Harmonics and Torque Ripple Reduction of Brushless Dc Motor by Using Cascaded H-bridge Multilevel...

Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1

CHAPTER 1

INTRODUCTION

1.1 Introduction:

Brushless DC motor have characteristics of simple structure, large torque, no need

to change the phase based on brush, it has long use time and good speed regulation. Due

to the above mentioned advantages now electric vehicles and micro electric motor cars

in the market mostly adopt BLDCM. To drive the motor the traditional BLDC

controlling system requires hall sensor signals. When there is disturbance on the hall

sensor, the fault actions on the main circuit prompts the BLDCM action unsteady, the

whole controlling system reliability is greatly reduced, the controller cost is also

increased. In recent years, for the speed control of BLDC Motors some of these

developments like Sinusoidal Pulse width Modulation Controller have been

implemented. To control BLDC motors Neural network control has also been used. It is

not Satisfactory as its performance under load disturbance and parameter uncertainty due

to the non linearity . Sliding control Techniques is originated from Soviet literature , For

Designing of robust system Performance it have advantages like order reduction,

disturbance rejection and invariance to parametric variations. By applying the proposed

technique, stability of the entire loop and the smoothness of the converging process of

the system are better than classical PI controller. At the same time sliding surface can be

reached quickly and the system chattering can be reduced , facilitating the design of

variable-structure control.

Permanent magnet brushless dc motors are mostly used. Over other motor types

Permanent magnet motors have several advantages with trapezoidal back EMF and

sinusoidal back EMF. Compared to dc motors due to the elimination of the mechanical

commutator they are lower maintenance and also they have a high-power density which

makes it ideal for high torque- to weight ratio applications. Compared to induction

machines, allowing for faster dynamic response to reference commands they have lower

inertia. Also, due to the permanent magnets they are more efficient which results in

virtually zero rotor losses. For Servo Applications Permanent magnet brushless dc

(PMBLDC) motors could become serious competitors to the induction motor.

Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 1


Because of its high efficiency, high power factor, high torque, simple control and

lower maintenance the PMBLDC motor is becoming more popular in various

applications. Higher cost and relatively higher complexity introduced by the power

electronic converter used to drive them is the major disadvantage with permanent magnet

motors. In the development of a torque/speed regulator the added complexity is evident.

In various drive applications due to High efficiency, high power density and wide

range speed controllability of BLDC motors make them suitable. For fast data access and

for high speed characteristics spindle motors are used in computer hard disk.

Brushless Direct Current (BLDC) motors are one of the motor types rapidly

gaining popularity. In industries such as Appliances, Automotive, Aerospace, Consumer,

Medical, Industrial Automation equipment and Instrumentation BLDC Motors are used.

Brushes are not used for commutation because as the name indicates it brushless motor;

instead, they are electronically commutated. Over induction motors and Brushed motors

BLDC motors have many advantages. A few of these are:

Better speed versus torque characteristics

High dynamic response

High efficiency

Long operating life

Noiseless operation

Higher speed ranges

In application the ratio of torque delivered to the size of the motor is higher,

making it useful where space and weight are critical factors. In this application note, the

construction, working principle, characteristics and typical applications of BLDC motors

are discussed.

1.2 MAIN CHARACTERISTICS BLDC MOTOR

The two coaxial magnetic armatures of BLDC Motor are separated by an air gap. In

certain types of motor,

The external armature, the stator, is fixed.



The internal armature, the rotor, is mobile (the rotor can also be external in certain

cases).

The induced part of the machine is Stator.

Inductor of the machine is Rotor.

The internal armature, the rotor, is a permanent magnet in Brushless DC Motors.

The constant Current (DC) supply is given to the armature .

Poly phased external armature (stator) and is covered by poly- phased currents (3

phases in our cases).

Permanent magnet type rotor is used in Brushless DC motor; It has almost the

same properties and physical laws as a DC current machine.

An electric motor transforms electrical energy into mechanical energy. Two main

characteristics of a brushless DC motor are:

It has an electromotive force proportional to its speed

The stator flux is synchronized with the permanent magnet rotor flux.



CHAPTER 2

LITERATURE SURVEY

2.1 Basic Structure Of BLDC Motor

Modern brushless motors and AC Motor construction is very similar, known as the

permanent magnet Synchronous Motor. Typical three-phase brushless dc motor is

illustrated in Fig.2.1. Poly phase AC Motor and Brushless DC Motor stator windings are

similar, and the rotor is composed of one or more permanent magnets. Brushless dc

motors are different from ac synchronous motors in that the former incorporates some

means to detect the rotor position (or magnetic poles) to produce signals to control the

electronic switches as shown in Fig.2.2. The most common position/pole sensor is the

Hall element, but some motors use optical sensors.

Fig 2.1: Disassembled view of a brushless dc motor

Fig: 2.2 Layout of BLDC Motor



Although the most orthodox and efficient motors are three-phase, two-phase brushless dc

motors are also very commonly used for the simple construction and drive circuits.

Fig.2.3 shows the cross section of a two-phase motor having auxiliary salient poles.

Fig 2.3 Two-phase motor having auxiliary salient poles

Comparison of conventional and brushless dc motors:

Although it is said that brushless dc motors and conventional dc motors are

similar in their static characteristics, they actually have remarkable differences in some

aspects. When we compare both motors in terms of present-day technology, a discussion

of their differences rather than their similarities can be more helpful in understanding

their proper applications. Table 2.1 compares the advantages and disadvantages of these

two types of motors. In a conventional dc motor, commutation is undertaken by brushes

and commutator in contrast, in a brushless dc motor it is done by using semiconductor

devices such as MOSFETs & IGBT”S etc.

A Bipolar-Starting and Unipolar-Running Method to Drive a Hard Disk Drive

Spindle Motor at High Speed With Large Starting Torque. This report presents a method

to drive a hard disk drive (HDD) spindle motor at high speed with large starting torque by

utilizing a bipolar-starting and unipolar-running algorithm.

2.2 Bipolar and Unipolar Drive of a BLDC Motor

A Topology of a Bipolar and Unipolar Drive One of the popular windings in a

three-phase BLDC motor is Y-winding. It can be classified into unipolar or bipolar

driving method as shown in Fig. 2.4. Unipolar and bipolar driving methods energize one



phase and two phases out of three phases at each commutation period, respectively. And

their commutation periods are 60 and 120 electrical degrees, respectively. The unipolar

drive in Fig. 2.5 has fewer electronic parts and simpler circuits than a bipolar drive, and

the commutation frequency is half of a bipolar drive. But it has high torque ripple and

dead spots like a single-phase motor, because it cannot invert the direction of the current

flowing through the phase winding.

Table 2.1: Comparison of Conventional and Brushless DC Motors.

Fig. 2.4 Conventional bipolar and unipolar drive. (a) Bipolar drive. (b) Unipolar drive.



Fig. 2.5 Unipolar drive with six transistors.

Fig. 2.5 shows another topology of a unipolar drive with six transistors. It can

invert the direction of the current flowing through the phase-winding with the ON–OFF

operation of additional three transistors and additional power supply, so that it has small

torque ripple, no dead spot, and the same commutation period as a bipolar drive. Table

1.2 shows the commutation sequence of a bipolar and unipolar drive with six transistors.

Both bipolar and unipolar drives have a commutation period of 60 electrical degrees to

produce a maximum torque. the ideal torque curves that correspond to the energized

phase on the rotor position for 360 electrical degrees of each driving method. There is a

phase difference of 30 electrical degrees between the commutation sequence of a bipolar

and unipolar drive as shown in Fig. 2.6 and Table 2.2.

Table 2.2 Commutation Sequence of Bipolar and Unipolar Drive



Fig 2.6 Torque curves of bipolar and unipolar drive with six transistors.

(a) Bipolar drive. (b) Unipolar drive.

2.3 Torque–Speed–Current Relationship of Bipolar and Unipolar Drive

Torque–speed–current relationship of a BLDC motor operated in linear regions

can be explained by the following equations.

Table 2.3 shows the major design variables of the BLDC motor driven by a

bipolar or unipolar drive in the case of the application of the same voltage.



Table 2.3 Major Design Variables of a BLDC Operated by Bipolar and Unipolar Drive.

Fig. 2.7. Theoretical inverter circuit for bipolar-starting and unipolar-running drive.

(i) Negative back EMF would drive current.

Driver because larger input current flows in the former case so that the starting torque of

a unipolar drive is much smaller than that of a bipolar drive in practice.



2.4 Bipolar-Starting and Unipolar-Running Method of a BLDC Motor

This project proposes a bipolar-starting and unipolar-running method to run the

motor at high speed with large starting torque, which takes advantage of the large starting

torque of a bipolar drive, and the high operating speed of a unipolar drive. Fig. 2.6 shows

the theoretical inverter circuit that can be used as either a bipolar or a unipolar drive, It

has two additional switches, N and N, which are composed of transistors and diodes, and

they are connected to the neutral point of the BLDC motor.

In the case of operating as a bipolar drive, they are off so that the inverter circuit

is exactly the same as the bipolar drive. It does not have to have additional power supply

as in the conventional unipolar drive as shown in Fig. 2.4, because additional switches

can change the direction of the phase current. This negative back EMF may drive the

current, which is shown as the This current rapidly builds up, and it contributes to

negative torque and loss, this phenomenon may prevent the BLDC motor from

accelerating to high speed. Fig. 2.7 shows the proposed novel inverter circuit which can

be operated as a bipolar drive, unipolar drive, or bipolar-starting and unipolar-running

drive.

Fig. 2.8 Novel inverter circuit for bipolar-starting and unipolar-running drive.



Table 2.4 States of the Novel Inverter Circuit for Bipolar Starting and Unipolar

Running Drive

In the case of operating as a bipolar drive, they are on so that the inverter circuit is

exactly the same as the bipolar drive. During this period, the transistor A is on so that the

freewheeling current during the off-period of PWM can be dissipated by flowing through

the closed loop along the phase-A, transistors of N and A. Table 2.4 shows the state of

the novel inverter circuit in Fig.2.7when the phase-AB of the bipolar drive and the phase-

A of the unipolar drive are energized, respectively. After the motor is accelerated

sufficiently by the bipolar drive, switching from the bipolar to unipolar drive can be

achieved by operating the additional transistor s in N and N. They switch the neutral

point of the BLDC motor to ground or to the supplied voltage depending on the firing

sequence of the unipolar drive. Then, this controller allows the motor to run to the high

speed that can be obtained by a unipolar drive. There is a phase difference of 30 electrical

degrees between the commutation sequences of the bipolar and unipolar drive as shown

in Fig. 2.5, so that switching to the unipolar drive should take place in the middle of one

commutation period of the bipolar drive.

Fig. 2.8 shows the developed system configuration of a BLDC motor controller.

A hall sensor detects the rotor position, and DSP controls the switching of the inverter

circuit and the speed of a motor using the PI control method. Speed, phase voltage, phase

current, and back-EMF are directly monitored on a PC through the communication

circuits and a user-interface program. It has three timers and two analog-to-digital (A/D)

converters.

This paper presents a bipolar-starting and unipolar-running method of a BLDC

motor. It proposes a novel inverter circuit to switch the BLDC motor from bipolar drive



Fig. 2.9 Developed DSP-based BLDC motor controller

to unipolar drive at any speed, without using additional power supply, and it verifies the

effectiveness of the proposed method by the experimentation. It runs the motor to high

speed with a large starting torque, and it also protects the inverter circuit by reducing

large input current during start up. The proposed method can be effectively applied to

drive a BLDC motor under large load conditions to high speed, and it can also drive a

BLDC motor in the wide range of operating speeds.

2.5 SPEED CONTROLLER:

Many applications, such as robotics and factory automation, require precise

control of speed and position. Speed Control Systems allow one to easily set and adjust

the speed of a motor. The control system consists of a speed feedback system, a motor,

an inverter, a controller and a speed setting device. A properly designed feedback

controller makes the system insensible to disturbance and changes of the parameters.

The purpose of a motor speed controller is to take a signal representing the

demanded speed, and to drive a motor at that speed.



Speed controller calculates the difference between the reference speed and the

actual speed producing an error, which is fed to the PID controller. PID controllers are

used widely for motion control systems. Block diagram of the PID controller is shown

in figure 2.9

Fig 2.9 PID Controller Block Diagram

The action of the proportional part of the controller can be summarized as giving

an immediate response to a difference between the reference and the feedback the action

of the proportional term will reduce, unless perturbations in the system appear. The

integral term, conversely, uses past as well as present values of the error. Because past

and present errors are integrated, a steady-state error will result in an increasing

compensating action of the controller. This, in turn, will make the difference between

the reference and the measured value to converge towards zero. The derivative term

improves the stability of the system and reduces overshoot in the response.

2.6 BLDC MOTORS

This chapter describes the typical construction and operation of a BLDC motor and

derives a mathematical model that can be simulated efficiently in Matlab and Simulink.

CONSTRUCTION:

A BLDC motor is a permanent magnet synchronous that uses position detectors

and an inverter to control the armature currents. The BLDC motor is sometimes referred

to as an inside out dc motor because its armature is in the stator and the magnets are on

the rotor and its operating characteristics resemble those of a dc motor. Instead of using a



mechanical commutator as in the conventional dc motor, the BLDC motor employs

electronic commutation which makes it a virtually maintenance free motor.

There are two main types of BLDC motors: trapezoidal type and sinusoidal type.

In the trapezoidal motor the back-Emf induced in the stator windings has a trapezoidal

shape and its phases must be supplied with quasi-square currents for ripple free operation.

The sinusoidal motor on the other hand has a sinusoidally shaped back – emf and requires

sinusoidal phase currents for ripple free torque operation. The shape of the back – emf is

determined by the shape of rotor magnets and the stator winding distribution.

The sinusoidal motor needs high resolution position sensors because the rotor

position must be known at every time instant for optimal operation. The trapezoidal

motor is a more attractive alternative for most applications due to simplicity, lower price

and higher efficiency.

BLDC motors exist in many different configurations but the three phase motor is

most common type due to efficiency and low torque ripple. This type of motor also offers

a good compromise between precise control and number of power electronic devices

needed to control stator currents. BLDC motor transverse section is shown in Fig. 2.10.

Position detection is usually implemented using three Hall - an effect sensor that detects

the presence of small magnets that are attached to the motor shaft.

Fig. 2.11 BLDC motor transverse section.

OPERATION

Typically, a three phase inverter is fed to the Brushless dc motor with what is

called six-step commutation. The conducting interval for each phase is 120o by electric



angle. The commutation phase sequence is like AB-AC-BC-BA-CA-CB. Each

conducting stage is called one step. Therefore, only two phases conduct current at any

time, leaving the third phase floating. In order to produce maximum torque, the inverter

should be commutated every 600 so thet current is in phase with the back EMF. The

commutation timing is determined by the rotor position, which can be detected by Hall

sensors as shown in fig 2.11(H1, H2, H3).The figure also shows ideal currents and back

emf waveforms.

Figure 2.11 shows a cross section of a three phase star connected motor along

with its phase energizing sequence. Each interval starts with the rotor and stator field

lines 1200 apart and ends when they are 600 apart. Maximum torque is reached when the

field lines are perpendicular. Current commutation is done by inverter as shown in a

simplified from in figure. The switches are shown as bipolar junction transistors but

MOSFET switches are more common. Table 2.5 shows the switching sequence, the

current direction and the position sensor signals.

Fig. 2.12 Ideal back-emf’s phase currents, and position sensor signals.



Fig. 2.13 BLDC motor cross section and phase energizing sequence

Fig. 2.13 Simplified BLDC drive scheme

2.7 Applications

Consumer Electronics

Brushed DC motors perform many functions which can also be fulfilled by BLDC

motors, but Brushed motors are better in case of cost and control complexity as compared

to BLDC motors. In many applications particularly devices such as computer hard drives

and CD/DVD players BLDC Motors are dominating. In Small cooling fans the electronic

equipment are exclusively powered by BLDC motors.

Transport

In electric vehicles and hybrid vehicles high power BLDC motors are found. These

BLDC motors are essentially synchronous AC motors with permanent rotor magnets. The

BLDC technology is used in Segway Scooter and Vectrix Maxi-Scooter.



BLDC motors are used in a number of electric bicycles. Standard bicycle

transmission with pedals, sprockets are present in electric bicycle, and that chain can be

pedaled along with, or without, the use of the motor as need arises.

Heating And Ventilation

Instead of using various types of AC motors there is a trend to use BLDC motors in

the HVAC and refrigeration industries, now many fans are run by using a BLDC motor.

In order to increase overall system efficiency some fans uses BLDC motors.

Certain HVAC systems use BLDC motors for higher efficiency because the built-in

microprocessor allows for programmability, better control over airflow, and serial

communication.

INDUSTRIAL ENGINEERING

This section needs development. See Stepper motor, Servo motor.

MODEL ENGINEERING

For model aircraft including helicopters currently use most popular motor i.e

BLDC motor.

2.8 What is a Harmonic?

The harmonic typical definition is “a sinusoidal component of a periodic wave

having a frequency that is an integral multiple of the fundamental frequency.” Some

times in many applications pure sinusoidal wave is required it means power without any

harmonics refer to “clean” or “pure” power. But such clean waveforms typically only

exist in a laboratory. Harmonics will be continued for a longtime to do so. Due to

Harmonics (called “overtones” in music) trumpet sound like a trumpet, and a clarinet like

a clarinet sound will be appeared. At fundamental frequency of the voltage the Electrical

generators try to produce electric power. In the North America, this frequency is 60 Hz.

This frequency is usually 50HZ in other parts of the world. In Aircraft the fundamental

frequency is 400 Hz. At 60 Hz, it means sixty times a second the waveform of voltage

increases to maximum positive peak and then back to zero, further it reaches to

maximum negative value, and then back to zero. The rate at which these changes occur is

the trigonometric function called a sine wave, as shown in figure 2.14. In many natural



phenomena this function occurs, such as the way in which the string on a violin vibrates

when plucked.

Figure 2.15. Fundamental Sine Wave

Depending on the fundamental frequency the harmonic frequencies are different. For

example, the 2nd harmonic on a 60 Hz system is 2*60 or 120 Hz. At 50Hz, the second

harmonic is 2* 50 or 100Hz. 300Hz is the 5th harmonic in a 60 Hz system, or the 6th

harmonic in a 50 Hz system. Figure 2.15 shows how a signal with two harmonics would

appear on an oscilloscope-type display, which some power quality analyzers provide.

Figure 2.16. Fundamental with two harmonics

A number of mathematical methods were developed one of the most popular

method is fourier transform to analyze complex signals that have many different

frequencies present.



2.9 WHY WORRY ABOUT THEM

The presence of harmonics does not mean that the factory or office cannot run

properly. Like other power quality phenomena, it depends on the “stiffness” of the power

distribution system and the susceptibility of the equipment. Due to high harmonic voltage

and current levels failures should takes place in different types of equipments. Some

typical types of equipment affected due to harmonic pollution include: - overheated

neutrals are exist due to Excessive neutral current. In three phase wye circuits odd triplen

harmonics are present which are additive in nature. This is because the harmonic number

multiplied by the 120 degree phase shift between phases is an integer multiple of 360

degrees. In Fig 2.16 we can see the harmonics from each of three phase legs are in phase

with each other in the neutral. Due to this the meter reading will be incorrect like

induction disc W-hr meters and averaging type current meters.

Reduced true PF, where PF= Watts/VA.

some losses will increase as the square of harmonic value (such as eddy current

losses and as skin effect). This is also true for lighting ballasts and solenoid coils.

Figure 2.17 Additive Third Harmonics.



In motors and generators zero,negative sequence voltages are present. The voltage

harmonics in a balanced system,are either positive ( 4th, 7th,...), negative (2nd,

5th, 8th...) or zero (3rd, 6th, 9th,...) sequencing values. The motor rotate s either

forward or backward depends on the voltage at particular frequency or neither

(just heats up the motor), respectively. Similar to transformer the heat in motor

increase dueto increased losses.

Table2.6. Harmonic Sequencing Values in Balanced Systems.

2.10 What is Total Harmonic Distortion?

Total harmonic distortion is a complex one but it can be understand easily by the

following concept, it becomes easy by the basic definitions of harmonics and distortion.

Figure 2.18: Power System with AC source and electrical load

Now assume that the load will be taken in any one of the two basic types: linear

or nonlinear. Depends on the type of the load power quality of the system will be affected

because current is drawn for each type of load. The current drawn by the linear load is

sinosuidal in nature it doesn’t distort the waveform (Figure 2.17),for example household

appliances. The current drawn by the non linear loads is not perfectly sinusoidal (Figure

2.17) Since voltage waveform distortions are created.



Figure 2.19: Ideal Sine wave

Figure 2.20: Distorted Waveform

From figure 2.18 we can see how the sinosuidal wave is distorted due to the

nonlinear load harmonic distortions. For example for a 60Hz fundamental waveform, the

2nd, 3rd, 4thand 5th harmonic components will be at 120Hz, 180Hz, 240Hz and 300Hz

respectively. hence, harmonic distortion is the degree in which due to the summation of

all these harmonic elements a waveform deviates from its pure sinusoidal value.Where as

in ideal sine wave zero harmonic elements are present in that case, there is nothing to



distort the perfect wave. In comparison of fundamental component to harmonic

component the summation of all voltage or current waveformes of harmonic elements is

known as Total harmonic distortion or,THD its equationis given below.

After calculating THD we obtain the result in percentage and comparing the harmonic

components to the fundamental component of a signal. If THD% is more, then more

distortions are present in the mains signal.

2.11 Multi level Inviters:

A three level voltage is considered to be as a smaller one in multilevel converter

topologies. The multilevel VSC can work in both rectifier and inverter modes due to the

presence of bi-directional switches the name itself multilevel indicates it can switch at

multilevel at either input or output current or voltage nodes. If the number of levels

increases the THD% will be decreases to zero. The number of the achievable voltage

levels, however, is limited by voltage-imbalance problems, voltage clamping

requirements, circuit layout and packaging constraints complexity of the controller, and,

of course, capital and maintenance costs. in industrial applications three different major

multilevel converters are used : cascaded H-bridges converter with separate dc sources,

diode clamped, and flying capacitors. Their Operation and structure can be discussed in

the following sections.

In Fig. 2.19 we can see the schematic diagram of one phase leg of inverters with

different number of levels, for which the power semiconductors action is represented by

an ideal switch with several positions. A two-level inverter generates an output voltage

with two values (levels), while the three-level inverter generates three voltages, and so

on.



Fig. 2.20 One phase leg of an inverter with (a) two levels, (b) three levels, and

(c) n levels.

2.12 Advantages of MLI(Multi level inverter):

The switching looses are high in high power circuits if we switch at higher

frequency .

Particularly in Low power & low voltage circuits Mosfets are used

In Mosfets the total losses are sum of the conductions losses 70% and switching

losses 30 % .

So at high switching frequency the Mosfets does not effects the total losses much.

IGBT’s are used in case of High power high voltage circuits.

The most attractive features of multilevel inverters are as follows.

1) The output voltages can be generated with extremely low distortion and lower dv/dt.

2) The input current drawn is having very low distortion.

3) It will operate at low switching frequency.

2.13 Why Cascaded H-bridge multilevel inverter:

In high-power AC supplies Cascaded H-Bridge (CHB) configuration has recently become

very popular. The H-bridge (single-phase full bridge) inverter units are connected in

series in each of its three phases. The different level inverters ac terminal voltages are

connected in series. By using different combination of four switches s1,s4 each converter



level can generate three different voltage outputs, +Vdc, -Vdc and zero. The different full-

bridge converters which are connected in series in the same phase such that the sum of

the individual converter ac output voltage waveform is obtained. In this topology, the

number of levels of output-phase voltage is defined by m= 2N+1, where N is the number

of DC sources. A seven-level cascaded converter, for example, consists of three full

bridge converters and three DC sources. Minimum harmonic distortion can be obtained

by controlling the conducting angles at different converter levels. For each 180° (or half

cycle) the switching device always conducts regardless of the pulse width of the quasi-

square wave.

2.14 Cascaded H-Bridge Multilevel Inverter

Fig 2.21: Single phase structures of Cascaded inverter (a) 3-level, (b)5-level, (c) 7-level

In 1975 the series H-bridge inverter is appeared one more alternative for a

multilevel inverter is the cascaded multilevel inverter or series H-bridge inverter.

Cascaded multilevel inverter was not fully realized until two researchers, Lai and Peng.

They patented it and presented its various advantages in 1997. Since then, the CMI has

been utilized in a wide range of applications., CMI shows superiority in high-power

applications with its modularity and flexibility especially shunt and series connected

FACTS controllers. A three-phase CMI topology is essentially composed of three



identical phase legs of the series-chain of H-bridge converters, which can possibly

generate different output voltage waveforms and offers the potential for AC system

phase-balancing. This feature is impossible in other VSC topologies.

2.14.1 Operation of CMLI

Based on the series connection of single-phase inverters with separate dc sources

the converter topology is classified. Fig. 2.20 shows the power circuit for three level five-

level and seven-level cascaded inverter which is having one phase leg. The resulting

phase voltage is synthesized by the addition of the voltages generated by the different

cells. In a 3-level cascaded inverter each single-phase full-bridge inverter generates three

voltage levels at the output: +Vdc, 0, -Vdc (zero, positive dc voltage, and negative dc

voltage). This is made possible by connecting the capacitors sequentially to the ac side

via the power switches. The resulting output ac voltage swings from -Vdc to +Vdc with

three levels, -2Vdc to +2Vdc with five-level and -3Vdc to +3Vdc with seven-level

inverter. The staircase waveform is nearly sinusoidal, even without filtering.

For a three-phase system, the output voltage of the three cascaded converters can

be connected in either wye (Y) or delta (Δ) configurations. For example, a wye-

configured 7-level converter using a CMC with separated capacitors is illustrated in the

fig. 2.21

2.14.2 Features of CMLI

For real power conversions, (ac to dc and dc to ac), the cascaded-inverter needs

separate dc sources. The structure of separate dc sources is well suited for various

renewable energy sources such as fuel cell, photovoltaic, and biomass, etc. Connecting

separated dc sources between two converters in a back-to-back fashion is not possible

because a short circuit will be introduced when two back-to-back converters are not

switching synchronously.

2.14.3 Advantages and Disadvantages of CMLI.

Advantages :

1) The regulation of the DC buses is simple.

2) Requires the least number of components among all multilevel converters to

achieve the same number of voltage levels,



3) Modularity of control can be achieved. Unlike the other inverters where the

individual phase legs must be modulated by a central controller the full-bridge

inverters of a cascaded structure can be modulated separately.

Fig 2.21 Three-phase 7-level cascaded multilevel inverter (Y-configuration).

4) Soft-switching can be used in this structure to avoid bulky and lossy resistor-capacitor-

diode snubbers.

DISADVANTAGES

i) Communication between the full-bridges is required to achieve the synchronization of

reference and the carrier waveforms.

ii) Needs separate dc sources for real power conversions, and thus its applications are

somewhat limited



CHAPTER 3

PROPOSED CONCEPT

3.1 Introduction

There are several inherent advantages for Brushless DC motors which is having

trapezoidal Back-EMF. Most prominent among them are high efficiency and high power

density due to the absence of field winding, in addition high reliability, low maintenance

and high capability is obtained due to the presence of brushes.. However in a practical

BLDC drive, significant torque pulsations may arise due to the back emf waveform

departing from the ideal. As well as commutation torque ripple, pulse width modulation

(PWM) switching. Due to the current commutation the Torque ripples are caused by the

mismatches between the applied electromotive force and the phase currents with the

motor electrical dynamics. It is one of the main drawbacks of BLDC drives. Especially at

Low speeds these torque ripples produces noise and degrade speed-control

characteristics. Due to the power electronic commutation, the usage of high frequency

and switching of power devices, there will be Imperfections in the stator and the

associated control system. The Various harmonics components are present in the input

supply voltage of the motor. Electromagnetic Interference (EMI) problem is appeared

during its operation, because of the presence of high frequency component in the input

voltage. Now a day’s researchers are trying to reduce the torque ripple and harmonic

component in the BLDC motor. An active topology to reduce the torque ripple is

synchronous motor. This PROJECT discusses the hysteresis voltage control method. The

torque ripple is minimized using SPWM switching is presented in PROJECT, this

scheme has been implemented using Relational operator to generate modified Sinusoidal

pulse width modulation (SPWM) signals for driving power inverter bridge. In this Project

SPWM controller method is used for reducing the torque ripple and harmonics. This

method is based upon the generation of the square wave armature current. To reduce

torque ripple the indirect position detection, it is based on the detection of the zero

crossing points of the line voltage measured at the terminal of the motor. The proposed

method based upon the SPWM controlled technique. The armature current is measured



and compared with the reference value to produce the gate pulses for the multilevel

inverter.

3.2 Functional Units

The BLDC Motor requires a power electronic drive circuit and a commutation system for

its operation. The Fig.3.1 describes the functional units present in the drive circuit and the

associated commutation controller for the BLDC Motor. A 4 pole BLDC motor is driven

by the inverter for 120 degree commutation. The rotor position can be sensed by a hall-

effect sensor, providing three square wave signals with phase shift of 120o. These signals

are decoded by a combinational logic to provide the firing signals for 120o conduction on

each of the three phases. The operation of the system is as follows: as the motor is of the

brushless dc type, If we give DC Suply to the Cascaded H-Bridge Multilevel Inverter

then inverter will convert DC Supply to AC Suppply and this supply is given to the

BLDC Motor the Output of the BLDC is Sensed by HallEffect Sensor(i.e. Shaft position

sensor) this sensor will send signals to the SPWM Controller in this relational operator

will compare the sinosuidal signal with referrnce triangular wave and generate sinosuidal

pulses These pulses are send to CHB Multilevel inverter and this inverter will send

desired supply to the BLDC Motor Hence the Motor will run at desired speed.


CASCADED H-BRIDGE MLI

DC SUPPLY

3-PHASE

BLDC

MOTOR

POSITION

SENSOR

SPWM


Fig.3.1 Closed loop Diagram of BLDC Motor with MLI

Fig. 3.2 Relational Operator Block of SPWM Controller

3.3 Cascaded H-Bridge Multilevel Inverter

The clamped inverter, also known as a neutral clamped converter is difficult to be

expanded to multilevel because of the natural problem of the DC link voltage

unbalancing. Moreover, the clamped inverter, also known as a neutral clamped converter

is difficult to be expanded to multilevel because of the natural problem of the DC link

voltage unbalancing. It consists of two capacitor voltages in series and uses the center

tap as the neutral. Each phase leg of the three level converters has two pairs of switching

devices in series. The center of each device pair is clamped to the neutral through

clamping diodes. The waveform obtained from the three level converters is a quasi-

square wave output. The Switching sequence of three phase five level MLI is represented

in Fig.3.3


SPWM pulses


Table 3.1 Switching table for Full H-Bridge of seven level inverter3.4 New Multilevel Topology:

3.4.1 General Description:

The power semiconductor switches are combined to produce a high-frequency

waveform in positive and negative polarities, in conventional multilevel inverters.

However, for generating bipolar levels there is no need to utilize all the switches.By the

new topology this idea has been put into practice .This topology is a hybrid multilevel

topology which separates the output voltage into two parts. One part is named level

generation part and is responsible for level generating in positive polarity. This part

requires high-frequency switches to generate the required levels. The switches in this part

should have high-switching-frequency capability. The other part is called polarity

generation part and is responsible for generating the polarity of the output voltage, which

is the low-frequency part operating at line frequency.

The RV topology in seven levels is shown in Fig. 3.4. As can be seen, it requires

ten switches and three isolated sources. The principal idea of this topology as a multilevel

inverter is that the left stage in Fig. 3.4 generates the required output levels (without

polarity) and the right circuit (full-bridge converter) decides about the polarity of the

output voltage. This part, which is named polarity generation, transfers the required

output level to the output with the same direction or opposite direction according to the

required output polarity. It reverses the voltage direction when the voltage polarity

requires to be changed for negative polarity.



Fig.3.3. Three-phase seven level proposed multilevel topology.

This topology easily extends to higher voltage levels by duplicating the middle

stage as shown in Fig. 3.4. Therefore, this topology is modular and can be easily

increased to higher voltage levels by adding the middle stage in Fig. 3.4. It can also be

applied for three-phase applications with the same principle. This topology uses isolated

dc supplies.

Therefore, it does not face voltage-balancing problems due to fixed dc voltage

values. In comparison with a cascade topology, it requires just one-third of isolated



power supplies used in a cascade-type inverter. In Fig. 3.4, the complete three-phase

inverter for seven levels is shown with a three-phase delta connected system.

According to Fig.3.4, the multilevel positive voltage is fed to the full-bridge

converter to generate its polarity. Then, each full bridge converter will drive the primary

of a transformer. The secondary of the transformer is delta (Δ) connected and can be

connected to a three-phase system. This topology requires fewer components in

comparison to conventional inverters. Another advantage of the topology is that it just

requires half of the conventional carriers for SPWM controller. SPWM for seven-level

conventional converters consists of six carriers, but in this topology, three carriers are

sufficient.

The reason is that, the multilevel converter works only in positive polarity and

does not generate negative polarities. Therefore, it implements the multilevel inverter

with a reduced number of carriers, which is a great achievement for inverter control. It is

also comparable to single-carrier modulation, while this topology requires the same

number of signals for SPWM. However, this topology needs one modulation signal

which is easier to generate as opposed to the single-carrier modulation method which

needs several modulation signals. Another disadvantage of this topology is that all

switches should be selected from fast switches, while the proposed topology does not

need fast switches for the polarity generation part. In the following sections, the

superiority of this topology with respect to SPWM switching and number of components

is discussed.

Table 3.1

SWITCHING SEQUENCES FOR EACH LEVEL



3.4.2 Switching Sequences:

Switching sequences in this converter are easier than its counter parts. According

to its inherent advantages, it does not need to generate negative pulses for negative cycle

control.. This topology is redundant and flexible in the switching sequence.

In order to avoid unwanted voltage levels during switching cycles, the switching

modes should be selected so that the switching transitions become minimal during each

mode transfer. This will also help to decrease switching power dissipation. According to

the aforementioned suggestions, the sequences of switches (2–3-4), (2-3-5), (2-6-5), and

(1, 5) are chosen for levels 0 up to 3, respectively. In order to produce seven levels by

SPWM, three saw-tooth waveforms for carrier and a sinusoidal reference signal for

modulator are required as shown in Fig.3.4.



Fig.3.4. Switching Sequences for Different Level Generation



Fig.3.5.SPWM Carrier and Modulator for Proposed Topology.

Table.3.Switching Cases in Each State According to Related Comparator Output

According to this definition, the switching states and switching modes are described in

Table II. As illustrated in Table II, the transition between modes in each state requires

minimum commutation of switches to improve the efficiency of the inverter during

switching states. The number of switches in the path of conducting current also plays an

important role in the efficiency of overall converter. For example, a seven-level cascade

topology has 12 switches, and half of them, i.e., six switches, conduct the inverter current

in each instance. However, the number of switches which conduct current in the proposed

topology ranges from four switches (for generating level 3) to five switches conducting

for other levels, while two of the switches are from the low-frequency (polarity

generation) component of the inverter. Therefore, the number of switches in the proposed

topology that conduct the circuit current is lower than that of the cascade inverter, and

hence, it has a better efficiency. The same calculation is true in a topology mentioned in.

The least number of switches in the current path for a seven-level inverter according to is

five (for generating level 3), which requires one switch more in the current path

compared to the proposed topology which requires only four conducting switches.. The



gating signal for the output stage, which changes the polarity of the voltage, is simple.

Low-frequency output stage is an H-bridge inverter and works in two modes: forward

and reverse modes. In the forward mode, switches 8 and 9 conduct, and the output

voltage polarity is positive. However, switches 7 and 10 conduct in reverse mode, which

will lead to negative voltage polarity in the output. Thus, the low-frequency polarity

generation stage only determines the output polarity and is synchronous with the line

frequency.



CHAPTER 4

MATLAB MODELLING

4.1 The Role of Simulation in Design

electromechanical devices like motors and generators and electrical circuits are

combinations of Electrical power systems. The performance of the systems are constantly

improving by Engineers work in this discipline. For Requirement of drastically increased

efficiency power system designers are forced to use power electronic devices and

sophisticated control system concepts that tax traditional analysis tools and techniques.

Further complicating the analyst’s role is the fact that the system is often so nonlinear

that the only way to understand it is through simulation.

To achieve their performance objectives hydroelectric, steam power generation

and other power generation plants not only use the devices of power systems and also a

common attribute of these systems is their use of power electronics and control systems.

4.2 Introduction to Matlab/Simulink

MATLAB is a software package for computation in engineering, science, and

applied mathematics. It offers a wide range of expert knowledge and powerful

programming language, excellent graphics. MATLAB is published by a trademark of The

Math Works, Inc.

MATLAB focus is on computation, not mathematics: Manipulations and

Symbolic expressions are not possible. All results are numerical and also inexact, thanks

to the rounding errors inherent in computer arithmetic.The numerical computation

Limitation can be seen as a drawback, but it’s a source of strength too: MATLAB is

much preferred to Maple, Mathematical, and the like when it comes to numeric’s.

4.2.1 Simulink

Simulink (Simulation and Link) is an extension of MATLAB by Math works Inc.

It works with MATLAB to offer modeling, simulating, and analyzing of dynamical

systems under a graphical user interface (GUI) environment. with click-and-drag mouse

operations the model construction is simplified. comprehensive block library is present in

Simulink which consists of toolboxes for both linear and nonlinear analyses. Models are

hierarchical, which allow using both top-down and bottom-up approaches. As Simulink is



an integral part of MATLAB, it is easy to switch back and forth during the analysis

process and thus, full advantage of features offered in both environments can be taken by

the user.

New proposed project can be designed by using this simulink and Project

performance can be easily verified under different operating conditions. After checking

the performance of the system a hardware circuit can be developed.

The proposed project is created and simulated in MATLAB environment. MATLAB

circuits and their simulation results for following are discussed

1) SPWM Controller

2) Cascaded H-Bridge Multilevel Inverter

1. SPWM CONTROLLER:

A sinusoidal waveform can be produced by the sinusoidal pulse-width modulation

(SPWM) technique by filtering an output pulse waveform with varying width. better

filtered sinusoidal output waveform can be formed by high switching frequency. By

varying the frequency and amplitude of a reference or modulating voltage the desired

output voltage can be achieved. The variations in the amplitude and frequency of the

reference voltage change the pulse-width patterns of the output voltage but keep the

sinusoidal modulation. As shown in Figure 2.1, a low-frequency sinusoidal modulating

waveform is compared with a high-frequency triangular waveform, which is called the

carrier waveform. when the sine waveform intersects the triangular waveform the

switching state will be changed. Variable switching times between states can be

determined by the crossing positions. In three-phase SPWM, a triangular voltage

waveform (VT ) is compared with three sinusoidal control voltages (Va, Vb, and Vc),

which are 120◦ out of phase with each other and the relative levels of the waveforms are

used to control the switching of the devices in each phase leg of the inverter. A three-step

inverter is composed of six switches S1 through S12 with each phase output connected to

the middle of each inverter leg as shown in Figure 2.2. The output of the comparators in

Figure 2.1 forms the control signals for the three legs of the inverter.



Figure 4.1: Control Signal Generator for SPWM

Figure 4.2: Three-Phase Sinusoidal PWM Inverter

Two switches in each phase make up one leg open and close in a complementary

fashion. That is, when one switch is closed, the other is open and one switch open other

switch will be closed. Vao, Vbo, and Vco are the output pole voltages of the inverter

switch between -Vdc/2 and +Vdc/2 voltage levels where Vdc is the total DC voltage. The

peak of the triangle Carrier voltage waveform is always greater than the peak of the sine

modulating waveform. When the the triangular waveform is less than the sinusoidal

waveform, the upper switch is turned on and the lower switch is turned off. Similarly,

when the triangular waveform is greater than the sinusoidal waveform, the upper switch



is off and the lower switch is on. Depending on the switching states, either the positive or

negative half DC bus voltage is applied to each phase. The switches are controlled in

pairs ((S1,S4), (S3,S6), and (S5,S2)) and the logic for the switch control signals is:

◦ S1 is ON when Va>VT S4 is ON when Va<VT

◦ S3 is ON when Vb>VT S6 is ON when Vb<VT

◦ S5 is ON when Vc>VT S2 is ON when Vc<VT .

Figure 2.3: Three-Phase Sinusoidal PWM: a). Reference Voltages (a,b,c) and

Triangular Wave b). Vao, c) Vbo, d) Vco e) Line-to-Line Voltages

2. Cascaded H-Bridge Multilevel Inverter

Usually the multilevel inverter designers objective is to get a purely sinusoidal

wave out of a Constant or variable DC Voltage source or current source. Inverters using

DC Source Voltage are called VSI i.e. Voltage Source Inverters, while the inverters made

using source that is current are called CSI i.e. Current Source Inverter. Inverters using

DC Voltage Source are called VSI, There will be a same circuit for both the types

of inverters. In CSI an extra inductor is present that is the only difference between both

types. By making the changes in switch firing delay we can obtain a square wave output

of 1 level or we can obtain an output of up to 2 level using VSI.



With simple H-bridge it is difficult to get a pure sinusoidal wave based on single

phase inverter. So we use the concepts of multilevel inverters where in the

whole inverter system combined together will give an output in the form of steps which

looks similar to sine wave inverters. we can get a purely sinusoidal wave by using large

number of levels, and by adjusting the delays and by using a proper capacitor at the end,.

In my model, I have used the Cascaded multilevel Inverter topology which is far better

than others when cost of implementation is concerned. Using this topology, we can

obtain n level using (n-1) H-bridges, By varying the switching timing in the ‘pulse

generator’ block, I obtained 7 levels in the output. The Simulink block diagram is shown

below:

Fig.4.4 Simulink Diagram of Pulse Generator Block

I have created subsystem for each H-bridge and used the subsystems to avoid clutter in

the workspace. The models present inside each H-bridge is shown in the figure given

below:



Fig 4.5 Subsystem of H-Bridge Inverter

The 7 level voltage output obtained is shown in the figure given below

Fig 4.6 7-Level Output of Cascaded H-Bridge Multilevel Inverter



It can be inferred that the output is nearly close to the sinusoidal wave, but a lot can be

done to improve. If the pulse width is adjusted properly, a nearly perfect sine wave can be

obtained.

4.3 MATLAB/SIMULINK RESULTS:

The simulation is carried out in Matlab/Simulink software and results are presented in

different cases

4.3.1 Case-1 proposed five level inverter:

The output of the inverter is given to the BLDC motor. The motor currents are sensed and

it is given to the rectifier and the obtained value is compared with the reference value and

the error value is processed using Relational Operator. The obtained value is compared

with the triangular wave to generate controlled SPWM signals

Fig.4.7: Matlab/Simulink Model Of Proposed Five Level Inverter



Fig. 4.8: Simulated output wave form of the inverter voltage

Fig.4.9: Simulated output of Hall Effect sensors

The motor outputs are sensed and it is given to the Halleffect sensor and the obtained

value is compared with the reference value and the error value is processed using

Relational operator. The obtained value is compared with the triangular wave to generate

controlled SPWM signals. The obtained pulses are taken from position sensor signals of



the motor to give pulses to multilevel inverter.Fig.4.3 shows the output signal from the

Hall Effect sensor of the Brushless DC motors

Fig.4.10: Torque waveform of BLDC motor

Fig. 4.11: FFT analysis of phase current A is 5.44%

The Fig.4.5: which shows the FFT analysis of the Phase A current of the Brushless DC motor. it is found that THD is 5.44%



Fig.4.12: FFT analysis of phase voltage

The Total Harmonic Distortion (THD) which tells the amount of harmonics present in the current or voltage. In above figure shows the FFT analysis of the Phase-Phase voltage of the Brushless DC motor. The amount THD also calculated for this waveform. is 12.79%

Fig. 4.14: speed of BLDC motor

Fig.4.7 shows the speed waveform of Brushless DC motor. The harmonics and the torque ripple can be reduced by smoothening the current waveform.

4.3.2 Case-2 seven level inverter fed BLDC motor



Fig.4.14: Seven Level Inverter Fed BLDC Motor

Fig.4.15: Simulated Output Wave Form Of The Seven Level Inverter.



Fig. 4.10: FFT analysis of phase current A is 0.7%

The Fig.4.10: which shows the FFT analysis of the Phase A current of the Brushless DC motor. it is found that THD is 0.7%

Fig. 4.11: FFT analysis of phase voltage is 19.77%

The Total Harmonic Distortion (THD) which tells the amount of harmonics present in the current or voltage. In above figure shows the FFT analysis of the Phase-Phase voltage of the Brushless DC motor. The amount THD also calculated for this waveform. is 19.77%



Fig. 4.12: Three phase output voltage of 7-level BLDC motor



CHAPTER 5

CONCLUSION & FUTURE SCOPE

5.1 CONCLUSION:

This paper has given a brief summary of different types of multilevel inverters

and their circuit topologies. Based on multilevel inverter structure today more number of

and more commercial products are there. This paper has proposed harmonics and torque

ripples have been reduced using multilevel inverter with the SPWM Controller controlled

technique. For a BLDC motor the harmonic contents of current and voltage are analyzed

and the amount of torque ripple and the THD also calculated. The main advantage of this

method is it uses one SPWM controller for the three phases. Finally a generalized

expression for highest order harmonic based on switching frequency and number of

levels is derived. Matlab/Simulink models are developed.

5.2 FUTURE SCOPE:

(i). For further reduction of the harmonic content selective harmonic or space vector

modulation technique can be used.

(ii). A SRM motor can be used which can be also used in regenerative mode of operation,

to which a battery energy storage system can be placed.


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Harmonics and Torque Ripple Reduction of Brushless Dc Motor by Using Cascaded H-bridge Multilevel...

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