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Thread: What happens to a motor's amp draw when you decrease the supplied voltage?

  1. #1
    Join Date
    Dec 2018
    Location
    IL
    Posts
    3

    Default What happens to a motor's amp draw when you decrease the supplied voltage?

    I'm confused about what happens to the amp draw of a DC boat motor if I change the voltage being supplied to it. If I decrease the voltage, does the amp draw *also* decrease (because the prop is spinning slower, thus putting less physical load on the motor), or does the amp draw *increase* (because P=I*V, and the boat and propeller aren't changing)? I can't seem to wrap my head around this, even though I know that P=I*V, V=I*R, and RPM = KV*Volts. Oh yeah, and Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488.

    For example, this motor on Amazon has the following specs:
    KV(RPM/V): 2250KV
    Watts: 1800W
    Max Voltage: <22.2V
    Max Amps: 80A
    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

    That all makes sense: P=I*V, so 1800=80*22.2 (roughly). But if I want to run this motor off my 4S battery (14.8 volts), then how does this affect the current draw, and the ESC I will need to spec? I've been assuming that this motor would require an 80-amp ESC, but if I'm only supplying 14.8 volts... do I need a larger ESC, or can I get away with using a smaller one? And *how much* larger or smaller? I have no idea what happens to my current as I change the voltage in this context!
    - Technoid (Alex)

  2. #2
    Join Date
    Feb 2008
    Location
    GB
    Posts
    2,726

    Default

    If you drop the voltage whilst keeping the same prop the current will drop too. Having sad that an 80A ESC isn't enough, you need at least a 120A and preferably more.
    Paul Upton-Taylor, Greased Weasel Racing.

  3. #3
    Join Date
    Mar 2017
    Location
    VI
    Posts
    371

    Default

    Like the above reply said once the prop is the same and you reduce voltage the current will also decrease.

    As for ESC with a 2250kv an 150 to 180 amp ESC should be suitable for 4s.

    I would like to point out, that it is very easy to pull 80amps on 4s with a 2250kv motor.

    You would need to use a x440 size or smaller prop with that motor on 4s or 14.8v.

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