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88 punched circular shapes from a steel lamination sheet until the slotted shape and rotor dimensions have been achieved. That said, some SPM motors are also made with laminations to minimise losses in the backing that holds the magnet in place. The main advantage of IPMs over SPMs is the lower amount (and therefore expense) of magnetic material used. However, the method of constructing IPM inrunner rotors using lamination stacking can increase their cost because of the complexity required for precisely cutting the slots and inserting the magnets. At times or in regions where magnetic material is in short supply though, IPMs can be less expensive. Regardless of the motor configuration, is it imperative that the magnets are held tightly in place to ensure they do not come loose in flight, which can destroy the motor. They should also be mounted in such a way that minimises the flux loss and magnetic interference outside the motor housing (this is especially so for outrunners, which have the magnets closer to the outside). Magnet metrics A few key qualities of magnet materials should be kept in mind. The first is the magnetisation, or flux density, of a magnet, which refers to the magnitude of the magnetic flux given off by the magnet after the external magnetic field has been removed. This can determine the torque capability of the motor, and is typically measured in teslas, symbol T. A similar measure indicating the quality of the magnet is the energy product, or magnetic energy density (measured in kilojoules per cubic metre). The higher the energy product, the less volume of magnet material that needs to be used. Then there is the material’s Curie temperature, which is a measure of how much heat the magnet can withstand before losing its magnetism. This can indicate how much work and expense a motor manufacturer may have to put into a thermal management system with the material. A magnet’s coercivity (measured in amps per metre) should also be considered. This relates to its resistance to demagnetisation as a result of prolonged exposure to external magnetic fields or high temperatures. Materials with a high coercivity therefore make good permanent magnets. Alloys produced from rare earth elements make the strongest permanent magnets, especially those using neodymium, iron and boron. Neodymium is produced in a range of quality grades, and can be manufactured by sintering or by bonding (compression), the former giving higher quality. When produced by sintering, neodymium magnets have a magnetic flux density of 1.0-1.4 T (and typically higher than 1.26 T), and an energy product of 200-400 kJ/m ³ . Those produced by bonding tend towards 0.6- 0.7 T and 60-100 kJ/m ³ , and have far lower coercivity – 600-1200 kA/m as opposed to the 750-2000 kA/m offered by sintered neodymium. The more heat-resistant grades of neodymium can have a Curie temperature August/September 2018 | Unmanned Systems Technology Hand-winding can increase the quantity of copper wire inside a motor’s coils, which in turn increases the power-to-weight ratio (Courtesy of Neumotors) Most of the world’s neodymium, the permanent magnet material with the highest flux density, comes from China (Courtesy of T-Motor)