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Alternator Rules of thumb?

Discussion in 'General Electronics Discussion' started by MadMechanic, Mar 7, 2013.

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  1. MadMechanic

    MadMechanic

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    0
    Aug 28, 2012
    Hello All,

    I am designing a generation unit where a 12V automotive alternator will be coupled to a small engine. Exact efficiency losses unknown, I have a 30 to 35 amp load. The load will be drawing from a 12V battery and the battery will be recharged during the draw by the alternator. The alternator is rated (40A).

    Being that my load is potentially 35Amps...would I be o.k. using a 40Amp alternator? Or should I go with something like a 50 or 55 amp?

    Is there any general good practice or rule when selecting sufficient compensation such as this? I am not sure what my frictional or resistance losses are. Thanks,
     
  2. GreenGiant

    GreenGiant

    842
    6
    Feb 9, 2012
    is it a constant 30-35amp load or is it short bursts?
    at what RPM is the alternator rated at 40A?
    is the motor going to rev to that RPM automatically (computer?)

    at idle (assuming standard around 1000 RPM) it will only put out a few amps (maybe 15-20) and you also have to take into account the spark plug(s) that will be drawn from the battery as well

    You should probably get a good 65-75A rated alternator
     
  3. MadMechanic

    MadMechanic

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    0
    Aug 28, 2012
    Thanks for the input. I emailed the mfg. of the alternator to request ideal RPM @ 40A. The engine is capable of 7000rpm so I should be able to find the correct speed. The Load is an electric motor coupled to the battery and will be constant at times during full speed operation. The motor could be subject to high amperage jumps if it gets bogged down, but I was going to fuse it at its rating for those situations.

    My engine is capable of 1000W output @ 7000 Rpm which equates to 83.3 amps. If I am coupling this engine directly....would it be more efficient to fit the engine with an 80A alternator versus a 60A when I am drawing 35A? or would the 80A unit drain more power from the engine? Thanks,
     
  4. john monks

    john monks

    693
    2
    Mar 9, 2012
    According to Delco's white paper on alternators, typically a belt efficiency is about 98% and an alternator efficiency is 55%. But further down in the paper it mentions that alternator efficiencies range from about 55% to about 80%. And at 55% efficiency and fuel at $4.00/gallon the electric rate will be about $0.51/kWh.

    Now for what you are asking the paper says that the peak efficiency occurs at 30-40% of full load at 3000-2500 rpm. So you might consider about an 80A alternator. www.delcoremy.com/documents/high-efficiency-white-paper.aspx
     
    Last edited: Mar 8, 2013
  5. MadMechanic

    MadMechanic

    48
    0
    Aug 28, 2012
    Interesting, Thanks for the info. Would I be able to assume that a 40A alternator and an 80A alternator....in the same size housing, same shaft same pulley.. would have the same mechanical resistance?

    For example if my load was always 35A, And I was turning an 80A alternator, when I load the system with 35A...Would the strain of torque on the engine shaft be close to the same if the alternator was a 50A?
     
  6. john monks

    john monks

    693
    2
    Mar 9, 2012
    Probably not but with the numbers you choose probably would not make much difference. Looking at Delco's white paper, page 25 you see that the greatest efficiency is around 60% of the rated current of the alternator. So running 35A with an 80A alternator your running at about 44% of the capacity. And 35A with a 50A alternator you are running at about 70% capacity. Now looking at page 25 I see that the efficiency of both is about 70%. Which is better is too close to call. And I am assuming you would be running the alternator at about 2200rpm.
    The greatest efficiency would be around 60% of the rated capacity.
    I am assuming that the white paper is a close representation of your alternators.
     
    Last edited: Mar 8, 2013
  7. MadMechanic

    MadMechanic

    48
    0
    Aug 28, 2012
    Hi John,

    If my engine were to run at a higher speed than the normal RPM of the rating.. would I see a higher output than the noted efficiency? I can run up to 7000RPm. Peak HP at 7000Rpm is 1000Watts or so. Even if I do not match the draw of my load...If my load is drawing 12V and my alternator is providing 13-14 volts. Could I at least assume the system is under slow safe discharge? Thanks again.
     
  8. john monks

    john monks

    693
    2
    Mar 9, 2012
    From the white paper it appears that the maximum efficiency occurs around 2,200rpm. This gives you about 70% efficiency with their example. Going faster the efficiency drops to 65% then lower. Of course you can adjust your pulley sizes to give you 3:1 drop in rpm. So the diameter of the pulley on your engine will be about one third the diameter of the pulley on the alternator. From the chart page 25 it looks to me the most efficient input to your alternator is 2,200rpm putting out about 60% of the rated current.
    This is a bit awkward because there is a limit on how small a pulley can be. You might try to put a larger pulley on the alternator.
    Specifically, an alternator will put out more power with greater rpm but the efficiency, the mechanical power in verses the electric power out, will be less.
     
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