AVID Technology designs and manufactures highly efficient
and power dense motors and control electronics
for a wide range of applications.
From electric and hybrid vehicles to industrial robotics.
This video sets out to explain the key permanent magnet motor variants
and where they might be used.
Radial flux motors are by far the most common type of electric machine.
In this video we will focus on permanent magnet radial motors.
There are also switched reluctance
and induction machines
but these are deliberately excluded.
In a radial flux motor
the magnetic field is radial to the rotational axis of the shaft.
There are two key topologies and several sub-classes within that
In this type of motor the rotor spins inside the stator
AVID has extensive design and volume manufacturing experience
of all types of radial interior rotor motors.
The first kind of internal rotor motor we are going to look at
is known as a surface magnet or SPM motor.
In this type of motor
the magnets are mounted to the surface of the rotor
typically bonded in place
held in position by a composite tape
or manufactured as a ring that is wrapped around the rotor.
This style of motor is used in a wide range of applications
where high efficiency is required
and also for high-speed applications
This style of motor is typically highly efficient
because the torque is generated directly by the reaction
between the magnetic field of the surface magnets
and the fields generated in the stator teeth by the windings.
 There are minimal rotor loses as the magnetic field
from the magnets is directly acting in the air gap.
The key disadvantage of this kind of machine
is a relatively high proportion of magnetic material
for the torque of the machine compared to other types of motor
and the increased manufacturing costs
especially when considering filament winding
to provide a high strength retention of the magnets.
The second kind of IR motor is known as an interior magnet
 or IPM motor
In this style of motor simple bar magnets are slotted
or molded into punched holes in a rotor core
manufactured from a stack of steel lamination sheets.
IPM motors typically use less magnetic material
than an equivalent SPM motor reducing costs.
The magnets are also mechanically located in the slots
removing the need for any banding
In addition to the magnetic field generated by the permanent magnets
they also typically use
rotor reluctance to to enhance the magnetic field strength
at the surface of the rotor.
Whilst this improves the torque to magnetic material ratio
it reduces peak efficiency
under high torque when compared to SPM rotors.
Although no banding is required
the assembly of the rotor can be complex
due to the use of multiple layers of lamination steel
and magnet arrays.
The need to insert many small magnets
into the slots requires complex
automated machinery for volume production.
IPM motors are popular for electric vehicle traction applications.
This motor is known as an external rotor
or outrunner style motor.
sometimes shortened to ERPM.
In this style of motor the magnets are mounted
to the interior surface of the rotor
typically bonded in place.
The rotor spins outside the stator
which is fixed inside the rotor.
ERPM motors provide more torque at lower RPM
due to the improved mechanical advantage
ffered by the rotor/stator configuration
of the active magnetic surface when compared to an IR motor.
This makes them well suited to certain applications.
The weakness of an ERPM motor
is the difficulty cooling the stator and rotor
compared to an IR motor
where there is a good thermal pathway from the stator
to the motor housing.
On the ERPM motor
the stator is inside the machine
with no thermal pathway
and it’s difficult to route liquid cooling into it.
Sealing of the ERPM motor assembly
is also more difficult as the moving part
is external to the stationary part
which typically means that two sets of clearances
 need to be maintained in a housed motor.
Finally  the Rotor has a relatively high inertia
because the rotating mass
is mounted far away from the central axis.
Unlike a radial motor
axial flux motors feature a magnetic flux
that runs axial to the rotation of the motor rotor.
They are sometimes called pancake motors
due to their flat shape
which is possible because the active magnetic surface
is the face of the rotor rather than its outside diameter.
This allows an increased active magnetic surface area
compared to a radial motor
and typically improved performance
in terms of power to weight and torque density.
There are two key axial flux motor configurations
internal stator external rotor
and external stator internal rotor
The internal stator external rotor axial flux motor
sometimes called a Torus axial flux motor
has several key advantages.
The benefits of this type of motor are compact stator windings
that are easy to mass produce
and reduced motor copper content.
However there are some significant disadvantages
which include often complex rotor forms
leading to rotor stability issues
and requiring complex control.
These machines have a high rotor inertia.
It is often difficult to cool the motor
requiring separate oil cooling systems
that negate some of the power density benefits of the motor
and it can experience high levels of torque ripple from the segmented stator.
The double stator single rotor axial flux motor
sometimes called DSIR
is one of the most power and torque dense motors available on the market
The benefits of this design
stem from the simple rotor
which is very stable and compact
it has a reduced rare earth magnet content
compared to most other PM motors.
It also has a very large
heat rejection surface area
from the rear faces of the motor stator
meaning it is easy to cool
which delivers good real world performance
and high usable power and torque density.
The housing is also easy to fully seal
which makes it ideal for harsh environments
such as electric vehicle drives
hazardous area robotics and aerospace.
AVID’s patented EVO machine
also has an ultra-low inertia composite rotor.
This can significantly improve transient performance of this motor.
The key disadvantage of this style of motor
 is an increased copper content
compared to ERAF motor
and the need for slot inserted stator windings
meaning high volume manufacture
requires some very specialized machinery.
Here we can see the sizing equation for a radial flux motor.
The torque density stays the same for a given length of machine.
It can be seen that the radial machine
increases its torque to the power of two of the diameter.
Power density then comes from torque density times speed.
Here we can see an axial flux sizing equation.
For a given axial flux motor length
it can be seen that the axial machine increases
its torque to the power of three of the diameter
As the diameter increases
typically the max operating speed is reduced.
This graph shows how the sizing equations work in practice.
The graph shows how torque and power vary
when a radial and an axial machine of originally
150 millimeters diameter
with the same performance
have their diameters increased.
It can be seen that as the diameter increases
the power and torque of the axial machine
increase much more than in the radial machine
which is what leads to the higher torque density
of axial machines.
Axial and radial flux motors
have their advantages and disadvantages
meaning they’re both suited to a wide range of applications.
Deciding which type of motor
will perform best for a particular application
requires a more in depth study
of the application requirements.
If you are interested in learning more
about radial and axial flux motor design
 and manufacturing
or have a specific requirement
for a high power density and efficiency motor drive system
contact AVID today.
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