WHAT DO THE KV RATINGS ON MOTORS MEAN?
updated Jan 2015
Manufacturers use different wire winds to produce different Kv results.
Kv is a number that relates to how many RPMs a given motor will spin based
on applied voltage. You will see specs on motors where it says Kv=860.
That means that the motor will spin at 860 rpms if you apply one volt to it.
If you apply 7 volts it will span at 6020 rpms.
If the manufacturer takes the same motor he can wind it so that it will
have a lower Kv rating, which typically produces more torque, so these are
typically used with large propellers that will be turned slower. These are
very popular on gliders, for example, where climb angle and climb rate is
much more important than top speed.
Take the same motor and wind it differently and it will have a higher Kv
rating producing higher speeds for a given voltage. These are typically
used with smaller props for higher top speeds. Or they can be used with gear
boxes to handle those big props, providing a similar result to low KV
motors. Sometimes a gear box works better in the installation.
You would also take KV into consideration based on what battery you plan to
use. Let's look at a practical example looking at various kV motors on the same plane.
You have selected a 500 KV motor. The specs say this motor, using a 5S pack
spinning a 15X8 prop that will draw about 45 amps. But suppose you did not have
the ground clearance for a 15" prop. Suppose you could only use a 12" prop. Then
you might choose a 1000 kV motor which would be a better match for that 12" prop.
Another way to look at this would be to think of kV like gears in your car. The lower t
he kV number the lower the gear the greater the "pulling power" or in the case of the prop
the greater the ability to spin that big prop. This is good at start up and take-off, or perhaps for
towing another plane, but it will produce a lower top speed.
Think if a high kV motor like the high gear in a car. Terrible acceleration from a standing start
and not a lot of pulling power for a trailer, but a higher top speed, once you get there.
Since we can't change gear ratios on the airplane's motor we select a compromise that will give us
the best average performance over the performance envelope we desire.
Let's say you were looking for a more scale look and that required you to use a 9 inch prop. Then you might
select a 1500 kV motor on the same plane. The airplane would probably fly but you would need a
longer roll out on the runway to get off the ground, more like a full scale airplane.
If you look here you will see that a given motor is offered in several Kv
ratings. They make suggestions as to which motor is best matched with which
prop and which battery packs. If you click on a given motor you can see what
kinds of power is drawn based on which pack and which prop. If you click on
each of the motors within a model you can see the very different power
curves produced by the different battery/prop combos. Here you see the same
motor with a different wind producing a different Kv result, each optimized
for a different purpose.
http://www.maxxprod.com/mpi/mpi-262.html
So, how does this add to other information about motors?
I first set a watts/pound target for my plane depending on the performance I
want. I typically target between 70 and 100 watts per pound for sport
planes and gliders. I don't fly 3D.
Then I consider whether I am looking for high speed or high climb rate. A
glider or a 3D plane would be optimized more toward the climb rate side of
this discussion. A pylon racer would be optimized more for speed. A pattern
plane might be somewhere in the middle.
Now I get down to prop and battery. Wider prop for better climb, narrower
deeper prop for higher speed. Now look at the motor character based on
either battery target or prop target and choose the motor/battery/prop combo
that meets your objectives.
That is kind of high level but you get the idea.