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Thread: Sprockets...

              
   
   
  1. #11
    Señor Member podolefsky's Avatar
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    Quote Originally Posted by frodus View Post
    but..... remember that 200A limit is motor side. The amps are likely much less on the battery side. He should get a good meter or gauge and limit it such that he's not going over 5C on the battery side.
    We've been over both of these things before - Travis is the expert on CALBs, so you should listen to him. But, as long as you're not hitting LVC, you shouldn't need to limit to 5C.

    Travis is also right that 200A motor doesn't necessarily mean 200A battery. But motor side amps will only be more than battery when the controller is limiting current. If it goes to 100% duty cycle at full throttle, battery and motor current will be the same. If you raise the current limit so that the controller will put out as much as the motor asks for, you will have motor current = battery current under full throttle.
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  2. #12
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    Quote Originally Posted by podolefsky View Post
    If you use a higher gear ratio, say 1:3.5 or 1:3.7,....... You might actually notice acceleration from 30-50 get better.
    I don't think so. With the same current limit and a higher ratio, he'll have less wheel torque while in current limit and that will be almost all the way to top speed. The RPM vs load curve is fairly flat for this motor. He'll end up with a higher top speed because he will be loading the motor more therefore seeing more power.

    His other option to get higher top speed would be to add some cells increasing the voltage.

  3. #13
    Moderator Nuts & Volts's Avatar
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    Remove the current limit and test to see if you can go any faster (top speed). This will tell you if you need more torque from the motor to go faster. If you can not go any faster than you RPM or power limited. If you can go faster than you are current limited by your batteries it would seem.

    Then from there you can start experimenting with different sprocket sizes or more batteries.
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  4. #14
    Señor Member podolefsky's Avatar
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    You need to include tire size when you think about gear ratios. If you have a smaller tire, you'll go slower with the same gear ratio and RPM. (So compare your tire size to DRZ400.)

    I could be wrong, but doubt it's the current limit. The controller can limit the current, but power in = power out * efficiency, and you said you're seeing 100A at 72V, which is 7.2 kW. That should be enough to hit 55 mph unless you are losing power somewhere. When you get near the RPM limit of the motor, efficiency drops, so that could be part of the problem. If you gear higher, you'll get into a higher efficiency band and you'll actually get more power to the wheel with the same battery output.
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  5. #15
    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by podolefsky View Post
    I could be wrong, but doubt it's the current limit. The controller can limit the current, but power in = power out * efficiency, and you said you're seeing 100A at 72V, which is 7.2 kW. That should be enough to hit 55 mph unless you are losing power somewhere. When you get near the RPM limit of the motor, efficiency drops, so that could be part of the problem. If you gear higher, you'll get into a higher efficiency band and you'll actually get more power to the wheel with the same battery output.
    Good point. I should have looked at the power numbers closer. Then my guess is that you are running out of RPM due to voltage/motor spec
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  6. #16
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    Quote Originally Posted by podolefsky View Post
    When you get near the RPM limit of the motor, efficiency drops, so that could be part of the problem. If you gear higher, you'll get into a higher efficiency band and you'll actually get more power to the wheel with the same battery output.
    Motor efficiency will drop at higher RPM when running near no load or when near the physical limits of the materials, but that is not a factor in this case. Take a look at the motor curve http://www.electricmotorsport.com/st...ower_curve.pdf Use the 36V curve and double the RPM numbers. It will be a fairly flat RPM curve from 3400 RPM at 20A (30 in.lb.) out to 3000 RPM at 140A (240 in.lb.). The power curve is also linear, 800W to 8600W for that range. At 100A, it would be 3100 RPM, 170 in.lb. and 6200W.

    To go faster, he has to increase the ratio so it loads the motor more, say to 240 in.lb. Then it would draw 140A and put out 8600W at 3000 RPM. Motor efficiency would not change significantly.

    As long as he is near 3000 RPM and under 200A, current limit isn't a factor in top speed.

  7. #17
    Señor Member podolefsky's Avatar
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    You can see the 72V curve here.

    This plot only shows a small part of the RPM range, nothing below 3000 RPM. You can be below 3000 RPM and still pull 100A. The 100A billmi quoted is also battery amps, not motor amps. They'll only be equal at 100% duty cycle (which might be the case at full throttle, but we don't know for sure).

    He shouldn't need more than 100A to go 55 mph. Only if he is going up hill or has some major inefficiency somewhere.
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  8. #18
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    Quote Originally Posted by podolefsky View Post
    You can see the 72V curve here.

    This plot only shows a small part of the RPM range, nothing below 3000 RPM. You can be below 3000 RPM and still pull 100A.
    Thanks. That curve confirms what I said. And it shows motor performance at 72 volts on the motor. So RPM below 3000 RPM will be at less than 72V on the motor by virtue of the controller. When RPM is at or near or slightly above 3000, he is out of current limit and at 100% dc giving the motor full battery voltage, ie 72V. That's where his top vehicle speed will happen.

  9. #19
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    Quote Originally Posted by podolefsky View Post
    You can be below 3000 RPM and still pull 100A.
    Not at full throttle and a 200A current limit.

  10. #20
    Señor Member podolefsky's Avatar
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    Quote Originally Posted by lugnut View Post
    Not at full throttle and a 200A current limit.
    Why not? What happens when you go to full throttle from a stop?
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