there are three types of motors inside a model servo. These are iron
(Fe) core motors, coreless motors, and brushless motors. In a
conventional 3-pole Fe-core motor, the three field windings are wrapped
around a small chunk of iron. This assembly is what rotates and it's
called an armature. Sequentially turning the 3 fields on-and-off is what
makes the motor rotate. Clear?
since turning fields on-and-off requires a switch (just like switching a
light bulb on-and-off), this process of switching in motors is called
commutation. How is it done? Simple, really because if you look closely
at one end of a motor there's a copper ring. And if you look really
closely you'll see the ring is actually split into three parts (one for
each winding). Using mechanical brushes to actually make the contact
with the commutator ring segments, the motor rotate as each field is
turned on and off sequentially. Thus, as the motor rotates the brushes
make then break contact thereby energizing the next field to turn on the
electromagnetic force. The brushes making and breaking contact with
each of the three segments of the copper commutation ring is why the
motor rotates. Clear?
an effort to make an electric motor reach operating RPM more quickly
(both motors may hit the exact same RPM, so we're just talking about
getting the motor up to operating speed more quickly), a savvy engineer
figured out how to form the copper windings without the iron core (hence
the term coreless). Presto, by reducing the rotating mass of the
armature (since it doesn't have a small chunk of iron inside), the
coreless motor's armature is lighter and this means the coreless motor
reaches its operating RPM more quickly than an iron core motor.
Otherwise, you can have the same mechanical characteristics, e.g. same
bearings, KV, same torque, same everything between a coreless and Fe
core motor. This is why the speed specs of a coreless servo are better
(faster) than those for a servo with a conventional motor. If you need
the speed to achieve a certain maneuver, then the equation is simple,
you pay more for servos. Where the confusion comes in is with regards to
brushless-motor servos. Let's try to clear this up.
it's important to understand that brushless motors are coreless also.
The only difference is a brushless-motor is simpler because it uses
semi-conductor (electronic) switches to achieve commutation while the
coreless-motor servo uses the same old-fashioned brushes and commutator
ring to switch the fields. The advantage of the brushless motor vs. the
coreless motor is there are no bruishes to wear Do you know what's meant
by commutation? It's simple, a commutator is just a switch. And if you
disassemble an electric motor and see this copper ring with three
grooves cutting across it, it just means you have a 3-pole motor with
three separate windings. Thus, and the motor rotates and the brushes
jump the gap in the commutator ring, the contact is broken form one
field and made to the next field as the brushes make contact with the
next segment of the commutator ring. This process of switching current
from one field to the nest is termed commutation.
windings are switching from one to the other. Understand? one In a
conventional DC-motor (either core or coreless), a commutator switches
the current from winding to winding via brushes thus forcing the rotor
to turn. This is how DC motors work, OK?
is, as the brushes make and break contact with the commutator surface,
there's a tiny electric spark, or arc (yes, like a miniature arc
welder). As the tips of the brushes spark molecules of the metal are
evaporated away - poof - as in gone! Of course, these bits of metal
being worn away don't disappear because that would violate the laws of
physics (conservation of matter). Instead these particles become a very
fine metallic dust. And this metallic dust begins to coat the inside of
the motor, which insulates, e.g. leads to heat retention, which is bad.
Worse, the dust gets into the bearings too and while all these things
don't get bad all at once, they do begin from day one - fact!
to this, over time the brushes wear. Eventually they become too worn to
make good contact with the commutator surface. Also, at the same time,
the tips of the brushes wear a groove in the surface of the commutator.
This is when a coreless servo begins to develop the jitters, or dead
spots on the commutator, plus the other issues because these all lead to
less than optimal performance. Electronic commutation eliminates all
this bads stuff; here's how.
a brushless motor replaces the mechanical components (brushes and
commutator) with electronics - semiconductor switches. Thus, brushless
motors feature electronic commutation and by ditching both the brushes
and commutator we bring your servo-motor technology up to date from the
1920s because the stator windings are turned on-and-off at the
appropriate time by magic. OK, it's not magic, but via electronics . . .
and electrons don't wear out! Neither do they coat the inside of the
motor, or otherwise lead to all the crap building up on the inside of
the motor and inside the tiny bearings. Oh, and by the way, it's not
that coreless motors are bad (we offer a coreless servo for those on a
budget and JR gets $200 for 8711HV coreless servos) it's just that
brushless motors are better . . . in our opinion, a lot better, which is
why this is our top of the range offering.
there a downside to a brushless-motor servo? Well, sure there is . . .
brushless-motor servos costs a little bit more. Is it worth the
difference? That's up to you but we're not talking about a heck of a lot
more so if you can afford that then by all means you definitely want
brushless-motors instead of coreless-motors inside the servos
controlling your pride and joy.
Now you're in the know. Next time you hear someone confused about the difference . . . go ahead and clue them in.