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Can I run a 12v alternator with a 12v motor?

  1. Nov 21, 2011 #1
    The situation is this, I would like to power a Delco 1 wire 100 amp alternator with a 3/4HP 12v DC motor, I would have an automobile battery to start the motor, and for the alternator to maintain, plus I would like to have enough power to run an additional 30 amps (max) of accessories. I can afford a slow drain on the battery, due to the fact I can charge it externally if needed, but I would like to limit how often an external charge source is needed. The motor I am looking at using draws 58 amps at full load, @ 1800 rpm. I realize I would have to gear the pulleys approximately 2 to 1. I am not sure if this is a possibility, or if this runs in to perpetual motion problems. I have laid out the switching needed to cut power to motor so that I don't run into a continuous loop problem.
    Thanks in advance for your replies.
  2. jcsd
  3. Nov 21, 2011 #2
    Hi waugj and welcome to physics forums!

    I'm not sure I fully understand the setup, but note that if all your accessories require 12V, then the best you can do is just run them straight from the battery and don't have the motor or alternator involved. Energy is conserved. You can't create it or destroy it (at least as far as we know). There is energy in the battery and you want to send that energy to the accessories without wasting any. The motor and alternator will get warm and they will make noise. Heat and sound are forms of energy. So the warmth and noise are an indication that energy is being wasted. The only reasons one might use a motor driving an alternator is to electrically isolate the accessories from the battery (and I can't think of a reason you'd want to do that generally) or to change the voltage (but I'll assume that's not your goal). As always, only mess with electrical systems if you know how and know the safety issues. Even 12V batteries can be dangerous if connected incorrectly.
  4. Nov 21, 2011 #3


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    Staff: Mentor

    Welcome to PF! If you're concerned about the possibility of running into perpetual motion problems, why not just do a conservation of energy analysis? You have the voltage and amperage for the input and output - so calculate the input and output power and see how they line up!

    P.S.: We get this question several times a month...
  5. Nov 21, 2011 #4
    Thank you for your welcomes, and your replies.

    fleem, my load will be a small fan and a pump, plus the motor to drive the alternator, I would like to run these loads without depending on an outside charger, generator, etc. Noise and heat in this application are not important, but I will not be able to charge/replace the battery, or refuel a generator very often compared to how quickly it would drain. I considered multiple batteries in parallel, but to get the ability for them to last as needed, they would add up to excessive weight, so I thought that I might be able to put that weight to better use in a regenerative system. The loads, battery, and alternator would all be in the same circuit, so I am not trying to isolate anything, I would just like the alternator to maintain the charge on the battery, while it is powering the loads, including the motor driving the alternator. The alternator I will be using should put out about 13.5VDC @ 3600 to 4000 RPM, and once again it is 100A, which I know is it’s max, and not continuous output, I am not sure of the continuous, but I think it should be in the 80A range.

    The motor I mentioned above is rather expensive, so before purchasing it, I would like to have as much confidence as possible that this system would work.

    Mr. Watters, I am familiar with Ohm’s Law enough to use it for conversions, but I am not sure how to do the conservation of energy analysis you mentioned, if you could give me more detail I would deffinately do so. I read numerous posts before adding mine, but I couldn’t find one similar to mine. If I am repeating another post, I apologize.

    Thanks again.
  6. Nov 22, 2011 #5


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    Staff: Mentor

    Power in watts is volts times amps. If the output is greater than the input, then you have a violation of conservation of energy.

    I only mentioned that we get this question a lot so you could search the forum to see similar conversations if you wish.
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