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I am looking at 2 types of bldc motors for a project, a 9096 225 kv outrunner & a 100125 230kv inrunner. The rated torque of the outrunner is 17.8 nm with a Kt value of .0491 and peak current of 400 amps. The rated torque of the inrunner is 13.5 nm with a Kt value of .0526 and a peak current of 430 amps.

My question is, if the Kt value is supposed to be the torque created per amp why does the inrunner not produce more than the outrunner? I know the moment arm of the outrunner is greater, but shouldn't that reflect on the Kt value then?

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  • \$\begingroup\$ Which one costs more? \$\endgroup\$
    – vir
    Aug 23, 2022 at 17:46

3 Answers 3

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A couple of possibilities come to mind. First is that peak current rating may not correspond exactly to peak torque. Peak torque might be a peak continuous torque, and peak current could be the spike it's rated for.

Next, my recollection is that the internal torque needed to drive the shaft might be subtracted from your torque rating, and this can differ between motors.

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Are these motors even of similar weight? I have never seen an outrunner and inrunner similar weight with remotely similar kV ratings, let alone the near identical values you are listing.

Remember that a "rated" value is a value chosen by the designers; As in, the designers of the motor deemed this value as appropriate for the motor to operate at based on some criteria and this criteria is not always the same between motors (i.e. power vs efficiency).

From what I've seen, outrunners tend to be rated for maximum power output (a.k.a as much power and torque as you can get from the motor without overheating it) while inrunners tend to be rated at the point of maximum efficiency.

This is because half the reason of using an outrunner is its high direct drive torque and low RPM which often allows you to forego the cost and complexity of a gearbox. But an outrunner's efficiency-speed-torque characteristics still aren't as well matched as one would hope for many applications, and the result is that running an outrunner at maximum efficiency will result in a motor that produces an unreasonably low torque and power for its size, weight, and cost.

Whereas an inrunner's RPM is so inherently high anyways that you pretty much always require a gearbox for its power output to be usable. Therefore it is best to operate the inrunner at maximum efficiency since you always have a gearbox to adjust the power distribution between RPM and torque to match the application needs.

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I suspect the outrunner has a lower torque rating due to it's cooling capabilities - the stator surrounded by magnets have less surface area to reject heat than the stator that is also the housing of the motor. Both motors will make waste heat as a function of current squared, so that's a LOT of heat that must be shed.

As for the moment arm: that's been "accounted for" in the motors target BEMF coefficient. Essentially it's been designed for a certain kv, and windings are added or subtracted (at design time) until this requirement is met.

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