September 2010 43
Clean Horsepower
Learn how brushless
motors work
by Lee Estingoy
BRUSHLESS DCMotoRS are simple enough. Magnets attached to
a shaft are pushed and pulled by electromagnetic fields that are
managed by an ESC.
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 motors.
That’s the view from 40,000 feet, and it’s sufficient for most
aeromodelers. However, a deeper understanding of brushless motors’
operation can go a long way toward helping a user select the right
power system for an 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: The most common brushless motors for
RC airplane and helicopter use is an “outrunner.” It has permanent
magnets arranged around the inside of a can that is attached to the shaft.
Electrical coils are located in the center of the motor, with the can and
its magnets running on the outside, hence the term “outrunner.”
The functional opposite is the inrunner type, which is by far the
most popular in RC car applications. Inrunners have magnets attached
directly to the motor shaft, and motor coils surround the shaft and
magnets. Outrunners and inrunners look different, but their operational
principles are the same.
Take a brushless unit apart; you’ll see several loops of shiny copper
wire running parallel to the motor shaft. There will likely be many
more than three, but the number will be divisible by three; each coil is
actually part of one of three winding circuits in the power plant.
The coils are distributed so that every third one is attached to the
same motor wire. The steel structure around which coils are wound is
the stator stack, which serves to focus the magnetic field that the coils
generate.
Permanent magnets in a brushless motor are arranged so that their
poles run perpendicular to the motor shaft. They are aligned in such a
way that the pole presented to the electromagnetic coil is either a north
or a south. These alternate around the rotor circumference.
This alternating sequence of poles is convenient, because the same
magnetic field pushes and pulls adjacent poles. A two-pole motor
might be made from a single magnet “wrapped” around the shaft with
the north pole at one side and the south pole at the other.
Inrunners generally have permanent magnets bonded to the shaft.
Two pole rotors have the magnet surrounding the shaft.
The integrity of these attachments determines a motor’s max rpm.
Some power plants use a Kevlar or carbon wrap to increase the strength
of the attachment to the rotor.
Brushless-Motor Wiring: All hobby brushless units have three 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
those look different, the manner of termination does not change the job
of the ESC in running the motor.
With no outside influence (such as a moving magnetic field), these
motor winding circuits are dead shorts, which is exactly what the ESC
has to deal with during startup or under heavy load.
Apply power to a single winding, or phase, by connecting one motor
wire 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.
The outrunner brushless is the most popular type of motor today. It develops a large amount of power, yet maintains the quiet
attribute of direct drive.
09sig2x_00MSTRPG.QXD 7/22/10 9:47 AM Page 43
44 MODEL AVIATION
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 must be used to
drive one rotor magnet pole around a single revolution in our simplified
motor diagram.
Electric Phraseology: The term “Kv” (pronounced “kayvee”) does not
mean “kilovolt” in this context; it refers to the number of rotations per
minute (rpm) the unloaded motor will spin per volt applied. A 500 Kv
motor will “try” to turn at 5,000 rpm when 10 volts is applied (500 x 10
= 5,000).
A motor will try to turn to its predicted rpm, even
under load. It will effectively draw more
power to move the load to get to that
predicted speed.
Resistance is a critical concept
in understanding the operation of
a motor. There are several types
of resistance that come into play
in a motor.
All conductors have some
level of resistance to electric
currents. That causes a portion of
the electrical energy passing
through the wire to be wasted as
unwanted heat instead of creating
the desired magnetic field. This
type of resistance is known as
“copper loss.”
A conductor’s copper loss
goes up with the square of the
current. A conductor carrying 10
amps has one-quarter the copper loss of that same conductor when it
carries 20 amps.
Wattage lost (watts) = current (amps) x current (amps) x resistance
(ohms), although resistance is tiny (20 x 20 = 400 and 40 x 40 = 1,600).
So doubling the current in a motor results in four times the heat.
Iron loss is another kind of resistance. To understand it, it helps to
understand the relationship between current and magnetic fields.
When current flows through a wire, it generates a magnetic field.
This is good, and it is what causes these motors to turn.
The inverse of that is also true. When a wire is close to a moving
magnetic field, a current is generated in the wire (similar to the
alternator in your car).
Iron loss happens when the magnets moving near
the stator generate electric currents. These
currents in the steel stators cause heat for
the same reason as with the copper
losses, except steel has a much
higher resistance than copper.
To reduce this effect, many
“thin” stator laminations are used
instead of one solid chunk. Each
stator is electrically insulated
from the next, reducing the
induced currents.
Motors are commonly
identified by the number of turns
that make up the wraps of each
coil. Varying the number of wraps
of wire in the coils varies the
magnetic field.
More wraps can yield stronger
magnetic fields, but thinner wire
An outrunner uses electromagnetic coils
in the center of the motor to drive
permanent magnets attached to the
rotating exterior can. This design yields a
larger rotor diameter and can generate
more torque than a similar-size inrunner.
Inrunner motors generally have permanent
magnets bonded to the shaft, such as in the
four-pole rotor (L). Two-pole rotors (R)
have the magnet surrounding the shaft. The
integrity of these attachments determines
the max rpm of a motor. Some use a Kevlar
(L) or carbon wrap to increase the strength
of the attachment to the rotor.
These screen shots show how changes to the shape of the stator teeth tips and increased “back iron” allowed the stator to perform at
much higher power levels before reaching magnetic saturation and suffering the corresponding loss of efficiency. Such tweaks can make
a huge difference in performance characteristics.
The motor leads are connected to bundles of wire inside the power
plant. Those bundles are wrapped around the “teeth” in the steel
stator. Notice how the bundles are inserted into every third space.
This NeuMotors ORK uses a larger-thannormal
rotor diameter and a high pole
count to generate more torque at the shaft
than a traditional inrunner. Motors with
higher torque can often drive propellers
without the use of a gearbox.
Photos by the author
09sig2x_00MSTRPG.QXD 7/22/10 9:52 AM Page 44
September 2010 45
Above: With no outside influence (such as a moving magnetic field), these motor
winding circuits are dead shorts, which is exactly what the ESC has to deal with
during startup or under heavy load.
Right above: A precise sequence of power application and reversal is required to turn the shaft one full revolution. Each of three winding
coils is energized in two opposing ways using the three motor input leads: A, B, and 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.
must be used to fit more wraps into the same space as a larger wire
making fewer wraps. There’s always a tradeoff.
Thinner wire cannot handle as high of a current as thicker wire.
Motors made with more turns generally have a lower Kv and higher
copper losses because of the higher resistance of the winding wire than
motors with lower turns and larger winding wires.
Permanent magnets used in brushless motors have varying degrees
of magnetic strength and temperature resistance. These factors are
related to their composition and manufacture.
Permanent magnets generally lose magnetic strength as they are
heated, and there is a temperature beyond which the magnet suffers
permanent loss of magnetism. Demagnetized motors will allow the
motor to draw significantly more current and generate more heat while
producing progressively less power.
Popular magnetic materials for hobby brushless motors include:
• Neodymium: This substance has excellent magnetic strength and is
heat-tolerant to roughly 150 degrees Celsius, depending on grade. The
outside of the can will be much cooler than the magnets. A good rule of
thumb is that the outside of the motor is too hot if it exceeds 100
degrees Celsius.
• Samarium Cobalt: This material is not as strong, magnetically, as
Neodymium, but it is much more tolerant to heat.
Magnetic qualities can be selected by the manufacturer, and there
is often a need to strike a compromise between affordability and
performance. Hobbyists are generally reliant on, and trusting of, the
maker to disclose the material used. It is often difficult for a hobbyist
to verify manufacturers’ claims, so be wary of fantastic assertions in
this area.
A motor with the windings wrapped around “teeth” in the stator is
known to be “slotted.” Windings might also be wrapped completely
over the stator ring. Power plants built in that manner are known as
“slotless.”
An easy way to tell them apart is to turn the motor shaft. A slotted
motor’s rotor poles are magnetically attracted to the “teeth” on the
stator plates. You’ll feel a detent, slip, then detent in the shaft as it
turns; that is known as “cogging.”
A slotless motor has no teeth, so there’s nothing for the magnets to
pull on, thus no cogging.
A motor’s performance is usually measured in terms of efficiency.
A change of just a few percentage points in efficiency can make a
huge difference in the amount of power a motor can handle.
A motor that is 80% efficient running at 100 watts produces 80
watts of work and 20 watts of heat. (That’s heat equivalent to most
hobby soldering irons.) An otherwise identical unit that is 90%
efficient produces half the heat, or only 10 watts.
If heat dissipation is the only factor limiting the power in the
motor, and 20 watts of heat is the maximum amount of heat that it can
dissipate because of its surface area and cooling air, the 90%-efficient
motor can handle 200 watts of power before it generates the same 20
watts of heat.
The design and materials quality of motor components have a direct
effect on the performance of the unit. 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 stator
laminations.
• The material and strength of the permanent magnet.
• Bearings.
• Rotor balance.
Many aeromodelers can make a motor; however, careful attention
to engineering principles can make the difference between a power
plant that is efficient and reliable and one that performs poorly. MA
Lee Estingoy
[email protected]
(Editor’s note: Lee Estingoy is a valued and long-time employee of
Castle Creations. His experience with his employer and, especially, as
an active aeromodeler are direct assets to the information provided in
this article, which is in no way intended to endorse or recommend any
type or specific brand of motor or accessory.)
Models Weighing Less Than 16 Ounces
Watts Per Ounce Type/Performance Expectation
1.0 Barely leave the ground
1.25 Powered sport sailplanes, indoor RC
2.0 Small-field or backyard flyers
3.0 Aerobatic aircraft
5.0 3-D models
Models Exceeding 1 Pound
Watts Per Pound Type/Performance Expectation
30 Barely leave the ground
40-50 Sunday flyers, sport sailplanes, Old-Timers
60-70 Mildly aerobatic aircraft
80-100 Aggressive aerobatic airplanes
100 plus Ducted-fan-powered models, competition
sailplanes, 3-D aircraft
Electric Power System Selection Guide
–Bob Aberle
[email protected]
09sig2x_00MSTRPG.QXD 7/22/10 9:55 AM Page 45
Edition: Model Aviation - 2010/09
Page Numbers: 43,44,45
Edition: Model Aviation - 2010/09
Page Numbers: 43,44,45
September 2010 43
Clean Horsepower
Learn how brushless
motors work
by Lee Estingoy
BRUSHLESS DCMotoRS are simple enough. Magnets attached to
a shaft are pushed and pulled by electromagnetic fields that are
managed by an ESC.
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 motors.
That’s the view from 40,000 feet, and it’s sufficient for most
aeromodelers. However, a deeper understanding of brushless motors’
operation can go a long way toward helping a user select the right
power system for an 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: The most common brushless motors for
RC airplane and helicopter use is an “outrunner.” It has permanent
magnets arranged around the inside of a can that is attached to the shaft.
Electrical coils are located in the center of the motor, with the can and
its magnets running on the outside, hence the term “outrunner.”
The functional opposite is the inrunner type, which is by far the
most popular in RC car applications. Inrunners have magnets attached
directly to the motor shaft, and motor coils surround the shaft and
magnets. Outrunners and inrunners look different, but their operational
principles are the same.
Take a brushless unit apart; you’ll see several loops of shiny copper
wire running parallel to the motor shaft. There will likely be many
more than three, but the number will be divisible by three; each coil is
actually part of one of three winding circuits in the power plant.
The coils are distributed so that every third one is attached to the
same motor wire. The steel structure around which coils are wound is
the stator stack, which serves to focus the magnetic field that the coils
generate.
Permanent magnets in a brushless motor are arranged so that their
poles run perpendicular to the motor shaft. They are aligned in such a
way that the pole presented to the electromagnetic coil is either a north
or a south. These alternate around the rotor circumference.
This alternating sequence of poles is convenient, because the same
magnetic field pushes and pulls adjacent poles. A two-pole motor
might be made from a single magnet “wrapped” around the shaft with
the north pole at one side and the south pole at the other.
Inrunners generally have permanent magnets bonded to the shaft.
Two pole rotors have the magnet surrounding the shaft.
The integrity of these attachments determines a motor’s max rpm.
Some power plants use a Kevlar or carbon wrap to increase the strength
of the attachment to the rotor.
Brushless-Motor Wiring: All hobby brushless units have three 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
those look different, the manner of termination does not change the job
of the ESC in running the motor.
With no outside influence (such as a moving magnetic field), these
motor winding circuits are dead shorts, which is exactly what the ESC
has to deal with during startup or under heavy load.
Apply power to a single winding, or phase, by connecting one motor
wire 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.
The outrunner brushless is the most popular type of motor today. It develops a large amount of power, yet maintains the quiet
attribute of direct drive.
09sig2x_00MSTRPG.QXD 7/22/10 9:47 AM Page 43
44 MODEL AVIATION
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 must be used to
drive one rotor magnet pole around a single revolution in our simplified
motor diagram.
Electric Phraseology: The term “Kv” (pronounced “kayvee”) does not
mean “kilovolt” in this context; it refers to the number of rotations per
minute (rpm) the unloaded motor will spin per volt applied. A 500 Kv
motor will “try” to turn at 5,000 rpm when 10 volts is applied (500 x 10
= 5,000).
A motor will try to turn to its predicted rpm, even
under load. It will effectively draw more
power to move the load to get to that
predicted speed.
Resistance is a critical concept
in understanding the operation of
a motor. There are several types
of resistance that come into play
in a motor.
All conductors have some
level of resistance to electric
currents. That causes a portion of
the electrical energy passing
through the wire to be wasted as
unwanted heat instead of creating
the desired magnetic field. This
type of resistance is known as
“copper loss.”
A conductor’s copper loss
goes up with the square of the
current. A conductor carrying 10
amps has one-quarter the copper loss of that same conductor when it
carries 20 amps.
Wattage lost (watts) = current (amps) x current (amps) x resistance
(ohms), although resistance is tiny (20 x 20 = 400 and 40 x 40 = 1,600).
So doubling the current in a motor results in four times the heat.
Iron loss is another kind of resistance. To understand it, it helps to
understand the relationship between current and magnetic fields.
When current flows through a wire, it generates a magnetic field.
This is good, and it is what causes these motors to turn.
The inverse of that is also true. When a wire is close to a moving
magnetic field, a current is generated in the wire (similar to the
alternator in your car).
Iron loss happens when the magnets moving near
the stator generate electric currents. These
currents in the steel stators cause heat for
the same reason as with the copper
losses, except steel has a much
higher resistance than copper.
To reduce this effect, many
“thin” stator laminations are used
instead of one solid chunk. Each
stator is electrically insulated
from the next, reducing the
induced currents.
Motors are commonly
identified by the number of turns
that make up the wraps of each
coil. Varying the number of wraps
of wire in the coils varies the
magnetic field.
More wraps can yield stronger
magnetic fields, but thinner wire
An outrunner uses electromagnetic coils
in the center of the motor to drive
permanent magnets attached to the
rotating exterior can. This design yields a
larger rotor diameter and can generate
more torque than a similar-size inrunner.
Inrunner motors generally have permanent
magnets bonded to the shaft, such as in the
four-pole rotor (L). Two-pole rotors (R)
have the magnet surrounding the shaft. The
integrity of these attachments determines
the max rpm of a motor. Some use a Kevlar
(L) or carbon wrap to increase the strength
of the attachment to the rotor.
These screen shots show how changes to the shape of the stator teeth tips and increased “back iron” allowed the stator to perform at
much higher power levels before reaching magnetic saturation and suffering the corresponding loss of efficiency. Such tweaks can make
a huge difference in performance characteristics.
The motor leads are connected to bundles of wire inside the power
plant. Those bundles are wrapped around the “teeth” in the steel
stator. Notice how the bundles are inserted into every third space.
This NeuMotors ORK uses a larger-thannormal
rotor diameter and a high pole
count to generate more torque at the shaft
than a traditional inrunner. Motors with
higher torque can often drive propellers
without the use of a gearbox.
Photos by the author
09sig2x_00MSTRPG.QXD 7/22/10 9:52 AM Page 44
September 2010 45
Above: With no outside influence (such as a moving magnetic field), these motor
winding circuits are dead shorts, which is exactly what the ESC has to deal with
during startup or under heavy load.
Right above: A precise sequence of power application and reversal is required to turn the shaft one full revolution. Each of three winding
coils is energized in two opposing ways using the three motor input leads: A, B, and 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.
must be used to fit more wraps into the same space as a larger wire
making fewer wraps. There’s always a tradeoff.
Thinner wire cannot handle as high of a current as thicker wire.
Motors made with more turns generally have a lower Kv and higher
copper losses because of the higher resistance of the winding wire than
motors with lower turns and larger winding wires.
Permanent magnets used in brushless motors have varying degrees
of magnetic strength and temperature resistance. These factors are
related to their composition and manufacture.
Permanent magnets generally lose magnetic strength as they are
heated, and there is a temperature beyond which the magnet suffers
permanent loss of magnetism. Demagnetized motors will allow the
motor to draw significantly more current and generate more heat while
producing progressively less power.
Popular magnetic materials for hobby brushless motors include:
• Neodymium: This substance has excellent magnetic strength and is
heat-tolerant to roughly 150 degrees Celsius, depending on grade. The
outside of the can will be much cooler than the magnets. A good rule of
thumb is that the outside of the motor is too hot if it exceeds 100
degrees Celsius.
• Samarium Cobalt: This material is not as strong, magnetically, as
Neodymium, but it is much more tolerant to heat.
Magnetic qualities can be selected by the manufacturer, and there
is often a need to strike a compromise between affordability and
performance. Hobbyists are generally reliant on, and trusting of, the
maker to disclose the material used. It is often difficult for a hobbyist
to verify manufacturers’ claims, so be wary of fantastic assertions in
this area.
A motor with the windings wrapped around “teeth” in the stator is
known to be “slotted.” Windings might also be wrapped completely
over the stator ring. Power plants built in that manner are known as
“slotless.”
An easy way to tell them apart is to turn the motor shaft. A slotted
motor’s rotor poles are magnetically attracted to the “teeth” on the
stator plates. You’ll feel a detent, slip, then detent in the shaft as it
turns; that is known as “cogging.”
A slotless motor has no teeth, so there’s nothing for the magnets to
pull on, thus no cogging.
A motor’s performance is usually measured in terms of efficiency.
A change of just a few percentage points in efficiency can make a
huge difference in the amount of power a motor can handle.
A motor that is 80% efficient running at 100 watts produces 80
watts of work and 20 watts of heat. (That’s heat equivalent to most
hobby soldering irons.) An otherwise identical unit that is 90%
efficient produces half the heat, or only 10 watts.
If heat dissipation is the only factor limiting the power in the
motor, and 20 watts of heat is the maximum amount of heat that it can
dissipate because of its surface area and cooling air, the 90%-efficient
motor can handle 200 watts of power before it generates the same 20
watts of heat.
The design and materials quality of motor components have a direct
effect on the performance of the unit. 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 stator
laminations.
• The material and strength of the permanent magnet.
• Bearings.
• Rotor balance.
Many aeromodelers can make a motor; however, careful attention
to engineering principles can make the difference between a power
plant that is efficient and reliable and one that performs poorly. MA
Lee Estingoy
[email protected]
(Editor’s note: Lee Estingoy is a valued and long-time employee of
Castle Creations. His experience with his employer and, especially, as
an active aeromodeler are direct assets to the information provided in
this article, which is in no way intended to endorse or recommend any
type or specific brand of motor or accessory.)
Models Weighing Less Than 16 Ounces
Watts Per Ounce Type/Performance Expectation
1.0 Barely leave the ground
1.25 Powered sport sailplanes, indoor RC
2.0 Small-field or backyard flyers
3.0 Aerobatic aircraft
5.0 3-D models
Models Exceeding 1 Pound
Watts Per Pound Type/Performance Expectation
30 Barely leave the ground
40-50 Sunday flyers, sport sailplanes, Old-Timers
60-70 Mildly aerobatic aircraft
80-100 Aggressive aerobatic airplanes
100 plus Ducted-fan-powered models, competition
sailplanes, 3-D aircraft
Electric Power System Selection Guide
–Bob Aberle
[email protected]
09sig2x_00MSTRPG.QXD 7/22/10 9:55 AM Page 45
Edition: Model Aviation - 2010/09
Page Numbers: 43,44,45
September 2010 43
Clean Horsepower
Learn how brushless
motors work
by Lee Estingoy
BRUSHLESS DCMotoRS are simple enough. Magnets attached to
a shaft are pushed and pulled by electromagnetic fields that are
managed by an ESC.
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 motors.
That’s the view from 40,000 feet, and it’s sufficient for most
aeromodelers. However, a deeper understanding of brushless motors’
operation can go a long way toward helping a user select the right
power system for an 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: The most common brushless motors for
RC airplane and helicopter use is an “outrunner.” It has permanent
magnets arranged around the inside of a can that is attached to the shaft.
Electrical coils are located in the center of the motor, with the can and
its magnets running on the outside, hence the term “outrunner.”
The functional opposite is the inrunner type, which is by far the
most popular in RC car applications. Inrunners have magnets attached
directly to the motor shaft, and motor coils surround the shaft and
magnets. Outrunners and inrunners look different, but their operational
principles are the same.
Take a brushless unit apart; you’ll see several loops of shiny copper
wire running parallel to the motor shaft. There will likely be many
more than three, but the number will be divisible by three; each coil is
actually part of one of three winding circuits in the power plant.
The coils are distributed so that every third one is attached to the
same motor wire. The steel structure around which coils are wound is
the stator stack, which serves to focus the magnetic field that the coils
generate.
Permanent magnets in a brushless motor are arranged so that their
poles run perpendicular to the motor shaft. They are aligned in such a
way that the pole presented to the electromagnetic coil is either a north
or a south. These alternate around the rotor circumference.
This alternating sequence of poles is convenient, because the same
magnetic field pushes and pulls adjacent poles. A two-pole motor
might be made from a single magnet “wrapped” around the shaft with
the north pole at one side and the south pole at the other.
Inrunners generally have permanent magnets bonded to the shaft.
Two pole rotors have the magnet surrounding the shaft.
The integrity of these attachments determines a motor’s max rpm.
Some power plants use a Kevlar or carbon wrap to increase the strength
of the attachment to the rotor.
Brushless-Motor Wiring: All hobby brushless units have three 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
those look different, the manner of termination does not change the job
of the ESC in running the motor.
With no outside influence (such as a moving magnetic field), these
motor winding circuits are dead shorts, which is exactly what the ESC
has to deal with during startup or under heavy load.
Apply power to a single winding, or phase, by connecting one motor
wire 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.
The outrunner brushless is the most popular type of motor today. It develops a large amount of power, yet maintains the quiet
attribute of direct drive.
09sig2x_00MSTRPG.QXD 7/22/10 9:47 AM Page 43
44 MODEL AVIATION
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 must be used to
drive one rotor magnet pole around a single revolution in our simplified
motor diagram.
Electric Phraseology: The term “Kv” (pronounced “kayvee”) does not
mean “kilovolt” in this context; it refers to the number of rotations per
minute (rpm) the unloaded motor will spin per volt applied. A 500 Kv
motor will “try” to turn at 5,000 rpm when 10 volts is applied (500 x 10
= 5,000).
A motor will try to turn to its predicted rpm, even
under load. It will effectively draw more
power to move the load to get to that
predicted speed.
Resistance is a critical concept
in understanding the operation of
a motor. There are several types
of resistance that come into play
in a motor.
All conductors have some
level of resistance to electric
currents. That causes a portion of
the electrical energy passing
through the wire to be wasted as
unwanted heat instead of creating
the desired magnetic field. This
type of resistance is known as
“copper loss.”
A conductor’s copper loss
goes up with the square of the
current. A conductor carrying 10
amps has one-quarter the copper loss of that same conductor when it
carries 20 amps.
Wattage lost (watts) = current (amps) x current (amps) x resistance
(ohms), although resistance is tiny (20 x 20 = 400 and 40 x 40 = 1,600).
So doubling the current in a motor results in four times the heat.
Iron loss is another kind of resistance. To understand it, it helps to
understand the relationship between current and magnetic fields.
When current flows through a wire, it generates a magnetic field.
This is good, and it is what causes these motors to turn.
The inverse of that is also true. When a wire is close to a moving
magnetic field, a current is generated in the wire (similar to the
alternator in your car).
Iron loss happens when the magnets moving near
the stator generate electric currents. These
currents in the steel stators cause heat for
the same reason as with the copper
losses, except steel has a much
higher resistance than copper.
To reduce this effect, many
“thin” stator laminations are used
instead of one solid chunk. Each
stator is electrically insulated
from the next, reducing the
induced currents.
Motors are commonly
identified by the number of turns
that make up the wraps of each
coil. Varying the number of wraps
of wire in the coils varies the
magnetic field.
More wraps can yield stronger
magnetic fields, but thinner wire
An outrunner uses electromagnetic coils
in the center of the motor to drive
permanent magnets attached to the
rotating exterior can. This design yields a
larger rotor diameter and can generate
more torque than a similar-size inrunner.
Inrunner motors generally have permanent
magnets bonded to the shaft, such as in the
four-pole rotor (L). Two-pole rotors (R)
have the magnet surrounding the shaft. The
integrity of these attachments determines
the max rpm of a motor. Some use a Kevlar
(L) or carbon wrap to increase the strength
of the attachment to the rotor.
These screen shots show how changes to the shape of the stator teeth tips and increased “back iron” allowed the stator to perform at
much higher power levels before reaching magnetic saturation and suffering the corresponding loss of efficiency. Such tweaks can make
a huge difference in performance characteristics.
The motor leads are connected to bundles of wire inside the power
plant. Those bundles are wrapped around the “teeth” in the steel
stator. Notice how the bundles are inserted into every third space.
This NeuMotors ORK uses a larger-thannormal
rotor diameter and a high pole
count to generate more torque at the shaft
than a traditional inrunner. Motors with
higher torque can often drive propellers
without the use of a gearbox.
Photos by the author
09sig2x_00MSTRPG.QXD 7/22/10 9:52 AM Page 44
September 2010 45
Above: With no outside influence (such as a moving magnetic field), these motor
winding circuits are dead shorts, which is exactly what the ESC has to deal with
during startup or under heavy load.
Right above: A precise sequence of power application and reversal is required to turn the shaft one full revolution. Each of three winding
coils is energized in two opposing ways using the three motor input leads: A, B, and 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.
must be used to fit more wraps into the same space as a larger wire
making fewer wraps. There’s always a tradeoff.
Thinner wire cannot handle as high of a current as thicker wire.
Motors made with more turns generally have a lower Kv and higher
copper losses because of the higher resistance of the winding wire than
motors with lower turns and larger winding wires.
Permanent magnets used in brushless motors have varying degrees
of magnetic strength and temperature resistance. These factors are
related to their composition and manufacture.
Permanent magnets generally lose magnetic strength as they are
heated, and there is a temperature beyond which the magnet suffers
permanent loss of magnetism. Demagnetized motors will allow the
motor to draw significantly more current and generate more heat while
producing progressively less power.
Popular magnetic materials for hobby brushless motors include:
• Neodymium: This substance has excellent magnetic strength and is
heat-tolerant to roughly 150 degrees Celsius, depending on grade. The
outside of the can will be much cooler than the magnets. A good rule of
thumb is that the outside of the motor is too hot if it exceeds 100
degrees Celsius.
• Samarium Cobalt: This material is not as strong, magnetically, as
Neodymium, but it is much more tolerant to heat.
Magnetic qualities can be selected by the manufacturer, and there
is often a need to strike a compromise between affordability and
performance. Hobbyists are generally reliant on, and trusting of, the
maker to disclose the material used. It is often difficult for a hobbyist
to verify manufacturers’ claims, so be wary of fantastic assertions in
this area.
A motor with the windings wrapped around “teeth” in the stator is
known to be “slotted.” Windings might also be wrapped completely
over the stator ring. Power plants built in that manner are known as
“slotless.”
An easy way to tell them apart is to turn the motor shaft. A slotted
motor’s rotor poles are magnetically attracted to the “teeth” on the
stator plates. You’ll feel a detent, slip, then detent in the shaft as it
turns; that is known as “cogging.”
A slotless motor has no teeth, so there’s nothing for the magnets to
pull on, thus no cogging.
A motor’s performance is usually measured in terms of efficiency.
A change of just a few percentage points in efficiency can make a
huge difference in the amount of power a motor can handle.
A motor that is 80% efficient running at 100 watts produces 80
watts of work and 20 watts of heat. (That’s heat equivalent to most
hobby soldering irons.) An otherwise identical unit that is 90%
efficient produces half the heat, or only 10 watts.
If heat dissipation is the only factor limiting the power in the
motor, and 20 watts of heat is the maximum amount of heat that it can
dissipate because of its surface area and cooling air, the 90%-efficient
motor can handle 200 watts of power before it generates the same 20
watts of heat.
The design and materials quality of motor components have a direct
effect on the performance of the unit. 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 stator
laminations.
• The material and strength of the permanent magnet.
• Bearings.
• Rotor balance.
Many aeromodelers can make a motor; however, careful attention
to engineering principles can make the difference between a power
plant that is efficient and reliable and one that performs poorly. MA
Lee Estingoy
[email protected]
(Editor’s note: Lee Estingoy is a valued and long-time employee of
Castle Creations. His experience with his employer and, especially, as
an active aeromodeler are direct assets to the information provided in
this article, which is in no way intended to endorse or recommend any
type or specific brand of motor or accessory.)
Models Weighing Less Than 16 Ounces
Watts Per Ounce Type/Performance Expectation
1.0 Barely leave the ground
1.25 Powered sport sailplanes, indoor RC
2.0 Small-field or backyard flyers
3.0 Aerobatic aircraft
5.0 3-D models
Models Exceeding 1 Pound
Watts Per Pound Type/Performance Expectation
30 Barely leave the ground
40-50 Sunday flyers, sport sailplanes, Old-Timers
60-70 Mildly aerobatic aircraft
80-100 Aggressive aerobatic airplanes
100 plus Ducted-fan-powered models, competition
sailplanes, 3-D aircraft
Electric Power System Selection Guide
–Bob Aberle
[email protected]
09sig2x_00MSTRPG.QXD 7/22/10 9:55 AM Page 45
