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Thread: So what is better for top speed?

              
   
   
  1. #51
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    I have the options of
    72 volts at 450 amps (Motor can take that)
    96 volts at 300 amps (motor can take that too)

    0r

    96 volts at 800 amps, (and yes the motor will survive that for a while), but that is a whole different system, and I do not think the batteries can take that for too long.
    c'mon Ed, give us some parameters here.
    What are you talking, 1/8 .. 1/4 mile drag, rolling start, prolonged top speed ??

  2. #52
    Old EV Racer EVcycle's Avatar
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    Now you want parameters? I was just trying to start a discussions and you all want rules.....
    If I do that all I get is info on ONE project. I really wanted a broader chat...which we have had.

    The discussions was for top speed. 1/4 mile would be acceleration. Top speed would be Bonneville stuff. (No I am not going there)

    Top speed with a rolling start. No MPC required. After a certain speed drag will play a role. (actually Drag plays a role at any speed)
    Once you get past 80 MPH (128KPH), the effect will put a greater load on the motor/battery.
    Last edited by EVcycle; 02 January 2011 at 1924.
    EV Ed
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  3. #53
    Not to be taken seriously DaveAK's Avatar
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    If there were no rules then there would be no difference between a 1/4 miler and a street bike, and the budget would be limitless.

    Without parameters the question becomes how long is a piece of string.

  4. #54
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    The discussions was for top speed. 1/4 mile would be acceleration. Top speed would be Bonneville stuff. (No I am not going there)
    Exactly. So for prolonged top speed, an 800A capable supply is pointless for a 200A continuous motor as would be a 300A limited supply in a drag scenario where the 800A could be used briefly. TTXGP circuit racing and TT Zero scenarios different again.
    Similarly 96V may rev the boll**ks off a particular motor while 72V might not achieve max.

    It's like asking what's the best camouflage colour!
    Last edited by magicsmoke; 02 January 2011 at 1945.

  5. #55
    Old EV Racer EVcycle's Avatar
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    There is that box again......


    Everyone keeps putting parameters in.

    Why would I put 800 amps into a motor that would only take 200?


    I think we have killed this subject.




    Class ....dismissed.
    Last edited by EVcycle; 02 January 2011 at 2009.
    EV Ed
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  6. #56
    Not to be taken seriously DaveAK's Avatar
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    I think it's a little unfair to say that some of us aren't thinking outside the box. I'm sorry if that's how it comes across.

  7. #57
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    Why would I put 800 amps into a motor that would only take 200?
    Because in post #2 you said "for this example we can use the Mars 1003 motor as the baseline."

    Anyway, before the end of class bell stops ringing, a definitive answer to your question is ..

    Pack Volts = Volts to achieve max safe unloaded rpm + (Current desired at max rpm x Armature resistance)

    In the above, choose 'current desired' to be peak / continuous / whatever.

  8. #58
    teddillard
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    Yeah, I'm sorry, but this question still makes no sense to me. I can't figure out what you're asking or what you're trying to get at. If you're looking for the best configuration or tuning within a system you have on hand, fine, what are the choices- we need the parameters- and if not, then it's not really much of a question. Even I know the answer...

    I still can't figure out if you can actually program the volts/amps on the controller you're using, and if that was one of the things you were trying to get at... would you enlighten me?

  9. #59
    Empulse R #24 frodus's Avatar
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    Quote Originally Posted by EVcycle View Post
    I have the options of
    72 volts at 450 amps (Motor can take that)
    96 volts at 300 amps (motor can take that too)
    0r
    96 volts at 800 amps, (and yes the motor will survive that for a while), but that is a whole different system, and I do not think the batteries can take that for too long.
    pick the one with the highest power then. None of those are equivalent power, but that is going against your first question. You need to have some constants, i.e. power is constant, which is better, higher voltage lower amps, or higher amps lower voltage. If you have the same exact battery pack, same controller, same motor but its a question of configuration, then I think that you've seen that a higher voltage would be best for getting that higher RPM.

    96V and 800A (77kw) is obviously the higher power one, but after that, 72V and 450A (32.4kw) is the next highest power. The lowest power of those three is 96V and 300A (28.8kw).



    Now.... back to the original question:

    All that has been said about power being the limiting factor and the back emf controlling RPM....etc are all valid. In order to overcome the back EMF once RPM increases you need to increase the voltage to get any more RPM.

    But lets look at a different motor for instance (because Mars, Agni curves are over a really narrow range and i can't stand their graphs), where the torque curve is something we can see over a wider RPM range.

    http://www.evfr.net/synkromotive/com...motors/K91.pdf
    (sorry its hard to read)

    look at page 2. Note that there are 4 different RPM curves. This shows the effect of increasing voltage on RPM. The HP on the graph is for 75V only, but you can calculate the HP and see how it would be effected (TxN/5252 = HP). Watch RPM decrease as the load on the motor (torque) is increased. Now, the HP does keep going, but it will trail off and start decreasing. The Torque is a function of current, but as you increase RPM, your Back EMF comes into play..... but its still directly proportional to input current.

    So the trick is, if you're working within the parameters of the motor, to maximize the HP, since in the end, that is what controls your ability to overcome air resistance and thus effect your top speed. Anything over about 40mph and air resistance really starts to effect your HP requirements.

    Now lets use that K91 motor and 2 scenarios, and a few constants, since I like comparing apples and apples and I know it drives you &%$#ING nuts

    lets say same controller set to different limits. Same batteries in two configurations (72V and 96V). They're the same power, just different voltage and amp configurations. Same motor (my K91). Lets say the power is 21.6kw. At 96V that's ~225A. At 72V that's 300A.

    Look at the power curve, and draw horizontal lines at two points. One in red at 225A and another in blue at 300. Now, see where that red line hits 225A, lets say its about 32ftlbs of torque. Now do the same for the blue line, and it apears to be about 45ftlbs. At 225A and 32ftlbs, the RPM for 96V is ~3500rpm. At 300A and 45ftlbs, the RPM for 72V is ~2250rpm.

    So:
    HP at 72V 300A = (45ftlbs x 2250)/5252 = 19.27HP (which roughly lines up with the graph)
    HP at 96V 225A = (32ftlbs x 3500)/5252 = 21.32HP

    So for the same input parameters (input power), the higher voltage lower current scenario would give you a higher output power.
    Travis

  10. #60
    Old EV Racer EVcycle's Avatar
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    Thank you Travis, (and every one else that added to this thread!).

    That was a good way for us mere mortals to understand it.



    Now back to our regularly scheduled programming.
    EV Ed
    Never let being a responsible adult get in the way of passion and curiosity.
    Motenergy.com
    Ev album 2456, 2458, 2457, 2681, 3456, 3630

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