# How do I calculate motor efficiency from voltage, current and RPM?

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.

• "Useful work" is whatever is possible to get out of the motor, not what is actually gotten out of the motor. At least, that's a reasonable interpretation. The efficiency of a motor should not suffer just because you forgot to hook it up to something right? – levitopher Jul 27 '14 at 5:36
• I think "useful work" just means measurable output. I guess there needs to be some work done over time to find the power output of the motor. If there's some translational motion, this is easy, but if there is rotational motion, you'd need to know the torque of the motor (which I don't). This website helped a little: engineeringtoolbox.com/work-torque-d_1377.html – 1overcosine_c Jul 27 '14 at 11:35