Review of Electrical Motor Drives for Electric Vehicle Applications

EVs (Electric Vehicles) have been rejuvenated over the last decades while the motor drive technologies are still evolving. This paper provides a review of electrical motor drive technologies used in EV applications, with a performance comparison of candidate machines and their drive topologies. EV applications demand high efficiency, high torque density, high reliability, and wide speed range while reducing weight, complexity, total costs and environmental impact. In the literature, DC (Direct Current) motors, IMs (Induction Motors) and PM (Permanent Magnet) motors can be generally found in marketplace whilst RMs (Reluctance Motors) have been researched for some time and are nearing commercial availability. This paper evaluates the performance of these four main types of electrical motor drives for EV propulsion applications using analytical methods. PM motors may offer the best performance in terms of torque density and compactness but the cost is the highest (primarily dominated by rare-earth permanent magnets), limiting their widespread application in mass production EVs. DC motors have their own merits but suffer from limited power density and necessity for maintenance. Induction motor drives are a mature and proven technology. In particular, squirrel-cage IMs are robust, reliable and inexpensive, striking a balance between system cost and complexity, power density and extended speed range. Reluctance motors can provide a good torque density and cost effective EV drive solutions. Their drawbacks can also be overcome by the use of power electronic converters and advanced control strategies. Induction and reluctance motor drives are well suited for cost sensitive mass production EV applications. Looking to the future, increased hybridization may be a way forward in industry which combines attractive features of different electrical machines and control algorithms and still offer much promise in performance and total cost. At last, reliability study on EVs requires historical information and driving patterns, demanding research expertise in eco-sociology, human behaviors as well as human-machine interface.


DRIVES
EM drives as propulsion are at the heart of EVs. They need to operate in a harsh environment with the humidity of up to 85% and the ambient temperature between -40 and 135 celsius degree. This is particularly true for HEVs where EMs are in proximity to the ICE. Other specifications include high driving duty cycle, wide constant-power range and high torque density.
These requirements for EV drives can be depicted in Fig. 1 [9] and should be matched by the output characteristics of an EM drive. Typically, an EV motor needs to generate a peak torque (during starting, acceleration or hill climbing) which is 5-10 times the torque for cruising [13][14][15]. In Fig. 1, the constant power region represents high speed cruising where the maximum speed may be five times more than the base speed. Gearbox costs indicate a maximum motor speed around 10,000 rpm.

As reported in literature, DC motors, IMs and PM motors have been widely used in EV propulsion drives while
RMs have begun to emerge in the automotive market.
The feasible machines are illustrated in Fig. 2 and their cross-sections are shown in Fig. 3(a-d) [16] for comparison.
The features of each candidate motor drive are further described as follows: DC Motors: The earliest EVs used almost exclusively DC motor drives [4] simply because the DC supply was available from the battery in vehicles. This covered the period when DC machine technologies were developed and gradually perfected.
Overall, PM motors are suited for EV propulsion owing to their high torque and power density, high controllability, high torque/inertia and torque/volume ratios, low weight and size. They are particularly suited for in-wheel direct drive applications [26][27][28][29][30]. However, the main magnetic field in PM motors is fixed (due to PMs) so that their high speed range for constant power operation is rather limited.
With field weakening, it is possible to extend speed range to 3-4 times the base speed [16,[30][31][32][33][34][35][36][37][38]. When a vector control scheme is used for field weakening operation, a high d-axis demagnetizing current is applied. This would increase not only the conductor power loss but also the risk of demagnetizing the PMs. Fundamentally, limited reserve of rare earth materials and high costs of rareearth PMs limit the widespread of PM motors for massproduction cost-sensitive markets such as EVs.
PM BL motor drives are widely found in Japanese EVs such as Tino of Nissan, Insight of Honda and Prius of Toyota [16].

MOTOR DRIVE CONSIDERATIONS
A commercial successful EVe must have an electrical motor drive coordinated with power train, battery and power electronics in an appropriate system configuration [53].
Choice of the Powertrain: Typically, an electric power train comprises a geared drive motor, a power converter, and a battery for EVs. For HEVs, they also have an ICU, an energy storage for HEVs, as shown in Fig. 5.
EMs can be designed to operate at high speed. This can significantly reduce the mass and volume of the geared motor drive at the expense of increased mechanical power losses associated with the clutch, reduction and differential gears. For example, a HEV using a two-stage planetary gear and differentials is favored in [54]. On the  [16] other hand, the direct drive configuration would eliminate the gears and power transmission system but the EM can be very large and heavy, calling for motor drives with a very high power density. This needs almost exclusively a high-performance PM motor. Arguably, a one-stage geared motor drive (without differentials) may be a very good compromise in this regards.

Choice of the Battery System:
The performance of a battery system has always been a major limiting factor in any EVs. The choice of the power supply is dictated by the battery used. Currently, a lithium-ion (Li-ion) battery is capable of providing a voltage output of 3-4V, a maximum power of 1.5 kW, and an energy density of 100-120 Wh/kg. Its voltage level is nearly three times more than a nickel-cadmium battery [55][56] and its maximum energy storage is three times more than a copper-acid battery. A detailed comparison of the three major EV batteries is tabulated in Table 2.
With battery power, the usable charge voltage ranges  Table 3.
As the cost of power electronics continues to reduce, the full utilization of these devices are no longer an obstacle.
There is an ongoing trend to integrate electrical motors with power electronic controller, named the "integrated motor drive". However, reliability of these key components is critical to the drive system's healthy operation. The

FOR ELECTRIC VEHICLES
In spite of significant R&D effort in developing EVs, it will be a long way to achieve all EVs. As the core element in electrical drives, EMs have been improved in many different ways. One attempt is to re-arrange the magnetic flux configuration and the other is to integrate different machine features. These will continue to be developed for many years to come. between two stators is termed the Kaman geometry [66].
In principle, the Torus geometry can use both disk surfaces of the stator and thus provide more torque and better system efficiency than a Kaman geometry.
As discussed previously, PM motors are well suited for direct drive in-wheel PM motors. The transverse flux machines offer the prospect of very high specific loadings (25Nm/kg has been measured on prototypes). They derive much of their benefit from high pole number but this limits maximum speed. Asdirect drives their promise is clear, but more development is necessary before they can compete with higher speed conventional machines.
Transverse flux motors have high torque density and favorable characteristics in terms of maximum torque and efficiency.
An example of transverse flux PM machine is shown in Fig. 6(a-b). The transverse flux motor is compact and of low cost. Compared to PM motors, they use much less magnets for the same output torque. However, high temperature magnets would need to be used to allow operation of the same coolant circuit as the ICE. It would be beneficial to develop ferrite-based transverse flux motors to lower the cost. In general, transverse flux motors have a good geometry for in-wheel mounting for direct drive. But they suffer from low inductance, poor power factorwhich increases the power rating of the associated power converter [67].   Fig. 8.