The very first electric machines were built in the 1820s (Fig. 1a). Since then, inventors and worldwide scientists have been working on the motor’s performance and its size and weight, to make them more compact. Another milestone in the electric motor history was the invention of an induction motor by Galileo Ferraris and Nikola Tesla in the 1880s (Fig. 1b,c). Machine size and weight are a significant issue nowadays because we expect everything to be lighter and smaller. Technical specialists employ special ratios to compare motors’ performance (and any other devices’). The ratios are defined by two quotients: motor performance to motor mass and motor performance to motor volume. Motor performance usually means peak power and torque, continuous power and torque and rotational speed.

Fig 1. Jedlik’s “electromagnetic self-rotor” (1827) a), modern small power induction motors b,c)

An electric motor usually has two winding systems: in its rotor and in its stator. One of them can be replaced by permanent magnets – the other source of the magnetic field. Neodymium magnet technology developed in 1982 by General Motors and Sumitomo Special Metals brought one of the most relevant changes to the size reduction of an electric machine. Neodymium and other rare element-based magnets replaced excitation winding in alternating current synchronous- and direct current machines.

Motors excited by the high-energy permanent magnet (called PM motors) changed the world of motion significantly. PM motors are generally divided into two main groups: Surface Mounted- (SPM) and Interior Permanent Magnet motors (IPM). Both of them have pros and cons concerning: overall performance, manufacturability and overall cost. In an IPM, magnets are embedded into a rotor laminated stack. A rotor core structure and permanent magnets’ arrangement inside that core define a magnetic pole. A magnetic pole shapes the magnetic field distribution in a motor magnetic circuit, including an air gap between stationary- and rotating part.

A popular arrangement of magnets in the IPM group is so-called V‑pole (Fig. 2b,c). A minimum two magnet segments create V‑pole (like a letter V or U). By changing V‑side length and V‑angle engineers change magnetic field inside the motor and finally the motor performance.

Fig 2. 10-poles surface-mounted magnet rotor a), 10-poles interior magnet rotor b), d- and q-axis in IPM rotor c), IPM motor torque components

The V‑pole IPM motor rotor has two symmetry axes: d‑axis in the centre of the pole and q‑axis between two poles (Fig 2c). That feature of the magnetic structure contributes to the total torque developed by a motor. The torque component caused by different magnetic properties of a rotor core is named reluctance torque (Fig 2d). Surface-mounted SPM motors do not generate reluctance torque.  Engineers aim to design the best electromagnetic structure of a motor within all the technological and financial circumstances. At first sight, a PM motor rotor build looks simple. There are mainly two materials: permanent magnets and a laminated stack of a silicon steel in the IPM motor rotor (Fig 2b). By changing the mechanical build of a rotor, engineers can change the magnetic shape which is different from the mechanical shape. All the tiny technological non-magnetic gaps (Fig. 3a) and a magnetic saturation phenomenon derive a magnetic shape and hence motor performance. It is not a rarity that the 0.2-0.8mm adjustments in the mechanical rotor structure of 20-30kW motor (17kg, 230x220mm) improve the motor performance significantly (of about 10%).

Fig 3. IPM rotors: BMW i3 Traction Motor a), 2006 Toyota Prius b), i-DCD motor Honda c)

Currently, IPM motors are popular as traction motors for all the propulsion types (cars included) due to their smaller size and weight. IPM motors are also used in elevators, conveyors, light excavators, basket lifts and so on.