52 Electric and Hybrid Vehicles Electrochemical batteries have been the traditional source of energy in EVs. Lead/acid batteries were used in the first commercially available EV Saturn EV1 in 1996, but since then, the technol- ogy has progressed towards NiMH and Li-ion batteries. The batteries need a charger to restore the stored energy level once its available energy is near depletion due to usage. The limited range problem of battery-driven EVs prompted the search for alternative energy sources, such as fuel cells and flywheels.
Prototypes, limited number of models and fleet vehicles have been developed with fuel cells over the last two decades, but at the same time battery technology have also progressed significantly. Several different types of Li-ion battery chemistry have emerged that are being used in both EVs and HEVs. The EVs developed in recent years are based on induction machines or permanent magnet machines. In the early developmental stages of EVs, DC machines were used, but their many disad- vantages turned EV developers to look into various types of AC machines. The rare-earth material- free, maintenance-free and low-cost nature of induction machines is appealing to some developers.
High-speed operation of induction machines comes with a penalty in size and weight, although recent studies on re-rating of induction motors for EVs are opening new perspectives even in the high-speed range. The high efficiency together with high power density features of permanent mag- net machines makes them an attractive solution for EV applications, although the cost of permanent magnets can become prohibitive in the future unless low-cost options for high-energy permanent magnets are developed. The high power density and potentially low production cost of switched reluctance machines make it ideally suitable for EV applications.
However, the acoustic noise prob- lem has so far been the deterrent for the use of switched reluctance machines in EVs. The electric motor design includes not only the electromagnetic aspects of the machine but also the thermal and structural considerations. The motor design tasks of today are supported by the finite element stud- ies and various computer-aided design tools making the design process highly efficient. The electric motor is driven by a power electronics-based power processing unit that converts the fixed DC voltage available from the source into a variable voltage, variable frequency source controlled to maintain the desired operating point of the vehicle.
The power electronics circuit comprises power semiconductor devices that saw a tremendous development over the past three decades. The enabling technology of power electronics is a key driving force in developing efficient and high-performance powertrain unit for EVs. The advances in power solid-state devices and VLSI (very large-scale integra- tion) technology are responsible for the development of efficient and compact power electronics circuits and electronic control units. High-power devices in compact packaging are available today enabling the development of lightweight and efficient power processing units known as power electronic motor drives.
The developments in high-speed digital signal processors or microprocessors enable complex control algorithm implementation with high degree of accuracy. The controller includes algorithms for both the motor drive in the inner loop as well as system-level control in the outer loop. 3.2 HYBRID ELECTRIC VEHICLES The term “hybrid vehicles” in general usage refers to vehicles with two or three different types of sources delivering power to the wheels for propulsion. The most common hybrid vehicles have an IC engine and one or more electric machines for vehicle propulsion. The IC engine can be used to generate electric energy ‘on-board’ to power the electric machines.
An energy storage device buffers the electrical energy flow between the electric machine operated as a generator and the electric machine operated as a motor. An electric machine can be operated both as a motor and as a generator. Vehicles architectures with only one electric machine use it both for electric power generation and for vehicle propulsion power; the constraint being that electric power generation and power delivery through electric propulsion are not possible simultaneously. In hybrid vehicles, the traction electric motors can operate independently or in association with the IC engine to power the wheels depending on the type of vehicle architecture.
The subject of treatment in this section is the basic architectures of HEVs.