What causes the back emf in motors (brushless DC motors to be exact). First thing is the rotating magnetic field of the rotor, which cuts the stator coils inducing an emf in the opposite direction. This would increase as speed increases. This is just a generator. Second, what about self inductance. You are switching the stator coils at a higher rate to gain more speed. Self inductance would limit the voltage. Do both together contribute to back emf?