Modeling of BLDC motor Powered from Li-ion Battery for Electric Bicycle Application

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1 Moeling of BLDC motor Powere from Li-ion Battery for Electric Bicycle Application Priyaarshini J Patil M. tech, (Power Electronics) Dept. Electrical & Electronics Engg. Dr.Ambekar Institute of Technology Bangalore, Inia arshini_jsp@reiffmail.com Abstract The Power-Assiste bicycle is a form of transportation system that attempts to merge both health an environmental benefits. It offers cleaner an eco-frienly alternative to travel short-to-moerate istances. These features prove their performance better than gasoline powere vehicles. This paper iscusses the suitability of BLDC hub motor base electric propulsion system in an electric bicycle to achieve better ynamic response. Dr. Jyoti P Koujalagi, Professor Dept. Electrical & Electronics Engg. Dr.Ambekar Institute of Technology Bangalore, Inia jyotipkoujalagi@gmail.com characteristics with respect to time are shown in results consiering its simulink moel. II. COMPONENTS OF ELECTRIC BICYCLES MOTOR CONTROLLER & DRIVER CIRCUIT MOTOR Inex Terms electric propulsion, inverter, brushless, battery, controller, sensor, MATLAB/SIMULINK. BATTERY ROTATION OF WHEELS W I. INTRODUCTION ith the price of fossil fuels rising significantly an no turning back at lower prices. The environment has also been more of a focus throughout the worl in the past few years, an it seems that cleaner alternatives have been steaily on the rise with no en in sight.these scenarios are making one to think about electric powere vehicles to promote both cleaner technology as well as a lesser epenence on oil. To start with such invention an electric bicycle can be a basic to begin for learner/researcher. The main avantages of electric bicycles are - It oes not cause air pollution, noise pollution as gasoline powere vehicles o, oes not require license, insurance, parking an registration fees, is not boun to roa network. It also reuces traffic issues of urban areas, an reuces the cost of travel compare to gasoline powere vehicles. An extra benefit to builing the electric bicycle is that it can also show the general public how much cheaper it woul be to convert their regular bicycle into an electric bicycle rather than riving solely in their gas-powere vehicles[1]. This paper proposes the electric bicycle construction using brushless DC motor powere from Li-ion battery. The mathematical moel of Li-ion battery along with its charging & ischarging characteristics is illustrate using its simulink moel. The suitability of brushless DC motor for electric bicycle an its mathematical moel is iscusse. The performance of the same in terms of torque, spee Fig 1 Block iagram of an Electric bicycle An electric bicycle essentially has four components: motor, power transmission system, an control system an power source as shown in Fig.1. The blocks shown in Fig.1 become essential to construct an electric bicycle, in which an electric motor is powere from a battery which is a DC source [2] an the spee accoring to rier's eman is varie using controller an river circuit. The spee comman is provie through throttle whose output is given to input of motor controller an river block. W ith rier s mechanical input at crank peals can increase the rive capacity, reuce torque require an rises spee. A Selection of Motor THROTTLE CONTROLLER RIDER FORCE ON CRANK PEDALS Choosing a motor is the first step in creating an appropriate system for the electric bicycle. Three ifferent motor types are commonly use for electric vehicles. Brushe DC motor, Inuction motor an Brushless DC motor [3]. Among three brushless DC motor is selecte keeping account of three key features such as higher efficiency, minimum maintenance an higher power ensity, high ynamic response; noise less operation an higher Page 47

2 spee ranges reflecting the characteristics of brushless DC motor. The power & voltage rating can be selecte base on calculations obtaine from mathematical moe of electric bicycle. B. Power Transmission System The electric rives ten to perform poorly in wet roa surface conition an ebris interferes with power transfer. A lighter bicycle with a reliable electric motor rive without compromising the appearance of the conventional bicycle is the preferre choice [4]. Hence irect rive is the most preferre rive metho. The motor is enclose in the rear or front wheel hub an usually such motors are calle Hub motor. Since there is no gearing an coupling in the rive system, it saves space an is noiseless & maintenance-free. In aition, this metho has higher operational reliability an higher efficiency. C. Power Source Parameters Li-ion Ni-MH Working voltage(v Leaaci Gravimetric energy ensity(wh/kg) Volumetric energy ensity (Wh/L) Cycle life(cycles) Capacity self-ischarge rate (% per month ) 5% 30% 10% Energy efficiency)c icharge/c charge) 99% 70% 75% Weight comparison for same capacity Size comparison for same capacity Reliability High Low High The Power source for electric bicycle will be battery as BLDC motor requires DC power source. This can be charge from 230V, 50Hz utility supply an even it can be charge from solar energy. While selecting batteries primary concern is cost, urability, energy ensity, an the number of recharge cycles. The most common batteries use are seale lea aci batteries ue to their low costs, but its relatively heavy weight an lesser life cycle this cannot be suitable for electric bicycle application. Table.1 shows the performance comparison of three electro-chemistry namely Lea-Aci, Ni-MH an Li-ion. TABLE.1 PERFORMANCE COMPARISION OF THREE BATTERY TECHNOLOGIES As it is clear from Table.1 that, among these three battery technologies, Lithium-ion Battery is the best power solution for Electric Vehicle (EV) applications, because of its high energy ensity an long cycle life. Lea-aci Battery as an outate technology has being use less an less for EV applications, ue to its heavy weight, large size an nonenvironmental frienly for battery materials such as lea an sulfuric aci [5]. The other han Li-ion battery technology has higher voltage rating an high reliability of 3.7V, which is three times greater than Ni-MH battery an hence requires less number of cells in series to increase voltage rating an hence increases its reliability compare to Ni-MH. It also gives 2 times longer istance rie than the Ni-MH battery can o. III. MATHEMATICAL MODEL OF LI-ION BATTERY The mathematical representation iscusse in this section is similar to Shepher moel in which an equation escribe the electrochemical behaviour of a battery irectly in terms of terminal voltage, open circuit voltage, internal resistance, ischarge current an state-of charge. Fig.2 gives equivalent circuit of battery. A. The ischarge moel Fig 2 Typical Li-ion battery moels The ischarge moel represent accurately the voltage ynamics when the current varies an takes into account the open circuit voltage (OCV) as a function of State-of-charge (SOC). A term concerning the polarization voltage is ae to better represent the OCV behaviour an the term concerning the polarization resistance is slightly moifie. The battery voltage obtaine is given by V batt = E 0 K i t i t R. i + Ae B.i t K i t. i (1) Page 48

3 Where V bat t = battery voltage (V) E 0 = battery constant voltage (V) K = Polarization constant (V= (Ah)) i t = Polarization Resistance (Ω) in ischarge moel = battery capacity (Ah) i (t) = i. t = actual battery charge (Ah) A = exponential zone amplitue (V) B = exponential zone time constant inverse (Ah 1 ) R = internal resistance (Ω) i = battery current (A) i = filtere current (A) B. The charge moel The charge behaviour of Lea-Aci an Li-Ion batteries have the same en of charge (EOC) characteristics, because the voltage increases rapily when the battery reaches the full charge [6]. This phenomenon is moelle by the polarization resistance term. During charging of battery the polarization resistance keeps on increasing an uner goes abrupt increase once battery is fully charge i.e. when i(t)=0. So unlike ischarge moel, the polarization resistance is valiate experimentally as in equation (2) Polarization Resistance = k. i t 0.1. The battery voltage obtaine uring charging is given by V batt = E 0 R. i k i t 0.1. i K. i t + i t Ae B.i (t)) (3) The above iscusse moel has a limitation of minimum capacity 0Ah & the maximum capacity of. so; even if the battery is overcharge its SOC cannot be greater than 100%. IV. BLDC BASED ELECTRIC PROPULSION A Brushless DC motor Operation (2) A brushless DC (BLDC) motor is an insie-out permanent magnet DC motor, in which the conventional multi-segment commutator, which acts as a mechanical rectifier, is replace with an electronic circuit to o the commutation. In a BLDC motor permanent magnets are mounte on the rotor with the armature winings being house on the stator with a laminate steel core, as illustrate in Figure.3 B Inverter for BLDC motor Fig 4 Three Phase Hex-brige Inverter Rotation in BLDC is initiate an maintaine by sequentially energizing opposite pairs of pole winings/phases i.e voltage stokes are applie to the two phases of 3-phase wining system so that the angle between the stator flux an the rotor flux is kept close to 90 egrees in orer to generate maximum torque from the motor. This is one by an electronic switching using H-brige inverter as shown is Figure 4. This stage consist of six semiconuctor switches usually power MOSFET is preferre ue to its high switching frequency >MHz, voltage rating up to 600V with lower switching an conuction losses. It is critical to know the rotor position for energizing appropriate winings of stator to sustain motion. The rotor position information can be obtaine from Hall Effect sensors. Usually three hall sensors are use an place 120 egree mechanically apart such that each sensor outputs a high level for 180 egree of an electrical rotation, an a low level for the other 180 egree [7]. The three sensors have 60 egree relative offset from each other an this ivies the rotation in to six-phase. Table. 2 show the relationship between the hall sensor input coe, the require active motor winings. TABLE.2 SENSOR INPUT BY ACTIVE SWITCH TABLE Theta_elec Hall sensor Commutation no: Active rive Fig 3 Permanent magnet BLDC construction (PWM1) 6(PWM6) Page 49

4 Theta_elec Hall sensor Commutation no: Active rive (PWM1) 6(PWM6) (PWM1) 5(PWM5) (PWM3) 5(PWM5) (PWM3) 4(PWM4) (PWM2) 4(PWM4) (PWM2) 6(PWM6) A. Moeling of BLDC motor.bldc motor can be realize mathematically in two ways: abc-phase variable moel an -q axis moel. In a BLDC motor, the trapezoial back EMF implies that the mutual inuctance between stator an rotor is nonsinusoial, thus transforming to -q axis oes not provie any particular avantage, an so abc phase variable moel is preferre. In this electrical & mechanical system of BLDC motor is iscusse. Following series of equations expresse for electrical system is for abc-phase variable moel & it is note that phase inuctance Ls is assume constant an oes not vary with rotor position. 1 i t a = (2v 3L ab + v bc 3R s i a + λ pω r ( 2φ a + φ b + s φ c )) (4) t i b = 1 3L s (-v ab + v bc + 3R s i b + λ pω r (φ a 2φ b + φ c )) t i c = ( t i a + t i b) (6) T e = pλ (φ a.i a + φ b. i b + φ c.i c ) (7) Where Ls = Inuctance of the stator winings R = Resistance of the stator winings i a,i b,i c =a, b an c phase currents φ a,φ b, φ c =a, b an c phase electromotive forces v ab, v bc =ab an bc phase to phase voltages ω r =Angular velocity of the rotor λ = Amplitue of the flux inuce by the permanent magnets of the rotor in the stator phases P = Number of pole pairs T e = Electromagnetic torque The mechanical system is expresse using following equations (8) & (9) t ω r = 1 J (T e T f Fω r T m ) (8) θ t = ω r (9) (5) Where J =Combine inertia of rotor an loa F = Combine viscous friction of rotor an loa θ = Rotor angular position T m = Shaft mechanical torque T f = Shaft static friction torque Table.2 shows the moeling of motor incluing realization of hall sensors as a function of rotor electrical angle. V. CONTROL SYSTEM The control unit for brushless DC motor which makes its suitability to electric bicycle can be implemente by ajusting the switching frequency of MOSFETs use in H- brige Inverter/river circuit which rives BLDC motor. The switching frequency of MOSFET is varie by proviing variable uty cycle PWM signals at gate input of MOSFET. The flow chart of Fig. 5 shows how exactly control system operates. It essentially inclues all possible constraints that a rier experiences in his trip i.e parameter like charge status of battery, brake application in case of obstacles, acceleration & ecelerations, start & stop likewise similar factors that can help in smooth rie shoul be incorporate in controller assembly unit. PWM uty cycle =0% Start Ignition? SOC > SOC min Start Motor Brake? Close loop spee Control Stopping Signal? Page 50 Stop

5 output to generate require PWM signals. Stator resistance Rs=0.2Ω, stator phase inuctance Ls = 8.5e 3 H Fig.5 Flow chart of control system suitable for electric bicycle VI.. MATLAB/SIMULINK MODEL Simulink moel to escribe the charging & ischarging characteristics of Li-ion battery is shown in Fig.6. A 100V, 10Ah Li-ion battery connecte to a resistive loa of 100Ω. A DC machine is connecte in parallel with the loa an operates at no loa torque. When the State-Of-Charge (SOC) of the battery goes uner 0.4 (40%), a negative loa torque of 200 Nm is applie to the machine so it acts as a generator to recharge the battery. When the SOC goes over 80%, the loa torque is remove so only the battery supplies the resistive loa. Fig.7 Simulink moel of BLDC powere from Li-ion battery VII. SIMULATION RESULTS The charging & ischarging characteristics waveform corresponing to SIMULINK moel shown in Fig.6 is illustrate with voltage, SOC an current waveforms of battery in Fig.8. Fig.6 Simulink moel to show characteristics of battery The BLDC motor powere from Li-ion battery MATLAB/SIMULINK moel is shown in Fig.7, a 24V, 10Ah rate battery is selecte to serve the purpose to meet the power eman of motor. The spee control mechanism is incorporate in control unit by taking hall sensor input, rotor spee & actual current at output of inverter. The rotor spee is compare with reference spee in this case 200rpm is chosen an resulting error signal is fe to PID controller. The output of PID controller is synchronize with hall sensor coe an converte to reference current. This reference current is compare with actual current at inverter Page 51

6 Fig.8 Waveforms of voltage, SOC an current from Li-ion battery It is note that for a given nominal voltage higher the Ah rating lesser will be the cycles of battery an for given Ah rating lower the nominal voltage then, ischarge time is more than charging time. Fig.9 Waveforms of voltage, SOC an current from Li-ion battery connecte to BLDC motor The current rawn from battery shoots to 1200A as soon as motor is powere for an instant to meet the torque requirement an after 0.2s, its value settles between A the value of SOC slowly rops from 100% to % in 2s. Fig.11 Waveforms of three back-emf inuce, power output of BLDC motor Fig.10 gives the three back emf inuce in each phase which is isplace at 120 egrees from each other with peak-peak value of 25volts. The output power curve from the motor shown below back-emf has shoot between 0-0.2s interval ue to which high current was rawn from the battery as iscusse in battery simulation waveforms an then raws power varying from 40W to 70W ue to torque ripples. CONCLUSION & FUTURE WORKS Fig.10 Waveforms of three stator phase currents, rotor spee an electromagnetic torque of BLDC motor Stator phase currents i a, i b an i c shown in Fig.10 are 120 egrees apart from each other with motor torque applie externally is of 3 N-m. Rotor spee is at 170 rpm an attains after 0.2s which is shown below phase currents. Though there is ripple in electromagnetic torque Te but, this is not reflecte at rotor spee an helps in smooth operation of motor. The results illustrate in this paper can help in esigning an electric bicycle to the ratings of one s requirement & can be extenable to tricycle application for physically challenge society. Here suitability of BLDC motor with smooth running operation is shown & in future sensor less operation can be aopte to overcome limitations & expenses of sensors. The system performance can be improve if renewable energy sources or fuel cell technology is integrate as a source of power. REFERENCES [1] Http/www. Electric bicycle guie.com/ electric bicycle history.html [2] Robert Cong, Roney Martinez, Mark Casilang, Peter Vong Electric Bicycle System -Senior Project California Polytechnic State University, San Luis Obispo, June [3] Mehra Ehsani, Yimin Gao, Ali Emai- Moern Electric, Hybri Electric, & Fuel cell Vehicles- Funamentals, Theory, & Design -secon aition-2010, CRC Press [4] S. I. Bran, N. Ertugrul an W.L. Soong Investigation of an Electric assiste bicycle an etermination of performance characteristics University of Aelaie. School of Electrical an Electronic Engineering, Aelaie, Australia. Page 52

7 [5] General Electronics Battery Co., Lt. Comparison of Different Battery Technologies [6] Olivier Tremblay1, Louis-A. Dessaint Experimental Valiation of a battery Dynamic Moel for EV Applications Worl Electric Vehicle Journal Vol. 3 - ISSN , May 13-16, [7] NXP Semiconuctors- Brushless DC motor control using the LPC2141- AN Rev October [8] Vinatha U, Swetha Pola, Dr K.P.Vittal- Simulation of Four uarant Operation & Spee Control of BLDC Motor on MATLAB / SIMULINK MIEEE Dept., NIT Surathkal, Inia Page 53