Motor terminology - Kv

[learningrc]

In my world we don't talk about Kv, we talk about Ke - the back-emf constant. This is just the back-emf voltage divided by RPM. As I've been getting into the RC world, I assumed Kv was just the inverse of that - (RPM)/(back-emf voltage). However, everything I read seems to treat Kv as (RPM)/(applied voltage to the motor). When RC motor manufacturers classify their motors as a certain Kv, are they talking about back-emf voltage or applied voltage?

 

[Motopreserve]

As I understand it - it's the latter. Applied voltage.

 

[Hexacrafter]

Yep... I agree with Moto

 

[econfly]

I'll admit I don't really get "Kv" and its practical use. In reality the RPM/volt will depend on the prop load. Do manufacturers just specify Kv for the smallest rated prop? In any case, it seems much more useful just to look at the motor specs for a given prop to see what RPM is produced at various power levels (and the resulting thrust). I am missing something?

 

[Motopreserve]

That definitely makes sense - but would it effect the difference of whether they are incompletely referring to back-emf, or applied voltage? Or are they referring (incompletely) to applied voltage only???

 

[econfly]

I have no idea -- my understanding of this is that, all else equal, a lower Kv motor spins more slowly. But make all else not equal and I just ignore Kv and look at the watts/thrust/RPM specs for a given prop size and voltage. This is why in another thread I was so focussed on the reliability of spec sheets. Without that, we are really just guessing when trying to pick the right motor.

 

[Motopreserve]

Yep, I was thinking about your other thread.

I guess we just need to err on the side of caution when picking a motor. Seems there are variables that are just untrustworthy.

 

[econfly]

Or just order a few, test them, etc. "Trust but verify".

 

[Motopreserve]

Or just order a few, test them, etc. "Trust but verify".

Perfect!

 

[SleepyC]

View attachment 24497

 

[learningrc]

Do manufacturers just specify Kv for the smallest rated prop?

Kv is a property of the motor that doesn't change with what prop you have. If manufacturers use applied voltage, then the best way to specify it would be to apply 1 V, measure the RPM, then apply 2 volts, measure the RPM, etc until you get enough points to make a best fit line of RPM vs voltage.

If manufacturers use back-emf, then the best way is to back-drive the motor with another motor (or a drill or anything else that spins at a reasonable rate) and then measure the voltage that is generated between 2 of the leads.

 

[econfly]

Kv is a property of the motor that doesn't change with what prop you have. If manufacturers use applied voltage, then the best way to specify it would be to apply 1 V, measure the RPM, then apply 2 volts, measure the RPM, etc until you get enough points to make a best fit line of RPM vs voltage.

If manufacturers use back-emf, then the best way is to back-drive the motor with another motor (or a drill or anything else that spins at a reasonable rate) and then measure the voltage that is generated between 2 of the leads.

OK, so the back emf measure is, by definition, one that implies no load on the motor. So the applied voltage measure must be similar -- the rpm (1000s) per volt absent load. And, I guess this further means that the rpm/volt curve is linear or very near it.