(I'm pretty sure I did say that, speaking of permanent magnet DC motors back in the day.)
So what changed?
Improvements in technology and magnetic
composites that could be made at a reasonable cost brought per-mag
motors back into play for electric vehicles...
Magnets and magnetic material got
better, with more powerful magnets that could resist corrosion and
breakage while at the same time delivering the power needed for
light, fast motors that could deliver the torque; and
Electronic control of motors got
better and cheaper, making it possible to control cogging and other
sources of torque ripple which made them noisy.
...and rightly so, because they are
fantastic! The torque and power density of an AC permanent magnet motor
is the best on the market, and this is just what you want for an EV.
Let's see what permanent magnet motors are all about, and why we are
going to be seeing a lot of them in electric cars from now on.
“compact form with high torque
density and less weight,” - you get the most torque for the space
and weight your motor is adding;
“higher continuous torque over a
wider range of speeds,” - your ability to accelerate doesn't fall
off as you approach highway speed;
“lower rotor inertia,” - so less
muscle needed to start it, keep it going, and stop it. This also
contributes to less heat generated by motor operating, which is part
of the torque density secret sauce;
“higher dynamic performance under
load,” - another heat-management design marvel;
“higher operational efficiencies with
no magnetizing current,” - the AC induction motor requires
magnetizing current to create a magnetic field in the air gap
(between the stator and rotor). A permanent magnet motor doesn't.
Something has to create the induction motor's magnetizing current
(another motor perhaps?), so a motor which doesn't need this can save
the weight and space;
“the corresponding absence of heat
due to current in the rotor, low torque ripple effect, more robust
performance compared to dc motors.” - DC motors (including
permanent magnet DC motors) have commutators with brushes which
mechanically carry the current to and from the commutator to the
windings on the armature. They produce torque by changing the
direction of the current every half turn, which creates a periodic
torque interruption known as “torque ripple”, which adds noise
and vibration. The brush/commutator/armature combination itself, even
under ideal conditions, generates heat and wear.
Permanent magnet AC motors have solved
all these DC motor problems to a large degree through improvements in
motor control and design.
Comparison of Permanent Magnet Motors
Types of Permanent Magnet Motors:
Radial flux motors are characterized by
magnetic flux running perpendicular to the rotational axis.
Surface-mounted Permanent Magnet Motors
If it has surface-mounted magnets on
the rotor, it's called an SPM motor. These are very efficient, high
torque, easy cooling.
Downside of SPM motors: uses more rare
earth magnetic material (such as neodymium and samarium cobalt) than
other types of motors, which costs more.
Internal Permanent Magnet Motors (IPM)
If the motor has its magnets on the
inside of the rotor, it's called an IPM motor. Features of IPM
multiple layers of laminated steel with
pretty designs (apparently not done because it's pretty, but because
the shape of the designs affects the flux: );
Magnets are located in slots so no
bonding or banding is required;
Also called “outrunners”. These
have the rotor on the outside and the stator on the inside.
These have the advantage of higher
torque at lower rpm, but the tradeoff is that they are hard to cool
because there's no path to get coolant in there and let heat out; and
the magnets are bonded into place which adds weight and material
Types of Permanent Magnet Motors: Axial
Axial flux motors are characterized by
magnetic flux running parallel to the rotational axis. They have
higher torque and power density than the radial flux type per-mag
Torus motor (Internal stator, external
This type of axial flux motor is characterized by...
Compact stator windings with reduced
copper content are cheaper and easier to mass produce;
Complex motor forms which often have
stability issues and require complex control;
Harder to cool;
High torque ripple;
High rotor inertia because the rotor's
mass is a longer distance from the axis.
Double Stator Internal Rotor (DSIR)
These axial flux motors are characterized by...
One of the most power and torque dense
Reduced rare earth magnet content;
Easy to cool;
Easy to fully seal and protect from
(This would be my choice if I wanted to
design an electric car with in-wheel motors, I think.)
Higher copper content, and
Need specialized machinery if you want
to produce these in high volume
Chevrolet Bolt EV Traction Motor –
Still curious? (I know, me too: )
To learn more about the
permanent magnet AC synchronous motor that is currently being used in
the Chevy Bolt, check out this video from Professor John D Kelly at
Weber State University who takes an in-depth look at the drive unit
so you can see everything that goes into it.