63 Vehicle Architectures and Design This section first presents the sizing of the propulsion component for an EV. Then, the sizing of powertrain components for a hybrid vehicle is presented followed by a design example. 3.5.1 eV p owerTrain s izing The major components in the powertrain of an EV are the electric motor and the energy storage system. The sizing of the electric motor involves finding the power rating of the motor and the operating speed range. The voltage rating and the dimensional packaging constraints must also be specified at the system-level design stage.
The design of the machine itself is handled by the electric machine designed to meet the requirements set at the system level. The discussion below addresses the power rating of the electric machine to meet vehicle performance requirements. The sizing of the energy storage device will be addressed later in the hybrid vehicle design section. Electric motors have three major segments in its torque–speed characteristics: (i) Constant torque region, (ii) constant power region and (iii) natural mode region. The envelop of the electric motor torque–speed characteristics is shown in Figure 3.12.
The motor delivers rated torque up to the base speed or rated speed of the motor when it reaches its rated power condition. The motor rated speed is defined as the speed at which the motor can deliver rated torque at rated power. The motor operates in a constant power mode beyond the rated speed where torque falls off steadily at a rate that is inversely proportional to speed. Electric motor can operate at speeds higher than rated using field weakening in the constant power region. There is a third natural mode region for very high motor speeds where the torque falls off rapidly being inversely proportional to the square of the speed.
The natural characteristic region can be an important part of the overall torque speed curve of certain motors that can be used to reduce the power rating of the motor. However, in most cases, the vehicle’s maximum speed is considered to be at the end of the constant power region. Note that the curves in Figure 3.12 show the envelope, i.e., the operating torque and speed limits in different regions. The electric motor can operate at any point within the envelope through the feed from a power electronics-based motor drive component.
The salient feature of wide-operating speed range characteristics of an electric motor makes it possible to eliminate multiple gear ratios and the clutch in EV and other applications. A single-gear ratio transmission is sufficient for linking the electric motor with the driveshaft. Electric motors with extended constant power region characteristics are needed to minimize the gear size in EVs. The size of an electric motor depends on the maximum torque required from the machine. The higher the maximum torque required, the larger will be the size of the motor. In order to minimize the size and weight, electric motors are designed for high-speed operation for a given power rating.
Gears are used to match the higher speed of the electric motor with the lower speed of the wheels. Typical motor speeds can be in the vicinity of 15,000 rev/m for typical wheel speeds of around T r # 1 Constant Power Region Natural Characteristics Region Torque Constant Torque Region # 2 # 3 pm ω ω rm Rotor Speed FIGURE 3.12 Electric motor torque–speed envelope.