BLDC Sepic Motor Torque Control

8/12/2019 BLDC Sepic Motor Torque Control

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CHAPTER 1

INTRODUCTION

1.1GENERALA brushless DC motor (BLDC) is a synchronous electric motor which is

powered by direct-current electricity (DC) and has an electronically controlled

commutation system; instead of a mechanical commutation system with brushes. In

such motors, current and torque, voltage and rpm are linearly related. The permanent

magnet brushless DC (BLDC) motors are increasingly used in computer, automotive,

industrial and household equipments because of its high power density, compactness,

high efficiency, low maintenance and ease of control. BLDC motor is inherently

electronically controlled and requires six commutation points per cycle.

Conventional BLDC motor drive is generally implemented via six-switch three-

phase inverter and three Hall Effect position sensors that generate proper signals for

current commutation. On the other hand, it is important to lower the manufacturing cost

of the BLDC motor drive for many applications. Cost reduction of BLDC motor drive

is accomplished by topological approach and the control approach. From a topology

point of view, minimum number of switches and eliminating the mechanical sensors

are required for the inverter circuit. In the control approach, using high performance

processors, algorithms are designed and implemented to produce the desired

characteristics. In this paper, a low cost BLDC motor drive, both reducing the number

of power switches and elimination of the position Hall sensors is introduced.

Cost reduction of BLDC motor drive is obtained by reducing the number of

power switches and also eliminating the sensors. On the other hand, prediction of an

electric motor performance is necessary for the evaluation characteristics of motor

designs and motor modeling. Available simulation softwares for electronic circuits or

dynamic systems can be classified into two main categories: (1) circuit simulation

programs such as PSpice (2) equation solver programs such as Matlab. These programs

are not designed specifically for power electronic systems so that the users have to

develop their own models to fulfill their needs.


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1.2 OBJECTIVE

The implementation of sensor less control involves a lot of challenges and

requires the knowledge in the field of electric drives, electric machines, control

systems, power electronics and DSP.

The main intention for doing this project is to thoroughly understand modeling

of the BLDC machine, to thoroughly understand and learn the sensing unit, control unit

and the power processor used in the drive and finally to thoroughly understand the

sensor less control

1.3OUTLINE OF THE PROJECT

Chapter 1 gives the introduction to the project work. Chapter 2 describes the

literature survey. An introduction to the basics of BLDC motor and its various features

is given in Chapter 3. An introduction to the four switch topology of the inverter to

drive the BLDC motor is given in Chapter 4. Chapter 5 gives the simulation works

done in MATLAB Simulink tool. The Hardware implementation is carried out for the

Trapezoidal BLDC motor using the PIC controller for generating the gate pulses to the

inverter is explained in chapter 6. Chapter 7 gives the conclusion of the project.


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CHAPTER 2

LITERATURE SURVEY

PM motor drives have been a topic of interest for the last 20 years. Different

authors have carried out modeling and simulation of such drives.

In 1986, Sebastian T, Slemon G. R. and Rahman M A Modelling of permanentmagnet synchronous motors, Magnetics, IEEE Transactions on, vol. 22 ,[12]

reviewed permanent magnet synchronous motor advancements and presented

equivalent electric circuit models for such motors and compared computed

parameters with measured parameters.

In 1988, Pillay and Krishnan, Modelling of permanent magnet motor drives,Industrial Electronics, IEEE Transactions on, vol. 35 [13], presented PM motor

drives and classified them into two types such as permanent magnet

synchronous motor drives (PMSM) and brushless dc motor drives (BLDC)

drives. The PMSM has a sinusoidal back emf and requires sinusoidal stator

currents to produce constant torque while the BLDC has a trapezoidal back emf

and requires rectangular stator currents to produce constant torque. Because of

the no sinusoidal variation of the mutual inductances between the stator and

rotor in the BLDC, it is also shown in this paper that no particular advantage

exists in transforming the abc equations of the BLDC to the d,q frame.

T.Lowand, Mohammed A Jabbar and [14] describe design considerations ofpermanent-magnet motors intended for brushless operation. PM motors

operated as brushless dc (BLDC) drives have received wide attention as their

performance can be superior to conventional brushed dc motors and ac motors.

A BLDC drive system is described, and the performance of a neodymium-iron-

boron based magnet excited PM motor with an imbricate rotor in a BLDC drive

is presented.

In 2008 , Halvaei Niasar, H. Moghbelli and A. Vahedi ,in paper titled A NovelSensorless Control Method for Four-Switch, Brushless DC Motor Drive without

Phase Shifter, IEEE Transactions on Power Electronics, Vol. 23, No. 6, [10]

presents the analysis, design, and implementation of a cost-effective sensorless

control technique for a low cost four-switch, three-phase inverter brushless dcmotor drive. The proposed sensorless technique is based on the detection of


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zero crossing points (ZCPs) of three voltage functions that are derived from the

filtered terminal voltages and. Six commutation instants are provided that

coincide to ZCPs of voltage functions. Hence, there is no need for any 30 or 90

phase delay that is prevalent in conventional sensor less methods. Two low-pass

filters are used for elimination of high-frequency noises and calculation of

average terminal voltages. Also, a direct phase current control method is used to

control the phase currents in the four-switch inverter. The performance of the

developed sensor less technique is demonstrated by simulation.

In 2008, in the paper titled, A Low-Cost Sensor less Control for Reduced-Parts, Brushless DC Motor Drives, IEEE Transactions on Industry

Applications, Halvaei Niasar, H. Moghbelli and A. Vahedi, explained the

design and implementation of a reduced parts BLDC motor drive. Part reducing

is achieved by elimination of three Hall Effect position sensors and reducing the

number of power switches to four switches. For current commutation, a low

cost sensor less control based on line voltages is developed. Two second-order

Butterworth low-pass filters with little phase delay are designed to eliminate of

high frequency PWM and calculation of average terminal voltages. Proposed

sensor less control doesn't need to any 30oor 90ophase shift that is prevalent in

other sensor less methods. Moreover, to make the rectangular phase current

waveforms, direct phase current control is used in which the currents of two

phases A and B are controlled independently. The performance of the

developed algorithms is verified via simulation and implementation. It is shown

that the main source of the estimation error is drop voltage on the stator

impendence in which at low speeds and for heavy loads it increases.

Mingyao Lin, Weigang Gu, Wei Zhang,Qiang Li, Design of Position DetectionCircuit for Sensor less Brushless DC Motor described a back electromotive-

force(EMF) detection circuit for position sensor less brushless DC

motor(BLDCM) drive systems is presented, in which the second-order

Butterworth low-pass filter is used. The detecting circuit is composed of a

voltage divider and an active low-pass filter. First, voltage dividing circuit is

designed, and two design principles should be followed. Second, the selections

of the structure, order and the parameters of the active filter are investigated indetail. Last, the theoretical analysis, simulation and experiment are all done


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with the proposed position detecting circuit. The investigation results show that

designed circuit can operate well in BLDCM drive is to design a position

detecting circuit

In the paper titled A Novel sensor less control method for Four Switch,Brushless DC motor Drive without any 300phase shifterHalvaei Niasar, H.

Moghbelli and A. Vahedi, introduced a novel and low cost brushless DC motor

drive.The proposed drive is a four switch inverter for three phase BLDC

motor,without any mechanical hall sensors.The proposed concept is based on

the fact that in Four switch topology , the zero crossing points of the stator

terminal voltages Vao,-Vboand Vao-Vbocoincide to six commutation instants and

can be used to commutate the current in phases A.B and C respectively.These

stator voltages are actually line to line voltages and are measured respect to

point o(middle point of DC bus) easily.Hence it is not necessary to delay 300

after zero crossing points of the measured voltages which are prevalent in the

other conventional six-switch and four switch BLDC motor drives.This

approach made it possible to detect rotor position in relatively wide range of

speed variation.Also Direct Phase Current Control(DCC) method is used to

control the phase current in four switch inverter.


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CHAPTER 3

BRUSHLESS DC MOTOR

3.1 INTRODUCTION

When a trapezoidal back emf PMSM running at self controlled mode, it behaves

similar like a DC motor, but without brushes and commutator, So this kind of motors

are known as BLDC motor. In such motors, current and torque, voltage and rpm are

linearly related. BLDC motors are a type of synchronous motor. This means the

magnetic field generated by the stator and the magnetic fields generated by the rotor

rotate at the same frequency. BLDC motors do not experience the slip that is

normally seen in induction motors.

3.2 BLDC MOTOR CONSTRUCTION

BLDC motors come in single-phase, 2-phase and 3-phase configurations.

Corresponding to its type, the stator has the same number of windings. Out of these, 3-

phase motors are the most popular and widely used. The construction details of a

BLDC motor is shown in Fig.3.1

Fig.3.1- Constructional details of a BLDC Motor

3.2.1 Stator

The stator of a BLDC motor consists of stacked steel laminations with windings

placed in the slots that are axially cut along the inner periphery as shown in Fig.3.2.

Traditionally, the stator resembles that of an induction motor; however, the windings

are distributed in a different manner. Most BLDC motors have three stator windings


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connected in star fashion. Each of these windings are constructed with numerous coils

interconnected to form a winding. One or more coils are placed in the slots and they are

interconnected to make a winding. Each of these windings are distributed over the

stator periphery to form an even numbers of poles.

Fig.3.2- Inner details of the stator of a BLDC Motor

There are two types of stator windings: Trapezoidal and Sinusoidal. This

differentiation is made on the basis of the interconnection of coils in the stator windings

to give different types of back Electromotive Force (EMF). A trapezoidal back emf is

shown in Fig.3.3 and Fig.3.4 shows a sinusoidal back emf.

Fig.3.3- Trapezoidal back emf


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Fig.3.4- Sinusoidal back emf

3.2.2 Rotor

The rotor is made of permanent magnet and can vary from two to eight pole

pairs with alternate North (N) and South (S) poles. Based on the required magnetic

field density in the rotor, the proper magnetic material is chosen to make the rotor.

Ferrite magnets are traditionally used to make permanent magnets. As the technology

advances, rare earth alloy magnets are gaining popularity. The ferrite magnets are less

expensive but they have the disadvantage of low flux density for a given volume. In

contrast, the alloy material has high magnetic density per volume and enables the rotor

to compress further for the same torque. Also, these alloy magnets improve the size-to-

weight ratio and give higher torque for the same size motor using ferrite magnets.

Neodymium (Nd), Samarium Cobalt (SmCo) and the alloy of Neodymium, Ferrite and

Boron (NdFeB) are some examples of rare earth alloy magnets. Continuous research is

going on to improve the flux density to compress the rotor further. Fig 3.5 shows cross

sections of different arrangements of magnets in a rotor.


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Fig.3.5- Rotor magnet cross-sections

3.2.3 Hall Sensors

These kinds of devices are based on Hall-effect theory, which states that if an

electric current- carrying conductor is kept in a magnetic field, the magnetic field exerts

a transverse force on the moving charge carriers that tends to push them to one side of

the conductor. A build-up of charge at the sides of the conductors will balance this

magnetic influence producing a measurable voltage between the two sides of the

conductor. The presence of this measurable transverse voltage is called the Hall-effect

because it was discovered by Edwin Hall in 1879. Unlike a brushed DC motor, the

commutation of a BLDC motor is controlled electronically. To rotate the BLDC motor,

the stator windings should be energized in a sequence. It is important to know the rotor

position in order to understand which winding will be energized following the

energizing sequence. Rotor position is sensed using Hall Effect sensors embedded into

the stator.

Most BLDC motors have three Hall sensors embedded into the stator on the

non-driving end of the motor. Whenever the rotor magnetic poles pass near the Hall

sensors, they give a high or low signal, indicating whether the N or S pole is passing

near the sensors. Based on the combination of these three Hall sensor signals, the exact

sequence of commutation can be determined. A transverse section of the BLDC motor

is given in Fig 3.6

The back-EMF is the voltage induced in a winding by the movement of the

magnet in front of this winding. It is independent of the energy supplied to the motor.

The back-EMF is directly proportional to the rotation speed, the rotor flux and the


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number of turns in the corresponding winding where we want to calculate the back-

EMF.

Fig.3.6-Transverse section of BLDC Motor

In one turn of the winding, the back-EMF equation is:

E = -d/dt (3.1)

Where is the rotor flux

In a complete winding, the back-EMF equation is:

E = nN (3.2)

Where E is in V, N is the speed (rotation per second), is in Wb

3.3 THEORY OF OPERATION OF BLDC MOTOR

Each commutation sequence has one of the windings energized to positive

power (current enters into the winding), the second winding is negative (current exitsthe winding) and the third is in a non-energized condition. Torque is produced because

of the interaction between the magnetic field generated by the stator coils and the

permanent magnets. Ideally, the peak torque occurs when these two fields are at 90 to

each other and falls off as the fields move together. In order to keep the motor running,

the magnetic field produced by the windings should shift position, as the rotor moves to

catch up with the stator field. A typical speed-torque characteristics of the BLDC motor

is shown in Fig.3.7

3.3.1 Commutation

The Commutation is based on the information of the actual rotor position.

Rotor position encoder is necessary. Different types of commutation used are:-

HALL-sensors--standard, block commutation

Encoder (opt. / magn.)--High-End, positioning, sine commutation

Back-EMF (no sensor)--rotational drives

3.3.2 Torque

The torque equation for the motor is:


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T = KI (3.3)

Where I is the current in the motor, is the rotor flux and K is a constant giving the

direct proportionality of the torque to the current and the flux.

The power of the motor is then:

Pm = T (3.4)

Where is the angular speed of the rotor in radians/sec

Fig.3.7- Speed-torque characteristics

3.4 DIFFERENCES BETWEEN BLDC AND PMSYNCHRONOUS MOTORS

The characteristic of a brushless DC motor is dependent on its internal

construction. Brushless DC motors are a variant of permanent magnet DC motors

(PMDCM). PM DC motors are simply synchronous motors in which the rotor field is

driven with a constant current. By driving the rotor winding with a constant current, a

constant magnetic flux is developed within the motor. This can also be achieved by

replacing the rotor winding with a permanent magnet. Such motors are called brushless

DC motors. Brushless DC motors dont require slip rings, so motor maintenance is

reduced and reliability is increased. Slip rings create dust as they wear. The dust needs

to be periodically cleaned from the motor housing. When slip rings wear past a certain

length, they need to be replaced. The difference between a synchronous motor and a

BLDC motor is evident from the internal diagrams of these machines shown in

Fig.3.8and Fig.3.9


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Fig.3.8- Internal diagram of a 3 phase Synchronous motor

Fig.3.9- Internal diagram of a 3 phase brushless DC motor

The stator windings of BLDC motors contain a multi-phase winding. Small

power motors are usually 2-phase, while medium and large power motors are 3-phase.

Some washing machine motors have 4 or 5 phase windings to reduce torque ripple. The

windings may be either wye connected or delta connected. Most motors have

ungrounded wye connections. Brushless DC motors require the motor controller

perform the commutation function. Commutation is a function of rotor position. The

appropriate stator windings of the motor need to be energized when the rotor pole lines

up with winding. It is possible to drive a BLDC motor by simply forcing the

commutation intervals to a preset value. The problem with this type of control is that

the applied phase voltage may not be proportional to the speed forced by the controller

commutation sequence. The generated stator flux interacts with the rotor fluxes, which

is generated by a rotor magnet, defines the torque and thus speed of the motor. The

voltage strokes must be properly applied to the two phases of the three-phase winding


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system so that the angle between the stator flux and the rotor flux is kept close to 90 to

get the maximum generated torque. Due to this fact, the motor requires electronic

control for proper operation. Most BLDC motors have internal sensors to provide

position information. The most common type of sensor is the Hall Effect sensors. When

the rotor pole lines up with a particular phase, the sensor output goes high and when the

rotor has passed, the output goes low. 3-phase motors typically have three hall sensors.

The sensors are placed in the centre of each phase winding. They may be spaced at 60

or 120 electrical degree intervals. 120-degree spacing is common. Encoders may also

be used. They are used on servomotors. The encoders are usually mounted on the end

of a gearbox. It is also possible to drive a BLDC motor without sensors.

3.4.1 Advantages of BLDC motor High Speed OperationA BLDC motor can operate at speeds above 10,000 rpm

under loaded and unloaded conditions.

Response & Quick AccelerationInner rotor Brushless DC motors have lowrotor inertia, allowing them to accelerate, decelerate, and reverse direction

quickly.

High Power Density BLDC motors have the highest running torque percubic inch of any DC motor.

High Reliability - BLDC motors do not have brushes, meaning they are morereliable and have life expectancies of over 10,000 hours.

3.4.2 Disadvantages of BLDC motor Requires Complex Drive Circuitry Requires additional Sensors Higher Cost Some designs require manual labor (Hand wound Stator Coils)

3.4.3 Applications of BLDC motorApplications that are best suited for BLDC technology can take full

advantage of its unique operating characteristics - accurately control connected

loads and variable-speed drive capability. Motor speed, applied voltage, and

torque share a linear relationship. Widespread use and acceptance of brushless


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DC motors in residential products (where motors are small and efficiency

advantage is most significant) has prompted greater competition in the market.

In home appliances, washers, dryers and compressors are good examples. In

automotive, fuel pump control, electronic steering control, engine control and

electric vehicle control are good examples of these. In aerospace, there are a

number of applications, like centrifuges, pumps, robotic arm controls,

gyroscope controls and so on. With market interest comes further research and

development, ultimately reducing first cost and increasing application flexibility.

Fig.3.10 - Brushless DC motor application

Fig.3.10 shows a BLDC motor powering a micro remote-controlled airplane.

The motor is connected to a micro processor controlled BLDC controller. This 5gm

motor is approximately 11 watts and produces about two times more thrust than the

weight of the plane.


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CHAPTER 4

THE FOUR-SWITCH INVERTER TOPOLOGY

4.1 MODELING OF BLDC MOTOR

The modeling of BLDC motor drive system is based on the following

assumptions:

1. All the stator phase windings have equal resistance per phase and constant selfand mutual inductances.

2. Power semiconductor devices are ideal.3. Iron losses are negligible.4. The motor is unsaturated.

Based on the above assumptions, the three phase input voltages can be written as:

Val= Ria+Ladt

dia+ea (4.1)

Vb= Rib+Lbdt

dib+eb (4.2)

Vc= Ric+Lcdt

dic+ec (4.3)

The Electromagnetic torque is expressed as

Te=

1(eaia+ebib+ecic) (4.4)

The electromagnetic torque can also be expressed as

Te=

EI2 (4.5)

The electromagnetic torque can be expressed in terms of mechanical parameters as

Te=TL+J dt

d

+B

(4.6)


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where va, vb, and vcare the stator phase winding voltages of phase a, b and c

respectively, ea, eb, and ecare the back-emfs of phase a, b and c respectively, i a, ib, and

ic are the phase currents of phase a, b and c respectively, TL is the load torque, J is

inertia, is angular speed, B is viscous damping coefficient. The voltage equation can

be written in matrix form as:

c

b

a

V

V

V

=

R

R

R

00

00

00

c

b

a

i

i

i

+

L

ML

ML

00

00

00

dt

d

c

b

a

i

i

i

+

c

b

a

e

e

e

(4.7)

BLDC motor needs quasi square current waveforms, which are synchronized

with the back EMF to generate constant output torque. Also, at every mode only twophases are conducting and another phase is inactive. However, in the four-switch

inverter, the generation of 120 conducting current profiles is inherently difficult.

Hence, the direct phase current (DPC) control method is used . Therefore, the currents

of phase A and B in modes 2 and 5 are controlled independently and the current

profiles are the same as the currents of a conventional six-switch inverter BLDC motor

drive. The current and back-emf profiles of a BLDC motor is shown in Fig.4.1

Fig.4.1: Profile of current and back emf of a BLDC motor


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4.2 SIX SWITCH INVERTER TOPOLOGY

The basic six switch inverter topology is shown in figure. It comprises six

power switches together with six associated reactive feedback diodes. Each of the three

inverter legs operates at a relative time displacement (phase)

Fig.4.2-Conventional six-switch three phase BLDC motor drive system

180 conductionIn 1800 conduction each switch conducts for 180, such that no two

semiconductors switches across the voltage rail conduct simultaneously. Six patterns

exist for one output cycle and the rate of sequencing these patterns specifies the bridge

output frequency. The conducting switches during the six distinct intervals are shown.

The three output voltage waveforms can be derived by analyzing a resistive star load

and considering each of the six connection patterns. Effectively the resistors

representing the three-phase load are sequentially cycled anticlockwise one at a time,being alternately connected to each supply rail. The output voltage is independent of

the load, as it is for all voltage source inverters.


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Fig.4.3-180 Degree conduction wave forms

120 conductionThe basic three-phase inverter bridge in figure can be controlled with each switch

conducting for 120. As a result, at any instant only two switches (one upper and one

non-complementary lower) conduct and the resultant quasi-square output voltage

waveforms are shown in figure. A 60 (), dead time exists between two seriesswitches conducting, thereby providing a safety margin against simultaneous

conduction of the two series devices (for example T1 and T4) across the dc supply rail.

This safety margin is obtained at the expense of a lower semiconductor device

utilization and rms output voltage than with 180 device conduction.


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Fig.4.4- 120 Degree conduction wave forms

4.3 FOUR-SWITCH THREE-PHASE BLDC MOTOR DRIVE

According to the working of BLDCM, at a time only two phases are

conducing.So the costs and losses can be reduced by minimizing the number of

switches. In this project inverter with a three phase four switch topology is used. In

Figure the four switch topology is shown.

Fig.4.5- The four switch topology


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A BLDC motor needs quasi-square current waveforms, which are synchronized

with the back-EMF to generate constant output torque and have 120 degree conduction

and 60 degree non-conducting regions. Also, at every instant only two phases are

conducting and the other phase is inactive. However, as mentioned earlier, in the four-

switch converter, the generation of 120 degree conducting current profiles is inherently

difficult. This can be explained as follows: In the four-switch configuration, there are

four switching status as shown in Fig.4.6, such as (0, 0), (0, 1), (1, 0), and (1, 1), in

which the motor load is replaced by a resistive load and the switches are replaced by

simple ideal switches. 0 means that the lower switch is turned on and 1 the upper

switch is turned on.

The two switches never turn on and off simultaneously. In the case of the six-switch

converter, switching status (0, 0) and (1, 1) are regarded as zero-vectors, which cannot

supply the dc-link voltage to the load, so that current cannot flow through the load.

However, in the four-switch converter, one phase of the motor is always connected to

the midpoint of the dc-link capacitors, so that current is flowing even at the zero-

vectors, as shown in Fig.4.6(a) and (b). Moreover, in the case of (0, 1) and (1, 0), the

phase which is connected to the midpoint of dc-link capacitors is uncontrolled and only

the resultant current of the other two phases flow through this phase. If the load is

ideally symmetric, there is no current in the (0, 1) and (1, 0) vectors. Therefore, in order

to use the four-switch converter topology for the BLDC motor drive, a new control

scheme should be developed.


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Fig.4.6- Voltage vectors of four-switch converter: (a) (0,0) vector (b) (1,1)

vector (c) (1,0) vector (d) (0,1) vector

The two-phase currents need to be directly controlled using the hysteresis

current control method by four switches. Hence, it is called the direct current controlled

PWM scheme.The different modes of operation of four-switch BLDC motor with

PWM strategy is shown in Fig.4.7


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TABLE 4.1

DETAILED CURRENT EQUATIONS ACCORDING TO THE OPERATING

MODES

Mode I (00300) Ib+Ic=0 and Ia=0

Mode II (300900) Ia+Ib=0 and Ic=0

Mode III (9001500) Ia+Ic=0 and Ib=0

Mode IV (15002100) Ib+Ic=0 and Ia=0

Mode V (21002700) Ia+Ib=0 and Ic=0

Mode VI (27003300) Ia+Ic=0 and Ib=0

Sequence of switching of the four-switch converter is shown in Table 4.2.

TABLE 4.2

SWITCHING SEQUENCES OF THE FOUR-SWITCH CONVERTER

Modes Active phases Silent phases Switching devices

Mode I Phases B and C Phase A S4

Mode II Phases A and B Phase C S1 and S4

Mode III Phases A and C Phase B S1

Mode IV Phases B and C Phase A S3

Mode V Phases A and B Phase C S2 and S3

Mode VI Phases A and C Phase B S2

Table 4.3 shows the voltage and current equations in different modes of

operation for certain conditions


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TABLE 4.3

VOLTAGE AND CURRENT EQUATIONS

4.4 COMPARISON OF THE CONVENTIONAL SIX-SWITCH TOPOLOGY &

THE PROPOSED FOUR-SWITCH TOPOLOGY

The cost reduction of variable-speed drives is accomplished by two approaches.

One is the topological approach and the other is the control approach. From a topology

point of view, minimum number of switches is required for the converter circuit. In the

control approach, algorithms are designed and implemented in conjunction with a

reduced component converter to produce the desired speedtorque characteristics. Until

now, the reduced part converters have been applied mainly to ac induction motor drives

However, these days, the BLDC motor is attracting much interest, due to its high

efficiency, high power factor, high torque, simple control, and lower maintenance.

Thus, the possibility of the reduced part converter for BLDC motor drives with

advanced control techniques is getting investigated. Consequently, it is found that one

switch leg (two switches) in the conventional six-switch converter, as shown in Fig.4.2,

is redundant to drive a three-phase BLDC motor. It results in the possibility of the four-

switch configuration instead of the six switches, as shown in Fig.4.5. Compared with

the four-switch converter for the induction motor [1], it is identical for the topology

point of view. However, in the four-switch converter, the generation of 120 conducting

current profiles is inherently difficult due to its limited voltage vectors. This problem is

well known as asymmetric voltage pwm. It means that conventional pwm schemes


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for the four-switch induction motor drive cannot be directly used for the BLDC motor

drive. Therefore, in order to use the four-switch converter topology for the three-phase

BLDC motor drive, a new control scheme should be developed. The solutions can be

obtained from a modification of the conventional voltage controlled pwm strategies,

such as the space vector pwm. The current control block becomes much more

complicated. Moreover, in order to handle the complicated calculations in one sampling

period, a high-speed digital processor is also necessary, which increases the

manufacturing cost. Therefore, for the low cost BLDC motor applications, voltage

vector pwm schemes cannot be regarded as a good solution for cost effectiveness.

4.5 METHODOLOGY

In this work, a low cost BLDC motor drive with reduced parts that is by

reducing the number of switches from six to four is to be developed. The

implementation of a low cost, reduced parts BLDC motor is desired with high system

reliability. Sensorless algorithm via back-emf method will be used. Also the developed

sensorless algorithm should eliminate the motor neutral voltage, the fixed phase shift

circuit and low starting speed.

4.6 REVIEWS ON SENSORLESS METHODS

Manufacturing cost of a BLDC motor drive can be reduced by elimination of

position sensors and by developing feasible sensorless methods. Further, sensorless

control is the only choice for some applications where these sensors cannot function

reliably, especially in hostile environments. If low cost is a primary concern and low

speed motor operation is not a requirement and the motor load is not expected to

change rapidly, then sensor less control may be the better choice.

The sensor less technique generally used can be grouped into 5categories:

using measured currents, voltages, fundamental machine equations and algebraicmanipulations

using observers using back-emf methods sensorless starting techniques novel techniques not falling into the previous four categories.


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4.6.1 Methods Using Measurables and Math

This method is based on

1. The voltages and currents to calculate flux linkages2. The difference between the prediction of a voltage or current of a model and the

actual value

3. The machine equations, measurables, known as machine parameters andalgebraic

manipulations to calculate position and speed.

Using voltages and currentsThe voltage equation of any machine can be written as:

V=RI+ dt

d

(4.8)

Where,

Vis the voltage vector,

I is the current vector,

R is the resistance matrix, and

is the flux linkage vector.

This equation is then manipulated to obtain,

(V-RI) d (4.9)

Knowing the initial position, machine parameters, and the relationship of flux

linkage to rotor position, the rotor position is estimated. Determining the rate of change

of the flux linkage from the integration results, the speed is determined. A variation

includes using the previous position data and polynomial fitting, extrapolating to obtain

the next step position prediction. An advantage of the flux-calculating method is that

line-line voltages may be used in the calculations.

Using the machine modelIn this method, a d-q model of the machine, the actual dq transformed currents and

voltages, and those on a hypothetical axis offset from the dq axis by a small angle ,

the output voltages of the model, on the hypothetical axis, and those on the actual dq

axis are compared.

Using machine parameters and equationsThis method uses machine parameters and equations, measurables, and algebra,

reference frame theory and transformations to calculate position and speed. Initially themeasured voltages and currents are transformed to rotor and stator d-q reference frame


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variables. Relation between stator and rotor reference frames, denoted with superscripts

s and r, respectively, are

vrq = vsqcosr - v

sd sinr (4.10)

vrd = vsqsinr + v

sd cosr (4.11)

irq = isqcosr - i

sd sinr (4.12)

ird = isqsinr - i

sd cosr (4.13)

By substituting the rotor and stator reference frame equations in terms of stator

variables, the rotor angle and so the rotor speed can be calculated.

4.6.2 Methods using observers

An observer provides a mathematical model of the brushless DC motor, which

takes measured inputs of the actual system and produces estimated outputs. The error

between the estimated outputs and measured quantities is fed back into the system

model to correct the estimated values, such as the rotor position and speed, as would be

the actually measured variables in a closed-loop system control. Although most of the

observer-based methods are used for PMAC motors, which have sinusoidal back-EMF

and need continuous rotor position, for the BLDC motors, which require just six

position points for one electrical cycle, the continuous position information from the

observer is not necessary typically. But, for special purposes, such as flux weakening

operation based on advanced angle control, the positions between commutation points

are required.

Sliding-Mode Observer (SMO)For controlling BLDC motor, it is necessary to know an absolute position of the

rotor, so an absolute encoder or resolver can be used for sensing the rotor position. But,

these position sensors are expensive and require a special arrangement for mounting.

Also, the state equation of BLDC motor is nonlinear, so it is difficult for the linearcontrol theory to be applied and the stability of position and velocity estimation have

not been clarified. To improve the mechanical robustness and to reduce the cost of the

drive system, several estimation techniques eliminating the encoder or resolver can be

applied. Some relevant methods have been developed using the sliding-mode observer,

which are briefly explained next.

In the Direct Torque Control method (DTC), the state equation of the BLDC

motor is utilized to achieve a relationship between the angle of the stator current vector

and the back-EMF vector angle, obtaining minimum error angle estimation and


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reducing the torque ripple in com-mutation regions. In this control method, the proper

voltage vector is selected from a look-up table using the rotor flux vector position and

torque error, which is led to the predefined hysteresis . However, DTC methods based

on hysteresis controllers have some serious drawbacks such as a high amount of torque

and flux pulsations and variable switching frequency of the inverter. Also, in the direct

torque control of brushless DC motor, the stator flux linkage observation is needed, and

the accuracy of the observed stator flux linkage is affected by the variation of stator

resistance, electric interference, magnetic interference, measurement error and so on.

These drawbacks are solved with the DTC Space Vector Modulation (DTC-SVM)

scheme, which uses a constant switching frequency.

However, the DTC-SVM scheme needs a transformation from stationary

reference frame to stator flux field orientation frame and vice versa, therefore it has a

high computation time and could be an erroneous cumulative scheme . Also, with the

introduction of DTC technique and the advances of speed sensor less systems, the

interest in stator resistance adaptation came to scene for an optimal performance of

speed sensor less systems in low speed region.

Recently, and commented above, low speed operation with robustness against

parameter variations remains an area of research for sensor less systems, taking into

account that an accurate value of stator resistance is of utmost importance for its correct

operation in low speed region. As in the upper speed range, the resistive voltage drop is

small as compared with the stator voltage; hence the stator flux and speed estimation

can be made with good accuracy. At low speeds the stator frequency is also low, but

stators voltage reduces almost in direct proportion and the resistive voltage drop

maintains its order of magnitude and becomes significant. This greatly influences the

estimation accuracy of the stator flux and hence the speed estimation. An estimationalgorithm based on SMO in conjunction with Popovs hyper-stability theory can be

used to calculate the speed and stator resistance independently, which can guarantee the

global stability and the convergence of the estimated parameters.

The SMO is widely studied in the field of a motion control, and it can be

applied to nonlinear systems, such as BLDC motors. This technique applied to control

systems encounters restrictions in practice, due to the high voltage values of the power

supply needed and severe stress given to the static power converters. On the other hand,

the sliding mode has been shown very efficient in the state estimation due to its salient


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features, i.e., robustness to parameter variations and disturbances including the

measurement noise. The use of sliding mode in state observer does not present physical

restrictions relative to the convergence condition (the estimation error moves toward

zero) and does not subject the system to undesirable chattering. These problems can be

alleviated using a binary observer with continuous inertial Coordinate-Operator

Feedback.

Extended Kalman Filter (EKF)The extended Kalman filter algorithm is an optimal recursive estimation

algorithm for nonlinear systems. It processes all available measurements regardless of

their precision, to provide a quick and accurate estimate of the variables of interest, and

also achieves a rapid convergence. This is done using the following factors: the

knowledge of the system dynamics, statistical description of the system errors (noises,

disturbances, etc.), and information about the initial conditions of the variables of

interest. The algorithm is computationally intensive, thus an efficient formulation is

needed rather than a straightforward implementation. Moreover, for a practical

application of the filter in real time, different aspects of implementation have to be

addressed, such as the computational requirements (processing time per filter cycle,

required memory storage, etc.) and the computer constraints (cycle execution time,

instruction set, arithmetic used, etc.).

This method can be used to estimate the rotor position and speed. Motor state

variables are estimated by means of measurements of stator line voltages and currents,

and applying EKF next. During this process, voltage and current measuring signals are

not filtered, and rotor position and speed can be estimated with sufficient accuracy in

both steady state and dynamic operations. Unlike the deterministic base of other

studies, the model uncertainties and nonlinearities in motors are well suited to thestochastic nature of EKFs, as well as the persistency of excitation due to the system and

measurement noises. This is the reason why the EKF has found wide application in

speed-sensorless control, in spite of its computational complexity. However, with the

developments in high performance processor technology, the computational burden and

speed of EKF has ceased to be a problem.

The block diagram of the system for speed and rotor position estimation of a

BLDC motor is shown in Figure 4.8. The system can be functionally divided in two

basic parts: the speed control system and the estimation system. The first one consists


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of a power circuit (DC supply, inverter and motor) and control circuits, which perform

three functions: current commutation, current control and speed control. The measured

speed (k) and phase currents (ik) as well as the estimated rotor position (^k/k) are

used as feedback signals. The main blocks of the estimation algorithm are the EKF and

the block for calculating average motor line voltages during sampling time. The

average line voltages vector, defined on the basis of average line voltages in the k-

sampling time (uk), is calculated at the beginning of the sampling time by means of

terminal voltages to neutral-point vector (uNk), the inverter transistors duty cycle (k),

the inverter DC voltage (U0), the estimated speed (^k/k), the rotor position (^k/k),

and measured currents vector (ik).

Figure 4.8- System configuration for speed and rotor position estimation of

a BLDCM

Among recent speed-sensorless studies using EKF based estimation, the

simultaneous estimation of the rotor angular velocity, the rotor flux and the stator

resistances, via a Kalman filter in combination with the model reference adaptive

system (MRAS), have been performed, but are sensitive to variations in the stator and

rotor resistances. Some innovative techniques have been currently developed, such as

the Bi Input-EKF (BI-EKF). This method utilizes a single EKF algorithm with the

consecutive execution of two different inputs, which are calculated from the two

extended models based on the rotor and stator resistance estimation, respectively. Thesetwo different inputs are used for the rotor flux based speed control both in the transient


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and steady-state over a wide speed range. Also, the load torque is estimated, including

viscous friction term, rotor angular velocity, rotor flux, and stator current components

without the need for signal injection.

Model Reference Adaptive System (MRAS)In some cases, the stator and rotor resistance estimation is not applicable when the

speed-sensorless control system is in transient state, such as operation under largely

varying load torque and/or changes in the speed command. In other cases, the rotor

time constant via high frequency signal injection, the stator resistance and the rotor

angular velocity can be estimated by using MRAS. However, the stator resistance

estimation is turned on for short time intervals when the rotor angular velocity

estimation has reached its steady-state; that is, both the stator resistance and rotor

angular velocity estimations are performed interchangeably.

The model reference adaptive system, developed using Popovs stability criterion,

is one of many promising techniques employed in adaptive control for estimating the

speed and stator resistance. Among various types of adaptive system configuration,

MRAS is important since it leads to a relatively easy-to-implement system with a fast

adaptation for a wide range of applications. The basic principle is illustrated in Figure

4.8, called parallel MRAS. The dynamic models are represented by the block

Reference Model, which is the actual system (for example, the motor, containing all

unknown parameters, i.e., motor speed, stator and rotor resistances) and the block

Adjustable Model, which has the same structure as the reference one (i.e., motor, but

with the adjustable or estimated parameters, instead of the unknown ones). An error

vector is derived using the difference between the outputs of two dynamic models and

is driven to zero through an adaptation law. As a result, the estimated parameter vector

will converge to its true value X. One of the most noted advantages of this type ofadaptive system is its high speed of adaptation. This is due to the fact that a

measurement of the difference between the outputs of the reference model and

adjustable model is obtained directly by comparison of the states (or outputs) of the

reference model with those of the adjustable model system. It is remarkable that the

error signal may be formulated with flux (F-MRAS), back-EMF (E-MRAS), reactive

power (Q-MRAS) and active power (P-MRAS).


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Figure 4.9- Basic configuration of a MRAS

For instance, a MRAS with instantaneous reactive power can be used for speed

estimation of sensorless vector controlled motor drive. This MRAS converts a vector

quantity (i.e., current vector) into a scalar quantity using the concept of reactive power,

and the reference model utilizes measuredcurrent vector. Also, the adjustable model

uses the estimated stator current vector, and the current, estimated through the machine

state equations, is configured in terms of reactive power. An active power MRAS based

scheme can also be used for rotor resistance identification, whose estimation is

effective in wider range of variations and could be applied in real time field-oriented

control (FOC) .

Adaptive ObserversThe interest in stator resistance adaptation came to scene much recently, with the

advances of speed sensorless systems and with the introduction of DTC technique. An

accurate value of the stator resistance is of crucial importance for correct operation of a

sensorless drive in the low speed region, since any mismatch between the actual value

and the set value used within the model of speed estimation may lead not only to a

substantial speed estimation error but also to instability as well. Therefore, to develop

online stator resistance identification schemes are of utmost importance for accurate

speed estimation in the low speed region. These estimators often use an adaptive

mechanism to update the value of stator resistance. Some of the most relevant are

MRAS, explained previously, and adaptive full-order flux observers (AFFO).

Adaptive Full-order Flux Observer (AFFO)The AFFO scheme has been developed using Lyapunovs stability criterion and

allows estimating the rotor speed and stator resistance simultaneously. Using this


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observer, the estimated quantities converge to their real values if the persistency of

excitation condition is also satisfied. Correct estimation of rotor flux space vector and

rotor speed is therefore possible through this observer according to the stator and rotor

resistances online adaptation. Jointly with MRAS, AFFO is not computationally

intensive, but with a non-zero gain matrix may become unstable. In such methods, the

stator resistance adaptation mechanism is determined with the difference between the

measured and observed stator currents. With a maximum torque per ampere (MTPA)

strategy, based on slip frequency (inverse of the rotor time constant in the rotor flux

oriented reference frame) adjustment, the stator current amplitude can be minimized for

each value of motor speed.

Apart from the variation of the stator resistance with temperature, other parameters

in the AFFO will change during operation as well, such as the rotor resistance due to

temperature changes, which will have an important influence on the speed accuracy of

the adaptive observer. The stator and rotor self-inductance and magnetizing inductance

vary due to magnetic saturation, being it possible to use a nonlinear magnetic model. In

steady state, it is known that a misestimating of the rotor resistance provides correct

estimations of the stator and rotor flux, but results in a misestimating of the speed.

4.6.3 Methods using back emf sensing

Terminal voltage sensingIn field-oriented operation of the BLDC, phase back emf is aligned with phase

current. Switching instants of the converter can be obtained by knowing the zero-

crossing of the back-emf and a speed-dependent period of time delay. Monitoring the

phase back-emf when the particular phase current is zero (the silent phase), the zero

crossing is detected. Low pass filters are used to eliminate higher harmonics in the

terminal voltages. With this method, a reduced speed operating range is normally used,typically around 1000-6000 rpm.

Third harmonic back emf sensingThe third harmonic based method is one of most relevant back-EMF sensing

schemes. It has a wider speed range and smaller phase delay than the terminal voltage

sensing method. However, at low speed, the integration process can cause a serious

position error, as noise and offset error from sensing can be accumulated for a

relatively long period of time . At lower speeds, detection of both the third harmonic

and the zero-crossing of the phase voltage become difficult due to the lower signal


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levels. In comparison, the conventional back-EMF control scheme is able to drive the

motor from 6,000 rpm to about 1,000 rpm, but the third harmonic control scheme is

capable to operate the motor from rated speed (6,000 rpm) down to about 100 rpm.

This does not introduce as much phase delay as the zero-crossing method and requires

less filtering . Then, the efficiency drop is more accentuated for the terminal voltage

sensing scheme, because the delay introduced by the low pass filter decreases with the

motor speed. This phase delay introduced by the filter is responsible for the loss of field

orientation and loss of the quadrature condition between rotor flux and stator current.

The immediate consequence is the reduction of the torque per current ratio of the

motor, which implies in larger copper losses . Also, the third harmonic back-EMF

method is applicable for the operation in flux weakening mode, and the methods based

on zero-crossing of the back-EMF are simple. However, it is only applicable under

normal operating conditions (commutation advance or current decay in free-wheeling

diodes lower than 30 electrical degrees)

Freewheeling diode conductionThis method uses indirect sensing of the zero crossing of the phase back-emf to

obtain the switching instants of the BLDC motor. In the 120 degree conducting wye-

connected BLDC motor, one of the phases is always open-circuited. For a short period

after opening the phase, there remains phase current flowing, via a freewheeling diode.

This open phase current becomes zero in the middle of the commutation interval, which

corresponds to the point where back-emf of the open phase crosses zero. By this

technique, 45-2300 rpm sensorless operation has been achieved. This technique

outperforms the previously mentioned back-emf methods at low-speeds

Back-emf integrationIn this method position information is extracted by integrating the back-emf of

the unexcited phase. The integration is based on the absolute value of the open phase

back-emf. Integration of the back-emf starts when the open phase back-emf crosses

zero. A threshold is set to stop the integration which corresponds to a commutation

instant. As the back-emf is assumed to vary linearly from positive to negative

(trapezoidal back-emf assumed), and this linear slope is assumed speed-insensitive, the

threshold voltage is kept constant throughout the speed range. If desired, current

advance can be implemented by changing the threshold. The integration approach is


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less sensitive to switching noise, automatically adjusts to speed changes, but the low

speed operation is poor.

4.6.4 Methods using starting techniques

To operate a BLDC without a rotor position sensor, requires some method to

start the motor. Several methods are used for starting namely

1. open-loop

2. with known initial position

3. Using a method for machine interrogation and signal processing

4. Using computationally complex methods

5. A method that relies on winding inductance which varies with rotor position due to

saliency.

Open-Loop StartingThe back-EMF detection methods cannot be applied well when the motor is at a

standstill or low speed, since back-EMF is zero. A starting procedure is needed to start

the motor from standstill. The open-loop

starting is accomplished by providing a rotating stator field which increases gradually

in magnitude and/or frequency. Once the rotor field begins to become attracted to the

stator field enough to overcome friction and inertia, the rotor begins to turn and the

motor acts as a permanent magnet synchronous machine with the disadvantage that the

initial rotor movement direction is not predictable. When the stator field becomes just

strong enough, the rotor could move in either direction. If the speed of the stator field is

slow enough and the load torque demanded does not exceed the pull-out torque the

motor will operate synchronously in the desired direction. The change over from open-

loop to sensorless method is made when sufficient back-EMF is generated, so that the

sensorless method should start generating the switching instants of all transistors.Taking all this into account, the procedure starts by exciting two arbitrary

phases for a preset time (for example, 0.5 s). At the end of the present time, the open-

loop commutation advancing the switching pattern by 120 is done, and then, the

polarity of the motor line current is altered. Then, the rotor turns to the direction

corresponding to the exited phases as is shown in Figure 4.10. Next, the commutation

signal that advances the switching pattern by 120 is given, as Figure 4.9b indicates,

and the open-loop commutation is immediately switched to the sensorless drive. After

the next commutation the position sensorless drive is attained (Figure 4.10), and the


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motor line current indicates that satisfactory sensorless commutations are performed by

the position-detecting method.

This method is simple but the reliability is affected by the load and it may cause

temporarily reverse rotation of the rotor during the start-up. This is not allowed in some

application, such as disk drives, which strictly require unidirectional motion. However,

it may be satisfactory in others such as pump and fan drives.

Figure 4.10- Open-loop starting procedure

Another problem exists if the stator field is rotating at too great a speed when

the rotor field picks up. This causes the rotor to oscillate, which requires the stator field

to decrease in frequency to allow starting.

The stator iron of the BLDC motor has non-linear magnetic saturation

characteristic, which is the basis for determining the initial position of the rotor. In

order to overcome the drawbacks mentioned above, the rotor position detecting and

speed up methods based on saturation effect of the stator iron can be applied, such as

the short pulse sensing technique. This scheme adopts a voltage pulse train composed

of the successive short and long pulses to generate positive torque to speed up the

motor, and it does not bring any reverse rotation and vibration during the start-up

process. The response speed of the stator current and the response peak value of the

current of the stator winding can be used to detect the rotor position .


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4.7 SENSORLESS TECHNIQUE FOR FOUR SWITCH THREE PHASE

INVERTER DRIVEN BLDC MOTOR [3, 11]

A BLDC motor can be driven by an inverter using six switches and four

switches as shown in Fig.4.2 and Fig.4.5 respectively. Most of the sensorless methods

for a six-switch inverter BLDC motor drive are not directly applicable to the four-

switch inverter. The main reason is that in the four-switch topology, some methods

detect less than six points, and other commutation instants must be interpolated via

software. This paper presents a novel sensorless method for four-switch BLDC motor

drive based on zero crossing points of stator line voltages.

In a four-switch inverter topology, as in Fig.4.5, terminal C is connected to the

middle point of DC bus (point O). With point O as reference, Fig.4.11 shows the three

line voltage waveforms, Vao, Vba and -Vbo. Zero crossing points (ZCPs) of these

voltages lag 30ofrom ZCPs of phase back EMF voltages and so are coincident to the

commutation instants. Therefore, by detecting the zero crossing points of three line

voltages, six commutation points are obtained. Three line voltages are derived from

terminal voltages Vao and Vbo. They have higher magnitude compared to back EMF

voltages (3 times phase voltages plus drop voltage on the stator impedance). The three

voltage functions are:

Fig.4.11Stator line and phase back emf voltage

VFa(Vao, Vbo) = Vao (4.14)

VFb(Vao, Vbo) = VboVao (4.15)

VFc = -Vbo (4.16)

Due to PWM control of the inverter, stator terminal voltages V ao and Vbo

contains high frequency switching signals and thus; detection of the zero crossings


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Va0 = RIa + Ldt

dia+ ean + Vno (4.17)

Vb0 = RIb + Ldt

dib+ ebn + Vno

(4.18)

0 = RIc + Ldt

dic+ ecn + Vno (4.19)

Because the drive employs the Direct Current Control method, the motor adopts

1200conducting mode and only two phases are energized at one time. So, the current in

the two phases has the same amplitude and opposite direction, while in the third phase,

the current is zero. As shown in Fig.4.13, in the four-switch inverter topology, phase

voltages Vaoand Vboare at a phase difference of 60. It results Vaoand Vboare 300

phase lag respect to eanand ecnrespectively. Moreover, Vbavoltage (or Vbo- Vao) vector

has 300delay respect to ecn. It means that the zero crossing points of VaoandVbocan

be used to commutate the current in phase A and C, and also, while two voltages Vao

and Vbobecome equal together, two commutation instants of phase B may be detected.

Fig.4.13Stator line to line voltage vectors of a FSTPI-BLDC motor drive


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CHAPTER 5

SIMULATIONS

A complete simulation of the system is done in Simulink of the MATLABenvironment. The system consists of a BLDC motor with the dc link voltage and

inverter.

5.1 MACHINE PARAMETERS

The Table 5.1 below gives the machine parameters used for all the simulation works

TABLE 5.1

MACHINE PARAMETERS

Pn 425 [W] Zp 16 [pole]

Tn 10 [Nm] n 700 [rad/sec]

R 0.64 [] J 5e-4 [Kg.m2]

Ls 1.0 [mH] M 0.25 [mH]

Kf 1.194 [Nm/A] Ke 0.0667 [V/rpm]

5.2 SIMULATION OF SENSORLESS FSTPI BLDC MOTOR

DRIVE

In sensorless control scheme, control is achieved via terminal voltage sensing. The

three voltage functions are used to get the commutation points. A voltage divider

circuit is used first, followed by low pass filter (second order Butterworth) and then a

zero crossing detection circuit to get the virtual hall signals.

The gate signals are obtained as required exactly as per the switching sequence of

four switch topology. Each switch has a 120 degree conducting period. Among the 6

modes of operation, only in mode 2 and mode 5, two switches are ON together. In rest

all diodes; its just one switch that is ON.


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SIMULATION CIRCUIT

Fig.5.1-simulation of FSTPI circuit


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OUTPUT WAVEFORMS

Fig.5.2- Gate signals vs time (sec)

The gate signals for four switches are obtained as shown above. The signals

have a magnitude of 1V

Fig.5.3- Virtual hall signals vs time (sec)

Virtual Hall signals for sensing the three phases of rotor windings each

displaced by a delay are obtained and shown above

Fig.5.4- Rotor speed (rad/sec) vs time (sec)The rotor speed implies that it follows the reference speed is shown above


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Fig.5.5- Back emf of phase c vs time (sec)

The trapezoidal back emf of phase c is obtained. Similarly for other phases

similar waveform can be obtained

Fig.5.6- Stator current of phase c vs time (sec)

Stator current of phase c is obtained and the waveform is slightly distorted


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CHAPTER 6

HARDWARE IMPLEMENTATION

6.1 PMBLDC DRIVE SYSTEMSA block diagram of Permanent Magnet Brush less DC Motor is shown in the

Figure 6.1. It consists of a three phase inverter, position sensor and a controller. The

inverter along with the position sensor arrangement is functionally analogous to the

commutator of a conventional dc motor. The commutation of a BLDC motor is

controlled electronically. To rotate the BLDC motor, the stator windings should be

energized in a sequence depending on the rotor position. So it is important to find out

the position of the rotor either by hall elements, light-emitting diodes, phototransistors

or encoders. Normally hall sensors are used to detect the rotor position. For every 60

electrical degrees of rotation, one of the hall sensors changes the state, so it takes six

steps to complete an electrical cycle

Fig.6.1-Block diagram of PMDC drive system


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6.2 PWM BASED MOSFET DRIVER

Pulse-width modulation (PWM) of a signal or power source involves the

modulation of its duty cycle, to either convey information over a communicationschannel or control the amount of power sent to a load. This circuit is mainly designed

to control the speed of the AC induction motor and DC motor. The MOSFET are used

to control the speed of the motor by varying the supply voltage to the motors. The

MOSFET is switched with very high speed with the help of PWM waves. The PWM

waves are generated by the PIC microcontroller. The PWM time period and duty cycle

is controlled by the software.

In the microcontroller we are generating two PWM waves with different time

period. They are used to drive the two set of MOSFET drivers through AND gate. So

the AND gate is used to change the switching time between the two set of MOSFET

drivers. When the duty cycle of both the PWM waves is high, the output of the AND

(IN1) gate is high which is given to transistor network. The transistor network is

consists of BC 547 and BC 557 transistor. Now the both the transistor is conducting,

due to that 12v is given to MOSFET Q1 and Q2 gates. So the MOSFET are switched

ON and delivered the output on the center tapped transformer.

In the center tapped transformer, the DC input is given to middle terminal and

other two end terminals are connected in the each of the MOSFET drivers Drain

terminal. The DC input negative terminal is connected in the source terminal. Similarly

in the next of duty cycle, another AND gate (IN2) output is high which drive another

set of MOSFET drivers.

Due to high switching speed the given DC input is converted to related sinewave which is step up through the transformer. This AC voltage is delivered in the

transformer secondary. This AC voltage can be used to drive the AC induction motor.

Suppose if you want to drive the DC motor the corresponding AC voltage is rectified

through bridge rectifier.


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Fig.6.3-Current measurement circuit

In this case, when the input is greater than zero, D2 is ON and D1 is OFF, so the

output is zero. When the input is less than zero, D2 is OFF and D1 is ON, and the

output is like the input with an amplification of R2 / R1. The full-wave rectifier

depends on the fact that both the half-wave rectifier and the summing amplifier are

precision circuits. It operates by producing an inverted half-wave-rectified signal and

then adding that signal at double amplitude to the original signal in the summing

amplifier. The result is a reversal of the selected polarity of the input signal.

Then the output of the rectified voltage is adjusted to 0-5V with the help of

variable resistor VR1. Then given to ripples are filtered by the C1 capacitor. After the

filtration the corresponding DC voltage is given to ADC or other related circuit.

6.4 Power supply

The ac voltage, typically 220V rms, is connected to a transformer, which steps

that ac voltage down to the level of the desired dc output. A diode rectifier then

provides a full-wave rectified voltage that is initially filtered by a simple capacitor filter

to produce a dc voltage. This resulting dc voltage usually has some ripple or ac voltage

variation.

A regulator circuit removes the ripples and also remains the same dc value evenif the input dc voltage varies, or the load connected to the output dc voltage changes.


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This voltage regulation is usually obtained using one of the popular voltage regulator

IC units.

Fig.6.4-Power supply circuit

6.5 DAC CIRCUIT

In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for

converting a digital (usually binary) code to an analog signal (current, voltage or

electric charge). Digital-to-analog converters are interfaces between the abstract digital

world and analog real life. An analog-to-digital converter (ADC) performs the reverse

operation.

DAC usually only deals with pulse-code modulation (PCM)-encoded signals.

The job of converting various compressed forms of signals into PCM is left to codes.

The DAC fundamentally converts finite-precision numbers (usually fixed-point binary

numbers) into a physical quantity, usually an electrical voltage. Normally the output

voltage is a linear function of the input number. Usually these numbers are updated at

uniform sampling intervals and can be thought of as numbers obtained from a sampling


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essentially unchanged over the full 4.5v to 18v power supply range power dissipation

is only 33mvw with +5v supplies and is independent of the logic input states.

The output of the DAC is current signal. So it is given to current voltageconverter which is constructed by the LM 741 operational amplifier. Finally the analog

voltage is given to Triac or SCR control circuit.

6.6 INTRODUCTION TO PIC :

The microcontroller that has been used for this project is from PIC series. PIC

microcontroller is the first RISC based microcontroller fabricated in CMOS

(complementary metal oxide semiconductor) that uses separate bus for instruction and

data allowing simultaneous access of program and data memory.

The main advantage of CMOS and RISC combination is low power consumption

resulting in a very small chip size with a small pin count. The main advantage of

CMOS is that it has immunity to noise than other fabrication techniques.

6.6.1 PIC (16F877) :

Various microcontrollers offer different kinds of memories. EEPROM,

EPROM, FLASH etc. are some of the memories of which FLASH is the most recently

developed. Technology that is used in PIC16F877 is flash technology, so that data is

retained even when the power is switched off. Easy Programming and Erasing are other

features of PIC 16F877.

I/O PORTS:Some pins for these I/O ports are multiplexed with an alternate function for the

peripheral features on the device. In general, when a peripheral is enabled, that pin may

not be used as a general purpose I/O pin.

PORTA AND THE TRISA REGISTER:PORTA is a 6-bit wide bi-directional port. The corresponding data direction

register is TRISA. Setting a TRISA bit (=1) will make the corresponding PORTA pin

an input, i.e., put the corresponding output driver in a Hi-impedance mode. Clearing a


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TRISA bit (=0) will make the corresponding PORTA pin an output, i.e., put the

contents of the output latch on the selected pin.

PORTB AND TRISB REGISTER:PORTB is an 8-bit wide bi-directional port. The corresponding data direction

register is TRISB. Setting a TRISB bit (=1) will make the corresponding PORTB pin

an input, i.e., put the corresponding output driver in a hi-impedance mode. Clearing a

TRISB bit (=0) will make the corresponding PORTB pin an output, i.e., put the

contents of the output latch on the selected pin. Three pins of PORTB are multiplexed

with the Low Voltage Programming function; RB3/PGM, RB6/PGC and RB7/PGD.

The alternate functions of these pins are described in the Special Features Section. Each

of the PORTB pins has a weak internal pull-up. A single control bit can turn on all the

pull-ups.

This is performed by clearing bit RBPU (OPTION_REG). The weak pull-

up is automatically turned off when the port pin is configured as an output. The pull-

ups are disabled on a Power-on Reset.

PORTC AND THE TRISC REGISTER:PORTC is an 8-bit wide bi-directional port. The corresponding data direction

register is TRISC. Setting a TRISC bit (=1) will make the corresponding PORTC pin

an input, i.e., put the corresponding output driver in a hi-impedance mode. Clearing a

TRISC bit (=0) will make the corresponding PORTC pin an output, i.e., put the

contents of the output latch on the selected pin. PORTC is multiplexed with several

peripheral functions. PORTC pins have Schmitt Trigger input buffers.

PORT D AND TRISD REGISTERS:This section is not applicable to the 28-pin devices. PORTD is an 8-bit port with

Schmitt Trigger input buffers. Each pin is individually configurable as an input or

output. PORTD can be configured as an 8-bit wide microprocessor Port (parallel slave

port) by setting control bit PSPMODE (TRISE). In this mode, the input buffers are

TTL.


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PORT E AND TRISE REGISTER:PORTE has three pins RE0/RD/AN5, RE1/WR/AN6 and RE2/CS/AN7, which are

individually configurable as inputs or outputs. These pins have Schmitt Trigger input

buffers.

The PORTE pins become control inputs for the microprocessor port when bit

PSPMODE (TRISE) is set. In this mode, the user must make sure that the

TRISE bits are set (pins are configured as digital inputs). Ensure ADCON1 is

configured for digital I/O. In this mode the input buffers are TTL.

PORTE pins are multiplexed with analog inputs. When selected as an analoginput, these pins will read as '0's. TRISE controls the direction of the RE pins, even

when they are being used as analog inputs. The user must make sure to keep the pins

configured as inputs when using them as analog inputs.

MEMORY ORGANISATION:There are three memory blocks in each of the PIC16F877 MUCs. The

program memory and Data Memory have separate buses so that concurrent access canoccur.

PROGRAM MEMORY ORGANISATION:The PIC16F877 devices have a 13-bit program counter capable of addressing 8K

*14 words of FLASH program memory. Accessing a location above the physically

implemented address will cause a wraparound. The RESET vector is at 0000h and the

interrupt vector is at 0004h.

DATA MEMORY ORGANISTION:The data memory is partitioned into multiple banks which contain the General

Purpose Registers and the special functions Registers. Bits RP1 (STATUS


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are General Purpose Registers, implemented as static RAM. All implemented banks

contain special function registers. Some frequently used special function registers from

one bank may be mirrored in another bank for code reduction and quicker access.

EEPROM:EEPROM (electrically erasable, programmable read only memory) technology

supplies Non volatile storage of variables to a PIC-controlled device or instrument.

That is variables stored in an EEPROM will remain there even after power has been

turned off and then on again. Some instruments use an EEPROM to store calibration

data during manufacture. In this way, each instrument is actually custom built, with

customization that can be easily automated. Other instruments use and EEPROM to

allow a user to store several sets of setup information. For an instrument requiring a

complicated setup procedure, this permits a user to retrieve the setup required for any

one of several very different measurements. Still other devices use an EEPROM in a

way that is transparent to a user, providing backup of setup parameters and thereby

bridging over power outages

The data EEPROM and flash program memory are readable and writable during

normal operation over the entire VDD range. A bulk erase operation may not be issued

from user code (which includes removing code protection. The data memory is not

directly mapped in the register file space. Instead it is indirectly addressed through the

special function registers (SFR).There are six SFRS used to read and write the program

and data EEPROM memory.

TIMERSThere are three timers used Timer 0, Timer1 and Timer2 .The Timer-0 module

is a 8-bit timer/counter.The timer-1 module is a 16-bit timer/counter consisting two 8-

bit register (TMR1H) and TMR1L), which are readable and writable. The TMR1

register pair (TMR1H:TMR1L) Increments from 0000h to FFFFH and rolls over to

0000h. The tmr1 interrupt, if enabled, is generated on overflow, which is latched in

interrupt flag bit tmr1IF. This interrupt can be enabled/disabled by setting/clearing tmr1

interrupt enable bit tmr1IE.


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Timer2 is an 8-bit timer with a pre scaler and a post scaler. It can be used as

the PWM Time-base for the PWM mode of the CCP module(s). The TMR2 register is

readable and writable, and is cleared on any device reset. The timer2 module has an 8-

bit period register PR2. Timer2 increments from 00h until it match PR2 and then resets

to 00h on the next increment cycle. PR2 is a readable and writable register. The PR2

register is initialized to FFh upon reset. Timer 2 can be shut off by clearing control bit

tmr2on to minimize power consumption.

ANALOG TO DIGITAL CONVERTER (ADC)There are two types of analog to digital converter is present in this IC. Here

using 10-bit ADC. The ADC module can have up to eight analog inputs for a device.

The analog input charges a sample and hold capacitor. The output of sample and hold

capacitor is the input into the converter. The converter then generates a digital result of

this analog level via successive approximation. The A/D conversion of the analog

input signal results in a corresponding10-bit digital number. The A/D module has high

and low voltage reference input that is software selectable to some combination of

VDD, VSS, and RA2 Or RA3.The A/D module has four registers. These registers are

1. A/D result high register (ADRESH)

2. A/D result low register (ADRESL)

3. A/D control register 0 (ADCON0)

4. A/D control register 1 (ADCO

INTERRUPTSThe PIC16F87X family has up to 14 sources of interrupt. The interrupt control

register (INTCON) records individual interrupt requests in flag bits. It also has

individual interrupt requests in flag bits. It also has individual and global interrupt

enables bits.


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6.7 HARDWARE IMPLEMENTATION

Fig.6.6. Hardware setup


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OUTPUT WAVEFORMSThe voltage and virtual hall signal outputs are taken at two conditions, as one in

lower speed region around 450 rpm and other at high speed region around 1000 rpm.

The voltage and virtual hall signals are shown below:-

PC interfacing is done using VB techniques and speed variations can be known

from the output waveforms. The motor speed depends only on the amplitude of the

applied voltage; the amplitude of the applied voltage is adjusted by using the PWM

technique.


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CHAPTER 7

CONCLUSION

The BLDC motor kit was designed to work with the SEPIC Converter. A Speed

control scheme with Torque Ripple minimization is executed by means of the sensors

to further reduce cost and increase reliability. Furthermore, the only choice for some

applications where those function reliably due to harsh environmental conditions and a

higher performance is required.

A circuit topology and control strategy has been proposed to suppress

commutation torque ripple of BLDCM in this work. A SEPIC converter is placed at the

input of the inverter, and the desired DC link voltage can be achieved by appropriate

voltage switch control. No exact value of the commutation interval T is required, and

the proposed method can reduce commutation torque ripple effectively within a wide

speed range and load

As future work the PI controller in simulation circuit can be replaced by fuzzy

controller which gives a smooth variation of speed without oscillations. The Hardware

implementation can be modified with DSP processor, such that wide speed range

applications are possible.


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REFERENCES

[1] Halvaei Niasar, H. Moghbelli and A. Vahedi, Implementation of four-switch

brushless dc motor drive based on TMS320LF2407 DSP, 2007 IEEEInternational Conference on Signal Processing and Communications (ICSPC

2007), 24-27 November 2007, Dubai, United Arab Emirates, PP 332-335.

[2] T.J.E. Miller, "Brushless permanent magnet and reluctance motor drive",

Oxford, 1989

[3] Halvaei Niasar, H. Moghbelli and A. Vahedi, A Low-Cost Sensorless

Control for Reduced-Parts, Brushless DC Motor Drives,IEEE Transactions on

Industry Applications, 2008, pp 662-667.

[4] B.K. Lee, T.H. Kim, M. Ehsani; On the feasibility of fourswitch three-

phase BLDC motor drives for low cost commercial applications: topology and

control, IEEE Transactions on Power Electronics, Vol. 18, No. 1, pp. 164-172,

January 2003.

[5] Halvaei Niasar, H. Moghbelli and A. Vahedi, Modeling, simulation and

implementation of four-switch brushless dc motor drive based on switching

functions,IEEE Transactions on Industry Applications, 2009, pp 682-687.

[6] Krause, P.C.: Analysis of Electric Machinery, New York, McGraw-Hill,

1986.

[7] C. Tsung Lin, C. Wen Hung, C. Wen Liu; Sensorless Control for Four-

Switch Three-Phase Brushless DC Motor Drives, in Proceeding of the Forty-

First IEEE Industry Applications Conference Meeting (IAS'06), Taiwan, Oct.2006, vol. 4, pp. 2049-2053

[8] J.P. Johnson, M. Ehsani, Y. Guzelgunler; Review of Sensorless Methods

for Brushless DC Motor, IEEE Industry Applications Conference, 1999, Vol.

1, pp. 143150.

[9] K.Venkataratnam: Special Electric Machines, Universities Press,

Hyderabad, 2008, pp 146-184


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[10] Halvaei Niasar, H. Moghbelli and A. Vahedi, A Novel Sensorless Control

Method for Four-Switch, Brushless DC Motor Drive without Using any 30o

Phase Shifter,Proceeding of International Conference on Electrical Machines

and Systems 2007, Oct. 8~11, Seoul, Korea, pp 408-413

[11] Halvaei Niasar, H. Moghbelli and A. Vahedi, A Novel Sensorless Control

Method for Four-Switch, Brushless DC Motor Drive without Phase Shifter,

IEEE Transactions on Power Electronics, Vol. 23, No. 6, November2008, pp

30793087.

[12] T.Sebastian, G.Slemon, and M.Rahman, Modelling of permanent magnet

synchronous motors, Magnetics, IEEE Transactions on, vol. 22, pp. 1069-

1071, 1986.

[13] P.Pillay and R.Krishnan, Modelling of permanent magnet motor drives,

Industrial Electronics, IEEE Transactions on, vol. 35, pp. 537-541,1988.

[14] T.Lowand, M A Jabbar, Permanent magnet motors for brushless

operation, IEEE Trans. Ind. Applicat, vol. 26, pp. 124-129, Jan/Feb 1990.

[15] Mingyao Lin, Weigang Gu, Wei Zhang,Qiang Li , Design of Position

Detection Circuit for Sensorless Brushless DC Motor


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DRIVE PARAMETERS Name/ Model : BLDC Motor Driver / AMB364 Supply Voltage : 12-24V DC Maximum Current : 10A Drive Power : 96W Speed Range : 3000 rpm to motor rated speed Functions : Internal / External Speed control Analogue Input : 0-5V DC

BLDC Sepic Motor Torque Control

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