Electric Motor used in EV’s:- A Comparative study
Electric Motor is an important part of an electric vehicle. An electric vehicle is vehicle that is run by electric propulsion system. Electric vehicle are of different type and can be classified as battery powered electric vehicle, hybrid electric vehicle or fuel based electric vehicle. With ever increasing demand of conventional fuel vehicles, there are eminent danger to our environment being more polluted in the years to come.
The electric motor was first invented in 1828 by Ányos Jedlik. The inventor Ányos Jedlik created a small model car powered by his new motor. Between 1832 and 1839, Scottish inventor Robert Anderson also invented a crude electric carriage.
These environmental concerns along with increasing fossil fuel energized engines have made researcher to look at other environment friendly options by which our vehicles can run. Electric vehicles or hybrid electric vehicles offer an option that will make our environment much cleaner and at same instant provides an efficient and more comfortable vehicle.
In electric vehicle either batteries or ultra capacitors are used as energy system. The overall performance of an electric vehicle is depend on type of motor drive being used. Different types of motor drives are used as a part of electric vehicle propulsion system.
Selection of traction motor for an electric vehicle is one of the most crucial step in designing the over all vehicle. Efficiency, cost, reliability, maturity of technology, power density and controllability like many criteria must be taken into account.
An electric motor is a device which converts electrical energy into mechanical energy. DC Motors, induction motor, permanent magnet synchronous motor, switched reluctance and Brushless DC motors are most common motors used in hybrid electric vehicle and pure electric vehicle from the industrial point of view. Cross sectional view of these motors are shown in fig.
Although motor used in electric vehicle and motor drive used in industrial process are similar but both can not be compared with each other. Since electric vehicle drive have to go through continuous variation of speed or frequent start/stop process along with being used in different environment. whereas industrial motor drive have to run at a rated speed.
Motor drive is one of the most important part of propulsion system for any electrical vehicle. For EV’s, the motor drive being adopted should have to satisfy some characteristics that are high power density and high instant power and high torque when it is at low speed at the time starting as well as for climbing. Also motor drive should be highly efficient when being used at different speed.
AC motor drive have some different advantage over DC motor Drive such as high power density, higher efficiency, effective regenerative braking, reliability, robustness and less maintenance cost. Attraction from DC motors are shifted towards AC motors. Switched reluctance motors are “the dark horse in the race”. Comparison of characteristics of different motor drives used in electric vehicle is presented.
In this section, advantage and disadvantage of different motor drives are discussed.
A. DC motor
Although DC motors are subject of interest sine old age due to its simple control and decoupling of flux and torque. DC motor consist of brushes and rings poses maintenance problem. Therefore after development of vector control of AC motors (synchronous and induction), the attraction towards DC for being use in traction application diminished. But DC motors have still good preference for low power application. Because of commutator of DC motor act as a robust inverter. Thus power electronics devise used in EV’s can be simple and cost efficient. “Dynavolt” is a HEV made by French company Peugeot in which DC motors are used as traction motor.
B. Induction Motor
Because of reliability, robustness, less maintenance requirement, and ability to efficiently work in hostile environment Squirrel cage induction motors have much preference for motor drive system. Among all other ac motors, induction motor has most mature technology. By using vector control method, torque and field control can be decoupled. Using flux weakening in the constant power region the speed range may be extended.
C. Permanent Magnet synchronous (PMS) Motor
Permanent magnet synchronous motors are also known as Brushless AC motor. PMS motor is one of the best competitor of Induction motor in traction application. PMS motor are already used by many car manufacturer such as Honda, Toyota and Nissan in their vehicle. Greater power density, higher efficiency and greater effective distribution of heat into the environment are advantages of these motors. However, power region of these motors are intrinsically narrow constant power region. Speed range of PMS motor and efficiency can be increased if conduction angle of the power converter is controlled at speed greater than base speed.
D. Switched Reluctance Motor
Attraction towards switched reluctance motor for being used in HEV is increasing day by day. Advantage of SRM is simple and rigid construction, simple control, greater torque-speed characteristics and fault tolerance. Operational feature of SRM is good as compare to BLAC motor and SRM can work under a large constant power region. But this motor has several disadvantage: high noise, greater torque ripple, electromagnetic magnetic interference and use of special convertor topology.
E. Brushless DC motor
Brushless DC motors are conceptually the result of reversing the stator and rotor of permanent magnet DC motor. In contrast to BLAC motors, BLDC motors are fed by rectangular wave. Deletion of the brushes, their compactness, greater efficiency and greater energy density are the advantages of BLDC motor.
In terms of maturity of technology, induction motor and dc electric motors has highest mature technology for being used propulsion system and maturity of technology of these motors are slightly greater than permanent magnet brushless electric motor and switched reluctance motor. Another factor should be considered while comparing different motor drive for use in propulsion system is reliability of the drive i.e drive requires less maintenance and breakdown should be minimum.
From reliability point of view, induction motor and switched reluctance motors are most reliable motor drive followed by permanent magnet brushless motor. DC drive have least reliability among all other drives. In terms of power density permanent magnet brushless motor have high power density as compare to both induction motor and switched reluctance motor. Again DC drive has least power density. Apart from these factors cost factor is also an important characteristics that should be taken under consideration. Cost factor is most important because its makes things commercially viable. In terms of cost factor, induction motor most cost efficient followed by dc and switched reluctance motors.