I have a setup where a motor is spinning at a constant (known) RPM, under no load. I know the power going into the motor (voltage * current), and I can find out the rotational kinetic energy of the rotor.
My question is, how do I calculate the efficiency of the motor from this? Is it even possible? I understand that the motor is not doing any "useful work", but it is still overcoming frictional forces to keep spinning, and there are definitely energy losses there.