Inside the Electronic Speed Control - 2010/11

58 MODEL AVIATION
by Lee Es t ingoy
Inside the
Electronic
Speed
Control
MYSTERIOUS 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!” The reason for
a component failure is a mystery to most
involved, but understanding a bit more about
brushless systems can go a long way toward
helping a hobbyist enjoy outstanding
reliability in an electric-powered airplane or
helicopter.
A brief description of the role of the
brushless Electronic Speed Control (ESC) is
that it must accurately make and break
connections between the three input leads of
the motor and the power source to drive the
rotor magnets around the arc of the power
plant. The most accessible way to describe
the operation of the ESC is to break it down
by functional sections.
A brushless ESC uses a microprocessor
to manage the operation of field-effect
transistors (FETs), using information from a
rotor position circuit. Let’s look at each of
these more closely.
Making the Connection: Before we go too
far, let’s make a few things clear about the
operation of a brushless motor. It uses three
sets of copper windings to push and pull
permanent magnets attached to the shaft
inside the power plant. It’s important to
understand that these windings are
connected at one end inside the motor.
There are two ways this connection is
made; one is the Delta, or D-wind, and the
other is the Y-wind. The controller doesn’t
care which is used; the windings need only
to be connected. The type of connection
does affect the torque curve of the motor.
Let’s call the three motor wires “A,”
“B,” and “C,” and their “free” ends, those
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 results 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 Metal Oxide
Semiconductor Field Effect Transistors
(MOSFETs, or FETs for short)—can turn on
and off in a fraction of a second and are
ideally suited for this application.
Let’s do a bit of math to get an idea of
the incredible activity going on inside the
ESC. An outrunner motor with 12 poles that
has a Kv (rpm per volt) of 1,500 and is
powered with 24 volts (6S Li-Poly) will spin
at 36,000 rpm (24 x 1,500 = 36,000).
The six 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 1/21,600 second!
FET Drive Circuitry: Turning an FET on
and off is not as easy as it might sound.
Each has three connections: gate, source,
and ground. To turn the FET on and create a
circuit, the gate leg has to be driven to a
point that is 5-10 volts higher than the
voltage of the source leg on the FET, which
November 2010 59
Even electronic horsepower needs a jockey
is connected to the motor power source.
Refer to the simplified ESC diagram. If
using a 4S Li-Poly battery, +IN will be
roughly 14.8 volts (3.7 volts x 4). The gate
requires 24.8 volts (14.8 + 10 = 24.8) for
proper operation. The ESC must therefore
be able to boost some of the power it takes
from the batteries to the increased voltage to
drive the FETs.
Motor Position Detection Circuitry: The
ESC has to know the precise location of the
rotor magnet(s) to accurately sequence the
connections that the FETs make. This is the
trickiest thing that the ESC has to do.
There are two main ways to go about
this: sensored and sensorless. Sensored
systems use electronic (Hall) sensors in the
motor to track the rotor. This requires
additional parts 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, because they
provide a slightly smoother motor start than
the sensorless controller. Sensored systems
were popular in the early days of RC
brushless aircraft power systems; however,
they are generally considered to be less
reliable and less efficient than sensorless
systems, so they are no longer popular for
such applications.
Sensorless/modern ESCs detect the rotor
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
spinning magnets in the power plant
generates a voltage in the third wire, and
sensorless ESCs detect and measure that
voltage to determine how far the rotor has
turned. Then the information is used to
switch FETs as needed to cause correct
magnetic push or pull in the phases.
The Microcontroller and Its Firmware: The
microcontroller is the “brain” that runs the
whole operation. Operating 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
might 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
helicopters.
Microcontrollers 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.
I’ve mentioned how the controller
switches FETs and keeps track of the motor
position. Don’t forget that the
microcontroller also has to process input
from the receiver to compute the desired
output power and flash indicator LEDs.
The user might not want to run at full
throttle all the time, so we have to be able to
Current can flow in either direction on each of the three motor wires, making six possible combinations of current flow. This
diagram shows one. The blue path traces 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 the PWM rate.
The purple path traces the “backflow” in the third motor wire (B) of current generated by the motion of the rotor magnets relative
to the windings. The rotor position circuitry measures voltage of this current to determine when to switch the FETs to drive the
rotor around inside the motor.
60 MODEL AVIATION
The two wind termination types are known as a Delta and a Y-wind. Delta
wind gets its name from the Greek symbol. It’s not much of a jump from
there to understand the name for the Y-wind. A Delta-wind motor generally
has nearly twice the Kv of a similar motor with a Y-wind.
This is a basic drawing of connections required to drive
a brushless motor. The three motor wires—A, B, and
C—can each be connected to positive or negative poles
of the power source by the ESC. The six possible
combinations are numbered, and color-coded letters
indicate connections and polarity at each point in the
process. Red indicates connection to positive; black
indicates connection to negative.
limit the output power by pulsing those
FETs between the usual positional pulses. If
that’s not enough, there may be special
routines that govern motor speed, record
data, monitor battery voltage, watch for
overcurrent or overtemperature conditions,
and manage activities of the switching BEC.
There is a lot going on here!
Input Capacitors: 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?
Remember 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 look like spurts of
voltage.
Each spurt starts at a higher level than at
which it ends 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 out this ripple to some
extent by using capacitors, but there is a
limit to how much the capacitors can fix.
The FET gate must be 10 volts higher
than source. If the source is crashing and
recovering a bit between each cycle, the
voltage in the gate circuitry might
unexpectedly meet/exceed the 10 margin
over the source voltage in the FET. That
causes the FETs to turn on unexpectedly—
and create nasty connections
in the controller that
typically lead to a bad day at
the field.
It’s not such a bad thing
if the FETs turn off. It is bad
when they all turn on at the
same time that the smoke
comes out.
Advanced topics in ESC design include the
following, any one of which would provide
plenty of material for an engineering
graduate paper. These are simple
descriptions.
• Controlling Speed: Running at partial
throttle is merely 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 rapidly pulsed on and off to
reduce the average power seen in the
winding.
At low throttles this second FET is
barely on, but it is on almost the whole time
near full throttle. The frequency (times per
second) at which we pulse the power for
speed control—not the polarity switches
that drive the motor—is called the PWM
rate, or switching frequency.
One of the paradoxes of brushless-motor
controllers is that partial throttle operation
Illustrations by the author Photos by the author and MA Staff
Improvements in FET packaging, the way
the internal silicon components are
connected to the circuit board, play a huge
role in the improvement of the ESC in the
past few years. The older S08 packaging (L)
connects with the tiny legs, while the huge
Drain pad on the newer Power Pack FET
(R) provides a much larger connection to
the circuit board. The net effect is that
much more of the heat generated in the
Power Pack FET can be transferred directly
to the circuit board.
There are four 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; it reduces the voltage of motor
batteries to a level that is useful to the radio system in the vehicle.
November 2010 61
Plug-and-play systems are noted for their
ease of use—no soldering. Electric-power
systems from E-flite are that easy and are
labeled with a system that correlates
with glow-power designations.
Typical ESC power boards designed for
the Power Pack FETs (top) and the older
S08 FETs (right). Two phases are color-coded; blue pads = motor wire connection,
green and red pads = FET drains and sources, and orange pads = gate connection.
Great Planes motors are sold under the
ElectriFly brand name. They feature plugand-
play electric-power systems for
models weighing 5 ounces to 50 pounds.
AXi motors can arguably be credited with making electric power available via mass
production and reasonable pricing. They are among the most efficient systems
available.
generates more ESC heat than full-throttle
operation. FETs have a small resistance
when they are fully on and current is
flowing through them. This generates a
relatively little amount of heat. As always,
there’s more to it.
FETs don’t simply go from an on to an
off state; there is a bit of a ramp to the
process in which 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, 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.
• Hardware Voltage Limitations—4S, 6S,
HV: Brushless ESCs are generally rated for
a specific range of voltage. This is due in
part to the voltage rating of the FETs
themselves. Generally, higher-voltage FETs
are usually more resistive than lowervoltage
FETs, so higher-voltage controllers
will require more FET capacity than lowervoltage
controllers to handle the same
amount of current. The drive circuitry must
also be modified to handle the higher
voltages.
The FET voltage limitation is a hard
number. Exceeding the FETs voltage limit
usually results in instant destruction of the
FET. Always pay attention to the voltage
limits recommended by the ESC
manufacturer.
• Hardware Amperage Limits—10 amps, 25
amps, 35 amps, etc.: Unfortunately
amperage limitations are not always blackand-
white. A number of considerations
determines the current an ESC can handle
successfully.
There 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 that
an ESC is seldom considered to be
expendable. It is difficult to anticipate high
currents and shut down the controller in
time to prevent the current spike from
damaging the controller.
Partial throttle operation generates more
heat, as does high PWM rates. The amperage
capability of an ESC is limited by the ability
of the device to dissipate heat generated by
the resistance of FETs and circuit boards. If a
controller is making more heat than it can
dissipate, a “runaway” condition occurs. This
can lead to thermal destruction of the
controller; solder holding the components to
the boards literally melts, and the parts are
free to float away.
A 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, because
the temperature rise depends on 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
indicate that the controller might be able to
handle higher currents for short periods, but
those periods are sometimes shorter than the
pilot would hope.
That is another area in which
manufacturers can rate their products based
on their own, often ridiculous, definition of a
controller duty cycle. Read the fine print.
Like the proverbial duck on 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. MA
Lee Estingoy
[email protected]
Sources:
Himax motors:
Maxx Products International
(800) 416-6299
www.maxxprod.com
E-flite
(800) 338-4639
www.e-fliterc.com
ElectriFly
(800) 637-7660
www.electrifly.com
AXi electronics:
Hobby Lobby
(866) 512-1444
www.hobby-lobby.com
Castle Creations
(913) 390-6939
www.castlecreations.com

58 MODEL AVIATION
by Lee Es t ingoy
Inside the
Electronic
Speed
Control
MYSTERIOUS 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!” The reason for
a component failure is a mystery to most
involved, but understanding a bit more about
brushless systems can go a long way toward
helping a hobbyist enjoy outstanding
reliability in an electric-powered airplane or
helicopter.
A brief description of the role of the
brushless Electronic Speed Control (ESC) is
that it must accurately make and break
connections between the three input leads of
the motor and the power source to drive the
rotor magnets around the arc of the power
plant. The most accessible way to describe
the operation of the ESC is to break it down
by functional sections.
A brushless ESC uses a microprocessor
to manage the operation of field-effect
transistors (FETs), using information from a
rotor position circuit. Let’s look at each of
these more closely.
Making the Connection: Before we go too
far, let’s make a few things clear about the
operation of a brushless motor. It uses three
sets of copper windings to push and pull
permanent magnets attached to the shaft
inside the power plant. It’s important to
understand that these windings are
connected at one end inside the motor.
There are two ways this connection is
made; one is the Delta, or D-wind, and the
other is the Y-wind. The controller doesn’t
care which is used; the windings need only
to be connected. The type of connection
does affect the torque curve of the motor.
Let’s call the three motor wires “A,”
“B,” and “C,” and their “free” ends, those
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 results 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 Metal Oxide
Semiconductor Field Effect Transistors
(MOSFETs, or FETs for short)—can turn on
and off in a fraction of a second and are
ideally suited for this application.
Let’s do a bit of math to get an idea of
the incredible activity going on inside the
ESC. An outrunner motor with 12 poles that
has a Kv (rpm per volt) of 1,500 and is
powered with 24 volts (6S Li-Poly) will spin
at 36,000 rpm (24 x 1,500 = 36,000).
The six 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 1/21,600 second!
FET Drive Circuitry: Turning an FET on
and off is not as easy as it might sound.
Each has three connections: gate, source,
and ground. To turn the FET on and create a
circuit, the gate leg has to be driven to a
point that is 5-10 volts higher than the
voltage of the source leg on the FET, which
November 2010 59
Even electronic horsepower needs a jockey
is connected to the motor power source.
Refer to the simplified ESC diagram. If
using a 4S Li-Poly battery, +IN will be
roughly 14.8 volts (3.7 volts x 4). The gate
requires 24.8 volts (14.8 + 10 = 24.8) for
proper operation. The ESC must therefore
be able to boost some of the power it takes
from the batteries to the increased voltage to
drive the FETs.
Motor Position Detection Circuitry: The
ESC has to know the precise location of the
rotor magnet(s) to accurately sequence the
connections that the FETs make. This is the
trickiest thing that the ESC has to do.
There are two main ways to go about
this: sensored and sensorless. Sensored
systems use electronic (Hall) sensors in the
motor to track the rotor. This requires
additional parts 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, because they
provide a slightly smoother motor start than
the sensorless controller. Sensored systems
were popular in the early days of RC
brushless aircraft power systems; however,
they are generally considered to be less
reliable and less efficient than sensorless
systems, so they are no longer popular for
such applications.
Sensorless/modern ESCs detect the rotor
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
spinning magnets in the power plant
generates a voltage in the third wire, and
sensorless ESCs detect and measure that
voltage to determine how far the rotor has
turned. Then the information is used to
switch FETs as needed to cause correct
magnetic push or pull in the phases.
The Microcontroller and Its Firmware: The
microcontroller is the “brain” that runs the
whole operation. Operating 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
might 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
helicopters.
Microcontrollers 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.
I’ve mentioned how the controller
switches FETs and keeps track of the motor
position. Don’t forget that the
microcontroller also has to process input
from the receiver to compute the desired
output power and flash indicator LEDs.
The user might not want to run at full
throttle all the time, so we have to be able to
Current can flow in either direction on each of the three motor wires, making six possible combinations of current flow. This
diagram shows one. The blue path traces 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 the PWM rate.
The purple path traces the “backflow” in the third motor wire (B) of current generated by the motion of the rotor magnets relative
to the windings. The rotor position circuitry measures voltage of this current to determine when to switch the FETs to drive the
rotor around inside the motor.
60 MODEL AVIATION
The two wind termination types are known as a Delta and a Y-wind. Delta
wind gets its name from the Greek symbol. It’s not much of a jump from
there to understand the name for the Y-wind. A Delta-wind motor generally
has nearly twice the Kv of a similar motor with a Y-wind.
This is a basic drawing of connections required to drive
a brushless motor. The three motor wires—A, B, and
C—can each be connected to positive or negative poles
of the power source by the ESC. The six possible
combinations are numbered, and color-coded letters
indicate connections and polarity at each point in the
process. Red indicates connection to positive; black
indicates connection to negative.
limit the output power by pulsing those
FETs between the usual positional pulses. If
that’s not enough, there may be special
routines that govern motor speed, record
data, monitor battery voltage, watch for
overcurrent or overtemperature conditions,
and manage activities of the switching BEC.
There is a lot going on here!
Input Capacitors: 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?
Remember 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 look like spurts of
voltage.
Each spurt starts at a higher level than at
which it ends 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 out this ripple to some
extent by using capacitors, but there is a
limit to how much the capacitors can fix.
The FET gate must be 10 volts higher
than source. If the source is crashing and
recovering a bit between each cycle, the
voltage in the gate circuitry might
unexpectedly meet/exceed the 10 margin
over the source voltage in the FET. That
causes the FETs to turn on unexpectedly—
and create nasty connections
in the controller that
typically lead to a bad day at
the field.
It’s not such a bad thing
if the FETs turn off. It is bad
when they all turn on at the
same time that the smoke
comes out.
Advanced topics in ESC design include the
following, any one of which would provide
plenty of material for an engineering
graduate paper. These are simple
descriptions.
• Controlling Speed: Running at partial
throttle is merely 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 rapidly pulsed on and off to
reduce the average power seen in the
winding.
At low throttles this second FET is
barely on, but it is on almost the whole time
near full throttle. The frequency (times per
second) at which we pulse the power for
speed control—not the polarity switches
that drive the motor—is called the PWM
rate, or switching frequency.
One of the paradoxes of brushless-motor
controllers is that partial throttle operation
Illustrations by the author Photos by the author and MA Staff
Improvements in FET packaging, the way
the internal silicon components are
connected to the circuit board, play a huge
role in the improvement of the ESC in the
past few years. The older S08 packaging (L)
connects with the tiny legs, while the huge
Drain pad on the newer Power Pack FET
(R) provides a much larger connection to
the circuit board. The net effect is that
much more of the heat generated in the
Power Pack FET can be transferred directly
to the circuit board.
There are four 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; it reduces the voltage of motor
batteries to a level that is useful to the radio system in the vehicle.
November 2010 61
Plug-and-play systems are noted for their
ease of use—no soldering. Electric-power
systems from E-flite are that easy and are
labeled with a system that correlates
with glow-power designations.
Typical ESC power boards designed for
the Power Pack FETs (top) and the older
S08 FETs (right). Two phases are color-coded; blue pads = motor wire connection,
green and red pads = FET drains and sources, and orange pads = gate connection.
Great Planes motors are sold under the
ElectriFly brand name. They feature plugand-
play electric-power systems for
models weighing 5 ounces to 50 pounds.
AXi motors can arguably be credited with making electric power available via mass
production and reasonable pricing. They are among the most efficient systems
available.
generates more ESC heat than full-throttle
operation. FETs have a small resistance
when they are fully on and current is
flowing through them. This generates a
relatively little amount of heat. As always,
there’s more to it.
FETs don’t simply go from an on to an
off state; there is a bit of a ramp to the
process in which 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, 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.
• Hardware Voltage Limitations—4S, 6S,
HV: Brushless ESCs are generally rated for
a specific range of voltage. This is due in
part to the voltage rating of the FETs
themselves. Generally, higher-voltage FETs
are usually more resistive than lowervoltage
FETs, so higher-voltage controllers
will require more FET capacity than lowervoltage
controllers to handle the same
amount of current. The drive circuitry must
also be modified to handle the higher
voltages.
The FET voltage limitation is a hard
number. Exceeding the FETs voltage limit
usually results in instant destruction of the
FET. Always pay attention to the voltage
limits recommended by the ESC
manufacturer.
• Hardware Amperage Limits—10 amps, 25
amps, 35 amps, etc.: Unfortunately
amperage limitations are not always blackand-
white. A number of considerations
determines the current an ESC can handle
successfully.
There 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 that
an ESC is seldom considered to be
expendable. It is difficult to anticipate high
currents and shut down the controller in
time to prevent the current spike from
damaging the controller.
Partial throttle operation generates more
heat, as does high PWM rates. The amperage
capability of an ESC is limited by the ability
of the device to dissipate heat generated by
the resistance of FETs and circuit boards. If a
controller is making more heat than it can
dissipate, a “runaway” condition occurs. This
can lead to thermal destruction of the
controller; solder holding the components to
the boards literally melts, and the parts are
free to float away.
A 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, because
the temperature rise depends on 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
indicate that the controller might be able to
handle higher currents for short periods, but
those periods are sometimes shorter than the
pilot would hope.
That is another area in which
manufacturers can rate their products based
on their own, often ridiculous, definition of a
controller duty cycle. Read the fine print.
Like the proverbial duck on 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. MA
Lee Estingoy
[email protected]
Sources:
Himax motors:
Maxx Products International
(800) 416-6299
www.maxxprod.com
E-flite
(800) 338-4639
www.e-fliterc.com
ElectriFly
(800) 637-7660
www.electrifly.com
AXi electronics:
Hobby Lobby
(866) 512-1444
www.hobby-lobby.com
Castle Creations
(913) 390-6939
www.castlecreations.com