Determining Motor Constant Value "k"

[mclovin]

Hi All,
I've been cheeking out motor performance graphs quite a bit latley and I have to say that I'm a bit dismayed by the data that I'm reviewing. Basically I don't trust the graphs-or my understanding of them. It's something about the layout - I need speed alog X and torque along Y or it makes me confused for some reason. I've looked at several online motor graph applications but they are focused on RC motors. This is cool but I don't always want to run an RC motor - and the whole propeller thing throws me off as well. I really, really want to generate my own motor graphs but I'm stuck on determining the motor constant value "k." As in Torque=k*I etc. Is there some empirical method short using a dynamometer to find the value ok "k?"

 

[Miles]

For brushed motors you can spin the motor at a known speed and measure the voltage generated.

For brushless, see:
http://homepage.mac.com/kmyersefo/M1-outrunners/M1-outrunners.htm#KV

 

[mclovin]

Ugggh! Of course. Omega*k=V..... I just have to remember to convert RPM to rad/sec.

Thanks, Miles.


**************EDIT******************

Whoops. I was under the mistaken impression that Kv and Ke (the back EMF constant) were the same. The equation above does not directly apply. Miles just schooled me that Kv and Ke are reciprocals of each other. It is Ke and Kt that are the same in SI units as Miles informs me.

Cheers, Miles.

 

[mclovin]

Miles,
I looked at the info in the link you posted and I have a question for you (or whomever) about Kv. I was under the impression that Kt and Kv were equal/equivelant with each other or is that only the true for a brushed motor?

 

[Miles]

In SI units the back EMF constant is equal to the torque constant.

The velocity constant is the reciprocal of the back EMF constant.

Hope I've got that the right way around....

It applies to any motor in this universe, I think