Electric motors are everywhere, from household applications such as your toothbrush, refrigerator and phone, to the planes, trains and automobiles we use on our daily commutes. Ever since the invention of the electric motor in 1834, engineers have striven to perfect the machine. Now, electric motor technology is more diverse than ever, with numerous categories, subtypes and individual motor topologies. But how do these motors work? What makes each type of motor so different? Moreover, why are some motors more popular in Electric Vehicles than others? These are a few of the questions which we will try to answer in this AVID Whitepaper.
First, let us start with the basics. All electric motors rely on magnets, and as everyone who has ever played with magnets knows, when you put a North pole (N) and South pole (S) together the magnets stick to each other, but when you put the N and N, or S and S together, they repel. In an electric motor, we are creating that same electromagnetic reaction force in the air gap between the motor’s rotor and stator. To do this, we need two magnetic fields; in fact, we have many magnetic poles inside the electric motor, creating many simultaneous reactions. In order to create continuous movement, we have to switch these magnetic fields on and off at high frequency, simultaneously attracting and repelling. This high-frequency switching creates the electromotive force (EMF), making the motor spin and creating our torque.
The first magnetic field comes from the motor stator, named so because it is stationary, in that it does not move. The stator is composed of an array of small electromagnets, created by wrapping conducting wire, normally copper around a specially shaped tooth made from magnetically conducting material. When an electric current is passed through these wires, this creates a magnetic field surrounding the electromagnet.
The second magnetic field is created by the motor rotor, which is often the main point of differentiation between electric motor types but will always serve as the main moving component in the motor. In low tech, ‘brushed’ electric motors, the motor rotors are mechanically commutated using carbon brushes that run against a commutator ring. As the motor rotates, the brush transfers electrical current through different coils wrapped on the rotor that form electromagnets. However, the problem with this kind of motor is that they are not very efficient and the brushes wear out. Therefore brushless motors are becoming more and more popular in the modern-day.
The most common brushless motor is the induction motor. Here, an electric current is induced in the motor rotor, in what is sometimes called a squirrel cage design (named after the copper bars that act as a coil in a transformer). The stator’s magnetic flux creates a lagging rotor magnetic field, creating EMF. Induction motors are ubiquitous in industrial applications, as the main principles have been around since the late 1800s when Galileo Ferraris successfully demonstrated his induction motor. Induction motors are also referred to as asynchronous motors, because whilst a magnetic field is setup in the stator that rotates synchronously to the AC supply, the rotation speed of the rotor is always lower than the frequency of the supply to the stator. As the rotor cage is effectively the secondary winding, a current is induced in the rotor, which creates an opposing magnetic field to that of the stator and generates torque. In order to induce a rotor current there must always be a differential in rotating speed between the rotor and the AC supply.
Despite all their positives, Induction motors are not that common for on-road EV traction applications, aside from their use by Tesla in their Roadster and Model S EVs. Induction machines are not widely used because they are relatively heavy and not as good as other motor types with low-speed torque, leading to relatively oversized machines. However, an advantage is that you can control the magnetic field strength in the motor rotor. If you can sacrifice mass and volume, induction motors are low cost and do not rely on rare earth magnets.
Next up are permanent magnet (PM) motors, in this type of motor, as the name implies, we embed rare-earth magnets into the rotor to create a set of permanent magnetic poles. We make a strong magnetic field with special magnets that use rare earth alloying elements to improve their performance. The magnets can be arranged either on the surface of the rotor (in SPM motors) or embedded into the rotor (in IPM motors). SPM motors are relatively simple to understand; however, IPM can be a little more complex. The rotor will be made of a ferrous material and used to concentrate the magnetic flux by cutting slots in it to create a flux path, the magnets are typically arranged in a V configuration which allows the field to be concentrated, allowing for a stronger, more concentrated magnetic field than would be possible with a surface magnet machine.
IPM motors have become by far the most common motors in EV powertrains because of their improved power density when compared to induction machines. Furthermore, the ability to control the reluctance torque improves driving cycle efficiency over a broader range of operating points (albeit for a small sacrifice in the peak efficiency of the motor). When brands such as Tesla talk about ‘hybrid reluctance motors’ or ‘partial permanent magnet motors’ in their Model 3s, this is what they are talking about. This is effectively the same motor technology/topology as can be found in a Nissan Leaf, Chevy Bolt and Renault Zoe, to name a few.
One downside of PM motors is that the rotor’s magnetic field is fixed, which is not always desirable in an application like an EV drive system where you have a wide range of operating speed and load conditions. However, one interesting possibility would be to create an additional magnetic flux using the magnetic reluctance effect of the rotor, in this case, additional stator current can be used to generate a corresponding additional magnetic field in the rotor working with the embedded permanent magnets. This reluctance effect can be controlled, giving an element of rotor magnetic field control.
Finally, there are full switched reluctance motors, in this type of machine there is no permanent magnet in the rotor, as all of the rotor’s magnetic field is created by magnetic reluctance effect. In theory, this type of machine can be low cost as it eliminates the need for rare earth magnets, but in practice, the performance trade-offs in terms of low power density and high torque ripple, along with manufacturing tolerance issues in the rotor design have limited their uptake.
Electric motor designs are being optimised and improved all of the time, and the higher power density for lighter more compact motors means manufacturers need to improve the cooling and thermal management within the motor due to the ohmic and iron losses caused by the heat from electric and magnetic fields within the rotor.
There is also a lot of focus on reducing manufacturing costs of the machine both through reduced materials content and improved manufacturing processes. This has led to the development of technologies such as hairpin or square bar winding and concentrated winding, both aiming to improve the copper fill factor in the motor and reduce the winding overhang on the stator. Not only are these technologies easy to automate for high volume mass manufacture, but through them, manufacturers can reduce both the weight and material cost of the motor.
In general, with the EV powertrain system, it is recognised that it is worth investing a little more in the piece-part cost of the motor and its inverter in order to improve performance and reduce weight, as this drives efficiency and ultimately battery range. So, whilst cost optimisation of a particular technology motor is important, it will not necessarily be the cheapest overall motor that is the most successful, but rather the one with the most competitive overall package.
If you want to learn more about electric motor technology, listen to episode 30 of the AVID Learning Podcast to hear AVID MD Ryan Maughan discuss the various motor technologies in today’s EVs!