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Thread: Something I've never really understood - battery vs. motor current

              
   
   
  1. #1
    Not to be taken seriously DaveAK's Avatar
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    Something I've never really understood - battery vs. motor current

    I'm sure this has been gone over many times in various threads, but I'm not confident I understand fully.

    Let's take a 72V battery, 72V motor and 72V/500A controller.

    First question - is peak current at stall speed? If so can we say that controller will impose a 500A limit, but because we're at stall speed it's only giving out say 10V?

    This gives

    10 * 500 = 5000W to the motor.

    For the sake of argument say we're 80% efficient at this load.

    5000 / 0.8 = 6250W from the battery.

    Assuming a voltage sag of 5V

    6250 / (72 - 5) = 93A draw on the battery.

    While the numbers might be way off, (I pulled 10V out my ass for example), is the principal described correct? How do I determine the peak draw on my batteries?

    This brings me on to another question. My batteries are 60Ah with a max discharge of 3C. This means I can pull up to 180A from them. I think this is the continuous rating but I can't remember. If I paralleled two of these cells would that give me the same as a single 120Ah cell, i.e. 360A max discharge? I'm pretty sure I know the answer to this one, but the more I think about it the more I doubt myself.

  2. #2
    Empulse R #24 frodus's Avatar
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    Yeah, you got it. 93A on the battery and 500A on the motor because the duty cycle is 10%, i.e. on 10% of the time, off 90%. During that 90%, current is flowing, and when you turn on, the batteries AND the caps inside are discharging.

    For instance, VFR 1.0 had 2 18Ah batteries in parallel and a Synkromotive contoller limited to 150 battery amps. We measured over 400A at the motor under full throttle at stall. The throttle doesn't manage speed, it manages torque, so it would regulate the switching according to what current limit you set. You can also limit the output of the controller.


    The second question, 60Ah with max of 3C for TS cells sounds about right. I'd suggest staying away from that continuously though, and I doubt you'll be cruising along straightaways at anything over 100A. If you paralleled, it'd give you twice the amps, but I'd derate a little, since the batteries may not share current exactly equally.
    Travis

  3. #3
    Not to be taken seriously DaveAK's Avatar
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    Hey, maybe I'm not as dumb as I think I am after all.

    My controller is actually (rated at) 425A, and it's set to maximum. But that's motor side not battery side. How am I going to figure out likely maximum battery draw, or do I just have to do some real world tests and measure it? I have a clamp meter which should measure and hold a peak value for me.

    I'm not actually planning to parallel cells, it was more of an educating myself question. Trying to get my head around all the ins and outs of this.

  4. #4
    Moderator Nuts & Volts's Avatar
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    Maximum battery current will occur at maximum power at the motor (max torque at max speed). This will put the controller duty at 100% meaning your motor sees full battery voltage and full battery current
    I think this works for any type of motor (brushed or brushless, ac or dc) that is being run with a PWM controller??

    But I think maximum battery current will be limited to (after the battery itself) the caps and switching devices (mosfet or igbt) of the controller being used.
    ^^^this is why i think kellys are overrated (they dont have the big enough capacitors)
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    Empulse R #24 frodus's Avatar
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    Yup dave, pretty much dead on.

    The max that the controller can supply would be 425A and that is motor side, but its also the limit of the battery side.... i.e. you COULD potentially have 425A and 72V going out to the motor, but its doubtful that'd ever happen.... battery amps and motor amps almost never reach max and equal eachother together, but as stated above, you can have them equal eachother at 100% PWM.

    Just use a clamp meter. Start with a low current limit like 300A, lock the tires and floor it and measure motor and battery current. Then creap up to the maximum you want the batteries to go.
    Travis

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    Quote Originally Posted by Nuts & Volts View Post
    Maximum battery current will occur at maximum power at the motor (max torque at max speed).
    You almost got it right. Maximum battery current will occur at maximum power which will be at maximum torque, but will be less than maximum speed (unless you're geared way too high (low numerical ratio) or going uphill). In most set-ups, you will continue to accelerate above this max power point and as you do, the current will reduce along with torque, meaning less power. With the DC system, at speeds above this maximum power point, the controller will be at 100% dc and motor current will equal battery current and motor voltage will be equal to battery voltage (minus the semiconductor drop). In the AC systems, this max power point occurs at base speed on the motor. And here again, if geared right, you can go faster, but with less torque and power, and therefore less battery current.

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    Seņor Member podolefsky's Avatar
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    Another way to get a rough estimate of motor current is to use the rated HP. P (in Watts) = HP*745. Power from battery has to equal power to motor / efficiency, then use I_battery = P / V_battery.

    This will be an upper bound since the HP rating is probably higher than reality.
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  8. #8
    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by lugnut View Post
    You almost got it right. Maximum battery current will occur at maximum power which will be at maximum torque, but will be less than maximum speed (unless you're geared way too high (low numerical ratio) or going uphill). In most set-ups, you will continue to accelerate above this max power point and as you do, the current will reduce along with torque, meaning less power. With the DC system, at speeds above this maximum power point, the controller will be at 100% dc and motor current will equal battery current and motor voltage will be equal to battery voltage (minus the semiconductor drop). In the AC systems, this max power point occurs at base speed on the motor. And here again, if geared right, you can go faster, but with less torque and power, and therefore less battery current.
    I understand that my statement doesnt work with an AC (3-phase system) but it should in correct a brushed dc system. If your controller is at 100% PWM your motor will be seeing 100% of battery voltage which means your motor is running at max speed. This may not be the maximum rated speed of the motor, but it is the max for the system.

    ok midpost revelation. At 100% PWM your controller can not usually send full current to the motor because of BEMF, so theoretically your max power will occur before max speed like you say. So max current does not occur at 100% PWM unless your supply voltage is much higher (not sure what this number should be) than your motors rated voltage at max speed.

    Anyone disagree?
    Last edited by Nuts & Volts; 16 February 2011 at 1728. Reason: dave corrected me below
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  9. #9
    Not to be taken seriously DaveAK's Avatar
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    I think Noah is right for what I need, which is battery current, (not motor current). Based on power out = power in then it doesn't matter if the motor is using 10V @ 500A or 50V @ 100A, it's still 5kW. Battery voltage is constant(ish) so then the answer I'm looking for would be 5kw/80V for example.

    Here's the motor spec:
    16 hp@ 200A
    19hp @ 300A
    25hp @ 425A

    So using these numbers gives 25*745/80=233A down to 16*745/80=149A. Working backwards from 3C (180A) gives about 19hp. So I should probably limit my controller to 300A as Travis suggests and see what I can peg out at. One good thing about the Sevcon is that it monitors the armature and field currents and records the max values for me. Then all I need to do is worry about measuring the battery current.

  10. #10
    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by DaveAK View Post
    I think Noah is right for what I need, which is battery current, (not motor current). Based on power out = power in then it doesn't matter if the motor is using 10V @ 500A or 50V @ 100A, it's still 5kW. Battery voltage is constant(ish) so then the answer I'm looking for would be 5kw/80V for example.

    Here's the motor spec:
    16 hp@ 200A
    19hp @ 300A
    25hp @ 425A

    So using these numbers gives 25*745/80=233A down to 16*745/80=149A. Working backwards from 3C (180A) gives about 19hp. So I should probably limit my controller to 300A as Travis suggests and see what I can peg out at. One good thing about the Sevcon is that it monitors the armature and field currents and records the max values for me. Then all I need to do is worry about measuring the battery current.
    Looks good man (i edited my post, wasnt thinking straight). That is a pretty neat feature on the sevcon.
    Also you could use a hall effect sensor to measure battery current http://search.digikey.com/scripts/dk...name=MT7185-ND
    set that up with your CAN project maybe?
    Whats under my tank may shock you!!! R6 Build, Blog/, [/URL] OSU Current webpage

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