Project on Electric Bike

Main Project 2012

Brushless DC Motor Based Electric Bike

Chapter 1

INTRODUCTION

1.1 BACK GROUND Electric vehicles have been around since the inception of the automobile but in the early days for dominance the internal combustion engine quickly won out as the best power system for cars. Although the electric vehicle was superior in many respects, a source of energy the battery was no match for the high energy content ease of handling, and cheap and abundant supply of motor fuel. Today nearly a century after the electric vehicle (EV) was forced into near oblivion; it seems that EV’S may actually become the ultimate winner. As easily recoverable petroleum deposits dwindle, automobile population soar, and cities become choked with combustion by products, the IC is increasingly becoming the victim of its own success. Automobile must become cleaner and energy efficient. Although electric vehicle have several advantages over exciting ICE vehicle availability of electric vehicle in the market is less due to its short range and high cost. The solution for these drawbacks are converting the existing vehicle into EV’s using low cost high efficient electric drivers. Existing drivers and battery system make this conversion very expensive. So a cheap as well as a compact drive is to be developed. This project is focusing on the design of DC motor drive for a low cost electric vehicle. The term 'electric bike' is bandied about a lot these days, and to the uninitiated it might be a somewhat nebulous term. For a classical bike, with a conservative number of speeds and

Dept. of EEE

1

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

none of the bells or whistles. Now, add three things: an electric bike motor, a battery, and a controller that activates and regulates the power and electric assistance. An electric bike is a standard bicycle that has electric components equipped on it. An electric bike will function normally like a regular bicycle. An electric bike will pedal like a regular bicycle, handle like a regular bicycle, and by and large an electric bike will use the same parts as a regular bicycle. An electric bike is meant to be thought of and used as a bicycle, not as a motorcycle. An electric bike is meant to augment human power, not completely replace it. An electric bike is not about travelling at high speeds. Imagine an electric bike as a regular bike that just happens to be easier. Yes, an electric bicycle is an easy bike. Styles of electric bikes Electric bike come in a variety of styles. Just like regular bikes, there are different frame sizes to accommodate different rider sizes, as well as riding styles. Electric bikes also differ when it comes to the three previously-mentioned main parts that make them electric: the bicycle motor, battery, and controller. Some electric bikes have an external, chain-driven motor mounted on the side of the bike, pulling the rear wheel. Most electric bikes have an internal motor, where the motor is located in the hub of the front or rear wheel. When the motor is located in the hub, it offers a more streamlined package, and also makes it so the bike is less conspicuously an electric bike. Motors also come in a wide variety of power ratings, from 200W to 700W. Depending on your potential usage of the electric bike, a more powerful motor may be necessary, or it might be completely unnecessary for you purposes. The controller is what lets you operate the electric assistance that your electric motor bike offers. Controllers come in a variety of styles, and all of them are located on the handlebar for ease of use. There are two main styles of controllers. There are throttle-based controllers, and pedal activated controllers. By and large, electric bikes are simple to use, ride, and maintain. They require little maintenance beyond what a standard bike requires. Electric bikes are used for a variety of purposes, ranging from effortless commuting to pure leisure. They attract riders from all backgrounds.

Dept. of EEE

2

Govt. College of Engg., Kannur

Main Project 2012

1.2

Brushless DC Motor Based Electric Bike

MINI PROJECT OVERVIEW In our Min Project, we designed, set up and tested a boost converter for an input voltage of 24V, and output voltage from a minimum of 40V to a maximum of 150V for a duty ratio between 0.4 and 0.84. We obtained a minimum output voltage of 44V and maximum output voltage 126V for duty ratio between 0.4 and 0.8. During testing we have obtained speed variations of motor between 2750 rpm to 9767 rpm on no load. We carried out no load and load tests on a universal motor. From the load tests we got the following results. V(Volt)

Ia(Amp)

N(rpm)

T(Nm)

Pin(W)

Pout(W)

ɳ(%)

115

0.91

5450

0.165

104

94

90.4

100.9

1.1

4133

0.247

110

106.99

97.3

93.3

1.4

2856

.357

130

106.77

81.7

72.3

1.64

913

0.41

118.5

40

33

68

1.7

275

0.49

115.6

14.11

12.2

From the load test it was observed that the efficiency of the universal motor decreased with increase in torque. Hence the speed and torque relations obtained for a universal motor where unsatisfactory for being used in a vehicle to propel it forward.

1.3

OBJECTIVE The electric vehicles are not accepted widely since the cost of electric vehicles and

conversion kits available in market is very high. As such vehicles use high capacity batteries with larger cost and size, the overall price of the vehicle increases. Our aim here is to design a suitable dc-dc convertor to boost the power from a single 12v battery to the power required by the motor. To start with we aim to design and fabricate a cheap brushless dc motor based drive for conversion of a bicycle into a electric bike.

Dept. of EEE

3

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

The main objectives can be summarized as follows. The main objective is to design and setup a dc-dc convertor and a controller for the motor.  Total capacity of the bike is 80 Kg.  Maximum speed of the bike is 30 km/h The design process include the following steps  Selection of motor and battery of appropriate ratings with minimum cost.  Design of dc-dc converter for selected motor and battery.  Fabrication and testing the circuits

1.4

SCOPE OF THE WORK The use of dc-dc convertor for boosting the power required limits the number of

batteries to one, So that there is a decrease in the price of the vehicle which makes it economical. Also here we use a brushless hub dc motor instead of a ordinary dc motor due to its several advantages discussed further in this report. Such type of motors can be easily controlled with the help of a convertor which comes along with the motor itself. Based on the same principle hub motor based electric cars and three wheelers can also be made.

Dept. of EEE

4

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

Chapter 2

BLOCK DIAGRAM FOR ELECTRIC BIKE

2.1 BLOCK DIAGRAM Block diagram of a general motor drive is shown below. The electric energy is processed using power electronic devices before giving it to load. A power converter along with a controller serves the processing system. The processed voltage used to drive a DC hub motor. The motor is directly connected to the rim of the wheel, hence there is no chain or belt drive for the transfer of mechanical power. The speed of the motor controlled manually.

Fig.2.1 Block Diagram of an Electric Bike Motor Drive

Dept. of EEE

5

Govt. College of Engg., Kannur

Main Project 2012

2.2

Brushless DC Motor Based Electric Bike

BLOCK DESCRIPTION

2.2.1 ELECTRIC ENERGY INPUT For an electric vehicle, considering the probability, weight, size and cost, batteries are the suitable source. Here we use a 12V battery to provide power to the dc motor. This power from the battery is given to a power convertor which is basically a boost convertor to produce the required 36V for driving the motor.

2.2.2 POWER CONVERTER A power converter modulates flow of power from the source to the motor in such a manner that motor is imparted speed torque characteristics required by the load. Here boost converter which uses MOSFET as the switching device is used as the power converter. The boost converter takes the dc voltage from the battery and steps it up to a higher dc voltage corresponding to the user selected speed. The stepped up voltage is then used to drive the motor. The change in the voltage is determined by the boost converter’s duty cycle which is managed by the controller block. Some description about Boost Converter and Power MOSFET are given below. 2.2.2.1 BOOST CONVERTER A boost converter is part of a subset of DC-DC converters called switch-mode converters. The circuits belonging to this class, including buck, fly back, buck-boost and push-pull converters are very similar. They generally perform the conversion by applying a dc voltage across an inductor or a transformer for a period of time (usually in the 100 kHz to 5 MHz range) which causes current to flow through it and store energy magnetically, then switching this voltage off causing the stored energy to be transferred to the voltage output in a controlled manner. The output voltage is regulated by adjusting the ratio of on-time to offtime. As this subset does not use resistive components to dissipate extra power, the efficiencies are seen in the 80-95% range. This is clearly desirable, as it increases the running time of battery-operated device The basic boost converter consists of only a switch (typically a transistor),a diode ,an inductor, and a capacitor. The specific connections are shown in Fig.2.2.

Dept. of EEE

6

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

Fig.2.2 Boost Converter

Fig.2.3 Typical Wave Forms of Boost converter

,

Dept. of EEE

7

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

2.2.2.2 POWER MOSFET For power electronic with voltage rating below 200V, power MOSFETs are the device of choice. Power MOSFETs have replaced traditional power bipolar junction transistors (BJTs) as the load switch in many applications including automotive system, because of their low on resistance, high switching speed, ease of control and superior safe operating area (SOA), and device ruggedness. In addition MOSFETs are preferred even for 200-600V applications, which may require a high switching speed but only a moderate power level.

Fig2.4 Power MOSFET: (a) symbol, (b) i-v characteristics, (c) idealized characteristics

A power MOSFET is a three terminal device where the gate (i.e., the control terminal) controls the main current flow between the two output terminals; the drain and source. The source terminal is usually common to gate and drain terminals. Power MOSFET output characteristics, that is the drain current Id as a function of drain-to-source voltage Vds with gate-to-source voltage Vgs as a parameter. The MOSFET is in off-state when the gate-source voltage Vgs is less than the threshold Vgs (th), which is typically a few volts.

2.2.2.3 FIRING CIRCUIT Controller unit provide control for the power MOSFET Nature of the controller unit depends upon the power convertor used. Controller generates the firing pulse for the MOSFET. The controller here used is 555 timer configured in astable multi vibrator. The duty ratio of the controller unit is varied by means of a potentiometer. Its working is as follows, at the beginning of a cycle the voltage across the capacitor will be very low. Hence it

Dept. of EEE

8

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

set the output to high and the discharge pin will be open .hence the capacitor will charge from supply through R1 Left part of pot P AND R2 .Hence the voltage across the capacitor will increase gradually. When it attains 0.67 times the Vcc it will reset the output and discharge pin is grounded. Now the capacitor will be discharger through R2 and portion of P. Hence the ON period and OFF period is given by T (on) = 0.693 *(R2+XP+R1)C1

----------------------------

(2.1)

T (off) = 0.693*(R2+(1-X)P)C1

-----------------------------

(2.2)

Fig.2.5 Firing Circuit

Dept. of EEE

9

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

2.2.3 MOTOR The available DC motor for an electric vehicle are DC shunt motor, DC series motor(universal motor ),PMDC, is the mostly used DC motor. But it is costly. DC shunt motor and DC series motor are not available for low ranges universal motors are less efficient and rated for high speed. Another group of motors called DC brushless motor are also available in the market. They are highly efficient and produce less losses and heat as the brushes in them are replaced by electronic sensors. In our project we use a Hub type dc brushless motor to run the cycle. Such motors have the rim of the wheel connected or attached to their rotors directly, so that when the rotor rotates the wheel also rotates. A 200W 36V dc brushless hub motor is fitted to the rear wheel which is controlled by a motor controller. A brushless motor (BLDC) has several advantages over normal dc motors. Such as : Electronic commutation based on Hall position sensors.  Less required maintenance due to absence of brushes.  Speed/Torque- flat, enables operation at all speeds with rated load.  High efficiency, no voltage drop across brushes.  High output power/frame size.  Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better.  Higher speed range - no mechanical limitation imposed by brushes/commutator.  Low electric noise generation. We have used hub type motors because it avoids the use of mechanical drives for transferring the mechanical power to the wheels. Welding and fixing of normal motors on the cycle frame is a difficult procedure and will also make the vehicle bulky. It will also cause more losses and hence draw more power from the battery. Moreover The torque –speed requirement of hub motors are far better than normal motors. A typical brushless DC hub motor has permanent magnets which rotate and a fixed armature, eliminating the problems of connecting current to the moving armature. The stator windings are attached to the axle, and the hub is made to rotate by alternating currents through these

Dept. of EEE

10

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

windings. In a DC hub motor, the magnets are on the axle, and the windings are actually spinning on the inside of the hub. An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the brush/commutator system. The position of the rotor at each rotation is sensed by the sensors and the controller reverses the current through the windings to prevent the reverse negative torque from developing on the rotor.

Fig 2.1 Rotor of a Hub Brushless DC Motor.

Fig 2.2 Stator of a hub Brushless DC Motor.

Dept. of EEE

11

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

2.2.4 CONTROLLER A brushless dc motor usually comes with a controller specially designed to control the motor rotation.Because the controller must direct the rotor rotation, the controller requires some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors, and therefore are often called sensorless controllers. A typical controller contains 3 bi-directional outputs (ie frequency controlled three phase output) , which are controlled by a logic circuit. Simple controllers employ comparators to determine when the output phase should be advanced, while more advanced controllers employ a microcontroller to manage acceleration, control speed and fine-tune efficiency. Controllers that sense rotor position based on back-EMF have extra challenges in initiating motion because no back-EMF is produced when the rotor is stationary. This is usually accomplished by beginning rotation from an arbitrary phase, and then skipping to the correct phase if it is found to be wrong. This can cause the motor to run briefly backwards, adding even more complexity to the startup sequence. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position.

Fig 2.3 A Typical Brushless Motor Controller.

Dept. of EEE

12

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

Chapter 3

CIRCUIT DIAGRAM AND DESGIN

3.1 CIRCUIT DIAGRAM OF BOOST CONVERTER

3.2 DESIGN OF BOOST CONVERTER •

Circuit diagram of boost converter is given

•

Take frequency =1KHz

Dept. of EEE

13

Govt. College of Engg., Kannur

Main Project 2012

•

Brushless DC Motor Based Electric Bike

Output voltage of the converter is given by V0=V/ (1-D) = 12/ (1-D) where D- duty ratio Or D=1-(12/V0)

•

For an output voltage=36v D=1-(12/36) = 0.67

•

The boundary current Ibis given by Ib = D*V/(2LF) Ib should be minimum motor current 1A applying minimum voltage output voltage condition i.e., D= 0.67 As we require here a constant output of 36V ,minimum Vo=36V. L= 0.67*12*/ (2*1*1000) = 4mH

•

Output voltage ripple is then by V= (1/C)*I0*D*T

-------------------------------- (2.8)

Taking voltage ripple as 10%, then C=1.7*0.67*0.001/15=75.93 uMFD

3.3 RATING OF MOSFET Maximum voltage across switch occurs when it is off i.e. When diode is on and is equal to output voltage ie, =36+ripple voltage=50v Maximum current through MOFET is the peak input current given by IL +D*V*T/2/L. (IL is the average inductor current) =I0/ (1-D) + D*V*T/2/L =1.7/ (1-0.67)+0.67*12*0.001/2/4*1000 =9A

3.3.1 RATING OF DIODE Maximum inverse voltage across the diode occurs when MOSFET is on. The maximum PIV of diode is 50V

Dept. of EEE

14

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

Maximum current through the diode is the maximum output current 1.7A

3.4 DESIGN OF TRIGGERING CIRCUIT •

T(on)=0 .0693*(R1+XP+R2)C1

•

T(off)=(R2+(1-X)P)C1

•

F=1000 HZ ;

•

C1(R1+2*R2+P)=1.44*0.01

•

At x=0 ;D=.4 ;Hence T(on)=0.693*(R1+R2)C1= T(on)=0.4*0.001

•

At x=1 ; output voltage should be maximum 150v; D=0.84 Hence

T(0n)+T(off ) =0.001 s

T(off)=0.693*R2*C2=.0.16*0.001 •

T(on) -T(off) =0.693*R1*C1=0.24*0.001

•

R1=3.2K ;C1=uF

Then R2=2.3K (p+5.5k)=14.44k p=10k

3.4.1 DESIGN OF GATE DRIVE The circuit diagram of an optocoupler based gated drive is shown. It will provide isolation between the firing circuit and power circuit.  From the data sheet of optocoupler MCT2E maximum forward voltage is 2v. Also the high level firing pulse is 12v i.e., 12-R1 (0.1+1/R2) = 1 Taking R2=1K Then R1=11/(0.1+.001)=110Ώ.  From the datasheet of MOSFET, gate voltage Vgs = 8V, to turn on the MOSFET. i.e., 12*R4/(R3+R4)=8 Taking R4=3.3K

Dept. of EEE

15

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

Then R3= [12*3300/ 8]-3300= 1650(take 1.6K) Given by T (ON) = 0.693*(R1+XP+R2) C1

---------------------

T (OFF) = 0.693*(R2+(1-X) P) C1

Dept. of EEE

------------------------------

16

(2.9) (2.10)

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

Chapter 4

RESULTS AND OBSERVATIONS

The cycle was tested by conducting the speed test and no load test test on it.

4.1 SPEED TEST The speed of the motor was tested for various positions of the throttle. The throttle was adjusted for 25%, 50%, 75% and 100% of its maximum rotation. The speed was measured by using a tachometer .the following observations were made.]

Dept. of EEE

Throttle Position (%)

Speed ( rpm)

25

765

50

931

75

1400

100

1631

17

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

The maximum speed observed was 1631 rpm and the minimum speed of motor was 765 rpm. The variation of speed with throttle position can be shown graphically as below.

Fig 4.1 Toque v/s Speed Graph.

4.2 NO LOAD TEST The no load test on the cycle was conducted by running the cycle on stand. The voltage across the motor and the current from the convertor were measured for various speeds of motor at no load. The following observations were made. Throttle (%)

I/P Voltage (V)

O/P Voltage ( V)

Current (A)

25

12

36

0.301

50

12

36

0.36

75

12

36

0.46

100

12

36

0.61

Dept. of EEE

18

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

PROTOTYPE

Dept. of EEE

19

Govt. College of Engg., Kannur

Main Project 2012

Brushless DC Motor Based Electric Bike

REFERENCES

[1]

Ned Mohan ,Tore M. Undeland and William P.Robbins, “Power Electronics Converters Application and Design”,Wiley and sons,Third edition,2009.

[2]

Gopal K Dubey, “Fundamentals Of Electric Drives”,Narosa publishing House,2001

[3]

Bimal.K.bose, “Power Electronics, and motor drives; Advances and Trends”, Academic press publication, USA.

[4]

Ali. Emadi, “Hand book of Automotive Power Electronics and Motor Drives” Taylor &Francis Group,LLC..

[5]

K.T. Chau and Zheng Wang, “Overview of Power Electronic Drives for Electric Vehicles ” HAIT Journal of science and Engineering B,vol2, Issues 5-6 pp.737761,2005.

[6]

Padmaraja Yedamale, Brushless DC (BLDC) Motor Fundamentals, Microchip Technology Inc.,2003.

[7]

Concept of Brushless Motors from http://en.wikipedia.org/ wiki/ Brushless_DC_electric_motor

[8]

Details about Control of Brushless Motors from http://www.bldcmotor.net

Dept. of EEE

20

Govt. College of Engg., Kannur