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VSI,CSI AND TO DETERMINE THD USING MATLAB
CHAPTER-1
INTRODUCTION
1.1DEFINITION
Higher-performance control such as V/f control, slip-frequency control and field-
oriented control require that the magnitude and frequency of the motor supply voltage
be varied simultaneously. Since AC mains voltage have fixed magnitude and
frequency, variable-voltage-variable-frequency supply can only be obtained by using
power-semiconductor-controlled inverters or cyclo-converters. The cyclo-converter
needs more number of power semiconductor devices and can only provide variable
frequency that is considerably lower than the frequency of AC mains. The most
favourable and commonly used one is the power semiconductor inverter. The basic
structure of a three-phase inverter is shown in Fig.1.4.
The three-phase inverter requires at least six power semiconductor power
devices that can be either BJTs, or MOSFETs, or IGBTs, etc. working in switching
mode. A DC source, which can be either voltage source or current source, is
connected to the inverter. For the voltage-source inverter (VSI), a capacitor is put
across the DC link to provide the inverter with constant DC voltage source. For the
current-source inverter (CSI), an inductor is connected in series with the DC link to
provide the inverter with constant current source.
Taking VSI as an example, the basic operation of the inverter can be
summarized as follows: (1) the upper and lower devices in the same phase are
switched complementarily, (2) each device works periodically, (3) devices of three
phases work sequentially with 120° phase difference. Under this mode of operation,
the inverter converts the DC voltage into three-phase AC square-wave voltages across
lines AB, BC and CA. It is obvious that the frequency of those AC square waves is
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equal to the switching frequency of each device, hence it is variable since the
switching frequency can be varied at will using electronic circuits. In the VSI, the
anti-paralleled diode is necessary for freewheeling of the inductive load currents.
Operation of the CSI is the same as that of the VSI except that the output of the
inverter is three-phase AC square-wave currents. For inductive loads such as the
induction motor, the output currents of the CSI cannot change as square waves but
still can rise or fall at a high rate, inducing superimposed spikes on the terminal
voltage of the motor at commutation transients.
The CSI has several advantages when compared with VSI [23]:
The power circuit is more rugged and reliable because over current due to short circuits
or commutation failure is prevented by the series inductor;
The power circuit has fewer components, hence are more efficient and less expensive;
It offers fast and direct control of motor torque over a wide range.
However, CSI has several limitations [23] when used for drive applications:
It cannot operate at high frequency because of the inductive load;
It cannot operate under no-load condition;
Large voltage spikes occur on the output phases causing high voltage stresses on the
power devices;
Response at light load is sluggish.
It causes torque pulsations and speed oscillations due to interaction of the stepped
stator current and nearly sinusoidal flux.
Moreover, the CSI requires a large-size DC-link inductor that is heavy and
bulky in comparison with the DC-link capacitor required by the VSI. Due to those
limitations and disadvantages mentioned above, CSI is not as widely used as VSI.
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1.2 PWM TECHNIQUES
The periodic AC square wave output from the inverter contains the fundamental
components and harmonics. The fundamental component dominates the running of
the motor and the harmonics contribute to torque/speed pulsations and copper/iron
losses. Frequency of the fundamental component is the same as that of the square
wave and hence can be varied by varying the switching frequency. Amplitude of the
fundamental component can be varied in two ways. A straightforward way is to vary
the amplitude of the DC-link source but a controlled bridge rectifier is then required.
Moreover, the square wave contains significant harmonics, causing significant
torque/speed pulsation and copper/iron losses. A better way is not to vary the
amplitude of the DC-link source but to shape the square wave into multiple pulses by
switching the power devices at higher frequency within the original ON/OFF cycle
and to vary the duty ratio of the pulses based on the amplitude of the desired input for
the motor. This method is called pulse-width modulation (PWM). By using PWM
techniques, simultaneous variations of the amplitude and frequency of the
fundamental component can be achieved by operating the inverter only, i.e., the
switching control of the power devices, and the DC-link source can be simply an
uncontrolled bridge rectifier. The sinusoidal PWM (SPWM) method varies the duty
ratio of the pulse as a sine function that represents the desired sinusoidal inputs for the
motor. It makes the low-frequency harmonics insignificant and pushes the harmonic
frequencies to higher values. This effect will be more significant if the switching
frequency is higher. High-frequency harmonics contribute very little to the torque
generation of the motor and can be filtered out using small filter components. In most
cases the leakage inductances of the stator windings of the motor are sufficient to
filter out high-frequency harmonics.
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PWM signals can be generated by using sub-oscillation methods [21], which use the
sinusoidal modulation signals to compare with the triangular carrier signals. This
method is preferred to implement in hardware using analogue components. When
digital signal processing methods based on microprocessors (e.g., DSP) is used, PWM
signals can be generated by using natural sampling techniques, which use the digitised
modulation signals to compare with the actual timer counts at high repetition rates to
obtain the required time resolutions. With either of the two methods, frequency of the
modulation signal determines the synchronous frequency of the motor while the
carrier frequency determines the switching frequency of the power device of the
inverter.
1.3 PROJECT DEFINITION
The word ‘inverter’ in the context of power-electronics denotes a class of power
conversion (or power conditioning) circuits that operates from a dc voltage source or
a dc current source and converts it into ac voltage or current. The ‘inverter’ does
reverse of what ac-to-dc ‘converter’ does. Even though input to an inverter circuit is a
dc source, it is not uncommon to have this dc derived from an ac source such as utility
ac supply. Thus, for example, the primary source of input power may be utility ac
voltage supply that is ‘converted’ to dc by an ac to dc converter and then ‘inverted’
back to ac using an inverter. Here, the final ac output may be of a different frequency
and magnitude than the input ac of the utility supply.
Irrespective of power flow direction, ‘inverter’ is referred as a circuit that operates
from a stiff dc source and generates ac output. If the input dc is a voltage source, the
inverter is called a voltage source inverter (VSI) similarly a current source inverter
(CSI), where the input to the circuit is a current source. The VSI circuit has direct
control over ‘output (ac) voltage’ whereas the CSI directly controls ‘output (ac)
current’. Shape of voltage waveforms output by an ideal VSI should be independent
of load connected at the output
Some examples where voltage source inverters are used are: uninterruptible power
supply (UPS) units, adjustable speed drives (ASD) for ac motors, electronic frequency
changer circuits etc. commercially available inverter units used in homes and offices
to power some essential ac loads in case the utility ac supply gets interrupted. In such
inverter units, battery supply is used as the input dc voltage source and the inverter
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circuit converts the dc into ac voltage of desired frequency. The achievable magnitude
of ac voltage is limited by the magnitude of input (dc bus) voltage. In ordinary
household inverters the battery voltage may be just 12 volts and the inverter circuit
may be capable of supplying ac voltage of around 10 volts (rms) only. In such cases
the inverter output voltage is stepped up using a transformer to meet the load
requirement of, say, 230 volts.
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CHAPTER-2
PROJECT OVERVIEW
2.1 PROPOSED METHODOLOGY
Voltage source inverters can be classified according to different criterions. They can
be classified according to number of phases they output. Accordingly there are single-
phase or three-phase inverters depending on whether they output single or three-phase
voltages. It is also possible to have inverters with two or five or any other number of
output phases. Inverters can also be classified according to their ability in controlling
the magnitude of output parameters like, frequency, voltage, harmonic content etc.
Some inverters can output only fixed magnitude (though variable frequency) voltages
whereas some others are capable of both variable voltage, variable frequency (VVVF)
output. Output of some voltage source inverters is corrupted by significant amount of
many low order harmonics like 3rd, 5th, 7th, 11th, 13th the desired (fundamental)
frequency voltage. Some other inverters may be free from low order harmonics but
may still be corrupted by some high order harmonics. Inverters used for ac motor
drive applications are expected to have less of low order harmonics in the output
voltage waveform, even if it is at the cost of increased high order harmonics. Higher
order harmonic voltage distortions are, in most ac motor loads, filtered away by the
inductive nature of the load itself. Inverters may also be classified according to their
topologies. Some inverter topologies are suitable for low and medium voltage ratings
whereas some others are more suitable for higher voltage applications. Voltages may
acquire either positive dc bus or negative dc bus potential. For higher voltage
applications it may not be uncommon to have three level or five level inverters.
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BLOCK DIAGRAM OF VSI
FIG2.1 BLOCK DIAGRAM OF INVERTER
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EXPLANATION OF BLOCK DIAGRAM
The switches in bridge configurations of inverters need to be provided with isolated gate (or base) drive signals. The individual control signal for the switches needs to be provided across the gate (base) and source (or emitter) terminals of the particular switch. The gate control signals are low voltage signals referred to the source (emitter) terminal of the switch. For n-channel IGBT and MOSFET switches, when gate to source voltage is more than threshold voltage for turn-on, the switch turns on and when it is less than threshold voltage the switch turns off. The threshold voltage is generally of the order of +5 volts but for quicker switching the turn-on gate voltage magnitude is kept around +15 volts whereas turn-off gate voltage is zero or little negative (around –5 volts). It is to be remembered that the two switches of an inverter-leg are controlled in a complementary manner. When the upper switch of any leg is on the corresponding lower switch remains ‘off’ and vice-versa. When a switch is on its emitter and collector terminals are virtually shorted. Thus with upper switch on the emitter of the upper switch is at positive dc bus potential. Similarly with lower switch on the emitter of upper switch of that leg is virtually at the negative dc bus potential.
Emitters of all the lower switches are solidly connected to the negative line of the dc bus. Since gate control signals are applied with respect to the emitter terminals of the switches, the gate voltages of all the upper switches must be floating with respect to the dc bus line potentials. This calls for isolation between the gate control signals of upper switches and between upper and lower switches. Only the emitters of lower switches of all the legs are at the same potential (since all of them are solidly connected to the negative dc bus) and hence the gate control signals of lower switches need not be isolated among themselves. As should be clear from the above discussion, the isolation provided between upper and lower switches must withstand a peak voltage stress equal to dc bus voltage. Gate-signal isolation for inverter switches is generally achieved by means of optical-isolator (opto-isolator) circuits. The circuit makes use of a commercially available opto-coupler IC, shown within dotted lines in the figure. Input stage of the IC is a light emitting diode (LED) that emits light when forward biased. The light output of the LED falls on reverse biased junction of an optical diode. The LED and the photo-diode are suitably positioned inside the opto-coupler chip to ensure that the light emitted by the LED falls on the photo-diode junction. The gate control pulses for the switch are applied to the input LED through a current limiting resistor of appropriate magnitude. These gate pulses, generated by the gate logic circuit, are essentially in the digital form. A high level of the gate signal may be taken as on command and a low level (at ground level) may be taken as ‘off’ command. Under this assumption, the cathode of the LED is connected to the ground point of the gate-logic card and anode is fed with the logic card output. The circuit on the output (photo-diode) side is connected to a floating dc power supply.
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The control (logic card) supply ground is isolated from the floating-supply ground of the output. In the figure the two grounds have been shown by two different symbols. The schematic connection shown in the figure indicates that the photo-diode is reverse biased. A resistor in series with the diode indicates the magnitude of the reverse leakage current of the diode. When input signal to LED is high, LED conducts and the emitted light falls on the reverse biased p-n junction. Irradiation of light causes generation of significant number of electron-hole pairs in the depletion region of the reverse biased diode. As a result magnitude of reverse leakage current of the diode increases appreciably. The resistor connected in series with the photo-diode now has higher voltage drop due to the increased leakage current. A signal comparator circuit senses this condition and outputs a high level signal, which is amplified before being output.
Thus an isolated and amplified gate signal is obtained and may directly be connected to the gate terminal of the switch (often a small series resistor, as suggested by the switch manufacturer, is put between the output signal and the gate terminal of the switch).
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VSI,CSI AND TO DETERMINE THD USING MATLAB
CHAPTER-3
VSI CIRCUIT DIAGRAMS ,MODELS,SUBSYSTSEM AND
WAVEFORMS
3.1 SINGLE PHASE HALF BRIDGE VSI WITH RESESTIVE
LOAD
FIG.-3.1 CIRCUIT DIAGRAM OF SINGLE PHASE HALF BRIDGE VSI
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FIG.-3.2 VOLTAGE AND CURRENT WAVEFORM
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In the power topology of a half-bridge VSI, where two large capacitors are required
to provide a neutral point N, such that each capacitor maintains a constant voltage
(Vi)/2. Because the current harmonics injected by the operation of the inverter are
low-order harmonics, a set of large capacitors (C+ and C-) is required. It is clear that
both switches S+ and S- cannot be ON simultaneously because a short circuit across
the dc link voltage source Vi would be produced. There are two defined (states 1 and
2) and one undefined (state 3) switch state as shown in Table 1. In order to avoid the
short circuit across the dc bus and the undefined ac output voltage condition, the
modulating technique should always ensure that at any instant either the top or the
bottom switch of the inverter leg is on.
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3.2 SINGLE PHASE VOLTAGE SOURCE INVERTER WITH
(RL) LOAD
Without pwm:-
FIG.-3.3 MODEL OF SINGLE PHASE HALF BRIDGE VSI WITHOUT
PWM
With pwm:-
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FIG.-3.4 MODEL OF SINGLE PHASE HALF BRIDGE VSI WITH
PWM
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PARAMETER
TABLE-3.1
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FIG.-3.5 THD VALUES
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3.3 SINGLE PHASE FULL BRIDGE VSI WITH RESISTIVE
LOADS
This inverter is similar to the half-bridge inverter. however, a second leg provides the
neutral point to the load. As expected, both switches S1+ and S1- (or S2+ and S2-)
cannot be on simultaneously because a short circuit across the dc link voltage source
Vi would be produced. There are four defined (states 1, 2, 3, and 4) and one undefined
(state 5) switch states as shown in Table 2. The undefined condition should be
avoided so as to be always capable of defining the ac output voltage. It can be
observed that the ac output voltage can take values up to the dc link value Vi, which is
twice that obtained with half-bridge VSI topologies. Several modulating techniques
have been developed that are applicable to full-bridge VSIs. Among them are the
PWM (bipolar and unipolar) techniques.
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FIG.-3.6 CIRCUIT DIAGRAM OF SINGLE FULL HALF BRIDGE VSI
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FIG.-3.7 VOLTAGE WAVEFORM
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FIG.-3.8 FFT ANALYSIS
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3.3 THREE PHASE FULL BRIDGE VSI WITH RESISTIVE
LOADS
FIG.-3.9 CIRCUIT DIAGRAM OF THREE PHASEFULL BRIDGE VSI
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Single-phase VSIs cover low-range power applications and three-phase VSIs cover
the medium- to high-power applications. The main purpose of these topologies is to
provide a three-phase voltage source where the amplitude phaseand frequency of the
voltages should alwaysbecontrollable.
Although most of the applications require sinusoidal voltage waveforms (e.g., ASDs,
UPSs, FACTS, VAR compensators), arbitrary voltages are also required in some
emerging applications (e.g., active filters, voltage compensators).The standard three-
phase VSI topology is shown in Fig eight valid switch states are given in Table . As in
single-phase VSIs, the switches of any leg of the inverter (S1 and S4, S3 and S6, or
S5 and S2) cannot be switched on simultaneously because this would result in short
circuit across the dc link voltage supply. Similarly, in order to avoid undefined states
in the VSI, and thus undefined ac output line voltages,the switches of any leg of the
inverter cannot be switched off simultaneously as this will result in voltages that will
depend upon the respective line current polarity. Of the eight valid states, two of them
produce zero ac line voltages. In this case, the ac line currents freewheel through
either the upper or lower components. The remaining states produce non-zero ac
output voltages. In order to generate a given voltage waveform, the inverter moves
from one state to another. Thus the resulting ac output line voltages consist of discrete
values of voltages that are Vi , 0, and -Vi for the topology shown in Fig. The selection
of the states in order to generate the given waveform is done by the modulating
technique that should ensure the use of only the technique to control the motor
armature, ac adjustable-speed induction motor drives employ mostly a voltage-source
inverter topology. Both scalar and vector control of induction motors are used in this
approach, the latter requiring current control of the. Although energy storage is more
practical and efficient in capacitors than in inductors, the use of VSIs may result in
reduced drive reliability due to the high of the pulse width modulated inverter output
voltage. However, the can be significantly lowered by filtering the VSI output
voltage. This can be achieved by adding filtering reactors, or better, LC filters.
The current-source inverter topology offers a number of inherent advantages,
including short-circuit protection, the output current being limited by the regulated
dc-bus current; 2) low output voltage .
AC drive CSI based on a phase-controlled front-end rectifier (50% nominal load and
60-Hz output). (a) Power topology. (b) Supply phase voltage and supply line current).
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(c) DC rectifier voltage (vr) and dc-link current) CSI line current and load line
voltage(vab)Load phase voltage (vla) and load line current tages over conventional
CSI motor drive implementations:
1) the gating signals are directly generated by the spacevectordigital modulator (extra
circuitry is only necessary toensure overlaps
2) the potential resonances are eliminateddue to the feedback-based voltage
controller; 3) the stresseson power switches and the overall losses are always
minimumdue to the minimum dc-link current operation; and 4) due tothe constant
inverter modulation index operation, the motorvoltage harmonic distortion is constant,
which minimizes the induction motor losses and allows an accurate output
filterdesign. These features make the CSI drive an interesting alternative to VSI-based
drives operating at similar switching frequency, when the requested fundamental
reactive power could be disregarded with respect to output power.A complete
comparison with standard CSI-based ac drivesis also presented. Key performance
indices, such as harmonic distortion (THD), PF, and time response are evaluated
andtabulated for both the standard and the proposed schemes. Experimental results are
given for a 2-kVA induction motor drive.
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3.4 THREE PHSE FULL BRIDGE VSI 180 DEGREE MODE WITH
RESISTIVE LOAD
FIG.-3.10 MODEL
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FIG.-3.11 SUBSYSTEM
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PARAMETER
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FIG.-3.12 PHASE AND LINE VOLTAGE AND CURRENT WAVEFORM
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FIG.-3.13 LINE VOLTAGE FFT ANALYSIS
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FIG.-3.14 LINE CURRENT THD AND FFT ANALYSIS
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3.5THREE PHASE FULL BRIDGE VSI 120 DEGREE MODE
WITH RL LOAD
FIG-3.15PHASE FULL BRIDGE VSI 120*MODE WITH RL LOAD
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FIG-3.16 SUBSYSTEM
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3.6THREE PHASE FULL BRIDGE VSI 120 DEGREE MODE
WITH R LOAD
FIG-3.17 MODEL
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FIG-3.18 SUBSYSTEM
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3.6THREE PHASE FULL BRIDGE VSI 180 DEGREE MODE
WITH RL LOAD
FIG-3.19 MODEL
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FIG-3.20 SUBSYSTEM
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3.7SINGLE PHASE FULL BRIDGE VOLTAGE SOURCE
INVERTER WITH (RL) LOAD
FIG-3.21 MODEL
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FIG-3.22 SUBSYSTEM
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FIG-3.23 FFT ANALYSIS CURRENT
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FIG-3.24 FFT ANALYSIS VOLTAGE
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FIG-3.25 VOLTAGE AND CURRENT WAVEFORM
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3.8 THREE PHASE FULL BRIDGE VSI 180 DEGREE MODE
WITH INDUCTION MOTOR LOAD
Model:
FIG.-3.26 MODEL OF THREE FULL BRIDGE VSI 180 DEGREE MODE
WITH INDUCTION MOTOR LOAD
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SUBSYSTEM OF VSI
FIG.-3.27 SUBSYSTEM OF VSI
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PARAMETERS OF IM
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FIG3.28 CHARACTERISTIC OF 120 MODE IM
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IM THD FOR TORQE
FIG3.29 FFT ANALYSIS FOR TORQE
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CHAPTER-4
CSI CIRCUIT DIAGRAMS ,MODELS,SUBSYSTSEM
AND WAVEFORMS
4.1 SINGLE PHASE FULL BRIDGE CSI WITH RESESTIVE
LOAD
FIG.-4.1 CIRCUIT DIAGRAM OF SINGLE PHASE FULL BRIDGE CSI
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FIG.-4.2 MODEL DIAGRAM OF SINGLE PHASE FULL BRIDGE CSI
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FIG.-4.3 VOLTAGE AND CURRENT WAVEFORM
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FIG.-4.4 CURRENT FFT ANALYSIS
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FIG.-4.5 VOLTAGE FFT ANALYSIS
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CHAPTER-5
ADVANTAGE
5.1 FFT ANLYSIS
There are numerous industries where the surroundings are unsafe for the employment
of human labor due to the existence of hazardous environments. Robots can be used
effectively in such environments where handling of radioactive materials is involved,
such as hospitals or nuclear establishments, where direct exposure to human beings
can be dangerous for their health.
5.2 IMPROVE POWER FACTER
Robots perform operations with superior exactitude, ensure uniformity of production
due to which rejections are minimized, and reduce losses. Measurements and
movements of tools being utilized are more accurate. Thus, the quality of the product
manufactured is improved manifold compared to the performance by human beings.
5.3 TO IMPROVE IFFICIENCY
Robots have the ability to work continuously without pause, unlike human labor for
which breaks and vacation are essential. Thus, production is increased by the
utilization of robots in industrial applications, and consequently profits of the
production unit are increased.
5.4 EASY TO SELECT LOAD ACROSS VSI AND CSI
In many production establishments work required to be executed is awfully boring,
being cyclic and repetitive, due to which it is difficult for the operators to remain fully
dedicated to their tasks and generate interest in their work. When tasks are
monotonous, workers tend to be careless, thereby increasing the probability of
accidents and malfunctions of machines. Utilization of robots has eliminated
problems associated with boredom in production.
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CHAPTER-6
APPLICATIONS AND LIMITATIONS
6.1APPLICATIONS
1. Monitoring and Security System
2. Tracking and navigation.
3. SURVIALLIANCE
4. NAVY AND ARMY
5. Rescuing purpose
6. Space mission
7. Find its path in difficult terrains and battle field
8. Use of renewable energy
9. Robotics and solarbotics
10. Obstacle detector
8.2 LIMITATIONS OF VSI AND CSI
An article about the advantages of sollarbotics wouldn't be complete without some
discussion of the limitations of sollarbotics. In spite of the very useful set of
advantages of sollarbotics discussed above, there are some tasks for which human
beings are better suited than robots. For example:
1. Robots are not suited for creativity or innovation
2. Robots are not capable of independent thinking
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3. Robots are not good at learning from their mistakes
4. Robots are not as suitable for making complicated decisions
5. Robots can't as readily adapt quickly to changes in the surroundings
CHAPTER-7
FUTURE SCOPE
CHAPTER-8
CONCLUSION
Project spy amphibious solla roller acronym as SASR is completed by the use of
theory explained in the previous chapters. We faced some problems in the completion
of this project like balancing in the water, proper rpm to the motors & finally we
tackled all the problems with the help of our project guide the ratings & their
parameters are kept in mind in all applications project is tested successfully on land as
well as on water and it has completed our objective i.e. It is moving on land and of
course on water and takes videos sending them to ground stations where they can be
analyzed with the help of display devices particularly TV
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VSI,CSI AND TO DETERMINE THD USING MATLAB
Thus project SASR is completed successfully which covers basic electrical &
electronics concepts and basic microcontroller programming skills.
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