Electric Vehicle technology and the role of Permanent Magnets

European legislation to phase out vehicles equipped with the internal combustion engine has led to a shift in the market – making way for Electric Vehicle technology to be embraced.

Permanent Magnet BLDC drives have been identified as the most promising technology to provide the attributes required for modern Electric and Hybrid Electric Vehicles.

Awareness of environmental problems is growing dramatically throughout the world. This is leading to a tremendous interest in developing non-polluting Electric Vehicles (EV) and Hybrid Electric Vehicles (HEV).

Vehicles equipped with the traditional Internal Combustion Engines have been in existence for over a hundred years. Although ICE vehicles have been improved by modern automotive electronics technology, they only offer around 20% efficiency [1]. Electric vehicles are one of the most promising technologies that can lead to significant improvements in vehicle performance, energy utilisation efficiency, and emission reductions.

Multiple countries have already acted on enforcing a ban on combustion engines, initially led by Norway for 2025. Other European countries include Sweden, France, Germany, Belgium, Switzerland and the Netherlands whom are considering a phase-out by 2030. This is not only in order to cut CO2 emissions but also to reduce the European Union’s (EU) dependence on Russia. Outside of the EU, phase-out of ICEs also seems likely in Japan which is home to the world’s top-selling electric car, the Nissan Leaf [2].

Electric Vehicles and Hybrid Electric Vehicles have been identified to be the most viable solution to address the problems associated with the internal combustion engine. Electric drivers are the core technology for Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs). These vehicles need to meet a number of requirements to be competitive in the automotive field, including: interior comfort, extended range, efficiency, low cost, reliability, and low maintenance [3]. These requirements lead to developing smaller efficient and higher-output motors. The basic characteristics of an electric drive for Electronic Vehicles are [4]:

  • high torque density and power density;
  • very wide speed range, covering low-speed crawling and high-speed cruising;
  • high efficiency over wide torque and speed ranges;
  • wide constant-power operating capability;
  • high torque capability for electric launch and hill climbing;
  • high intermittent overload capability for overtaking;
  • high reliability and robustness for vehicular environment;
  • low acoustic noise;
  • reasonable cost

 

The technologies of all component hardware are technically and markedly available on the market. At present, almost all of the major automotive manufacturers are developing either HEVs or fully EVs.

There is a large variety of electric motors that come in a number of topologies, the key being to balance high efficiency, reliability, and Size Weight and Power (Swap). Presently the three most common topologies used in EVs are Brushed DC, Brushless DC (BLDC), and AC induction.

With the advent of high-energy permanent-magnet (PM) materials, PM motors are becoming increasingly attractive. Being continually fuelled by new machine topologies and control strategies, PM BLDC drives have been identified to be the most promising to provide the characteristics needed for modern EVs and HEVs.

PM motor drives have the magnetic field excited by high-energy PMs. This means the overall weight and volume can be significantly reduced for given output torque, resulting in higher torque density, very quiet operation and almost never requiring any maintenance. Because of the absence of the rotor winding and associated rotor copper losses, their efficiency is inherently higher than that of induction motors and do not have the added concern of removing heat generated in the rotor by copper losses. Induction motors need a low pole number (two or four) which requires large copper end windings and considerable stator back iron. This results in a larger and heavier machine [3].

In recent years, attention has been drawn to Interior Permanent Magnet (IPM) synchronous motors. This is due to their simple structure, robust configuration, high power density, easy heat dissipation, and suitability for high speed operations. The IPM motor takes advantage of flux focusing, which is performed by angling the magnets to gain a higher flux density in the air-gap than that of the PM flux density. The IPM topology has the highest power density and highest efficiency among all types of motors. Through flux focusing flux leakage is minimised up to as much as 20%. Furthermore, IPMs have a more robust rotor than that compared to other PM motors without the need of additional retainment and lower cost rectangular magnets [4] [5].

Regenerative braking can be considered a requirement in EVs it can increase the vehicle range by up to 15% [6]. Regenerative braking is easier with a PM motor because the magnets do not need to be energised. This results in less rotor heat generated with PM motors compared with induction motors which require a well-suited cooling system [7]. This gives PM motors a higher efficiency for the amount of kinetic energy that can be recovered. Without the rotor winding PM motors also produce a unity power factor. It is however noted that attention must be given to demagnetisation of higher continuous temperatures in PM motors, when designed with NdFeB magnets [7].

Legislation throughout Europe has begun to force EV into the market, and it is expected that most of the world will follow suit. This is leading to a large production of EVs. Currently IPM motors have the highest efficiency, smallest size, lowest weight and are the most efficient in recovering the vehicle’s energy in the form of regenerative braking. Due to high efficiency and smaller lighter motors, the vehicle will have an extended range on the same size battery bank.

Electric Vehicles will also play a major role in the dynamics of how we generate, store and use energy in the future.

Electric Vehicles provide an added benefit to existing energy grids – Off Peak Recharging. An EV can be recharged during the night, when the energy demand on the grid is low. This is similar to the off-peak tariff used to heat residential hot water systems.

As we move towards a more distributed generation model in the future, the additional storage capacity of an Electric Vehicle has significant potential.  Peak demand as mentioned above can be capped, and we will have the ability to consume energy at off-peak times, thus removing the expensive energy peaks currently experienced.

[1] C. R. Ferguson and A T Kirkpatrick, Internal Combustion Engines: Applied Thoermosciences, Third Ed, Colorado University. John Wiley & Sons Ltd, 2016, pp. 2.

[2] Pedestrianobservations. (2016). Several European Countries to Follow Norway’s Lead, Ban Fuel-Powered Cars [Online]. Available FTP: https://pedestrianobservations.wordpress.com/2016/04/01/several-european-countries-to-follow-norways-lead-ban-fuel-powered-cars/

[3] M. Ehsani and Y. Gao, “Hybrid Electric Vehicles: Architecture and Motor Drivers” in Proceedings of the IEEE, Vol. 95, 2007, pp. 719-728

[4] K. T Chau and C.C Chan, “Overview of Permanent-Magnet Brushless Drives for Electric and Hybrid Electric Vehicles” in IEEE transaction on industrial electronics, Vol. 55, 2008, pp. 2246-2257

[5] J.R.Hendershot, “MotorSolve analysis of the 2010 Toyota Prius Traction Motor” 2015

[6] J.Cody and O.Gol, “Regenerative braking inn an electric vehicle” in Zeszty Rplemowe – Maszyny Elektryczne, Vol. 81, 2009, pp. 113-118

[7] R.Rapter and M.Prathaler, “Regeneration of Power in Hyrbid Vehicles” VTC 2002, pp. 2063-2068

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