BLDC Sepic Motor Torque Control

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    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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    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


    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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    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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    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.


    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-


    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


    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


    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


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


    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


    1(eaia+ebib+ecic) (4.4)

    The electromagnetic torque can also be expressed as


    EI2 (4.5)

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

    Te=TL+J dt




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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:











































    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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    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


    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



    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


    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




    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.


    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.


    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




    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


    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


    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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    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


    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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    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



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

    TABLE 5.1


    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]



    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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    Fig.5.1-simulation of FSTPI circuit

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    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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    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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    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


    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


    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


    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.


    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


    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


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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


    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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    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


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    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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    [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

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    [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

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

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

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


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

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

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    Method for Four-Switch, Brushless DC Motor Drive without Phase Shifter,

    IEEE Transactions on Power Electronics, Vol. 23, No. 6, November2008, pp


    [12] T.Sebastian, G.Slemon, and M.Rahman, Modelling of permanent magnet

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    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