Unlocking electric drive potential with SMCs

Electric powertrain design sits at the forefront of modern automotive engineering. With carmakers across the world now committed to electrification, the race is on to deliver increased range at an affordable cost. No stone is being left unturned when it comes to motor design, and the past few years have seen growing interest in the use of soft-magnetic composite (SMC) materials.

Engineering consultancy Drive System Design (DSD) has struck up a partnership with electric drive unit specialist Alvier Mechatronics and powder metallurgy giant Höganäs to develop this technology for use in high performance motors.

In the right applications, SMC components are said to increase the power density and efficiency of the motor, as well as potentially reducing cost and environmental impact. There are other benefits too – a recent shortage of electrical steel has pushed more manufacturers to consider SMCs as an alternative, while it’s also been claimed that the composite materials have superior NVH properties to conventional steel laminates.

To understand the significance of SMC materials, it’s worth taking a look at the traditional approach to motor design. Historically, the stator has been formed with layers of cold-rolled electrical steel, which are laminated together with an insulated coating between each sheet.

The stator helps to shape and enhance the electromagnetic field generated by the copper windings. By using a set of insulated layers, it’s possible to reduce the eddy current losses that would result from a solid core.

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But there are limitations to this approach. Although the laminates can be stacked on top of each other to create a three-dimensional structure they are, essentially, two-dimensional – as is the flux path of the magnetic circuit that they generate.

That’s where SMCs come in. Instead of flat sheets, these composite materials start with pure iron particles made from melted and water-atomised high-quality scrap iron, which are coated with an inorganic insulating material, similar to that used on the traditional laminates. Unlike the steel sheets, however, their physical and electromagnetic properties are equally strong in all directions.

“This material can be viewed as the equivalent of electric steel laminations but in three dimensions rather than two,” comments Lars Sjöberg, head of system design at Alvier Mechatronics. “In a conventional steel design, you have a set of punched laminate structures that are stacked together to form the stator part or the rotor parts of an electric machine. We can do the same but it’s an isotropic material, so it’s effectively laminated in all directions.”

Carefully optimised

The SMC material, developed by Alvier’s parent company Höganäs, starts with particles of high-quality scrap iron. These are coated with the insulating material and then fed into a die cavity, where the mixture is compacted using a single-axis punch. After that, the component is heat-treated to fine-tune its parameters, such as physical strength and magnetic performance.

This tuning process includes precise control over the size of the particles, which allows the core to be tailored to work most efficiently at a given frequency. At low frequencies – up to around 50 Hz – the core losses with an SMC component can be worse, but at high frequencies the efficiency of an SMC core is said to be considerably better.

“Being able to tailor the frequencies is very useful, especially if you’ve come up to slightly higher frequencies involved in electrical systems from power electronics,” comments Sjöberg. “There is a crossover where the SMC has lower iron losses compared to standard laminations and this is a trend going on when emerging new wide bandgap devices for power electronics like silicon carbide and so on. They all tend to go up in frequency.”

Greater control over the field shape and frequency response potentially allows SMC motors to be smaller and lighter for a given output than those using conventional steel laminates. The stamping process used to create them lends itself to complex three-dimensional structures that would be very difficult to create with laminates, and it’s also inherently suited to high-volume production.

(Above: Radial flux motor)

There are drawbacks, however. SMC materials are more brittle than steel, which can be an issue in certain applications (it’s also why SMCs aren’t generally used for rotors). They also have lower magnetic permeability, which means that higher currents are required to generate the same torque at low magnetic field strengths.

Generally, these limitations can be overcome with the correct design approach, and sometimes SMCs are even used in combination with steel laminates to give the best of both worlds.

“Typically we’re focusing on the stator, but it is sometimes possible to use SMC materials in the rotor,” comments Sjöberg. “These materials can be used in most of the places that you would traditionally find a steel laminate. The design has to be tailored to work with the constraints of the composite material – mainly the mechanical strength – so you wouldn’t just produce the same part in SMC, but the benefits can be considerable when it’s properly optimised.”

Systems approach

Finding the right applications for SMC materials is where DSD comes in. The Leamington-based company has spent the past few years developing a systems-level toolset known as ePOP (Electrified Powertrain Optimisation Process). This combines efficiency models for the various parts of the powertrain, making it possible to simulate the interaction between them and predict the system’s overall performance.

“In automotive applications, electrified powertrains are very different from internal combustion engine designs, in that they’re dominated by the energy storage system,” explains DSD’s principal engineer Bence Falvy. “The battery is a significant contributor to both the vehicle’s mass and its cost. So you can’t just combine the best motor, the best transmission and the best inverter and expect to have an optimal solution. Finding the right motor or transmission can reduce the size or the cost of the battery, or indeed gain more range from the same battery capacity.”

(above: Axial flux motor)

The total cost of ownership and the environmental impact of the system can also be estimated in ePOP, and these are other areas where it’s thought that SMC materials may prove advantageous.

“The use of SMC in electric machines hasn’t always been the industry’s preferred option on technical merits alone. But once you factor in specific application requirements and the sustainability aspect, it could become a new trend,” comments Falvy. “For us, SMC provides an additional solution type that we can offer to our customers, and it’s very favourable when it comes to sustainability.”

For Alvier, the partnership with DSD makes it possible to explore new application areas, Sjöberg explains: “We focus on electromagnetic design and the adaptation of these powder materials. Part of our job is to engage with the OEMs and the tier ones and feed back material development suggestions to Höganäs. But, being a start-up, we have limited capacity to do things like transmissions and electronics. That systems-level knowledge is a key element that DSD brings to the partnership.”

This holistic approach has become an integral part of powertrain design. It’s often pointed out that motors and inverters are now highly efficient, but there are still gains to be made, and as Falvy points out, the real benefits come from how efficiently the powertrain works as a whole. Factor in the potential cost and sustainability benefits, and it’s easy to see why SMCs are being given serious consideration.