A bidirectional DC-DC converter fed DC motor for electric vehicle application Idris Azizi and Hammoud Radjeai Automatic Laboratory (LAS), Setif-1- University Setif, Algeria
[email protected], hradjeai@ yahoo.fr Abstract—this work presents a digital simulation of an operation of a small electric vehicle. The traction chain consists of a battery, a bidirectional DC-DC converter and DC motor. The vehicle dynamics which represents the load torque applied on the motor shaft is taken into account. The two modes of operation; motor and regenerative braking mode are explained and the simulation model can simulate the two modes simultaneously. The energy resulted in the braking phase is stored in the battery, thus permits to increase the autonomy of the vehicle. Keywords—DC-DC converter; motor mode; generator mode; vehicle dynamics.
I.
INTRODUCTION
The electric vehicle (EV) is one solution to resolve the problems relevant to environment and the depletion of fossil resources, [1], [2]. The major drawback of this vehicle is the energy storage, many researches have been released to increase electric driving range, Improve efficiency, decrease cost and make EV competitive with conventional vehicles in the market[1], [3]. With revolutionary contributions of power electronics and Energy Storage Systems, electric drive trains totally or partially replace internal combustion engine (ICE) in this vehicle [1]. Regenerative braking can be used in EVs as a process for recycling the brake energy, which is impossible in the conventional internal combustion vehicles. Regenerative braking is the process of feeding energy from the drive motor back into the battery during the braking process, when the vehicle’s inertia forces the motor into generator mode. In this mode, the battery is considered as a load, thereby providing a braking force to EVs. It is shown that the use of regenerative braking of EVs can increase the driving range up to 15 % with respect to EVs without the regenerative braking system (RBS) [4]. Although they are more and more often replaced by AC motors, the DC motors stay widely used in the field of electric traction. To ensure the speed variation of these motors, choppers are used in the case of vehicles powered by battery or by a DC catenary. When supply is made from an AC catenary, we use again a chopper if the vehicle comprises a diode rectifier [5]. The superiority of these motors is that they lend
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themselves easily to a flexible control, continuous and almost instantaneous speed [6]. A bidirectional DC-DC converter can be used in the DC link of battery fed dc motor drives devoted to EVs applications. For motoring operation, the DC-DC converter is used to step down the battery voltage to a voltage level which allows the speed variation of DC motor. Furthermore, the bidirectional arrangement of DC-DC converter allows the reversal of power flow and the control of the motor braking current, so that a significant amount of the kinetic and/or potential energy of the vehicle can be recovered in the battery [7]. Among EV’s motor electric propulsion features, the energy efficiency is a basic characteristic that is influenced by vehicle dynamics and system architecture. For this reason, the EV dynamics are taken into account [8]. In this context, a traction chain is established which contains battery, DC-DC converter, DC motor and vehicle dynamics. The control of DC motor is done by tow loops using a cascade control configuration; one is the outer loop which controls the speed, the other is the inner loop which controls the current with a hysteresis regulator. II.
THE STRUCTURE OF THE PROPOSED SYSTEM
The block diagram of Fig.1 includes the battery, the DCDC converter, the DC motor and the vehicle dynamics.
i*
NOT
K1 U
D2 K2
L
D1
1/K
i
PI
ω*
v
Speed sensor
Vehicle Dynamics Fig. 1. The block diagram of the proposed system
A. THE BATTERY MODEL In SimPowerSystems library, there is a block implements a generic dynamic model parameterized to represent most popular types of rechargeable batteries. The equivalent circuit battery is shown in Fig.2 [9]. For this work we choose the lithium-ion battery for its advantages cited in the reference [1]. The following equations describe the discharge and charge of Lithium-Ion battery: Discharge Model (i*>0) * f 1 (it , i , i ) = E 0 − K
Q * Q i −K it + A exp(− Bit ) Q − it Q −it
(1)
Charge Model (i
