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Alternator Theory

  1. Mar 16, 2011 #1
    Hello all!

    Ive been thinking about car alternators...... and have a few questions!

    Does a car alternator have to be able to supply max power at engine idle speed? My thought being that you could turn on all electrical devices in your car at this speed. I do notice however that when you turn lights on etc at idle you can here the engine drop a few revs under the load.

    From this i have formed a theory in my head so would appreciate some comments on its accuracy.....!!!

    Power = 2 X PI X rpm x torque

    Therefore at engine idle the rpm is fixed and so if the power demand increases the only variable that can change is torque and so the alternator resisting torque increases to balance the equation.

    Do alternators work like this?! If so, if you had a second alternator in parralel, run from an imaginary power source then would the electrical power demand be shared equally between them?

    Many thanks for any help offered.

  2. jcsd
  3. Mar 16, 2011 #2
    Power (watts) = 2 X PI X (rpm/60) x torque(N-m)
  4. Mar 17, 2011 #3


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    The alternator exists partly to charge the battery.

    Depending on the car, the alternator may or may not produce enough voltage to charge the battery, even slightly, at low revs. This also depends on the state of charge of the battery.

    At higher engine revs, the output voltage of the alternator would be sufficient to forward bias the alternator diodes and much larger charging currents would flow.

    So, it isn't true to say the alternator must produce maximum power at low revs.

    If you turned on some devices in the car while the engine was operating at low revs, then, quite possibly, the current would come almost entirely from the battery.

    Putting alternators in parallel is generally a bad idea, because unless you can get the output voltages identical, one of the alternators will supply most or all of the power.
  5. Mar 28, 2011 #4
    Thanks for the response,

    So is it possible to add charge to the same battery from a second, possibly smaller alternator? Im reseaching an energy saving system in a vehicle if your wondering what im getting at!

    Also what happens in a car if/when the battery becomes fully charged? Where does the energy from the alternator go?

    Thanks again,


  6. Mar 28, 2011 #5
    The alternator converts energy from mechanical to electrical. The mechanical energy is supplied from the engine, whose input is chemical energy in the form of gasoline. The electrical output power of the alternator must be less than the mechanical input power from the engine, which must be less than the chemical power from gasoline combustion.

    The alternator's maximum output power requires maximum engine speed & max fuel consumption. But fortunately, alternators are equipped w/ regulators which provide constant voltage operation. By selectively adjusting the field current, the magnetic field strength can be varied & consequently a regulated output can be achieved despite varying loading & engine speed.

    If the engine is idling under light load, the regulator will adjust the field current for the needed load power w/ constant voltage, approx. 13.8V. If the load is then increased, the voltage out momentarily droops, so the field current is raised by the regulator. The voltage at the alternator output increases to the right value, & the new current value is obtained. But the power has increased as well. This increase in electrical power mandates a corresponding increase in mechanical power from the engine.

    If the fuel consumption is constant, i.e. no increase in fuel, than the idle speed will drop. Since the fuel consumed at the original engine idle speed did not increase, & the alternator demands more mechanical power from the engine, the engine must lose some of its mechanical power. The rpm drops as a result.

    Is this helpful?

    Last edited: Mar 28, 2011
  7. Mar 28, 2011 #6
    I did a little power test on my alternator in my old car a couple years ago. It was a 1992 Dodge spirit with a 60amp alternator. When the engine was idling at about 500rpm, it would output about 13.2volts which would increase to a maximum 13.7 volts at about 800rpm. Also, the maximum current output seemed to be about 800rpm. I connected a 100amp load to the battery and watched the output current from the alternator. It peaked at about 800 rpm, but at 700 rpm was supplying about 90% of peak. This all depends on your pulley ratio of course, but it goes to show that max power seems to be delivered just off idle.
  8. Jan 13, 2012 #7
    sorry to bump this old thread but my question specifically applies.

    ive seen some high performance alternators rate at a max rpm of 16-18k. what is the max rpm of regular 85 amp alternator? (in ft/lbs please)

    what is the max magnetic resistance of an 85 amp alternator at peak output?

    if you were to spin an alternator 2 or 3 times faster than the rpm of peak output, would the magnetic resistance continue to rise, or would it plateau?
  9. Jan 13, 2012 #8

    jim hardy

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    read up on a term "synchronous impedance"
    it's a function of frequency

    i think you'll find car alternator excels at low speed.
  10. Jan 13, 2012 #9
    thx for the response.

    i understand that an alternator works just fine at low speed, but what im attempting to figure out is will it perform just as fine at high speed and if at high speed will the magnetic resistance in the generating aspect reach a plateau where regardless of how much faster the alternator is turning it will retain for the most part the same amount of magnetic resistance.

    not so much the characteristics of electricity and its generation, but the actual physics of a working alternator as defined by the invention itself.

    x rpm == y output vs. 3x rpm == y output

    variable rpm vs. peak output and that's all it is going to push.

    once the alternator has achieved max output, has it also achieved max magnetic resistance.

    3000 rpm = max output = 15000 rpm = same output and same magnetic resistance ??????
  11. Jan 14, 2012 #10


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    I don't understand what you mean by "magnetic resistance". Does it refer to the torque needed to turn the alternator on load?

    A car alternator is tightly controlled by regulators that stop it producing excessive voltage or charging an already charged battery and doing damage.

    So, once it is producing 14 volts or so, then the regulator cuts in and stops it producing more voltage with higher RPM.

    Without regulators, I read that an alternator can produce many times the voltage that it produces in a car. I have never tried this, though.
  12. Jan 14, 2012 #11
    so i guess what you are saying that at higher rpms, the alternator does in fact plateau the magnetic resistance, therefore making it finite.

    i would assume that in the act of the regulator limiting its output, limiting the electricity being made, limiting the magnetic resistance from increasing, then once its achieved max output it has also achieved max resistance, regardless of rpm above whats required to achieve max output.

    i cant wait till i have some money where i can start testing my theories. :)

  13. Jan 14, 2012 #12

    jim hardy

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    what is magnetic resistance?

    it is important to define your terms.

    our mind is capable of believing in things that are not in line with how mother nature built the universe, or how Chrysler built their alternators.

    in principle, the car alternator is a current transformer operated at variable frequency with output current roughly equal to: (field current) X (ratio of armature turns to field turns.)

    the voltage regulator controls field current, hence output current.

    indeed as v6kr0 said, higher voltage is achievable if load current is low. i've seen over a hundred volts out of them.

    when you speed it up it becomes capable of more voltage but really not much more (if any more) current because of something called "leakage inductance"

    and leakage inductance is the only thing i can think of that somebody might call "magnetic resistance". its effect will be a function of speed.

    i think we are abusing words whose meaning we do not understand.

    i encourage your experiments. i buy alternators at my local metal scrap yard for twenty cents per pound - about two bucks. look through the pile for a clean one. a two dollar washing machine motor from same pile will spin it, look for one with a matching pulley.
    Last edited: Jan 14, 2012
  14. Jan 15, 2012 #13
    i definitely dont know all the proper terms and such for this, but i do have a reasonable understanding of the physics behind it. i was just wanting to clarify things.

    magnetic resistance - the way a generator or alternator makes current is achieved by electro-magnetism. if the magnetism was not there, it would just simply spin. the magnetic resistance i speak of is the resistance that causes you to need, for instance, a 1 horse motor to turn it to get the desired current, rather than being able to hand crank it and get 3 million volts. the force applied to the generator has to be greater than the magnetic resistance of the generator.

    it is from a simple physics stand point the same a the whole car. you cant drive a nascar over 200 mph with a lawnmower engine. lol you have to supply a greater force to the wheel than gravity, friction, wind, and the cars mass to get it to move forward at all.

    magnetic resistance similar to gravity, friction, wind, and the cars mass.

    what im asking about an alternator, is when the battery is charged, and there is no load, or when it is charging and there is a load, counting for the fact of the regulators in place, does an alternator get to the point when the alternator has achieved max output (other than friction and things like that) does the current creating magnetic field reach a point where it has reached the alternators maximum drag (electro-magnetic resistance)?

    or will it continue to increase the magnetic field even without sending more current?

    an alternator due to the fact that it has regulators can only put out so many volts and amps. this max output from what i have learned is at around 3-4000 rpm. does an alternator create more drag (other than friction and other such) the higher the rpms it goes, or does that magnetic drag reach a max of its own? and then it requires relatively the same amount of power to tun it at 4000 rpm as it does at 14000 rpm?

    this is like a smart student talking to a teacher, the student knows what he is asking but doesn't know the right terminology to ask it right. lol
  15. Jan 15, 2012 #14

    jim hardy

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    i think i understand your question better now.

    i'm sorry if my post seemd snide, that wasn't the intent - just wanted to push a little, hoping for the clarification that you gave.
    Nice job ! A problem well stated is half solved.

    your "magnetic resistance" is i think the torque required to spin the rotor.
    It's the force you'd feel opposing rotation were you spinning it by hand

    as posted way earlier in the thread, power into alternator, or any other rotating machine, equals torque X RPM X (some constant to account for units)
    in English units, hp = ( torque X RPM ) X (6.28/33000)

    so the faster it spins, the less torque required to make same power. that's why the loose belt squeaks worst at low speed.

    indeed there is a maximum output current from an alternator.
    The limit is from two effects
    1. how much field current is availanle to make the magnetic field ?
    Old Chryslers built their fields so that full battery voltage would cause only enough field to make the alternator's rated output current. Well, maybe a little more so it'd work at higher speeds.
    That's how they kept it from overheating its coils.

    2. Frequency effects
    Any coil opposes flow of alternating current, and the higher the frequency the more it opposes.
    Before your rectifiers the alternator makes alternating current, in its coils ,,
    in fact my friend's '59 Dodge had its rectifiers external, way up in front of radiator where they got good cooling. Three AC wires went back to alternator instead of one DC wire.

    SO - as you speed up alternator and frequency increases you'll reach a point where the output current starts to drop off from frequency effects.
    that is studied in this paper, see figure 2.

    so i think the answer to your question is, yes the alternator output flattens around 3000 RPM. That graph goes only to 6000 , and i expect it turns down at extreme rpm.
    now given small size of alternator pulley compared to crankshaft pulley that's much lower engine RPM...
    alternator's limiting rpm would be where centrifugal force starts slinging parts off the rotating field..

    that paper describes something i've wanted to do with motorcycles, where the field is a permanent magnet. thanks !

    old jim
  16. Jan 16, 2012 #15
    thank you for your excellent explanation.

    that and also that paper definitely confirm that my theories are at least possible. being that i am investigating a potentially new invention i cant go into details about my theories, but if it works, you will hear about it. :)
  17. Jan 16, 2012 #16


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    Control of an alternator is done by varying the current in the field coils. These are the rotating coils in a car alternator.

    This controls the field that the rotating coils move through the stationary coils.
    If the rotor moves faster, the output voltage would normally increase, but because the field strength is reduced, the output voltage is held constant.

    Incidentally, Wikipedia lists "magnetic resistance" as an alternative to "magnetic reluctance" which is quite different to the "torque needed to turn the shaft of the alternator".
    It refers to the difficulty magnetic field has passing through a magnetic material and it would be a fairly fixed property of the magnetic materials of the alternator construction.

    I'm sure I've never heard it used like this, though.
  18. Jan 16, 2012 #17

    jim hardy

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    ""I'm sure I've never heard it used like this, though.""

    indeed, a nonstandard use. it was a polite inquiry though.

    now it's got me wondering why those folded over flaps on the field have that particular shape. seems almost like a magnetic shunt to increase apparent Xa.??
  19. Jan 17, 2012 #18
    If you know the maximum resistance an alternator will exert, then you know the least amount of mechanical energy needed for maximum mechanical energy tranferance efficiency. Then it is a matter of playing with pulleys and types of belts and types of bearings to get maximum efficiency between mechanical energy and the electricity the alnernator converts.

    Then you have the ability to possibly make generators require less mechanical energy to produce the same if not more electricity understanding that you can never produce more than you are using to make the electricity in the first place.

    You can't get more energy back than what you put into it, but paying attention to what it takes to defeat the resistance of the alternator, you can manipulate horse power and torque to get the max out of what your putting into it. Possibly smaller motor, and get more bang for your buck.
  20. Jan 18, 2012 #19

    jim hardy

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    take careful readings in your experiments.

    look up "dynamometer", a machine to measure torque.
    it's basically a pivot and scale. you'll want to make one.
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