For a given motor quality, the bigger the motor mass the more torque the motor will have, and thus the bigger the prop it will be capable of swinging. If you choose to swing that bigger prop the motor will draw more amps.
For a given motor quality, the bigger the motor mass the Lower the RM (internal resistance) will be for a given KV, which means fatter wires and the motor will be capable of handling more power and will be more efficient under higher loads.
For a given motor quality, the bigger the motor mass the higher the IO (unloaded current draw) will be for a given KV, so even if you don't choose to swing the bigger prop the amp draw will be a little higher with the bigger motor (although not nearly as much higher as if you did swing the bigger prop), so bigger motor are less efficient under light loads.
I talk about mass rather than size as there are so many length and diameter options, and for practical purposes it makes little difference whether you get the motor's bigger mass from a longer can or a fatter can, For a given motor quality a 180g 28mm diameter long can will produce similar power and torque figures as a 180g 36mm short can, as will a 300g 36mm long can when compared to a 300g 40mm short can, and a 650g 40mm long can will when compared to a 650g 56mm short can. In most cases packaging constraints and commercial availability will dictate length to width ratio rather than any theoretical benefits to bigger diameter or longer cans.
I will explain KV as it is often misinterpreted, KV (velocity constant) is the RPM (usually quoted unloaded) that the motor will spin for each volt applied to it, for instance a 1000KV motor will spin at 10,000RPM at 10V and 20,000RPM at 20V.
As the KV increases the KT (torque constant) decreases proportionally, a lot of people misconstrue this as that low KV motors have more torque than high KV motors, but that is not quite the case. If you take 2 motors of similar quality and mass, one with high KV and one with Low KV, the one with the low KV will have a higher KT, but KT is not torque, it is torque per amp; While for a given current level the low KV motor will have more torque, the high KV motor will have fewer turns of thicker wire in order to achieve that higher KV, which decreases it's RM and increases its current handling capability, if you increase the current of the high KV motor to its limits its torque increases to the same as that of the low KV motor.
Last edited by NativePaul; 12-23-2020 at 10:04 AM.
Paul Upton-Taylor, Greased Weasel Racing.
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