Electronics and Semiconductors

Motors for electric vehicles

12 October 2022
The permanent magnet electric motor powers the Chevrolet Bolt. Source: General Motors

Classic vehicles are powered by internal combustion engines (ICEs), and as a result, they emit carbon dioxide by burning gas and gasoline. But to save the environment, researchers started focusing on renewable energies, which led to the development and growth of hybrid electric cars that employ both ICE and electric motors to move their wheels. After that, fully electric vehicles (EVs) came onto the market and are now widely accepted by users. Such vehicles are propelled by one or more electric motors, which will be reviewed in this article.

Faraday developed the idea of electromagnetic induction in the 18th century, which paved the way for the creation of electric motors. An electric motor generally contains a rotor, stator, air gap, windings and commutators/converters. Based on how these parts are arranged, several types of electric motors can be engineered. Some electric motors may not need brushes for energy conversion or commutation and some produce a back-electromotive force with a particular shape, which can be trapezoidal or sinusoidal. In order for an electric motor to be effectively implemented as the drive for EVs, it must be extremely efficient, have a high output power and be inexpensive. Nevertheless, the motors' specifications rely on their intended use. Such usage ranges from domestic to standard and heavy-duty automobiles. Moreover, the effectiveness of motors is largely determined by the duty cycle, thermal properties, and cooling mechanism of the vehicle.

Brushless DC motors

A brushless DC motor is an electric motor that is supplied by a DC power source and commutated electronically, as opposed to ordinary DC motors, which are commutated via brushes. In comparison to types of AC and DC motors, this system delivers greater torque, superior power density, lower operating and mechanical noise, and high efficiency without electromagnetic interference. As a result, due to its traction capabilities, this motor has been the most widely used in EV applications. It has wire-wound poles in the stator and a permanent magnet rotor. Permanent magnets are used to construct the rotor, which may change from two-pole to eight-pole pairs with alternating north and south poles. A uniform flux density is generated in the air gap between the rotor and stator. This allows a consistent DC voltage to be applied to the stator coils. Unlike its counterpart DC brushed motor, it has no maintenance requirements as it doesn’t need commutators or brushes.

Induction motors

In this type of motor, stator excitation with sinusoidal AC current creates a rotating field that generates current flow in the rotor, which then drives the generation of a magnetic field relative to the rotor. Because of the different frequencies of the rotor and the stator's magnetic fields, torque is generated. Induction motors lack brushes and commutators, are less expensive and need little maintenance. These characteristics make the induction motor an appealing option for EVs. However, the necessity to convert the power source from AC to DC necessitates additional circuitry and sophisticated control techniques.

Permanent magnet synchronous motors

Permanent magnet synchronous motors are comparable to brushless DC motors but are powered by a sinusoidal signal to reduce torque ripple. Unlike a brushless DC motor, which has a trapezoidal flux density, a multi-phase stator creates a sinusoidal flux density in the air gap. This motor has features of both a brushless DC motor and an induction motor. The rotor is composed of a permanent magnet, and the stator of the motor is wound. To further enhance its performance, the stator of this motor has been engineered to provide an induction-like sinusoidal flux density. This motor is extremely efficient, exhibits high starting torque and has a better power density than induction motors of equal ratings because the stator power is not allocated to the formation of a magnetic field. But as they are not self-starting, a drive is required to run them. It is also more costly than brushless DC motors. However, permanent magnet synchronous motors are used by the majority of automakers in hybrid and electric cars.

Switched reluctance motors

Reluctance motors having double saliency are known as switched reluctance motors (SRMs). The construction of these motors is straightforward, and the motors themselves are strong. The SRM's rotor is made up of laminated steel without any windings or permanent magnets. This reduces the rotor's moment of inertia, which aids in fast acceleration. The sturdy quality of SRM makes it suited for the application requiring high speed. SRM also provides great power density, which is required for EVs. It is also simple to cool them since the majority of the heat produced is contained within the stator. The most significant downside of the SRM is the need for complicated control systems. It is also a little noisy. However, in the future, SRM will be able to replace permanent magnet synchronous motors and induction motors.

[Discover more about electric motor technologies and manufacturers on GlobalSpec.com]

Conclusion

This article has discussed the popular motors being used for EVs. Brushless DC motors are a good option for two- or three-wheelers. For high-performance applications such as trucks, buses and cars, induction motors and permanent magnet synchronous motors are a good choice. However, EVs will have additional motor alternatives if synchronous reluctance and switched reluctance motors, which are now expensive, can be made more affordable as permanent magnet synchronous motors or induction motors.

To contact the author of this article, email engineering360editors@globalspec.com


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