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The term inrunner refers to an electric motor where the rotor (runner) is inside the stator. The term is in particular used for brushless motors to differentiate them from outrunners that have their rotor outside the stator.
Usage in drones and model aircraft
Compared to outrunner motors, inrunners tend to spin exceptionally fast, often as high as 11000 RPM per volt, far too fast for most aircraft propellers. However, inrunners lack torque. As a result, most inrunners are used in conjunction with a gearbox in both surface and aircraft models to reduce speed and increase torque
In many cases the inrunner is "ironless" in that there is no iron stator core to magnetize. The wire is run inside the can and held in place by epoxy or other resin material. Because there is no magnetic iron core, ironless in runners have no "cogging", in that they spin freely with no magnetic interaction when power is disconnected. A well-designed ironless inrunner is extremely efficient. This is because there is virtually no iron magnetization loss and very little windage loss in the motor. However, due to the lack of a magnetic stator core, the ironless motor has very low torque but also higher Kv (a unit that is defined as RPM per volt) when compared to an iron core motor.
Modern brushless inrunners can have iron core windings to address the lack of torque while maintaining high RPM capability, more commonly used in RC car and truck applications
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