Permanent Magnet Motors: Something Old, Something New

Now that Tesla has released their Model 3 with permanent magnet motors instead of their usual induction motors, you might be wondering “why?” Why did they (and Chevy, and BMW, and others) go with these motors? I thought we said per-mags were noisy and inefficient compared to other motor types.

(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.

What's Good about Permanent Magnet Motors?

Control Engineering touts the advantages of permanent magnet AC motors as...

  • “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

Radial flux motors are characterized by magnetic flux running perpendicular to the rotational axis.

Surface-mounted Permanent Magnet Motors (SPM)

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 machines include:

  • 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;
  • Less magnetic material, so less cost.

(This is the Chevy Bolt's motor type.)

External Rotor Permanent Magnet Motors (ERPM)

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 cost.

Types of Permanent Magnet Motors: Axial Flux

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 motor.

Torus motor (Internal stator, external rotor)  

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 motors available;
  • Reduced rare earth magnet content;
  • Easy to cool;
  • Easy to fully seal and protect from damage.

(This would be my choice if I wanted to design an electric car with in-wheel motors, I think.)

The downside:

  • Higher copper content, and
  • Need specialized machinery if you want to produce these in high volume

Chevrolet Bolt EV Traction Motor – Deep Dive

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.

Share this page:
Enjoy this page? Please pay it forward. Here's how...

Would you prefer to share this page with others by linking to it?

  1. Click on the HTML link code below.
  2. Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable.

Back to the Electric Car Motors page.