heli iq Feature
INSIDE AN ESC
UNDERSTANDING THE COMPONENTS
THAT MAKE IT WORK
words: Art Koral and Jonathan Feldkamp
Y
We probably underestimate
just how beneficial ESC
technology is. There
was a time not so many
years ago when ESCs for
military and commercial
applications were large and
expensive. Now, thanks to
improvements in solid-state
technology and FET quality,
ESCs are compact and
affordable. They are at the
core of the technology that
makes hybrid cars a reality.
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Basic component
layout and function
The job of an ESC is to take DC power
solder
from a battery and accurately control a
3-phase brushless, sensorless motor.
Before we talk about the components
A
and layout of the ESC portion, it helps
to understand what the motor looks
B
like electrically. Most brushless motors
C
have three wires or phases—we will
call them A, B, and C. They can be
connected in one of two basic ways:
Wye or Delta (shown to the right; the
squiggly lines represent the motor
A
coils [inductors electrically]). Although
these two look very different, it does
B
not change very much electrically. The
C
important thing to realize is anything
that we do to A and B is going to affect
C; the reason for this importance will
come later. Also note that without any outside influence (like a moving magnetic field)
these circuits are dead shorts—exactly what the ESC has to deal with during startup.
The ESC’s job is to take power from the battery and power the motor. This is
accomplished with MOSFETs, high-power switches that can turn on and off in a
fraction of a second. A very basic overview of a brushless power system would look
something like the diagram on the next page. The image shows that by turning on
the A and B switches (marked by the red *), current flows from +IN through the phase
connected to A and B and then to ground. The current flowing though this phase, or
coil, produces a magnetic field that either attracts or repels the magnet on the rotor
and causes rotation.
wye wind
technology
delta wind
That Prius looks mean!
ou have probably wondered what some
of the components on the board of an
electronic speed controller (ESC) are.
In this article, Jonathan Feldkamp of Castle
Creations details what those components do
and how they work together to accomplish
speed control. With this knowledge you will
have a greater respect for its technology, as
well as an understanding of how to size your
ESC and why amp and voltage limits exist.
MOSFETs (FETs):
Metal Oxide
Semiconductor
Field Effect
Transistors
simplified diagram of esc connection to a delta-wind motor
+ IN
+ IN
A
+ IN
B
C
Causing these little spurts of rotation is
easy—we can do the same thing with just a
battery by touching two of the motor wires
directly to the battery terminals. (Obviously,
GRN
GRN
GRN
don’t do this: It will either kill the battery or
the motor if left connected for more than a fraction of a second. One of the ESC’s biggest jobs is limiting how much current is allowed to
be pumped through a phase in this type of situation.) The trick to running a motor is turning the right two switches on at exactly the right
time and then back off again before things get out of hand. As current flows through a phase, the magnet is attracted to it (N to S or S to
N), pulling on it and rotating it toward the coil. But when the magnet has passed the coil, we change which FETs are turned on in order
to make current flow the other direction, repelling the magnet (N to N or S to S) and pushing it along in its rotation away from the coil.
Expand this idea to three coils, and it is pretty easy to see how you make the magnet spin all the way round—once we can do that, all
we have to do is repeat the process over and over again. For a real-life example, consider the speed controller in the next picture.
GRN
MOSFET
+IN
GRN
MOSFET
B
MOSFET
MOSFET Drive
Circuitry
GRN
MOSFET
C
MOSFET
+IN
In this picture, you can easily see the layout of the six MOSFETs
and their connections to power, ground, and the three motor
wires. You can also see the receiver cable and the large input
capacitor that acts as a reserve for the ESC. All of the rest of
the small parts are
filters of some sort
that are there to
A
ensure everything runs
GRN
+ IN
properly. The image
to the right shows a
B
CAD version of this
circuit board with
GRN
+ IN
the simplified ESC
C
diagram overlaid on
top of it where you
GRN
+ IN
can clearly see the
GRN
+ IN
large planes of copper
that make the highcurrent connections.
+IN
Clear view of six MOSFETs used to switch voltage on and off in each motor phase
on a Castle ESC.
A
MOSFET
Simplified functional diagram
of a standard ESC Circuit
LED
Microprocessor
RX
Motor Position
Detection
Circuitry
august 2009 | 77
And that’s the “simplified” one.
Now that we have a basic understanding of what it takes to make
a motor spin, we can start to look at the functional diagram of a
speed control. There are four main functional groups that we will
discuss: the power MOSFETs, the MOSFET driver circuitry, the
microprocessor, and the motor-position-detection circuitry. The
following image shows how these parts are connected together.
heli iq feature
inside and ESC
We now have a pretty good understanding of how the power portion
of an ESC works: The MOSFETs act as switches, turning on and
off to cause current to flow through the motor coils. Sometimes,
however, one MOSFET is not enough. For high-current controllers,
multiple FETs are used together in tandem to act as better switches.
Almost all of the heat generated in an ESC is caused by the
resistance of the FETs—every time we double the number of FETs,
the resistance is cut in half. Another option, besides adding more
FETs, is to use better ones. High-end controllers will use the best
FETs available.
Sensor yourself.
MOSFET
drive circuitry
Turning a FET on and off is not as easy as it may sound. If
we look at the electronic symbol, we see three connections.
The single connection that exits the part to the left is called
the gate—this is the FET’s switch. In order to turn the FET
on, the gate has to be driven to a voltage that is 5V to 10V
higher than the leg on the bottom of the FET. (This pin is
called the source.) For the low-side FETs, this is pretty easy;
we just need a 10V supply to turn them on. In order to turn
on the high-side FETs—those connected to +IN—a voltage
of 10V higher than +IN must be applied to the gate. (Think
about it: Once the FET is on, the drain is connected to +IN,
and in order for it to stay on, the gate has to stay at least 5V
above +IN). For example, if you’re using a 4S LiPo battery,
+IN will be around 14.8V, but to turn on the FET, a voltage
of 25V has to be applied to the gate. In order to accomplish
this, ESC designers can choose from off-the-shelf FET driver
chips or design the circuit themselves.
Motor-position
detection circuitry
In order to know when to turn the FETs on and off, the ESC has
to know exactly where the magnet is in its rotation process. This
is the trickiest thing that the ESC has to do—and this is also why
brushless motors and ESCs used to use separate sensors to
track the magnet. (This is still popular with car controllers today.)
Sensorless ESCs use a different method for this, however; while
the controller is using two of the motor wires to power the motor,
the third wire is left completely unconnected. We might expect
this third leg to be exactly halfway between the other two, but it is
not. The changing magnetic field caused by the spinning magnet
induces a current in the third phase that causes the voltage to
be slightly different than half. We can filter this signal and use it
to determine how far the rotor has turned and when we need to
switch the current FETs off and move on to the next two.
Here is a typical car
controller setup. Note the
extra sensor wire (black)
that tells the ESC where
the rotor is in its rotation.
Sensored setups are usefull
when lower and variable
RPM’s are required to
prevent “cogging.”
The microcontroller and its firmware
Without question, the microcontroller is
the brains of the whole operation. The
microcontroller runs software much
the same way that our computers run
programs: Developers write software,
compile it, and download it to the
microprocessors. Besides sending
signals to the FET drivers and keeping
track of the motor position, the
microcontroller also has to process the
input from the receiver, compute the
desired output power, and flash any
indicator LEDs. (Don’t forget that the
user may not want to run at full throttle
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all of the time, so we have to be able to
limit the output power.)
Processing the input from the receiver
is a pretty straightforward process. The
signal is a digital pulse whose length
determines the output power. A 1.0
millisecond pulse means full off (or
brake), while a 2.0 millisecond pulse
means full throttle, and everything in
between is partial throttle. As easy as this
process is, however, it is important that
we remember that everything that takes
away from watching the motor position
could cause us to miss the time that we
were supposed to change active FETs.
Running at partial throttle is just a
more complicated case of running a full
throttle. Instead of leaving two FETs on
and causing current to pull the magnet
along, we instead leave one turned on
and pulse the second on and off very
quickly. At low throttles, this second
FET is barely on at all, but as we
approach full throttle it is on almost the
whole time. The frequency (times per
second) that we do this pulse is called
the PWM frequency.
heli iq feature
inside an esc
Hardware limitations: 4S, 6S, HV, SHV, etc.
Besides the programmable features, hardware limitations also set controllers apart. Some are rated
for only 12V, while others can handle batteries all the way up to 90V. Not much changes as far as the
microprocessor is concerned; it is still a matter of turning on FETs as the magnet rotates. The most
obvious difference is the parts that are used on the circuit board. The FETs have to be rated at higher
voltages that usually mean they are not as good (and more resistive). The FET drive circuitry now has
to boost even higher and be able to drive even more FETs. The large input capacitors have to change
(and usually get a lot bigger). Basically, every part has to be evaluated to see if it will perform correctly
at the rated voltage. Besides the obvious, though, everything gets a little tricky at high voltages. Small
voltage spikes that didn’t cause any problems at 12V are now big enough to turn on FETs that are not
supposed to be turned on. (Imagine a high side and low side both being turned on at the same time—
this is the same thing as a direct short across the battery.) Great care has to be taken to ensure clean
signals and correct operation.
Got it all?
BEC: Battery
Eliminator Circuit
Another issue with high voltage
is the BEC. Back in the good
old days, everyone had a gas
engine for power and a small
battery pack to power the
receiver and servos (receiver
pack). As electronic speed
controls became popular, a
simple device called a linear
regulator was built into speed
controls to create a 5V power
supply that could replace the
receiver packs. This simple
BEC works great with servos
that do not draw much current
and especially well with low
input voltages. The problem
with the standard BEC is that it
operates by turning the
un-needed portion of the battery
voltage into heat. With a 12V
for an application. Let’s assume
that we have already picked a
battery and motor. Now we have
battery, 6V is being wasted; if
our servos use a total of 1A, that
is 6 watts of generated heat. But
with a 25V battery, 20V is being
wasted; at 1A we are generating
20 watts of heat—that is simply
too much for these linear
regulators to handle, and they
will shut down.
The next step in the evolution
of the BEC was the creation of
a switching BEC. The switching
BEC works in a way similar to the
speed control itself: It uses a FET
to turn the battery power on and
off very quickly and then filters
the output to create a constant
output voltage. The most notable
advantage is that the extra
voltage is not turned into
heat; a switching BEC
can easily be around 90
percent efficient.
to figure out how much current
our setup will draw. Choose a
speed control rated above the
estimated full-throttle current
(e.g., if 67A is considered
worst-case, then a 75 amp or
above would work just fine).
That being said, there is never
any problem with using a
controller that can handle a lot
more than you need. If it doesn’t
add too much weight, always
shoot for the larger controller.
A good practice is to make
a quick test flight, check the
controller temperature, go for
a longer flight, check again
the temperature to ensure
temperatures remain within safe
operating limits. Calculators can
be wrong, and every vehicle
is different; this is a hobby,
and caution should be taken,
especially during the first few
uses. The temperature of the
controller should stay below 212
degrees F—it should not sizzle
when you lick your finger and
quickly touch it.
Picking the rightsized controller
Castle is about to release a new highvoltage BEC that can operate at 12S and
deliver up to 20 amps of peak current.
Now that we know a little
more about how the ESC
works, it is a little easier
to pick the right controller
So, as you can see, ESCs are fairly simple in design; however, their programming and the quality of
their components is vital for proper operation. Choosing the right ESC for an application will determine
whether or not a model has a smoke feature you probably don’t want. Special thanks again to Jonathan
Feldkamp and the team at Castle Creations for their tremendous contributions to our hobby.
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why
running
at lower
throttle
settings is
harder on
the ESC
Before we get into
why partial-throttle
settings can generate
more current, we have
to briefly discuss back
EMF. When motors
rotate, not only do
they use power to
create motion, they
also act like generators
producing electricity
or back EMF. The
net result is a slip
condition where more
power is needed
than generated, and
the higher the load,
the greater the slip
condition. Slip is
needed to generate
motion, and the
current required is the
difference between the
power pulse and the
back EMF pulse.
Partial-throttle or
governed operation
at high load is harder
on the ESC than
full-throttle operation
because the speed
controller pulses
for shorter intervals
yet provides more
instantaneous current
when under load. At
lower RPMs, the back
EMF is reduced, and
the difference in current
between the back EMF
and the instantaneous
on pulse can be very
high. Watt meters
will not pick up these
spikes and will only see
the average current.