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Measure current from an alternator

  1. Jul 15, 2008 #1
    Hi, I have few alternators(one is 3phase), and I want to test the current generated for each alternator at a same rpm.

    however, what is the proper way of doing this? what is the connection should be?
    is there a need to convert them to DC, and charge to the battery while testing?

    or any other way?


    just wondering can I take a resistor as load instead of battery?

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  2. jcsd
  3. Jul 15, 2008 #2


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    Staff: Mentor

    I'm not sure of the best way, but one way would be to characterize the alternator with its Thevenin equivalent circuit. Measure the open circuit output voltage (RMS), then load it with a moderate load resistance (say a few Ohms to give you a few Amps), and measure the load voltage. This will give you what you need to calculate the output impedance of the alternator, and hence the output current capability at whatever reference voltage you want to call the min (hence the max current).
  4. Jul 15, 2008 #3


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    I think most altenators used in cars require it to be connected to a dc source since they don't have permanent magnets, just field windings. Do you know what types you have?
  5. Nov 3, 2009 #4
    just wanna know the current produce from alternator is AC or DC ?
  6. Nov 4, 2009 #5


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    Science Advisor

    A car alternator itself produces 3 phase AC but there are diodes inside the alternator case that turn this into DC.
  7. Nov 4, 2009 #6
    if i want the current from this alternator,should i use battery or just connect a menthol directly to alternator?
  8. Nov 4, 2009 #7


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    I have a car alternator and I measured the field coil resistance at 27 ohms. So, it would draw about 0.5 amps from a 12 volt battery.
    Whether you could get this from the alternator output depends on remanent magnetism. This is the magnetism remaining in the iron core of the field coil when there is no current flowing.

    If this was enough to get the alternator generating enough voltage to overcome the voltage drops of the diodes, the voltage out would magnetise the field coil a bit more which would give a bit more more output.... and so on.

    My alternator seems to have quite a lot of remanent magnetism but it is hard to tell if it would be enough to get the alternator producing output on its own.

    To be safe, you probably should have a battery on the field coils at least for startup. Then you could charge the battery from the alternator output.
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