diodes in the alternator charging circut Rube.
    When they start to go bad they make a slight squealing noise like Ned Beatty in "Deliverance".


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    Not many local shops rebuild altenators these days.
    Not sure I would trust a used one from Lamont at the junkyard.

    You did refer to the Haynes manual to see if it was putting out @13-14 volts?
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  16. #16
    Surf Bum
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    When you push start a car, you get the wheels rolling, let out the clutch and this connects the engines crankshaft to the spinning wheels (via the transmission) and therefore the crankshaft rotates. When crankshaft spins, it rotates the camshafts that control ignition and exhaust valve openings, drawing in fuel and air into the combustion champers.

    At the same time, the spinning camshaft also causes a belt to rotate that is connected to the alternator pulley and thus the alternator rotates, producing...you guessed it...electricity.

    I disagree with the post above that says that the alternator will not be revolving fast enough from a push start to produce electricity. One can easily push a car up to, say, 15mph. At that speed in 1st gear, the engine will rotate well faster than the 1000 rpm or so at which the alternator is putting out its typical 14v.

    Anyhow....as the alternator rotates and produces electricity, said electricity flows from the alternator to the battery to top off its charge and to the ignition coil which sends a spark to the distributor which distributes a spark to the appropriate spark plug and the appropriate time. The spark ignites the gasoline pushing the piston downwards which rotates the crankshaft which rotates the wheels of the car and...as mazda says...vroom vroom off you go.
    Last edited by pacificaslim; 09-01-10 at 07:17 PM.

  17. #17
    another retro grouch Mr IGH's Avatar
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    Quote Originally Posted by DannoXYZ View Post
    ...It's the battery that supplies all the juice to all the electronics. The alternator's only job is to re-charge the battery. Give it a test, go unplug the alternator from your car and try starting it....
    Yes, but start your car and disconnect the battery, your car will still run. The battery is a chemical capacitor, it smooths out the alternator's rectified current and stores energy to start the car.

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    Quote Originally Posted by pacificaslim View Post
    When you push start a car, you get the wheels rolling, let out the clutch and this connects the engines crankshaft to the spinning wheels (via the transmission) and therefore the crankshaft rotates. When crankshaft spins, it rotates the camshafts that control ignition and exhaust valve openings, drawing in fuel and air into the combustion champers.

    At the same time, the spinning camshaft also causes a belt to rotate that is connected to the alternator pulley and thus the alternator rotates, producing...you guessed it...electricity.

    I disagree with the post above that says that the alternator will not be revolving fast enough from a push start to produce electricity. One can easily push a car up to, say, 15mph. At that speed in 1st gear, the engine will rotate well faster than the 1000 rpm or so at which the alternator is putting out its typical 14v.

    Anyhow....as the alternator rotates and produces electricity, said electricity flows from the alternator to the battery to top off its charge and to the ignition coil which sends a spark to the distributor which distributes a spark to the appropriate spark plug and the appropriate time. The spark ignites the gasoline pushing the piston downwards which rotates the crankshaft which rotates the wheels of the car and...as mazda says...vroom vroom off you go.
    I'm going to disagree. I don't know about your experiences push starting cars, but in the several dozen times I've had to do it the car was never going anywhere near 15mph. Also, I've found that push starting in first doesn't seem to work as well as push starting in second. All this comes down to the fact that when push starting I've gotten the car just fast enough that when I quickly raise then depress the clutch the engine is spinning at its minimum start speed, close to 200 rpm. I don't think the alternator is going to be putting out much at an engine speed of 200rpm.

  19. #19
    You Know!? For Kids! jsharr's Avatar
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    Where is the magic blue smoke kept in modern alternators?
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    Quote Originally Posted by colorider View Post
    Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.

  20. #20
    Banned. ModoVincere's Avatar
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    Quote Originally Posted by jsharr View Post
    Where is the magic blue smoke kept in modern alternators?
    that got moved to the catalytic converter.

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    Quote Originally Posted by ModoVincere View Post
    that got moved to the catalytic converter.
    Au contraire, my friend's 1989 (actually ,maybe 1991) 2 door honda accord decided to let something short inside the hot short to ground in the alternator. For some reason there weren't any fusible links and the charge wire didn't burn out, so the entire battery shorted through the alternator case. Tons of magic blue smoke for about 30 seconds.

  22. #22
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by pacificaslim View Post
    At the same time, the spinning camshaft also causes a belt to rotate that is connected to the alternator pulley and thus the alternator rotates, producing...you guessed it...electricity.

    I disagree with the post above that says that the alternator will not be revolving fast enough from a push start to produce electricity. One can easily push a car up to, say, 15mph. At that speed in 1st gear, the engine will rotate well faster than the 1000 rpm or so at which the alternator is putting out its typical 14v.
    Couple of things:

    1. Camshaft spins at 1/2 engine-RPM.

    2. On most cars, where does the other end of the alternator belt attach?

    3. How many revs do you think you get out of the alternator when push-starting? Hint, count the distance the car moves after you release the clutch and work backwards through the tyre-diameter and gear-ratios.
    Last edited by DannoXYZ; 09-02-10 at 01:22 PM.

  23. #23
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by jccaclimber View Post
    I'm going to disagree. I don't know about your experiences push starting cars, but in the several dozen times I've had to do it the car was never going anywhere near 15mph. Also, I've found that push starting in first doesn't seem to work as well as push starting in second. All this comes down to the fact that when push starting I've gotten the car just fast enough that when I quickly raise then depress the clutch the engine is spinning at its minimum start speed, close to 200 rpm. I don't think the alternator is going to be putting out much at an engine speed of 200rpm.
    Yes, push-starting in 1st tries to spin the engine too fast and the car stops almost immediately. Push-starting in 2nd slows down the car less and gives it a couple of revs to try and get a couple of successful puffs going.

    I'll confirm one way or the other this weekend and try push-starting down a hill with alternator-only.

    Risky thing about without battery is it uses the battery-voltage for feedback on how much voltage to output. With zero-volts coming back from the disconnected battery, it'll increase voltage/current more and more to charge what it thinks is a dead battery. Could end up frying the VRM this way. And the ECUs as well if they get too much juice.

  24. #24
    don't be so angry clancy98's Avatar
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    unless your on a hill, how the hell can you push a car 15mph?

    wouldn't you have to be runnin around 15mph to keep pushing?

    But then again, I'm chubby, and I dont run that fast. More of a slow burn.
    Irregardless is not a word, and you do not sound more intelligent using it.

  25. #25
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    I've driven Fiats for many, many years: of course I'm well versed in push starting! I too usually use 2nd gear because it is a smoother start but 1st is doable if you're quick with the clutch. As for 15mph...well, I live in teh SF area so hills are everywhere so 15mph is not really that fast. Two decently fit people can easily push a car up to that speed, even on flat ground. But regardless, you're going to be spinning the engine at least 1,000 rpm (even in second gear). So let's say it continues to spin for only 2 seconds after you let the clutch out. That's 33 rpm (yes, crankshaft rotates twice per engine revolution, but the tach is showing you crankshaft rpm). If you look, you'll see the pulley on the alternator is much smaller in diameter than that of the crankshaft, meaning that the alternator will rotate more, probably at least twice as many times as the crankshaft.

    I'm guessing that plenty of "juice" is put out in 1 second of engine rotation to provide a spark to the sparkplug and thereby start the engine running.

    But there is really only one way to find out...jump start without the battery connected. I recommend making sure that the engine has a ground strap directly to the chasis somewhere if you are going to try to run a car without the battery installed since the battery ground cable will be disconnected.

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