Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Alternator output

  1. Jan 25, 2015 #1
    I have simple 18 pole rotor, which I drive using another motor. I have 9 coils of 0.6 ohm resistance each, which generate open voltage of 1 V each. When I short circuit individual coils, I see current of 1.60 amp in each coil. All of these are randomly out of phase. I cannot group them as in phase coils and then put them in series and make it as 3 phased, so you can think of this as 9 phase alternator. I can move coils from one location to another as they are not mounted on stator.
    I wonder what is the max output power I can get out of this alternator, considering I full wave rectify and get smooth DC o/p from each coil.
     
  2. jcsd
  3. Jan 26, 2015 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    If you want DC out then you can connect all nine poles in the same way that the three poles of an everyday car alternator are connected. See this link, which has a typical alternator diagram near the bottom. You will get even less ripple from your nine poles. It will be only one pole at a time that supplies current, the others are blocked from reverse current by their own personal rectifier.
    BTW, what was that alternator used for before you got hold of it?
     
  4. Jan 26, 2015 #3
    I made this alternator from scratch. Now I'm using only 12 magnets and 9 coils. I want to know/get max o/p from this alternator. Connecting all of them will only give me o/p from only one coil at a time. I was thinking rectifying all of them individually and then adding together, that would give me 9 times more power. Am I wrong?

    I just want to know what is the max power I can get in Parallel or series combination according to maximum power transfer theorem.
     
  5. Jan 26, 2015 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The answer to series or parallel connection question will depend upon what load resistance you want to serve (as with simple multiple battery connections). I would think in terms of working at the nominal output volts of a single pole as long as your load resistance is appropriate. You would need to know how much current a single pole can supply - without getting hot or sagging..
    Note: No generation systems work with 'Maximum Power' Loads because that would melt the generator (half the power would be dissipated in the windings). You mostly have a much lower source impedance (ideally Zero) than that of the load
    I understand what you want but you are not stating it quite right, imo. Your system will provide more Current than a single pole alternator - because there is always a pole producing near-maximum volts. The picture of the open circuit voltage variation for a three pole alternator - with a load of little bumps along the top - only tells part of the story. each of the poles near the one giving maximum volts will actually be providing current when the load is taking a lot of current.
    The actual current available from a single pole will depend upon the source resistance it provides - obviously there is a limit before the output volts start to sag.
    It is true to say that 9 independent alternators could supply 9 times the power of a single alternator - in principle. Say if each one fed a separate resistive heater. But that wouldn't be convenient if you had just one heavy load. You need to do the best you can by choosing and watching the load resistance to the alternator.
     
    Last edited: Jan 26, 2015
  6. Feb 6, 2015 #5
    Ok, thanks you.
    In general 3 phase generator, we can do Delta or Star winding connections. But I wonder, if there are 12 coils, then they are grouped as 4, 4, 4. So are these 4 coils are connected in series or in parallel?
     
  7. Feb 6, 2015 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I have to wonder whether there is, in fact, any real advantage in using more than three poles for an alternator. If there were any real advantage, then I should have thought that the hundreds of thousands of automotive alternators that are produced each year would incorporate more poles. Since good cheap silicon diodes became available, the cost of rectification would hardly count in the choice - it must surely be based on performance and 3 phase is how it's done.
    The point is that you are working with a given volume and mass and you need to divi it up between all the poles. It would be a question of whether the extra cost and bulk of your extra poles could not produce an even better output if it were a 3 Phase Alternator.
    I just don't know the answer to that one
     
  8. Feb 6, 2015 #7
    I guess I didn't phrase my last question correctly. Forget about my generator, which I said is 9 phase one. That was just experimental one, to understand the generator closely. My question is about 3 phase generators.
    If 3 phase generator have 12 Stator coils and we group them as 4, 4, 4. Do we arrange those 4 coils in series or on in parallel? Do we have any advantage there?
    Let's say we have 4 coils in parallel, if independently each coil generates short circuit current of 1 Amp and it's resistance is 0.6 ohm, then would they generate more current if joined in parallel, because the effective resistance of 4 coils in parallel is much less than 0.6 ohm = 0.15 Ohm.
     
  9. Feb 6, 2015 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It depends upon what voltage you want out of the alternator. If you put them in series then you will get more volts, if in parallel, you can draw more current, I would suggest that you are not so much interested in the short circuit current and more in the current the windings will supply without dropping significant volts. The details will depend on how the poles are arranged physically, I suppose but I would imagine that connecting offset coils in parallel might generate significant circulating currents, which you would't want.
     
  10. Feb 6, 2015 #9
    Got it. But may I know, What is circulating currents?
     
  11. Feb 7, 2015 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    If one coil is excited, slightly out of phase with another coil, connected in parallel, then current will flow around the coils and not out through the Load. Not a good idea because that current can be very high. The ultimate problem is when you get a short circuit between some of the turns in a motor (or transformer) and the induced current can soon cook the whole thing.
     
  12. Feb 7, 2015 #11
    I'm using Neo Magnets, so I guess that phenomenon won't occur in my generator.

    I plan to use below DC motor to use as load. Its resistance is 0.011 ohm. It says winding it 8T, which I'm not sure what it is.

    file.php?id=100565.jpg
     
  13. Feb 7, 2015 #12

    Averagesupernova

    User Avatar
    Gold Member

    Why do you think that since you are using permanent magnets that the dangers or circulating currents don't apply to you?
     
  14. Feb 7, 2015 #13
    I think if I use Diodes, current won't go in reverse direction or in other coil. I'll be rectifying each coil current and then supply it to the DC motor.
     
  15. Feb 8, 2015 #14

    Averagesupernova

    User Avatar
    Gold Member

    Circulating currents can exist when there are no diodes hooked up or if you are not taking any power from the alternator.
    The above quote explains it quite well.
     
  16. Feb 8, 2015 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    If you use diodes then you are not using the "4,4,4" arrangement. You are using a 1,1,1,1,1,1,1,1,1,1,1,1 setup - which is fine and with no circulating currents.
     
  17. Feb 8, 2015 #16
    I think I might be able to do 4,4,4 configuration too, if I use diodes for them in (1+D,1+D,1+D,1+D) + D and repeat. Only thing is I would end up using more diodes. But I don't know if the motor will run, if I provide 12 Amp to it.
     
  18. Feb 13, 2015 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I cannot decide what you mean by that but you need to draw a circuit diagram, showing precisely what you want to do. Then study it carefully to make sure there are no unintended short or open circuits. It's easy done and you don't want a smoke generator!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook