Tried a search - got nowhere.

I understand that the term kV applied to electric motoers means rpm per volt. Where did the term come from, is it under load or noload conditions, is it meaningful?

paul

RcPaul, the kv rating on a motor is under a no load.
If you know the kv of a motor it gives you an idea of what to expect from the motor on a given prop and voltage in.

This number is the motor voltage constant expressed in RPM/Volt. This indicates how fast the motor will turn for a given voltage (given there is no internal resistance). As an example, a motor rated at 4300Kv rating is 4,300 RPM/Volt. Therefore, if a 6.0-volt battery pack is used, the RPM would be 25,800 (6.0 x 4,300).

Also with knowing this info you can figure the Watts out. This will also help know how the plane will proform.

Paul, seems I'm just around the corner from you. I live over by Polaris. Just a hop and a skip. If you want to go fly together let me know. Always welcome.
Hope this helped to understand the Kv rating for you. BG

Thanks BgCatfish1!

If I only know the unloaded rpm per volt, how do I know either the power (watts) in or out? If I knew the power in, I could make some assumptions about the efficiency and the power out. It seems the spec tells me the point where the back emf equals the voltage in but I still don't see what that does for me. Also, why the letter k?

Just getting into this area of the hobby but I do have an electrical background so I shouldn't be too snowed!

Paul

Like they say up in New England, "Ya can't get there from here." kV by itself won't help you in calculating the Watts out. Maybe coupled with a half-dozen other parameters and at least as many assumptions, you could make something out of it... Truthfully, we don't often calculate Watts out because it's practically impossible to calculate accurately given the number of variables. Watts out can only be accurately measured, by a dynamometer. Normally we simply use Watts in, making rough assumptions as to the efficiency of the power system. Watts in is easy to calculate: Volts times Amps. This is hardly an exact science.

What good is kV by itself? It's useful for comparing motors of similar size and weight. A higher kV motor will spin a smaller prop faster, or will be "more powerful" given the same prop and battery, at the cost of higher Amp draw and shorter run time.

The "k" in kV stands for "konstant," either the German word for, or simply a play on the word "constant," as used in mathematical equations.