A hybrid approach to optimal electric drive train design

Abstract – This paper describes the methodology for developing algorithms for optimal design of an electric drive train for electric bikes and evaluating their performance. The design methodology is validated using simulation and experimental results. The behavioral model of the electric drive train is developed and the performance of the drive train subject to varying battery sizes and motor ratings are evaluated. The effect of reducing battery voltage on the vehicle speed and performance which is essentially the behavior of the Brushless DC motor is presented with simulation and experimental results.

This simulation model is very useful for optimizing the battery rating, motor rating and estimating the Range of electric bikes. Simulation and experimental results are presented.
Key words: Drive train design, Vehicle simulation, Range

Calculator, BLDC, Battery Performance, Electric vehicle

I. Introduction
Electric vehicles (EVs) store electricity in an energy storage device, such as a battery. The electricity powers the vehicle's wheels through an electric motor. EVs have limited energy storage capacity, which must be replenished by plugging into an electrical source

Fig 1: E Bike Block Diagram

The block diagram of electric vehicle is shown in Fig1. Main parts are motor-controller and battery which actually decides the performance of the vehicle. Motor can be compared with the engine of the normal vehicle so it is the most crucial part. The selection and evaluation of motor and battery are discussed.
This paper describes the Performance of electric vehicle with dropping battery voltage. First the voltage variation of battery with AH consumption is shown. Secondly the power and current requirement of vehicle at given speed is calculated and than the motor speed with varying battery voltage is discussed.
After the evaluation of above, the equations achieved are used to build the simulation model to evaluate the vehicle performance with different batteries and load conditions. Simulated results of vehicle are shown with the experimentally tested results.

II. BATTERY

The most significant technical barrier to developing commercially viable electric vehicles is the energy storage system. The challenge is to develop batteries that are able to meet both the requirements imposed by e bike system and market expectations of the system’s cost and life. In this context, a vehicle systems approach is needed to investigate the operational requirements specific to e- bike technology.
Batteries are the energy source for e bikes. Different types of batteries are available and can be used for the same. While selecting the batteries it’s life, voltage rating, capacity, current requirement of vehicle and weight of batteries has to be considered. Life of battery will depend on the battery technology, its charge and discharge current so one has to consider the vehicle current while selecting battery as the increase in discharge current decreases the battery life and its capacity. Increase in battery voltage will decrease the vehicle current so one has to evaluate different voltage and AH rating of battery to get the proper vehicle range and battery life.
Different types of batteries give different characteristics. Here analysis with AGM (absorbent glass mat) batteries is discussed. Battery is evaluated experimentally by discharging it at constant current of 7A and AH consumed is noted with actual battery voltage after every 5 minutes. Different battery sets of AGM batteries were tested and than the discharge curve is drawn with average % voltage Vs % Discharge and than trend line is drawn to achieve the equation for battery % voltage with % discharge.

Fig 2: Battery Voltage Vs Discharge
These batteries are AGM, VRLA (Valve Regulated Lead acid) batteries. The vehicle performance is a function of battery performance so it is suggested to do the elaborated experimental testing to achieve the battery performance curve.
The equation what is achieved from polynomial trend line is as follows:
y = -4.1126E-10x⁶ + 1.0197E-07x⁵ - 9.1399E-06x⁴

+3.4616E-04x³ - 4.9822E-03x² - 1.2110E -01x

+ 1.0574E+02 (1)
y = battery voltage in %

x = Battery AH discharge in %

III. Vehicle Power Requirements

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