لخّصلي

A Review of Electric Motor Drives for Applications in Electric and Hybrid Vehicles

             Abstract – This paper reviews several types of electric motor drives and their features in applications for electric vehicles (EV) and hybrid electric vehicles (HEV). The main driving forces for researching and producing EVs and HEVs are fast depletion of fossil fuels and deteriorating atmospheric conditions. Currently, with rapid development of power electronic devices and improved material quality, high performance electric machines employing induction motor (IM), permanent magnet (PM) motor, switched reluctance motor (SRM) or other novel motor design have been intensively researched. Various HEV/EVs have already been manufactured incorporating these motors to compete the conventional internal combustion engine (ICE) vehicles in the automotive market. In this paper, basic configurations for EVs and HEVs are introduced. Then the discussion of diverse types of electric motors including DC, IM, PM, SRM, and SynRM machines is presented. Moreover, future trend of EVs as well as electric machine developments are also envisaged.



INTRODUCTION



              Electric machines are playing indispensable role in modern industry. Nowadays, environmental protection and energy conservation are among major concerns in our world. Due to some salient features such as zero or low emission and high-power efficiency, Electric vehicles (EV) are gaining increased research efforts and booming dramatically in global automobile market. As indicated in Fig.1, the annual worldwide sales of Plug-in Electric Vehicles (PEV) are growing remarkably from 50,000 in 2011 to above 550,000 in 2015 [1]. Within the U.S., California is the largest domestic market with 52% of the total PEV sales (shown in Table I. According to the prediction in [1], with the annual growth rate of 8.6%, the EV sales in 2026 will reach 263,000 and the cumulative sales will be over 2.5 million.

                In general, there are three major types of EVs: battery electric vehicles (BEV), hybrid electric vehicles (HEV) and fuel-cell electric vehicles (FCEV) [2]. The characteristics of these vehicles are summarized in Table. II. Compared to conventional ICE vehicles, electric motor drives are the core parts in the propulsion system with batteries, ultracapacitors, and fuel cells serving as the energy source. In addition, plenty of electrical components are incorporated in EV such as power converters, electric machines, microcontrollers and sensors. The major challenges in this advanced propulsion system include design of powertrain component, energy storage and power management, hybrid control theory and control optimization, and selection of the optimal electric machine topologies.

                 This paper presents a review of electric motor drives for applications in electric and hybrid vehicles. Section II presents the introduction of typical EV/HEV configurations and system characteristics. Section III to Section VII analyze and compare five major types of motor drives, namely, DC motor drives, induction motor (IM) drives, permanent magnet (PM) motor drives, switched reluctance motor (SRM) drives, and synchronous reluctance motor drives (SynRM). Section VIII provides the concluding remarks for the paper and discusses the future trend of EV vehicles and electric machine design.





CONFIGURATIONS AND SYSTEM CHARACTERISTICS OF EV & HEV



               The typical system schematic of EVs is demonstrated in Fig. 2 [3]. It is composed of three subsystems – energy source, electric propulsion, and auxiliary. The electronic controller switches on and off the power devices in the power converter depending on the control from the brake and accelerator of the vehicle. Its main function is to regulate the power flow between the electric motor and energy source. Note that the battery or fuel cell is the only energy source to supply power to the vehicle’s electrical equipment such as lighting and audio devices. It also provides the required power to drive the vehicle by the electric motor.

                 Different from EVs, HEVs incorporate both electric motor and internal combustion engine (ICE) as the energy sources. Generally, four kinds of HEVs are classified, namely, the series hybrid, parallel hybrid, series-parallel hybrid, and complex hybrid, as shown in Fig. 3. The series hybrid is the simplest type of HEVs, and its only driving force comes from the electric motor. It is essentially an ICEassisted EV because the ICE supplies power directly to the generator, which either drives the motor or charges the battery. Unlike the series hybrid, both ICE and electric motor drive the wheels of the vehicle in the parallel hybrid. Inherently, it is an electric-assisted ICEV. It has several advantages such as reduced number of propulsion devices and smaller sizes of motor and ICE required for desired performance. In series-parallel hybrid, it combines the features of both series and parallel configurations due to the addition of an extra mechanical link between ICE and transmission devices. Hence it has more complicated structure and higher cost. For the complex hybrid, it is very similar to series-parallel type, but the key distinction lies in the bidirectional power flow of the electric motor in this topology and unidirectional power flow of the electric motor in the series-parallel hybrid. This feature allows for diverse operating modes, especially the unique multi propulsion power operating mode by joint action of the ICE and two motors. However, like series-parallel hybrid, it also suffers from higher complexity and cost.

                 Generally, three major aspects are involved in vehicle operation: 1) the initial acceleration; 2) cruising at vehicle maximum speed; and 3) cruising at the rated vehicle speed. The fundamental design constraints are generally set by these three conditions for the EV and HEV drivetrain. Once meeting these constraints, a drivetrain will usually function adequately in other possible operational regimes. In actual case, it is necessary to make further adjustments to these basic design constraints. The design emphasis is meeting all these constraints while minimizing the motor power [7].

                     Fig. 4 shows the torque-power-speed characteristics of electric motor in industrial application. The motor rated power Pm can be expressed by the following equation:

Pm=m/(2t_f )(〖v_rm〗^2+〖v_rv〗^2)
Where m is the mass of the vehicle, v_rm is the electric moto rated speed. v_rv is the vehicle rated speed. t_f is the time required for vehicle velocity increasing from 0 to v_vrv. The condition for minimum motor power can be found by differentiating Pm with respect to v_rm and setting it to 0, which results in:
v_rm=0
This condition sets the theoretical limit for minimum motor power. In this case, the electric motor operates entirely in the constant power region. Therefore, if the motor is accelerating from 0 to v_rv in t_f seconds in constant power region alone, the minimum power is achieved. However, if the motor operates entirely in the constant torque region during this period, then v_rm = v_rv. In this case, the required motor power is twice that of in the previous situation. Certainly, entire operation in constant power region is in no way achievable in the actual application. However, the significance of this theoretical discussion is evident: it reveals that the required motor power can be reduced by extending the constant power region.
The power architecture of several current EVs and HEVs is shown in Fig. 5. TABLE I provides some examples of electric machines adopted in automotive industry. As mentioned above, the electric propulsion system is the heart of EVs and HEVs, while the motor drives, consisting of electric motor, power converters, and electronic controller, is the core part of such system. Some major requirements for the EV motor drive include high instant power, high initial torque for accelerating and high power for cruising, fast torque response, high power density, wide constant torque and constant power regions, high efficiency over wide speed range, high reliability and reasonable cost. In the succeeding sections of the paper, five types of electric motors for EV/HEV application are introduced and analyzed, namely, the DC motors, the induction motors (IM), PM brushless motors, switched reluctance motors (SRM), and synchronous reluctance motors (SynRM).

DC MOTOR DRIVES

              DC motors are regarded as options for EV/HEV applications owing to their capability of achieving high torque at low speed. Their torque speed characteristics is fitting for traction requirement [4-5]. Also, the speed control is relatively simple, by varying the terminal voltage. Fig. 7 shows the basic model of the DC motor. Based on different topologies, DC Motors can be categorized into three types: series excited, shunt excited, and separately excited machine. Both shunt and series DC motor have only one voltage source hence the controls of flux and torque are simultaneously. On the other hand, the separately excited DC motors are intrinsically capable for field weakening operation since the torque and flux controls are decoupled. As a result, the extended constant power range can be realized. One the other hand, DC motors also suffer from several disadvantages such as bulky dimension, low efficiency, low maximum speed, low reliability and high demand for maintenance, especially for motors with mechanical commutators and brushes. Due to these drawbacks, brushless DC motor is more generally used with improved efficiency and reduced size, as shown in Fig. 8. Compared with conventional brushed DC motor, BLDC motors do not contain brushes. The commutation process is performed electrically by the electronic drive to excite the stator windings. Despite of aforementioned disadvantages, the DC motor drives are preferred in low power conditions, and they are more than the alternatives in automotive industry especially for HEV applications.



INDUCTION MOTOR DRIVES

             Induction motors are the most commonly used electric machines. Even without the DC-AC inverters, they can operate directly from an alternating current (AC) voltage supply. They have been widely used for constant-speed applications. IM motor drives are regarded as one of the most competent candidates as electric propulsion system for EVs and HEVs because of their robust construction, reliability, ruggedness, low maintenance demand, low cost and excellent peak torque capability. IM drives are also the most mature technology among all types of motor drives [8]. Without commutator and brush, the IM motors are free from brush friction hence the maximum speed limit is increased. As a result, the constant power speed range can be extended. By changing the frequency of the excited voltage, speed variation of the IM motor is achieved. In order to gain independent controls of the torque and the field flux, field orientation control (FOC) can be adopted, which is similar to the case of separately excited DC machine. Fig. 9 shows the typical characteristics of IM motor drives. It can be seen that in high-speed region, due to significant back EMF the stator current is reduced hence the torque is inversely proportional to square of the motor speed. For conventional IM, the critical speed is about two times the base speed. Any further attempt to operate the motor above this speed with maximum current will stop the motor from running.

              In general, IM drives suffers from several drawbacks such as high copper loss, low efficiency, low power factor, and limited constant power range. The controller cost of the IM motor drive is usually higher than that in DC motor. Extended speed range can be achieved by flux weakening. A novel type of IM drives with such feature are the vector controlled doubly fed induction motors (DFIM) [9-10], which can achieve field weakening range of 3-5 times the base speed. Even though such technique may extend the constant power operation of IM motors, the breakdown torque (maximum torque requirement) is increased thus results in the over-sizing issue for the motor.



PERMANENT MAGNET BRUSHLESS MOTOR DRIVES

            The PM motors is capable of utilizing both reluctance torque and magnetic torque to improve its efficiency and torque density. In these motors, a certain amount of magnets is added to the rotor of the machine. Typically, permanent magnet brushless motor drives have better performance compared to IM drives as candidates for the electric propulsion system. Their major advantages include 1) reduced overall weight and volume for given output power (high power density) due to the highly energized magnetic field from PM; 2) inherently higher efficiency owing to the absence of the rotor windings (hence the associated copper loss); 3) more efficient heat dissipation since it is mainly generated by the stator; 4) high reliability due to their immunity to overheating issue or mechanical damage by PM excitation. Considering the above advantages, PM motors are the prior choice for most of the HEVs and EVs manufacturers. However, this type of motors has relatively short constant power range due to their limited fieldweakening capability, which is mainly resulted from the presence of strong field flux produced by the PM, as shown in Fig. 11 (a). One possible method to increase the maximum speed range and improve the efficiency is controlling the conduction angle of the power converter at above the base speed (Fig. 11 (b)). Nonetheless, at very high-speed range, the efficiency may drop drastically due to the fact that the PM may suffer from demagnetization caused by significant back EMF and possible fault occurrence.

             Various configurations of PM brushless motor drives are commercially available. Based on the placement of PM, they can be categorized into two types: surface magnet mounted (SPM) and interior magnet mounted (IPM). The surface magnet designs usually produce only the magnetic torque (round rotor structure) and require less magnet, while interior magnet motor design yields both reluctance and magnetic torque with obtaining higher air-gap flux density. A modified type is the PM hybrid motor, in which both PM and field winding are used to produce the air-gap magnetic field. It may also include the motor whose topology is characterized by features of both PM motor and reluctance motor. An example of such motor is the permanent reluctance motor (PRM) developed by TOSHIBA for HEV and EV motors [11], which is shown in Fig. 13. In this design, by changing the magnet position and magnetic circuit design, the motor employs more portion of reluctance torque. The benefits of Increasing of reluctance torque lie in reducing required PM amounts and smaller back EMF. Hence, the motor can achieve a large variable speed range over 1:5, smaller flux weakening current and higher efficiency at high-speed operation region. An improved model under development next to PRM is so-called “memory motor” as shown in Fig. 14. The main feature of this motor is adaptive controlling the magneto motive force of the PM in response to variations of the load and speed of the motor. Compared to the IPM motor, it has two types of magnets: constant magnetized magnet and variable magnetizing magnet. The variable magnet is magnetized by increasing the d-axis current as shown in Fig. 15. A magnetic field, arisen from the d-axis current flowing in an armature coil, is in direction to change magnetization of variable magnets indicated by the black arrows in Fig. 14.

If d-axis current is increased in negative direction, there will be a reversion in polarity of the variable magnet as shown by the dark black arrows in Fig. 14. In this case, the total combined field flux is reduced remarkably. On the other hand, if positive d-axis current increases, again, reversion of the polarity happens, thus the variable magnetized magnet will be magnetized in the initial magnetizing direction. Fig. 16 demonstrates the efficiency map comparison between the conventional IPM motor and memory motor. Since the memory motor is capable of controlling the magneto motive force of a PM along with the motor speed variation, hence the flux-weakening current can be effectively minimized, which in turns reduces both copper loss and iron loss in a wide speed range.
Some major research and development challenges for PM motors include developing bonded magnets with high energy density with high temperatures endurance, as well as motor designs with increased reluctance torque, thermal management, and the temperature rating of the electrical insulation [4].

SWITCHED RELUCTANCE MOTOR DRIVES

            In recent years, switched reluctance motors (SRM) are gaining much interest from the academia and industry for its potential in EV/HEV applications. Originally, they are derived from single stack variable reluctance stepping motors. This kind of motors have numerous salient advantages including simple and rugged construction, high fault tolerance, low manufacturing cost, and remarkable torque-speed characteristics, as shown in Fig. 17. The SRM drive has very high-speed operation capability with wide constant power range. It also has high starting torque and high torque-inertia ratio [12]. The rotor structure is the simplest type compared to DC, IM, and PM motors with no rotor windings, magnets, commutators or brushes. An added benefit of this structure is simple external cooling of the motor, which makes SRM capable of operating at harsh ambient conditions (e.g. high temperature).

         In 1980s and 1990s, research works generally suggested using the SRM combinations such as 4-phase 8/6 pole, 3- phase 6/4 pole, and 3-phase 12/8 pole. The main purpose is to allow sufficient slot space for winding in order to lower the current density. Recent works, however, have revealed that having a high number of stator/rotor poles tends to be optimum for EV and HEV applications, like 18-stator pole in [13]. Another design of SRM motor drive competitive to 60kW IPMSM in third generation HEV is proposed in [14], which has 18/12 pole combination. Some major benefits for such configuration include higher generated torque, higher reliability and better fault tolerance. In [15], three different multiphase SRMs with exterior rotor for in-wheel EV application have been studied and experimented. The four-phase configuration are recommended considering the torque density, converter cost and control complexity. Another improved SRM topology is the so-called doublestator switched reluctance machine (DSSRM), which is shown in Fig. 19 [16]. Conventionally, the majority of the force is produced in the radial direction which does not contribute to rotation of the SRM motor. The main feature of this design is to balance the radial forces and optimize the motional forces, which results in high electromechanical energy conversion efficiency. Thus, higher power density and higher percentage of the motion forces are achieved.

          On the other hand, there are several major drawbacks for the SRM drives including high torque ripple, high acoustic noise, and complex controller design due to dependence of inductance profile on both rotor position and excitation current. Actually, noise, vibration, harshness (NVH) characteristics of a vehicle are among its major acceptance standards in terms of overall performance [17]. Compared to ICE, an electric machine like SRM motor produces much lower sound pressure levels but contains high frequency pure tones (up to 12 kHz), which can be unpleasantly perceived by the users. This issue may be the key drawback of SRM motor drives for large-scale implementation in automotive industry. Another significant issue with the conventional SRM motor is that it cannot be driven with a conventional three-phase power inverter [5]. Nonetheless, the inverter for a three-phase SR motor usually requires three diodes and three switching elements (e.g. the H-bridge inverter), as is the case with the traditional threephase power inverter.



SYNCHRONOUS RELUCTANCE MOTOR DRIVES

         Synchronous reluctance motor (SynRM) is a novel kind of synchronous machine, which combines the advantages of both IM motors and PM motors. The main feature of such motor is that it incorporates the robustness of IM motors with the comparable size, efficiency, and synchronous speed operation as PM motors [18]. The stator of the SynRM motor has distributed windings as IM and PM motors. The rotor is designed such that the smallest possible and the highest reluctance are achieved in the two perpendicular directions. In addition, the motor is fault tolerant as IM motors due to no flux in the rotor without energizing the stator windings. The control strategy is similar to that of PM motors due to identical stator structure.

              Typically, poor power factor is the major disadvantage for SynRM motors. Due to this fact, the required motor size potentially increases. Achieving high power factor demands higher saliency ratio in this machine. Such requirement can be satisfied by both axially and transversally laminated rotor structure. The axially laminated rotor is shown in Fig. 20 (a) which has high direct axis inductance Ld and low quadrature axis inductance Lq. As a result, the saliency ratio is high, and a better power factor is obtained. Some novel designs of SynRM motors for HEV applications have already been proposed in [19-21].

          In order to achieve higher power factor, a small amount of PMs can be added to the SynRM rotor, as shown in Fig. 20 (b). This structure is similar to IPM machine but with less PM usage and smaller PM flux linkage. Such design may improve efficiency without introducing large back EMF and changing the stator design. Moreover, such motor is free from high ambient temperature and demagnetization due to overloading, which are quite common in IPM motors. By judicious selection of the right amount of PM and efficiency optimization, the performance of PM-assisted SynRM machine can approach to that of IPM machine.



CONCLUSION AND FUTURE TREND OF ELECTRIC MOTOR DRIVES FOR HEV/EV APPLICATIONS

          Electric vehicles are the future of automotive industry with low or zero emission and high energy efficiency. Apart from battery-powered EV, solar-powered and wind-powered electric vehicles are also among popular researched area today dealing with application of renewable energy systems in EV/HEV vehicles [22]. In this paper, the basic configurations characteristics of HEVs and EVs have been introduced. Moreover, several types of motor drives like IM, PM, SRM, and SynRM, which are applicable to the EV design, have been briefly discussed.

        As suggested in [5], a proposal for alternatives and future works in HEV and EV development is to devote efforts in the following three areas: 

• finding new magnet materials for PM motor designs,
• developing and producing of PM motors with RE magnets,
• designing novel motor that use non-REMs or no magnets.
For the first area, PM magnet material development through the investigation of novel inter-metallic compounds may produce new types of magnets, which could potentially reduce the cost of existing RE PM magnets through reducing processing cost and heavy RE contents. If such new magnet is available, then the overall cost PM motor can be less expensive hence the second area is more preferable. Even though IPMSM is the most common machine in use, researchers and manufacturers are more interested in the third area, that is, developing motor drives with much lower magnet usage because of current high cost of magnetic material such as sintered neodymium–iron– boron (NdFeB) (the regular cost of NdFeB is over 150 US$/kg) and rapid depletion of the rare-earth magnet resources [23]. The power density and efficiency are the key issues for the magnet-free and reduced-magnet machine. Much effort has been devoted to the research and production of induction motors and SRM machines in recent years and their performances have been largely improved. Other machines such as SynRM motors, field-excitation flux switching synchronous machine (FEFSSM) and BLDC multiphase reluctance machines (MRM) are still in the early research stage and yet emerge in industrial production.