comparison of power. For example, a
. 15 brushless motor will have the
same power as a gas engine with a
. 15 displacement. Also be aware that
many times a . 15 gas or electric
motor will be referred to as a 15.
3. Stator Size. On an outrunner
brushless motor, the rotor is the outer
part of the motor. It has permanent
magnets attached to it. The rotor is
the part that turns. This is what you
connect your propeller to. The rotor is
wrapped around the stator. The stator
contains the coils which — when
energized in a particular order —
cause the rotor to turn. Many
manufacturers now use this when
describing their motors. For instance,
a 2826 motor will have a stator that is
28 mm in diameter and 26 mm long.
Along with the stator size, a kv rating
is given. The kv rating is the rpm per
volt of the motor. So, a motor with a
rating of 2,826-2,900 kv will turn
21,460 rpm with 7. 4 volts.
With these ratings, you
can start to compare motor
sizes. Even motors that are
rated using the can or glow
engine size will have kv
ratings. Some will also give
you the stator size, as well.
Some manufacturers will
provide the rotor size and not
the stator size. Take the
Turnigy 2211 motor shown in
Figure 5, for example. The
stator is actually 1809. It’s not
that big of a deal as long as
you know that they are doing
A lower kv rating means
lower speed and more torque. These
are good for driving large props. A
higher kv rating means less torque
and more speed. These are better at
driving small props. A motor will also
have a cell rating and max amperage
rating. If you exceed either of these,
you could burn up the motor. The
amp rating will give you an idea of
how much power the motor will draw.
If a motor draws 40 amps and you
have four of them running, you are
going to drain your battery quickly
unless you have a very large battery.
While there are many Electronic
Speed Controllers available, it is very
easy to choose one. First, since we
are using brushless motors, we need
a brushless ESC. You choose an ESC
based on its amperage rating. Two
ESCs are shown in Figure 8. The
larger one is 30 amps; the smaller
one is 12 amps.
If your motor’s maximum
amperage is 25 amps, you will need
the 30 amp ESC. If you have a small
craft that only pulls 10 amps, then the
30 amp ESC would be overkill and
add unnecessary weight to the craft.
In that case, you would choose
the 12 amp ESC.
Various ESCs have features that
you may want. For instance, on multi-rotors we use small computers for
control. We want an ESC that is
capable of responding quick enough
to keep up with the controller. The
type of connectors that come with the
ESC may dictate which one you
choose for a given project.
In order for your quadcopter to
get off the ground, it needs power.
We will be using Li-Po batteries as our
power source. A Li-Po's power-to-weight ratio is the best choice at this
point in time. They have more
capacity than their predecessors, so
for the same storage capacity you
have much less weight.
Li-Po batteries (like the ones
shown in Figure 9) come in many
different sizes, shapes, and
configurations. They use a rating
system that shows the number of cells
in the pack. A battery marked 3S1P
means that it is three cells connected
in series to make the battery. A 2S1P
will have two cells connected in series.
Li-Po batteries also have discharge
ratings. This will be marked as an XXC
number. A 25C means that it can be
discharged at a rate of 25 times its
capacity. So, in the case of a 2,200
mAh battery, you could discharge it at
a rate of 55,000 mAh or 55 amps,
without the battery being damaged.
Speaking of damage, Li-Po
SERVO 11.2012 39