Drives & Controls Magazine September 2023

MOTORS n presented a rare-earth-free motor with wireless power transfer to the rotor, thus eliminating the need for brushes. It has also developed a motor optimised for continuoustorque endurance and has combined the two technologies to create a “technology kit” for EV motors. Switched reluctance motors (SRMs) are potentially the easiest type of motor to manufacture, with their rotors being largely made of steel. This steel has a low reluctance compared to the air around it, so magnetic flux travels preferentially through the steel while attempting to shorten its flux path, thus driving the rotor. Despite their simplicity and reliability, SRMs have relatively low power and torque densities, and are affected by other issues such as torque ripple and acoustic noise. While SRMs have largely been confined to industrial or other heavy-duty applications, significant efforts are being made to develop them for use in EVs. Companies including the Californian motor developer Turntide Technologies have added more rotor and stator poles and come up with more sophisticated control systems to overcome some of traditional limitations. UK-based Advanced Electric Machines has developed a new type of SRM with a segmented rotor that remains simple in construction but is said to eliminate acoustic noise and torque ripple, while improving power and torque density. “Uniquely”for SRMs, AEM’s motors operate with the standard power electronics used by most EV drivetrains. AEM is also working a joint collaborative project with Bentley Motors and others to recycle rare-earth magnets. Alternative materials While many EV developers have been reducing the rare-earth content of their motors, Tesla attracted considerable interest earlier this year when it announced that its next-generation of drive systems will be based on PM motors containing no rare-earths. (Between 2017 and 2022, it already reduced the rare-earth usage in its Model 3 vehicles by around 25%.) Tesla is likely to combine several developments in magnetic alloys to improve field strengths and increase the size of its motors to provide similar outputs, while taking a hit in power density. Drivers probably won’t notice these changes unless they affect performance or vehicle space. There are several projects under way around the world to develop rare-earth-free magnets that can compete on magnetic performance with those containing the magnets. These are at differing levels of development and commercialisation. The problem with using alternative magnetic materials is that magnetic performance usually suffers. For example, some manufacturers that make both rareearth and ferrite magnet motors suffer a 5070% drop in power for the ferrite versions of similar-sized motors, so to match performance, much more magnetic material and/or a much larger motor are needed. The Japanese firm Proterial (formerly Hitachi Metals) has developed ferrite magnets with magnetic properties that it claims deliver the world’s highest output levels for magnets of this type. Motors using these materials will need only 20% more magnetic material to achieve a similar power density to machines using rare-earth magnets. The company has built a ferrite magnet motor prototype capable of producing an output of more than 100kW. A US company Niron Magnetics is developing “next-generation” magnets using iron nitride, and says that these magnets will compete on performance with those based on neodymium. A European project called Passenger is developing strontium ferrite and manganese aluminium carbon alloys. Materials with a truly comparable performance with rare-earth magnets are still some way off. But, with other changes to the motor design, they might not need to be. While the adoption of ferrite magnets would significantly reduce motor performance, optimising other aspects of motor designs could reduce this impact. An Australian technology company called Ultimate Transmissions has submitted a patent for a ferrite motor design that it believes is a route that Tesla could take to eliminating rareearths from its PM motors. Its design uses much larger ferrite magnets and higher speeds – around 20,000rpm – to In 2034, permanent magnet (PM) motors containing rare-earths will still dominate the EV market, but non-rareearth technologies including AC induction motors (ACIMs) and wound-rotor synchronous motors (WRSMs) will account for up to 30% of the market. Source: IDTechEx 4-6 June 2024 NEC Birmingham THE HOME OF ROBOTICS & AUTOMATION Part of: