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Thread: Motor size VS RPM VS power

  1. #1
    Associate Engineer
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    Dec 2012
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    Motor size VS RPM VS power

    Hi all, I have a real quick question; should I use a cheaper, smaller, LIGHTER, higher rpm motor from the RC hobby world or a larger, heavier, more expensive motor? Here are the details:

    For some type of small electric vehicle, think between the size and weight of an electric skateboard, and the size and weight of a go-kart. I have been wanting to build a small 4 wheeled kart that leans with steering input, so something along those lines.

    The options are:
    A semi-industrial 48V 1000W motor that can be found all over ebay. These are typically 4"-5" in diameter and 7"-8" long; quite a bit of heft compared to the smaller motors; and 3000 RPM.

    A large RC plane motor with continuous 60 amp rating at 48V, so nearly 3000 watts. But it is also nearly a 10,000 RPM motor at 48V.

    I have designed equipment for a few years that often gets 3HP-10HP drives for chain runs and I have also been into RC planes for a long time. I get that wiring a motor "hotter" with larger wire and fewer turns brings you more amps/power and higher RPM. And I understand the HP to torque relationship, HP = Torque x RPM 5252.

    So, a more refined version of my question would be, since a motors power is constant, but torque can be manipulated, why would anyone use the heavier motors with less power? True they have more torque at lower RPMs (if their power ratings are the same).

  2. #2
    Associate Engineer
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    Dec 2012
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    I feel like I need to clarify a couple of things on that post. I am only referring to small vehicle applications, like scooters, go-karts, and electric lawn mowers. I have a 1/3 HP motor and a capable inverter, I also have a 1/2 HP motor, but these motors are huge compared to some of my plane motors. For $70 I can get a 48V, 60A motor, that's 2,880 watts or 3.86HP. Why oh why do I see instructionals for electric moped conversions and those big DC motors, or those 1800 RPM single phase motors that weight close to 30 lbs? I am thinking there must be a legitimate reason, but the RPM differences really aren't that bad; it's usually a single chain or belt reduction to the drive wheels in all cases. Some of these RC motors can even be found as low as 7000 RPM. Now a high voltage system, that makes sense. To me though, I just can't find the reason for big motors and inverters for 24V and 48V scooters.

  3. #3
    Associate Engineer
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    Nov 2016
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    Find yourself a slow industrial motor with a lot of torque, then you don't even need gears/transmission. Connect the motor(s) directly to the wheel(s).
    Last edited by zedeneye1; 11-21-2016 at 12:51 PM.

  4. #4
    Associate Engineer
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    Quote Originally Posted by sickwayne77 View Post
    So, a more refined version of my question would be, since a motors power is constant, but torque can be manipulated, why would anyone use the heavier motors with less power? True they have more torque at lower RPMs (if their power ratings are the same).
    -A motor's power is not usually/always constant (Torque increase with rpm may not be linear). Refer to motor's data sheet to find out how it varies at different RPMs.

    -Heavier motors with lower power are obviously for industrial applications where a lot of torque is needed. Also, in such applications weight is usually not a concern, but cost is. Also, heavy duty parts to make it more reliable or long lasting.

    In the automotive world your equivalent would be asking "Why would anyone need a slow, big, heavy engine that produces so little power but has good torque on low end?". The answer would again be industrial applications (trucks, ships, generators, etc).

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