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Calculating DC Generator Output via Input

  1. Feb 27, 2012 #1
    My dad is an electrical engineer, I am not. I am just a dumb race car driver. That said, I want to not ask him too much about all this stuff since I want to not look like a complete idiot. I feel that some of this stuff should be somewhat common knowledge, but alas, I am drawing a blank.

    I am exploring the possibility of converting an unfinished Datsun 510 race car into something a little different. I'm thinking an "extended range" endurance race car-- that is to say there is a gasoline engine generating electrical power to be used by a motor, propelling the car. In this way the inefficiencies associated with change in load and RPM of the ICE will be minimized. Plus, some energy would be returned to the system through regenerative braking. In an endurance race, this could mean that I will have to stop for fuel much less often since fuel capacity is limited but usually similar between all the cars, say 10-15 gallons.

    I've got a lot figured out as far as the probable electric motor(s) to use and the gasoline engine, even the transmission and engine controller. My question comes down to a generator. I figure I can just use a similar one to the kinds powering the car, but want to know what I need specifically.

    How do I figure out how much power would be generated? If I have a gasoline engine that will produce around 100hp at probably around 6000-7000rpm, what sort of DC motor would I need to spin to efficiently transfer a majority of that back to the batteries? What do I need to know about running a DC motor in reverse to create electricity? I get that voltage usually shows how much RPM will occur powering a motor, and amperage will usually dictate the torque, but how do these things work as generators?

    Again, sorry my questions are vague and bit noob-ish, I just am having a time trying to wrap my head around what the numbers. Any help would be greatly appreciated!

  2. jcsd
  3. Feb 27, 2012 #2


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    Are we talking 'Hybrid' car here?
  4. Feb 27, 2012 #3


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    you're going to have a lot of added weight to support, if you plan to make it fast enough to race i don't know.....

    you're going to couple your engine to a 3 phase alternator, and have to rectify it to DC for your wheel motors... or you're going to have to transfer to dc, then invert to ac, in the case of ac wheel motors (which is what i would expect they would be...)

    The retarder is going to have to be low resistance, high power (which means a relative large size and weight)

    it's going to be difficult!

    the generators I have seen in mg sets for electric drive vehicles, are 3000 amp, 2000 volt. rectified to DC then through a full wave diode bridge. in the event of a dc wheel motor set, you will want to use some kind of contact / controlled coil as well to add the motor to the circuit, or also to drop out the motor and add the retarder.

    In the event of a dc wheel motor, you will need to keep the field portion of your motor powered, while in retard so a voltage is induced in your armature so you can actually connect the high power grid across it to provide braking action.

    In my experience.. an AC wheel motor (more expensive) is better for electric-automotive applications for both propel and retarding. (research 4 quadrant operation of ac motor)

    In the event of ac wheel motor, you will need to invert your DC back to ac once your diodes convert it... this will require a lot of weight and MONEY if you want to use something like an IGBT (we have, 26 phase modules (igbt's) 2 choppers for the retarder grid (no need for multiple contact/coils with IGBTs) and 24 for inversion... that's 12 for each wheel motor, and two sets of six for forward and reverse)

    Yeah you might get some practical ideas from a company called tesla motors... they manufacture the most likely widely consumable electrical drive practical automobile...but they don't use a generator, I believe they run solely off a well engineered battery bank.. and a drive inverter (they use ac wheel motors, which is probably where the name tesla comes from).

    Any way man... good luck with your project... let us know how it's going... but i think it's going to be hard to get a alternator with a big enough output to power wheel motors to race a car, without some serious power, weight, and MONEY involved.... but hey let us know!!
  5. Feb 27, 2012 #4
    Thanks for the help! I know it'll take money and add weight. I'm just exploring the possibilities.

    DC motors are immensely cheaper and easier to cool, it seems. Can I do this? Going back to the original question, how would I rate a DC motor for use as a generator and what would I need to make it work?
  6. Feb 27, 2012 #5

    jim hardy

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    A DC machine is either a motor or generator depending on whether it's delivering or accepting current and hence torque. One machine will do both jobs.

    So your propulsion motor can do the regenerative braking provided you have someplace to stash the energy, like a modest battery.

    You'd rate it same whether it's used as a generator or motor . (i think that was the question - i confuse easily)

    In 1920's the Dodge Brothers' innovation was their DC machine used as both starter and generator. They called them "Self Starters" back then.

    You might look at military surplus equipment - i've seen aircraft starters in my local metal salvage yard, 28 volts DC at 1000 amps - one at each drive wheel would be interesting and they'd probably do okay at higher voltage.. they'd cost just twenty cents a pound there.
    I have someplace among my junk a surplus army tank alternator about size of a big toaster oven, 24 volts at several hundred amps... it too would probably make more volts. I'll be mounting it to my pickup's engine for 28 volt light bar .....

    There's lots of 28 volt aircraft ground power units on surplus market with big alternators inside.

    keep an eye on this site
  7. Feb 27, 2012 #6

    jim hardy

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    now you've got me wishing i'd bought those aircraft starters.

    So many toys, so little time.......

    old jim

    PS i'll bet Dad would love to help out.
    Last edited: Feb 27, 2012
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