Harmonics and Torque Ripple Reduction of Brushless Dc Motor by Using Cascaded H-bridge Multilevel...
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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.
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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 com- mutated. 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.
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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.
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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
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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
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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.
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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
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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.
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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.
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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.
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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
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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.
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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
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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
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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.
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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.
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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
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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.
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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.
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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.
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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
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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.
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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
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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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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,
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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.
Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 28
CASCADED H-BRIDGE MLI
DC SUPPLY
3-PHASE
BLDC
MOTOR
POSITION
SENSOR
SPWM
Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 29
SPWM pulses
Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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.
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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
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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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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.
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
Fig.3.4. Switching Sequences for Different Level Generation
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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.
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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.
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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.
Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 40
Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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:
Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 41
Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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%
Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 45
Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
Fig.4.14: Seven Level Inverter Fed BLDC Motor
Fig.4.15: Simulated Output Wave Form Of The Seven Level Inverter.
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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%
Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 48
Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
Fig. 4.12: Three phase output voltage of 7-level BLDC motor
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Harmonics and Torque Ripple Reduction of BLDC Motor by using Cascaded H-Bridge Multilevel Inverter Chapter-1
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.
Dept of Electrical and Electronics Engineering, PE, S.J.C.E.T, Page 50