Module 2: Dynamics of Electric and Hybrid vehicles

Lecture 4 : Vehicle Power Plant and Transmission Characteristics

Electric Motor

The electric motors have are ideal for vehicle application because of the torque speed characteristics of the motors (Figure 5). Electric motors are capable of delivering a high starting torque. It is very important to select proper type of motor with a suitable rating. For example, it is not accurate to simply refer to a 10 h.p. motor or a 15 h.p. motor, because horsepower varies with volts and amps, and peak horsepower is much higher than the continuous rating.

 

Figure 5: Torque vs. speed and power vs. speed characteristics of electric motor

It is also confusing to compare electric motors to IC engines, since electric motors are designed for a continuous rating under load and IC engines are rated at their peak horsepower under loaded condition. The commonly used motors in EVs are:

•  AC motors

•  Permanent magnet (PM) motors

•  Series wound DC motors

•  Shunt wound DC motors

The DC series motors were used in a number of prototype Electric Vehicle (EVs) and prior to that mainly due to the ease of control. However, the size and maintenance requirements of DC motors are making their use obsolete. The recent EVs and Hybrid Electric Vehicles (HEVs) use AC, PM and Switched Reluctance motors. A classification of motors used in EVs is shown in Figure 6.

 

  • Figure 6: Classification of electric motors used in EVs

The AC Induction Motor (IM) technology is very mature and significant research and development activities have taken place in the area of induction motor drives. The control of IM is more complex than DC motors, but the availability of fast digital processors, computational complexity can easily be managed. The competitor to the induction motor is the permanent magnet (PM) motor. The permanent magnet motors have magnets on the rotor, while the stator construction is same as that of induction motor. The PM motors can be surface mounted type or the magnets can be inset within the rotor. The PM motors can also be classified as sinusoidal type or trapezoidal type depending on the flux density distribution in the air gap. Permanent magnet motors with sinusoidal air gap flux distribution are called Permanent Magnet synchronous Motors (PMSM) and the with trapezoidal air gap flux distribution are called Brushless DC (BLDC) motors.