The design of an electrical machine is difficult at best. It’s always a compromise between manufacturability and performance. Some might say it's also a balance of shape and performance.
In the design of an axial flux motor, theoretically the flux will move in the opposite direction, but at a significantly reduced level. So you’re settling in how you achieve that -- machining away the excess, wasting plenty of material in the process.
Today, axial flux motors are moving toward nontraditional designs. Whatever the case, the materials you’ll want in your next electric motor design depends on how the motor must function, where it will function, and so on.
When it comes to electric motor materials, your choices are either electrical steel, powder metal, or nothing at all. This article will focus on electrical steel (aka steel laminations) as well as two forms of powder metal.
There are other articles that do a fine job explaining material use for other motor components, such as the shaft. Today we’ll focus on three of the "core" elements.
Electric Motor Material: Designing Rotor & Stator Components
Industrial electric motors require high-performing materials to efficiently convert electrical energy into mechanical energy. Let’s examine where powder metallurgy stands with these three key electric motor components:
As you'll see, electric motor core material is already knee-deep in powder metal, or at least has the potential to leverage the advantages of powder metallurgy.
So, what are these motor component materials made from? Powder metal components for motors generally consist of iron, nickel, and cobalt.
- Iron is the cheapest of the three, so many designers turn there first.
- Cobalt is rarely used on its own, but is sometimes added to iron. Cobalt gives your part more saturation induction.
- Nickel is expensive but valuable to motor applications. It adds performance by making your component easier to magnetize.
Now, on to the bigger picture:
Electric Motor Stator Material
In traditional steel lamination stators, core losses are high. This may decrease their efficiency, depending on the usage of the motor and the frequency. If preventing core loss is important to your design, electrical steel may not be optimal.
Laminated stator core material also suffers from a two-dimensional personality. A stator lamination material might produce a nice, flat part, but what if your design isn’t flat, or requires other bells and whistles?
Fortunately, there’s a newer, more effective replacement. It’s possible to incorporate soft magnetic composite (SMC) to work efficiently in tandem with the rotor.
Soft magnetic composites are metal powders that can be easily magnetized and demagnetized compared with a hard magnet.
One unique possibility is combining soft magnetic composite with electrical steel laminations. There are so called “hybrid” situations where you get the benefits of both. Correctly designed, a combination assembly lets you use the advantages of electrical steel (lower production cost) while adding unique features with SMCs (through its awesome shape-making capability).
If your current electric motor design is running at 60-70% efficiency, can you improve that with SMC? Think of the long-term electric bill savings you could offer the end customer.
We have one more thought for those adding magnets to their rotor design. Can you create a completely powder metal-based motor, relying on sintered magnetic powders as the material to which you attach the magnets? Now you can merge two design concepts -- SMCs and a sintered powder metal -- taking full advantage of powder metallurgy.
More on that below.
Electric Motor Rotor Material
Rotor core materials are usually based on electrical steel laminations. The outer and inner parts of the motor -- the rotor and the stator -- are stamped at the same time to minimize waste. Traditionally, whatever you stamp the stator from, you stamp the rotor from too.
However, with newer motors, engineers are looking at performance magnets on the motor for better torque and tire performance.
Soft magnetic composite materials are NOT recommended for rotors as they’re currently designed. SMCs are not sintered and therefore lack the strength to hold up in these applications.
But sintered soft magnetic materials ... they can be great alternatives.
You may be wondering about the difference between sintered soft magnetic materials and SMCs. For now, just know that soft magnetic composites aren’t sintered. (We'll discuss more in a bit.)
Bearings are a staple of conventional powder metallurgy. They’re a high-volume, low-cost job, and are readily available in a wide variety of materials and shapes.
Powder metals have been in the bearing industry since the 1930s and are a proven material for many related applications. Initially they started out as bronze, but thanks to the innovations in powder metallurgy in later years, more cost-effective materials like iron can be used.
These small, metal components use sponge iron compacted to low density so you can infiltrate them with lubricating oil.
Motor bearings are what they are. The innovations are happening at the stator and rotor levels.
Radial Flux Motors? Or Something Else?
For conventional radial flux motors of 60 Hz, soft magnetic composites aren’t a great alternative. … But could we instead marry a hybrid design to optimize it?
What if you don’t want a simple radial design? What if you want other beneficial aspects from your electric motor material? That’s possible with lamination electrical steel, but it’s going to be a lot harder. Now you truly need to be all-in on soft magnetic composites because of their shape-making capability.
SMCs are ideal for new designs, or designs where you can combine SMCs and laminations to get performance advantages. Candidates include:
- Axial & transverse flux motors for simplified or 3D motor stator and rotor winding
- Wheel hub motors
- Low-speed motors
The image above is a classic example. This inverter direct drive motor in the LG Signature watch is right in the wheelhouse of soft magnetic composite. And as you develop new designs with the rotor, start asking yourself, “Can we switch these over to powder metal too?”
In the transportation space, SMCs are granting new shape-making and magnetic benefits to:
SMC can help bridge the gap by forming the shape that best suits your e-motor design.
The Role of Sintered Metal
This design transformation could involve not only SMCs, but also sintered soft magnetic materials. This SMC-adjacent branch of powder metallurgy offers higher strength than SMCs (in exchange for slightly less magnetic performance).
DC electromagnetic design is a growing application for sintered soft magnetics. Where else does it differ from other materials?
- Rapid-response solenoids
- Surface damage resistance
- Higher density for increased saturation induction
No More Settling for Traditional Electric Motor Materials
Electric motor components don’t have to be a compromise -- at least not in the way you’re used to.
Play around with the idea of combining silicon steel lamination, soft magnetic composite (for AC electric motor designs), and sintered soft magnetic materials (for DC motors). Make sure to communicate your specific design needs to your powder metallurgy manufacturer. The vendor should be able to determine the viability of powder-based components for your project.
You can learn more about SMCs and e-motor design by checking out the free Engineer's Hub of resources below:
Related AC Electric Motor Design Resources
(Editor's note: This article was originally published in September 2019 and was recently updated.)