SAE International Society of Automotive Engineers
Topology of electric traction motors (Equipmake)
In the new world of electric vehicles, a debate has never ended—which motor is the best?
In response, Ian Foley, Equipmake's operating director and expert in innovative electric drive system design and development, said, "It's like the computer wars of the early 1980s. The debate only subsided when IBM's PC became the industry standard. Now we have all kinds of motors, and the manufacturing philosophies of different motors are also different. We shouldn't just focus on the motor itself, but rather examine the overall system solution that includes the motor, battery, inverter, gearbox, and controller. Ultimately, the solution with the lowest cost is the winner."
Foley hopes that the final victory will belong to his company's APM200 spoke motor. He stated that the APM200 is highly efficient, currently the electric vehicle permanent magnet motor with the highest torque and power density (kW/kg), and it also has advantages such as low cost and light weight.
The APM200 motor weighs approximately 49 kg (108 lb), has a maximum speed of 10,000 rpm, and delivers a maximum power of 220 kW (295 hp) and a maximum torque of 450 N·m (332 lb·ft). It utilizes lower-cost neodymium iron boron (NdFeB) magnets, reducing the overall cost of the motor, and features a 5.5:1 integral planetary gearbox with the output shaft directly connected to the hub. Furthermore, Equipmake developed a dedicated inverter for the APM200, employing power electronics technology combining silicon carbide diodes and IGBTs (Insulated Gate Bipolar Transistors), enabling the motor to maintain high power operation at high frequency conversion.
Cross-sectional diagram of a spoke motor system (Equipmake)
Colder and colder
Cooling is crucial for motor performance. The lower the temperature of the motor magnets, the longer the motor can output peak power. However, simply achieving cooling is not enough; it is also essential to ensure that the cooling system is cost-effective, reliable, and efficient for mass production.
Foley stated, "The structure of the spoke motor meets these requirements. Traditional permanent magnet motors have V-shaped magnets pressed shallowly into the plates around the rotor. In contrast, the spoke motor's magnets are perpendicular to the aluminum rotor surface, like spokes, allowing them to be very close to the coolant (60°C water/glycol). In other words, traditional motor magnets are distributed on the plates, preventing them from reaching the coolant; while one end of the spoke motor's magnet is on the aluminum center hub, allowing the coolant to be close enough to the magnet for heat dissipation. Although spoke motors are more difficult to manufacture than traditional motors, we are confident that our designed motor is ready for mass production."
Foley stated that the key to achieving mass production of the motor lies in implementing design details, such as finding a way to mount the plates onto the center hub. "The center hub is basically a forged part. Our cooling is very efficient, so we can achieve the required high strength. The temperature of the aluminum center hub is controlled below 100°C, so we can use magnets that are low-cost but offer comparable performance, reliability, and lifespan. Thermal engineering is a key factor in making an electric vehicle motor stand out."
