of the revolutionary changes in electric flight was the introduction
of brushless motors and controllers to our hobby. Before brushless
motors arrived on the scene, we struggled with various sizes
of less efficient brushed motors that were often doomed to
a limited lifespan from our abuse.
an electric-powered R/C airplane is a common choice for the
hobbyist. The clean and quiet brushless power is often the
better choice for certain size models. The availability of
Ready-To-Fly electric-powered packages provides ease and convenience
for all ages and skills.
commonplace in today's market, understanding how a brushless
motor or controller works, is often a mystery to many users.
While there have been many articles written on this subject,
I recently enjoyed several that were well written by Lee Estingoy
of Castle Creations.
published in the AMA's Model
Aviation Magazine in November, 2010, we rejoin Lee as
he takes us through the brushless basics to inside the electronic
How do brushless motors work?
DC motors are simple enough: magnets attached to a shaft are
pushed and pulled by electromagnetic fields that are managed
by an electronic speed control. This differs from brushed
(DC) motors which use mechanical brushes rubbing on commutators
to time and energize the magnetic fields. It is also different
from alternating current (AC) motors which generally use the
cycle of the power itself to time the powering of the coils.
Brushless motors provide significantly higher power to weight
ratios and much better efficiency than traditional brushed
the view from 40,000 feet and it's sufficient for most modelers.
However, a deeper understanding of their operation can go
a long way to helping a user select the right power system
for his application. Each discovery on this road will inevitably
lead to more questions, but we have to start somewhere. Let's
begin with the basics of brushless motor operation.
Inrunners and Outrunners
most common brushless motors for RC airplane and heli use
are known as outrunners as they have permanent magnets arranged
around the inside of a can that is then attached to the shaft.
The electrical coils are located in the center of the motor
with the can and its magnets running on the outside, hence
the name outrunner.
Neumotors ORK on the left uses a larger-than-normal rotor
diameter and a high pole count to generate more torque at
the shaft than a traditional inrunner motor. Motors with higher
torque can often drive props without using a gearbox.
functional opposite would be the inrunner type, which is by
far the most popular in RC car applications. Inrunners have
their magnets attached directly to the motor shaft and the
motor coils surround the shaft and magnets. While they look
quite different, the operational principles are the same for
both types of motor.
a brushless motor apart; you'll see a number of loops of shiny
copper wire running parallel to the motor shaft. There will
likely be many more than three, but it will be a number divisible
by three as each of these coils is actually part of one of
three winding circuits in the motor. The coils are distributed
such that every third coil is attached to the same motor wire.
The steel structure around which the coils are wound is the
stator stack, which serves to focus the magnetic field generated
by the coils.
stator stack is made of hundreds of rings of special steel
which focus the magnetic fields generated by the copper wire
that is wrapped around the "teeth" in the stator.
More layers of thinner steel generally leads to a more efficient
motor -- less heat for a given power output.
motor leads are connected to bundles of wire inside the motor.
Those bundles are wrapped around the "teeth" in
the steel stator. Note how the bundles are inserted every
permanent magnets in a brushless motor are arranged such that
their poles run perpendicular to the motor shaft. They are
aligned such that the pole presented to the electromagnetic
coil is either a north or a south pole. These alternate all
the way around the circumference of the rotor. The alternating
sequence of poles is convenient as adjacent poles are pushed
and pulled by the same magnetic field. A two-pole motor may
be made of a single magnet "wrapped" around the
shaft with the north pole at one side and the south pole at
image on the left reveals a loss of efficiency at the inner
the right, we can see how changes to the shape of the tips
of the stator teeth and increased "back iron" allowed
the stator to perform at much higher power levels before reaching
magnetic saturation (no magenta color). Tweaks like this can
make a huge difference in performance characteristics.
Motors use electromagnetic coils in the center of the motor
to drive permanent magnets attached to the rotating exterior
can of the motor. This design yields a larger rotor diameter
and can generate more torque than a similarly sized inrunner.
motors generally have the permanent magnets bonded to the
shaft, such as in the four pole rotor on the left. Two pole
rotors (far right) have the magnet surrounding the shaft.
The integrity of these attachments determines the max RPMs
of a motor. Some motors use a Kevlar (right) or carbon wrap
to increase the strength of the attachment to the rotor.
Brushless Motor Wiring
hobby brushless motors have 3 wires that make up the motor
coils, let's call them 'A', 'B', and 'C'. One end of each
wire is connected to the controller and the other ends are
"terminated," or joined, in one of two ways; DELTA
or WYE. Although these two look very different, the manner
of termination does not change the ESC's job of running the
any outside influence (like a moving magnetic field) these
motor winding circuits on the left are dead shorts, which
is exactly what the ESC has to deal with during startup or
under heavy load.
power to a single winding, or phase, by connecting one of
the motor wires to positive and another to negative, and you'll
create a magnetic field that pushes or pulls the permanent
magnets on the rotor. Reverse the current's polarity and the
attraction or repulsion is also reversed. This serves to pull
and then push each magnet pole on the rotor moving the rotor
along on its path. There are six possible power combinations
and all six power combinations must be used to drive one rotor
magnet pole around a single revolution in our simplified motor
precise sequence of power application and reversal is required
to turn the shaft one full revolution. The three winding coils
above are each energized in two opposing ways using the three
motor input leads, A, B & C. The combinations are color
coded red for + and black for --.This drawing represents a
two pole motor with each phase having only a single coil.
design and material quality of motor components have a direct
effect on the performance of the motor. Design factors include:
the amount of copper squeezed into the motor
the air gap, or distance between the magnets of the rotor
and the windings
the material, thickness and shape of the steel used in the
the material and strength of the permanent magnets
bearings are important too, as well as rotor balance
about anybody can make a motor, however, careful attention
to engineering principles can make the difference between
an efficient and reliable motor and one that performs poorly.
Inside the Electronic Speed Control
events are often attributed to mystical causes and brushless
power systems are about as mysterious as things get in RC.
Some systems work and others don't. Why? The usual explanation
is something along the lines of "It's a mystery!"
Don't get me wrong, the reason for a component failure was
no doubt a mystery to most involved, but understanding a bit
more detail about brushless systems can go a long way to helping
a hobbyist enjoy outstanding reliability in an electric plane
quick explanation of the role of the brushless ESC is that
it must accurately make and break connections between the
three input leads of the motor and the power source in order
to drive the rotor's magnets around the arc of the motor.
The most accessible way to describe the operation of the ESC
is to break it down by its functional sections. A brushless
ESC uses a microprocessor to manage the operation of the FETs
using information from a rotor position circuit. Let's look
at each of these a bit more closely.
are 4 main functional groups in an ESC; the power MOSFETs,
the MOSFET driver circuitry, the microprocessor, and the motor
position detection circuitry. A Battery Eliminator Circuit
(BEC) is present in some controllers and it serves to reduce
the voltage of the motor batteries to a level useful to the
radio system in the vehicle.
we get too far, we need to understand a few things about the
operation of a brushless motor. Brushless motors use three
sets of copper windings to push and pull permanent magnets
attached to the shaft inside the motor. It's important to
understand that these windings are connected at one end inside
the motor. There are two ways that this connection is made,
one is called the Delta, or D wind and the other is the Y
wind. Surprisingly, the controller really doesn't care which
of the two is used, they just need to be connected. Keep in
mind that the type of connection does affect the torque curve
of the motor.
two wind termination types are known as either a Delta or
a Y wind. Delta wind gets its name from the Greek symbol,
delta. It's not much a jump from there to understand the name
for the Y wind. Generally speaking, a Delta wind motor will
have nearly twice the KV of a similar motor with a Y wind.
call the three motor wires A, B & C, and their "free"
ends, the ends that stick out of the motor, are connected
to the ESC. The ESC uses electronics to connect any of these
wires to positive or negative to achieve one of six possible
combinations that result in an electromagnetic field in a
precise location in the motor. The timing and duration of
these connections is critical, and unbelievably short. Mechanical
switches are simply incapable of the task, but high power
electronic switches, known as MOSFETs (MOSFETs - FETs for
short - Metal Oxide Semiconductor Field Effect Transistors)
can turn on and off in a fraction of a second and they are
ideally suited for this application.
drawing of the connections required to drive a brushless motor.
The three motor wires are A, B & C, and they can each
be connected to the positive or negative poles of the power
source by the ESC. The 6 possible combinations are numbered
and the color coded letters indicate connections and polarity
at each point in the process. Red indicates connection to
positive and black indicates connection to negative.
do a little bit of math to get an idea of the incredible activity
going on in the ESC. An outrunner with 12 poles that has a
KV (RPMs per volt) of 1,500 that is powered with 24V (6S Lipo)
will spin at 36,000 rpm (24 x 1,500 = 36,000 rpm). The 6 coil
combinations needed for a full magnetic rotation must be repeated
for every north pole in the motor. The example motor has 12
poles, so the controller must switch the FETs 36 times per
revolution of the shaft (6 north poles x 6 steps per magnetic
rotation). That means there are 1,296,000 electrical cycles
per minute (36,000 rpm x 6 winding phases x 6 poles = 1,296,000)
or 21,600 cycles per second. The controller must successfully
switch between the phases every 1/21,600th of a second!
can flow in either direction on each of the three motor wires
making six possible combinations of current flow. This diagram
shows only one. The blue path traces the current flow from
the battery through the FET controlling the "high"
side of the red motor wire (A), to the motor windings and
back through the black motor wire (C) and the FET controlling
that phase's low side. ESCs vary throttle by switching the
low side FET on and off rapidly during the period that a phase
is powered, this is known as the PWM rate. The purple path
traces the "back flow" in the 3rd motor wire (B)
of current generated by the motion of the rotor magnets relative
to the windings. The rotor position circuitry measures the
voltage of this current to determine when to switch the FETs
to drive the rotor around inside the motor.
FET Drive Circuitry and Packaging
a FET ON and OFF is not as easy as it may sound. Each FET
has three connections; the gate, source and ground. In order
to turn the FET ON and create a circuit, the gate leg has
to be driven to a voltage that is 5 to 10V higher than the
voltage of the source leg on the FET which is connected to
the motor power source.
example, if using a 4S LiPo battery, +IN will be around 14.8V
(3.7V x 4). The gate requires (14.8 + 10 = 24.8) 24.8 volts
for proper operation. The ESC must therefore be able to boost
some of the power that it takes from the batteries to the
increased voltage to drive the FETs.
Position Detection Circuitry:
The ESC has to know the precise location of the rotor magnet(s)
to accurately sequence the connections made by the FETs. This
is the trickiest thing that the ESC has to do. There are two
main ways to go about this,
sensored systems use electronic (Hall) sensors in the motor
to track the rotor. This requires additional parts in the
in the motor (sensors) and an additional wiring harness
to connect the motor sensors to the controller. Sensored
motors and controllers are popular in RC car applications
as they provide a slightly smoother motor start than the
second approach, the sensorless controller. Sensored systems
were popular in the early days of R/C brushless aircraft
power systems, but they are generally considered to be less
reliable and less efficient than sensorless systems, therefore
they are no longer popular for RC aircraft applications.
modern ESCs can detect the rotor's position through the
power wires by "listening" to the third wire for
signs of motor position while the power to the motor is
applied to the other two leads. The changing magnetic field
caused by the spinning magnets in the motor actually generates
a voltage in the third wire and sensorless ESCs detect and
measure this voltage to determine how far the rotor has
turned. This information is then used to switch the FETs
as needed to cause the correct magnetic push or pull in
Microcontroller and its Firmware:
The microcontroller is the "brain" that runs the
whole operation. Running a brushless motor takes tremendous
computing horsepower and better controllers use processors
that operate at 25 MIPS - 25 million instructions per second!
Controllers with less capable processors may be unable to
process the data quickly enough to run high pole count motors
at high speed because they hit a computational redline long
before the motor reaches its full rpm/power capability. This
is particularly true with high pole count outrunners in high
rpm (geared) applications, such as helis.
run software in much the same way that computers run programs.
The software must manage a number of processes taking place
simultaneously in the motor/controller system. We've already
discussed how the controller switches the FETs and keeps track
of the motor position, but don't forget that the microcontroller
also has to process the input from the receiver to compute
the desired output power and flash indicator LEDs. Of course
the user may not want to run at full-throttle all of the time,
so we have to be able to limit the output power by pulsing
those FETS in between the usual positional pulses. If that's
not enough, there may be special routines that govern the
motor speed, record data, monitor battery voltage, watch for
over current or over temperature conditions and manage the
activities of the switching BEC. There is a lot going on here!
in FET packaging, the way that the internal silicon components
are connected to the circuit board, play a huge role in the
improvement of ESC over the past few years. The older S08
packaging on the left connects with the tiny legs while the
huge Drain pad on the newer Power Pack FET on the right provides
a much larger connection to the circuit board. The net effect
of this is that much more of the heat generated in the Power
Pack FET can be transferred directly to the circuit board.
The large tubular devices that are an obvious part of most
ESCs are capacitors. These are essentially fast-acting reservoirs
for electrical power and ESC designers use them to smooth
out the power as it enters the controller, but why is this
an issue at all?
that the FET gates need to see a stable voltage to operate
properly. In practice, the voltage that comes from the battery
is not a constant value; a graph of battery voltage would
actually looks like spurts of voltage. Each spurt starts at
a higher level than it ends at during each power cycle of
the FETs, however incredibly brief. A graph of this would
look like a ripple. This changing voltage is called RIPPLE
VOLTAGE. ESC designers can smooth this ripple out to some
extent by using capacitors, but there is a limit to how much
the capacitors can fix.
FET gate must have a voltage that is 10V higher than source.
If the source is crashing down and recovering up a bit between
each cycle, there is a possibility that the voltage in the
gate circuitry may unexpectedly meet/exceed the 10V margin
over the source voltage in the FET - this causes the FETs
to turn on unexpectedly -- and create nasty connections in
the controller that almost always lead to a bad day at the
field. (It's not really such a bad thing if the FETs turn
off? it is when they all turn on at the same time that
the smoke comes out.)
power boards designed for the Power Pack FETS (left) and the
older S08 FETS (right). Two of the phases are color coded,
blue pads = the motor wire connection, green and red pads
= the FET drains and the sources, and the orange pads = the
Advanced Topics in ESC Design
one of the following topics would provide plenty of material
for an engineering graduate paper. The following are very
Running at partial throttle is just a more complicated case
of running at full throttle. Instead of leaving two FETs (positive
and negative) "on" for the entire period of the
motor pole's transit of the motor winding, one is turned "on,"
while the other is pulsed on and off very quickly to reduce
the average power seen in the winding. At low throttles this
second FET is barely on at all, but it is on almost the whole
time near full throttle. The frequency (times per second)
that we pulse the power for speed control - not the polarity
switches that drive the motor -- is called the PWM rate, or
of the paradoxes of brushless motor controllers is that partial
throttle operation actually generates more ESC heat than full
throttle operation. FETS have a very small resistance when
they are fully "on" and current is flowing through
them. This generates a relatively small amount of heat but
it is not a significant amount. As always, there's more to
it. FETs don't just go from an ON to an OFF state, there is
a bit of a ramp to the process, a period where the FET is
neither open nor closed. Electricity can flow through the
FET during these periods, but the resistance in the FET is
much higher than when the FET is fully ON. This leakage across
a high resistance generates a significant amount of heat.
At partial throttle, the FETs are required to cycle much more
rapidly than at full throttle, so a great deal more heat is
generated at partial throttle than at full throttle. Similarly,
more heat is generated in controllers set to run at high switching
rates than those set to run at lower switching rates.
Voltage Limitations - 4S, 6S, HV
Brushless ESCs are generally rated for a very specific range
of voltage. This is due in part to the voltage rating of the
FETs themselves. Generally speaking, higher voltage FETs are
usually more resistive than lower voltage FETs, so higher
voltage controllers will require more FET capacity than lower
voltage controllers to handle the same amount of current.
The drive circuitry must also be modified to handle the higher
FET voltage limitation is a hard number. Exceeding the FET's
voltage limit usually results in instant destruction of the
FET. Always pay attention to the voltage limits recommended
by the ESC manufacturer.
Amperage Limits - 10 amps, 25 amps, 35 amps?
Unfortunately, amperage limitations are not always black and
white. Here's a list of considerations that determine the
current an ESC can handle successfully:
is a current above which the silicon inside the FETs or
the metal legs or connections on the FET break down and
fail. Damage from excessive amp draw takes place in an instant.
Think of a fast acting fuse, except and ESC is not usually
considered to be expendable. It is very hard to anticipate
high currents and shut the controller down in time to prevent
the current spike from damaging the controller.
throttle operation generates more heat as does high PWM
amperage capability of an ESC is limited by the ability
of the device to dissipate the heat generated by the resistance
of the FETs and circuit boards. If a controller is making
more heat than it can dissipate, a "runaway" condition
occurs which can lead to thermal destruction of the controller
- the solder holding the components to the boards literally
melts and the parts are free to float away.
great way to rate a controller is to determine its "steady
state amperage." That is the maximum current it can carry
at its rated voltage without experiencing further temperature
rise. This can vary a bit as the temperature rise is dependent
on the ambient air temperature and the amount of cooling airflow
over the ESC. A dangerous way to rate a controller is to state
its "surge" or "burst" capabilities. These
are an indication that the controller may be able to handle
higher currents for short periods, but these periods are sometimes
shorter than the pilot would hope. This is another area where
manufacturers can rate their products' based upon their own,
often ridiculous, definition of a controller's duty cycle.
Always read the fine print.
the proverbial duck on the water, things look calm on top
but there's a whole lot going on inside a brushless motor
controller. A great deal of engineering goes into the physical
design and the software is surprisingly complex. Always
use a power system inside its performance envelope for best
performance and reliability.
often take the products in our hobby for granted. Electric
power systems allow us to simply flip a switch and go fly.
Although it is this simplicity that attracts new hobbyists
and keeps things fun for all, removing the mystery of how
things work can help a user select the right power system
for their application and only benefit the overall experience.
you fly electric, fly clean, fly quiet, and fly safe!
Special thanks for contributions
Lee Estingoy of Castle Creations, Inc. and Model Aviation
section of AMP'D covers some of the questions that our
readers have sent in and I thought would be interesting
I've read your stuff over the years and I still
don't understand how to select an electric motor.
What am I missing?
are not alone as many people seem to struggle
with this concept. Fortunitely, there are more
existing aircraft models sold today that are already
mated with appropriate electric power systems,
so the selection process has gotten much easier
for customers over the past few years.
still use the general rule of thumb for electric
powered flight, originated by Dr. Keith Shaw several
decades ago. The rule is based upon a power to
weight level in watts per pound and reads something
and longer means more power!
for Cub-like planes or Trainers
for Sport/Aerobatic/Pattern planes
for 3D, EDF, or other high performance planes
you are in the appropriate range of power to weight
for your application, then the plane will likely
fly to your expectations for power. Other simple
concepts to keep in mind is that a bigger or longer
motor size usually means it is capable of producing
more power. Larger motors also come with a weight
penalty so you need to balance the size with your
application. Outrunners have eliminated the need
for a gearbox because they can generate more torque
at the shaft than an inrunner motor, allowing
it to drive a larger prop directly. I almost always
use an outrunner motor for prop applications.
Although written over three years ago, most of
the concepts in issue 3 of AMP'D, What
motor do I use?, are still valid today. The
more you enjoy electric flight, the easier it
will become to select the right motor.