Electric motor topologies 101
Electric motors have been around since the 19th century and are used in a diversity of applications. They can be found anywhere, from small toys and electronics up to electric vehicles and industrial applications. Electric motors for electric vehicles are broadly classified into two categories:
The choice of a traction motor technology for an electric vehicle is crucial for the design of the whole system. Many criterions such as efficiency, cost, reliability and power density must be taken into consideration.
Within those two groups, there are numerous types of electric motors topologies. Here is a brief overview of what is available on the market:
In this article, we will briefly go over the main electric motor categories in regards to their possible usage as the main traction system of a vehicle. Not all motors topologies are suited to be used as the main electric car engine. A good example is the DC motor topology:
DC electric motors
DC motors are electric motors powered by direct current and can be classified into two sub-categories: Brush DC motors (BDC) and Brushless DC motors (BLDC).
They are inexpensive, easy to control and are readily available in all sizes and shapes. Every conventional vehicle utilizes many of them to power windows and seats, windshield wipers, electric power steering, pumps, auxiliaries and more.
Now when it comes to acting as the main electric car engine, BDC motors are not the ideal solution. All electric vehicles are designed with efficiency in mind, mainly as the result of the high cost of batteries and their limited storage of energy. As BDC motors use brushes to connect the rotor winding, mechanical wear occurs and they require a lot of regular maintenance. Also, friction and sparks created by brushes can overheat the system and limit the maximal rotation speed. Therefore, they are only typically 75-80% efficient in terms of energy conversion. From the perspective of maximizing the efficiency to limit batteries costs as well as low maintenance requirements of the auto industry, the BDC motors are are not the right candidates for on-road vehicles.
DC motors are better candidates for low power and low speed applications such as golf carts and forklifts.
AC electric motors
AC motors are electric motors powered by alternating current and can be broadly divided into two categories: Asynchronous motors (induction) and synchronous motors.
As seen in the diagram above, there are many topology variants in each category. To keep it short, we will only overview the two most popular technologies used in EV vehicles:
Asynchronous motors (induction)
Induction motors are widely available on the market: their design is quite simple, they are low priced and robust. The electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. The downside resides in the fact that they have a lower overall efficiency than permanent magnet motors (PMAC). This is in part due to the resistive losses happening in the copper bars of the rotor, which are not found in PMAC.
Synchronous motors – Permanent magnet
Permanent magnet AC motors (PMAC) do not rely entirely on current for magnetization. They use permanent magnet (embedded on or in the rotor) to create a magnetic field. They are a bit more efficient than induction motors and more compact. This is due to the fact that permanent magnets produce more flux for their size than the magnetic energy produced in an induction motor’s rotor. The downside is that they are usually costlier due to the use of rare earth materials in the magnets.
Compared to DC motor, both motor categories provide great energy conversion (efficiency range between 92-96% for induction motors and 95%-98% for PMAC motors), effective regenerative braking, high power density, and no mechanical wear between internal parts, leading to better reliability and less maintenance.
And the winner is?
It is commonly agreed that AC motors are more suited than DC motors to power electric vehicles. That being said, when it comes to deciding on a AC motor topology, it is not all black and white. There are pros and cons in every technology available and there are many aspects to consider before making the final choice.
What is truly important is that when an electric motor manufacturer decides on sticking with one motor topology, it is necessary to always pursue optimization of the motor within the limits of its topology. In this regard, TM4 has been working with AC synchronous permanent magnets since its inception. Over the years, lots of patented innovations have been combined to make the best out of this topology. Visit this section to explore TM4’s motor technologies.