Why do Motors Get Hot When Undergeared?

[ta_man]

Originally Posted by Revorocks

I want to know if and why a motor gets hot when it's undergeared?
I thought it was to do with a motor operates most efficiently when under a certain load, so least energy is lost as heat.
However this guy on YouTube is saying it's most efficient under no load, which I disagree with..

Basically, who's right, and why.

Thanks

On the face of it, that is in obviously untrue statement (which you are right to disagree with) because:

efficiency = power-out / power-in

At no load, the motor produces no power (power = torque * rpm) since it is producing zero torque at the output shaft (if it was still producing torque it would speed up, duh!).

Therefore power-out = 0 [zero] and thus efficiency at no load is zero.

If you extrapolate this down the rpm curve a little bit, you will see that when the motor is operating at very light load, it will have lower effeiciency than the max that it is capable of. And so it end up hotter than you would otherwise expect for the work that it is doing.

[Rant: on]

It is really hard to win an argument with someone that has faith in their position because they don't understand the science behind what they are talking about. It is rarely worth trying to educate such a person because they are usually pretty far down on the learning curve. It could be they are a youngster who hasn't had the opportunity to learn yet, or it could be another failure of our education system.

Arthur C. Clarke said "Any sufficiently advanced technology is indistinguishable from magic."

Unfortunately, for too many people today, current technology is magic.
[Rant: off]