ABSTRACT
Most developed countries that produce vehicles and automobile companies invest in lots of budgets for developing electric vehicle to reduce the use of fossil fuel. Particularly, as the battery technology has been made rapid progress, electric vehicles are able to operate with only battery system. In this context, battery charger connected with grid is required to charge the batteries attached with vehicles. Battery charger charges the battery bank through using the electrical energy of grid, and the operation is similar with that fossil fuel is injected into the gas tank of an internal-combustion engine. This system requires input power factor controller and energy conversion system for charging the battery bank, and the system generally consists of diode rectifier and DC/DC converter or single stage AC/DC PWM converter. In the case of the charger that consists of diode rectifier and DC/DC converter, based on the voltage level of battery, the battery is charged through using buck type or boost type DC/DC converter, and the input power factor is controlled as well. On the other hand, AC/DC PWM converter contains single stage circuit that consists of power semi-conductor switching devices instead of diode rectifier, and the converter charges battery and controls input power factor. Moreover, the structures of energy conversion system to control motor are classified into two types. The first type is that the voltage and capacity of battery bank is bulky. In this case, the battery bank is employed as the input of inverter. The inverter supplies energy for operating to motor and transports free-wheeling energy generated when vehicle brakes suddenly to the battery bank through simple rectifying operation. The second type is that the voltage and capacity of the battery bank is comparably low. When the vehicle is operating, the Bi-directional DC/DC converter boosts the battery energy until the voltage is same as that of inverter DC-link. On the other hand, when the vehicle is braking, the free-wheeling energy is charged into the battery bank through the Bi-directional DC/DC converter.
In this paper, battery charger used for electric vehicle as an energy conversion system, Bi-directional DC/DC converter, and three-phase voltage source inverter were designed. The inverter performs the vector control of interior synchronous permanent magnet motor for electric vehicle. Also, depending on the driving mode, simulation was performed through using the designed energy conversion system. The battery charger charges four 12[V] series connected lead battery bank with single commercial power, and the charger was designed based on AC/DC PWM buck converter. The capacity of the employed battery bank is 48[V]/100[Ah], and the battery bank was charged in constant current control condition with 0.2[C-rate], 20[A]. The Bi-directional DC/DC converter was designed based on three-phase interleaved type buck-boost DC/DC converter. The designed converter controls the output voltage constantly as 250[V] in operating condition, and the inductor current of each phase was controlled by the designed converter to have same average current value.
Particularly, the DC-link voltage of inverter is controlled in 230~270[V] range by the instantaneous boost and buck operations of Bi-directional DC/DC converter in free-wheeling mode. Due to the operational characteristic of the Bi-directional DC/DC converter, the DC-link voltage of the inverter is able to be stable, and the system shows better performance than vector controlled system since DC-link voltage of the inverter is controlled in definite range when free-wheeling mode is turned into driving mode. Previously mentioned, 1.2[kW] battery charger was designed for the 0.2[C-rate] constant current and 50.7[V] constant voltage control of 48[V]/100[Ah] battery bank, and 4[kW] Bi-directional DC/DC converter for boosting the charged energy of battery bank and three-phase voltage source inverter were designed. The designed energy conversion systems verified the validity of results through presenting the experimental results depending on the driving modes of electric vehicle.
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