# 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

## 1 Answer

If you know the rotational kinetic energy then cut the power and time how long it takes for the motor to come to a stop. Divide the kinetic energy by this time and you have the power dissipation as friction. The difference between this and the input power is then the power dissipated though non-frictional mechanisms.

Whether this is related to efficiency is debatable as the motor isn't doing any useful work so the efficiency is 0%. All your doing is splitting up the energy dissipation into frictional and non-frictional parts. It's still an interesting experiment though.

• Yeah, I guess there is a 100% loss as the power is just going into overcoming friction. I suppose I could just use the classic "lift a mass" experiment. – 1overcosine_c Jul 27 '14 at 5:19