By Rachel Gould - Engineering Student @ Jesus College, Cambridge
Electric cars are fantastically engineered machines. They are the future of road vehicles in the world's effort to cut fossil fuel usage and tackle the effects of global warming. Tesla is often considered a leading name in the electric vehicle sector, and in this article, I will explain the fascinating engineering behind the electric motor that drives one of its newest models, the Model 3.
First, it is important to understand how the motors in previous models work. This version is known as the Induction Motor, and it consists of a cylindrical rotor of conducting bars, which fits inside a larger tube full of windings. An AC input is passed through the windings, producing an RMF (rotating magnetic field). The fluctuating field causes an EMF in the rotor bars (due to faradays law), and consequently a current flow in the bars. When current flows, a magnetic field is produced around the bars. As this field interacts with the RMF, a force is produced, and the rotor turns. The induction motor was invented by Nikola Tesla, which is where the company gets its name. These are effective motors, but they can be improved.
Another type of motor, known as a Permanent Magnet Motor, has permanent magnets in the rotor instead of bars. The RMF in the outer tube therefore interacts with the permanent magnetic field, producing force and causing the rotor to turn. The issue with this is that the permanent field causes back EMF in the outer windings, an EMF which opposes and therefore reduces the AC input EMF. This is due to Faradays law, and the affect is larger as the rate of change of flux increases. Therefore, as the vehicles velocity increases, the motor performs even more poorly.
A motor that does perform very well at high speeds is a Synchronous Reluctance Motor (SynRM). Reluctance is a mediums ability to oppose magnetic fields, iron has low reluctance, whereas air has high reluctance. By cutting air slots into an iron rotor, the areas of high reluctance cause the rotor to rotate at the same frequency as the RMF as it tries to maintain a low reluctance state. The torque produced is known as reluctance torque. SynRMs perform well at high speeds as they eliminate the problem of back EMF caused by permanent magnets.
The Model 3 Motor combines the Permanent Magnet Motor and the Synchronous Reluctance Motor, by inserting permanent magnets into the air slots in the SynRM. The permanent magnets are aligned with the RMF to reduce the field form the permanent magnet, so at high velocities, back EMF is minimised, and the majority of the torque comes from reluctance. The resulting motor is named IMP-SynRM (Internal Permanent Magnet Synchronous Reluctance Motor). The permanent magnets are also segmented, to reduce eddy currents, and therefore reduce energy losses.
The development of electric car motors is aimed to output maximum efficiency as the world prepares to shift from petrol vehicles to electric ones. The IPM-SynRM motor is an incredible demonstration of the engineering that goes some way towards perfecting this technology.