source:physicsworld
I was recently in Newcastle to attend PEMD2022 – the 11th international conference on power electronics, machines and drives. What struck me was not only the huge performance improvements that have been happening in electric motors and generators but just how far we still have to go to make transport fully carbon-free.
Global sales of electric cars (including fully battery powered, fuel cell and plug-in hybrids) doubled in 2021 to an all-time high of 6.6 million. They now account for 5–6% of vehicle sales, with more being sold each week than in the whole of 2012, according to the Global Electric Vehicle Outlook 2022 report.
Each new electric vehicle will need at least one high-power electric motor.
Projections vary, but annual sales are expected to increase to 65 million electric vehicles by 2030 globally, according to market research firm IHS Markit. Annual sales of vehicles with internal combustion engines, in contrast, will decline from 68 million units in 2021 to 38 million by 2030.
What’s obvious is that each new electric vehicle will need at least one high-power electric motor. Almost all (about 85%) of these vehicles currently use motors with permanent magnet (PMs) as they are the most efficient (the record is 98.8%). A few use Alternating Current (AC) induction motors and generators, but they are 4–8% less efficient than PM motors, up to 60% heavier and up to 70% larger.
Still, these non-PM motors and generators are perfect for, say, trucks, ships and wind-turbine generators. They are also easy to recycle as they can, in principle, be made of one material (say aluminium) and then melted down when they come to the end of their life. Some firms, like Tesla Motors, are even combining the PM and electromagnetic approaches in ever more complex designs to optimize performance and range. None of the advances in electric vehicles would, however, be possible without the huge advances in solid-state power electronics.
Magnetic attraction
Magnets have come a long way since a shepherd in Magnesia in northern Greece noticed the nails in his shoe and the metal tip of his staff were stuck fast to a magnetic rock (or so legend has it). These “lodestones” were used for thousands of years in compasses to navigate but it was not until the early 1800s that Hans Christian Ørsted discovered that an electric current can influence a compass needle.
The first demonstration of a motor with rotary motion occurred in 1821 when Michael Faraday dipped a free-hanging wire into a pool of mercury, on which a PM was placed. The first DC electric motor that could turn machinery was developed by British scientist William Sturgeon in 1832. US inventors Thomas and Emily Davenport built the first practical battery-powered DC electric motor at about the same time.
These motors were used to run machine tools and a printing press. But as the battery power was so expensive, the motors were commercially unsuccessful, and the Davenports ended up bankrupt. Other inventors who tried to develop battery-powered DC motors struggled with the cost of the power source too. Eventually, in the 1880s, attention turned to AC motors, which took advantage of the fact that AC can be sent over long distances at high voltage.
The first AC “induction motor” was invented by the Italian physicist Galileo Ferraris in 1885, with the electric current to drive the motor obtained by electromagnetic induction from the magnetic field of the stator winding. The beauty of this device is that it can be made without any electrical connections to the rotor – a commercial opportunity seized upon by Nikola Tesla. Having independently invented his own induction motor in 1887, he patented the AC motor the following year.
For many years, though, PMs had fields no higher than naturally occurring magnetite (about 0.005 T). It wasn’t until the development of alnico (alloys of mostly aluminium, nickel and cobalt) in the 1930s that practically useful PM DC motors and generators became a possibility. In the 1950s low-cost, ferrite (ceramic) PMs appeared, followed in the 1960s by samarium and cobalt magnets, which were stronger again.
But the real game-changer occurred in the 1980s with the invention of neodymium PMs, which contain neodymium, iron and boron. These days, the N42 grade of neodymium PMs has a strength of some 1.3 T, although that’s not the only key metric when it comes to magnet and motor design: operating temperature is vital too.
Prices of some rare-earth materials have skyrocketed, prompting a huge amount of research into new magnet compositions.
That’s because the performance of PMs falls as they warm-up and once they go above “Curie point” (about 320 °C for neodymium magnets), they completely demagnetize – rendering the motor useless. Another important thing about all rare-earth magnets, including neodymium, cobalt and samarium, is that they have a high coercivity, meaning they don’t demagnetize easily when in operation. To make the highest coercivity and best temperature performance magnets you also need small amounts of other heavy rare earths such as dysprosium, terbium and praseodymium.
A question of supply
Trouble is, rare-earth elements are in short supply. It’s not because they are intrinsically rare, their name simply comes from their location in the periodic table. According to a report last year from Magnetics & Materials LLC, by 2030 the world will need 55,000 more tonnes of neodymium magnets than are likely to be available, with 40% of the total demand expected to come from electric vehicles and 11% from wind turbines.
China currently makes 90% of all the world’s neodymium magnets, which is why the US, the EU and others are all trying to develop their capabilities in the supply chain so as not to be disadvantaged. Prices of some rare-earth materials have skyrocketed, prompting a huge amount of research into new magnet compositions, recycling of existing magnets and advanced AC induction motors.
Whichever way you look at it, we’re going to need a lot of magnets if we are to green the economy.