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AC Current

  1. Apr 23, 2012 #1


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    I am new to this site and thought I would give this a shot.

    All my experience is in low volatge DC circuits and don't have a whole lot of experience dealing with AC but I am trying to learn.

    My boss and I are working on an experimental generator with 24VDC input but AC output.

    He has it built and now wants me to run some tests on it to find out A) AC output B) AC current output C) frequency of AC output D) Watts output.

    So far I have had a reading at output as high as 600V but stopped increasing RPMs because I didn't want to damage the meter.

    I've got readings of VAC at given RPMs and I know Ohm's law so D won't be a problem but how do I find max AC current without plugging anything into it??? We've plugged a few light bulbs into a power strip and I can get current readings using the standard method of interrupting circuit but this doesn't tell me max current output, just the load, as I am sure you know.

    We aren't sure if we should plug a device into it without knowing what it's putting out for fear of damaging something.

    Can someone help?
  2. jcsd
  3. Apr 24, 2012 #2


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    Current limits are decided by what you will tolerate in the way of voltage drop, and what the machine will tolerate in the way of temperature rise. It isn't an absolute figure but you must protect the equipment against long term damage.

    Usually most machines will come with a plate on the side that says how many amps the generator will supply or gives a volt-amp rating.
    If this isn't the case, you could contact the maker and ask.

    If you can't do this, then you drive the generator at a constant RPM that gives you a safe voltage for your load without the load connected and then you increase the load (by taking more current from the generator) and plot a voltage vs current curve.

    If possible, you also measure the temperature of the machine with a non-contact thermometer. Plot this, too.

    A machine that is too hot to touch would be hotter than 60°C (or 140°F). This would be a fair limit. It should be warm, but not too hot to leave your hand on.
    The machine will take time to heat up, so you need to run it until the temperature is steady.

    The voltage vs current curve will probably show a decline in voltage and you might decide that 10% decline is all that is acceptable.
  4. Apr 25, 2012 #3


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    Thank you for your reply vk6kro.

    Any suggestions for something adjustable that I can increase draw as I increase RPMs to plug into AC output of generator to start testing for maximum current output?

    We are looking to get 75 Amps out of this. Not sure how close we will get but I need something that can handle that, or at least close to 75 Amps.
  5. Apr 25, 2012 #4


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    Staff: Mentor

    You are powering this with 24VDC. What are you using to provide that? What is the maximum current the DC supply can provide? The maximum DC power IN will give you the upper limit of the AC power OUT.
  6. Apr 25, 2012 #5


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    It sounds like a 24 volt motor driving a generator, but it was generating 600 volts and now they want 75 amps out of it.

    That would be 45000 watts which is a crazy level of power to be supplying from 24 volts. 1875 amps plus losses.

    So, it is getting a bit confusing. What are you actually doing and what is this generator rated to do?
    What are you trying to power with it?
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