For a brushless electric motor, the KV rating tells you how many RPM that motor can turn per volt supplied to it. For example, if you have an 11.1 battery powering a 3500KV motor, your motor’s top RPM will be 3500 X 11.1 = 38,850 RPM. This speed can only be achieved if there is no load on the motor.
All electric motors top speed are limited in the same way. As the armature is spun by magnets being repulsed or attracted to the temporary magnetic field generated by the coils of wire in the motor, the moving magnets also induce their own voltage (called back EMF) in the coils. The top speed is reached when the back EMF equals the input voltage.
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