9 Electric and Hybrid Vehicles The power–torque relation in HP and ft · lbs is HP = . 5,252 Torque (ft l ⋅ Ч bs) rpm Figure 1.6 depicts that the IC engine does not produce any torque below a certain speed. A trans- mission is essential for an IC engine to match the vehicle speed with the narrow high-power speed range of the engine. On the other hand, the electric motor produces high torque even at zero speed and typically has constant power characteristics over a wide speed range. Therefore, the electric motor can be attached directly to the drive wheels with a single-gear transmission to accelerate the vehicle from zero speed all the way up to the top speed.
The fixed gear ratio is appropriately sized based on the operating speed range of the motor and the top speed of the vehicle. Single-stage transmission gears are designed to match the higher speed of the electric motor with the lower speed of the wheels, typically in the range of 10–15:1. Typical maximum electric motor speeds are 15,000 rev/m for wheel speeds around 1,000 rev/m. The power and torque densities of electric machines are typically much higher than that of a comparable-sized IC engine. The power and torque densities improve as the rated speed of the elec- tric machine increases. Electric machines are designed for high-speed operation to minimize size and weight.
The power and torque densities also depend on the type of electric machine used. The motor and, hence, the vehicle speed are controlled directly through the power electronic converter feeding the current into the motor. The electric motor is the sole propulsion unit in EVs, while in HEVs, the electric motor and the IC engine together in a series or parallel combination provide the propulsion power. The electric motor for propulsion in the EV/HEV is desired to have a high starting torque to meet acceleration require- ments, high power density to reduce the weight, and high efficiency to extend the battery range.
The ideal candidate motor drive should also have a wide operating speed range to facilitate a single-gear transmission stage. The important characteristics of an electric or hybrid vehicle motor include flex- ible drive control, fault tolerance, high efficiency and low acoustic noise. The motor drive must be capable of handling voltage fluctuations from the source. The requirements of an electric or hybrid vehicle motor, not necessarily in order of importance, are summarized in the following: • Ruggedness. T T • High torque-to-inertia ratio e . Large e results in “good” acceleration capabilities. J J • Peak torque capability of about 200%–300% of continuous torque rating.
P • High power-to-weight ratio e . w • Capability to operate with varying DC bus voltage. • Low acoustic noise, low EMI, low maintenance and low cost. • Extended constant power region of operation. 1.6 ELECTRIC AND HYBRID VEHICLE HISTORY The history of EVs is quite interesting. It includes the insurgence of EVs following the discovery of electricity and the means of electromechanical energy conversion, and later being overtaken by gasoline-powered vehicles [5].
The people digressed from a potential environmentally friendlier mode of transportation due to the lack of enabling technologies in the early years but are now revis- iting that prospect aided with significant technical developments over the years [6].