inverter based projects
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EXPERT SYSTEMS AND SOLUTIONS
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Cell: 9952749533www.researchprojects.info
PAIYANOOR, OMR, CHENNAI
Call For Research Projects Final
year students of B.E in EEE, ECE, EI,
M.E (Power Systems), M.E (Applied
Electronics), M.E (Power Electronics)
Ph.D Electrical and Electronics.
Students can assemble their hardware in our
Research labs. Experts will be guiding the projects.
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Inverter Introduction
An inverter is an electrical or electro-mechanical device that converts direct current(DC) to alternating current (AC); the resultingAC can be at any required voltage with the useof a transformer. Inverters are used in a wide
range of applications, from small switchingpower supplies in computers, to large electricutility applications that transport bulk power.Not to be confused with a logical inverter, the
electrical inverter is in effect a high-powerelectronic oscillator. It is so named becauseearly mechanical AC to DC converters weremade to work in reverse, and thus were"inverted", to convert DC to AC. The inverter
performs the opposite function of a rectifier.
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Applications
The following are examples of inverter applications.
DC power source utilization An inverter converts the DC electricity from
sources such as batteries, solar panels, or fuel
cells to AC electricity. The electricity can be atany required voltage; in particular it can operateAC equipment designed for mains operation, orrectified to produce DC at any desired voltage.
This energy can be fed back into the national
grid using solar inverters which synchronisetheir output waveform with that of the grid anddeliver a voltage slightly above that of the mainsin order to allow current to flow. They can alsoswitch off automatically in the event of ablackout.
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Uninterruptible power supplies
An uninterruptible power supply is adevice which supplies the storedelectrical power to the load in case of raw power cut-off or blackout. One
type of UPS uses batteries to storepower and an inverter to supply ACpower from the batteries when main
power is not available. When mainpower is restored, a rectifier is used tosupply DC power to recharge thebatteries.
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Induction heating
Inverters convert low frequency main AC power
to a higher frequency for use in inductionheating. To do this, AC power is first rectified toprovide DC power. The inverter then changesthe DC power to high frequency AC power.
High-voltage direct current (HVDC) powertransmission
With HVDC power transmission, AC power isrectified and high voltage DC power istransmitted to another location. At thereceiving location, an inverter in a staticinverter plant converts the power back to AC.
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V ariable-frequency drive
A variable-frequency drive controls theoperating speed of an AC motor bycontrolling the frequency and voltageof the power supplied to the motor. An
inverter provides the controlled power.In most cases, the variable-frequencydrive includes a rectifier so that DCpower for the inverter can be provided
from main AC power. Since an inverteris the key component, variable-frequency drives are sometimes calledinverter drives or just inverters.
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Electric vehicle drives
Adjustable speed motor control inverters arecurrently used to power the traction motor insome electric locomotives and diesel-electriclocomotives as well as some battery electric
vehicles and hybrid electric highway vehiclessuch as the Toyota Prius. Variousimprovements in inverter technology arebeing developed specifically for electricvehicle applications.[1] In vehicles withregenerative braking, the inverter also takespower from the motor (noe acting as agenerator) and stores it in the batteries.
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The general case
A transformer allows ac power to beconverted to any desired voltage, but atthe same frequency. Inverters, plus
rectifiers for dc, can be designed toconvert from any voltage, ac or dc, to anyother voltage, also ac or dc, at anydesired frequency. The output power can
never exceed the input power, butefficiencies can be high, with only a smallproportion of the power dissipated aswaste heat.
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Circuit description
Basic designs In one simple inverter circuit, DC power is
connected to a transformer through thecentre tap of the primary winding. A
switch is rapidly switched back and forthto allow current to flow back to the DCsource following two alternate pathsthrough one end of the primary winding
and then the other. The alternation of thedirection of current in the primary windingof the transformer produces alternatingcurrent (AC) in the secondary circuit.
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The electromechanical version of the switchingdevice includes two stationary contacts and aspring supported moving contact. The springholds the movable contact against one of thestationary contacts and an electromagnet pulls
the movable contact to the opposite stationarycontact. The current in the electromagnet isinterrupted by the action of the switch so thatthe switch continually switches rapidly back andforth. This type of electromechanical inverterswitch, called a vibrator or buzzer, was onceused in vacuum tube automobile radios. Asimilar mechanism has been used in door bells,buzzers and tattoo guns.
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As they havebecome available,transistors and
various other typesof semiconductorswitches have beenincorporated into
inverter circuitdesigns.
Simple inverter circuit shown with an
electromechanical switch and with a
transistor switch
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Output waveforms The switc h in t he simple inverter described above
produces a square voltage waveform as opposed tot he sinusoidal waveform t hat is t he usual waveformof an AC power supply. Using Fourier analysis ,
periodic waveforms are represented as t he sum of an infinite series of sine waves. The sine wave t hat has t he same frequency as t he original waveform iscalled t he fundamental component. The ot her sinewaves, called harmonics, t hat are included in t heseries have frequencies t hat are integral multiples of t he fundamental frequency.
The quality of t he inverter output waveform can beexpressed by using t he Fourier analysis data tocalculate t he total harmonic distortion ( THD). Thetotal harmonic distortion is t he square root of t hesum of t he squares of t he harmonic voltages divided by t he fundamental voltage:
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The quality of outputwaveform that is neededfrom an inverter dependson the characteristics of the connected load. Someloads need a nearly perfect
sine wave voltage supply inorder to work properly.Other loads may workquite well with a squarewave voltage.
Square waveform with fundamental
sine wave component, 3rd harmonicand 5th harmonic
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Advanced designs
There are many differentpower circuit topologiesand control strategies
used in inverter designs.Different designapproaches addressvarious issues that may
be more or less importantdepending on the waythat the inverter isintended to be used.
H- bridge inverter circuit with
transistor switches and antiparallel
diodes
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The issue of waveform quality can be addressedin many ways. Capacitors and inductors can beused to filter the waveform. If the designincludes a transformer, filtering can be appliedto the primary or the secondary side of thetransformer or to both sides. Low-pass filters are
applied to allow the fundamental component of the waveform to pass to the output whilelimiting the passage of the harmoniccomponents. If the inverter is designed toprovide power at a fixed frequency, a resonant
filter can be used. For an adjustable frequencyinverter, the filter must be tuned to a frequencythat is above the maximum fundamentalfrequency.
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Since most loads contain inductance, feedback
rectifiers or antiparallel diodes are oftenconnected across each semiconductor switch toprovide a path for the peak inductive loadcurrent when the switch is turned off. The
antiparallel diodes are somewhat similar to thefreew heeling diodes used in AC/DC convertercircuits.
Fourier analysis reveals that a waveform, like asquare wave, that is antisymmetrical about the180 degree point contains only odd harmonics,the 3rd, 5th, 7th etc. Waveforms that havesteps of certain widths and heights eliminate or
³cancel´ additional harmonics.
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For example, by inserting a zero-voltage stepbetween the positive and negative sections of the
square-wave, all of the harmonics that are divisibleby three can be eliminated. That leaves only the 5th,7th, 11th, 13th etc. The required width of the stepsis one third of the period for each of the positive andnegative voltage steps and one sixth of the period
for each of the zero-voltage steps Changing the square wave as described above is an
example of pulse-width modulation (PWM).Modulating, or regulating the width of a square-wave pulse is often used as a method of regulating
or adjusting an inverter's output voltage. Whenvoltage control is not required, a fixed pulse widthcan be selected to reduce or eliminate selectedharmonics. Harmonic elimination techniques aregenerally applied to the lowest harmonics because
filtering is more effective at high frequencies than atlow frequencies.
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M ultiple pulse-widt h or carrier based PWM
control schemes produce waveforms that arecomposed of many narrow pulses. Thefrequency represented by the number of narrowpulses per second is called the switc hingfrequency or carrier frequency . These controlschemes are often used in variable-frequencymotor control inverters because they allow awide range of output voltage and frequencyadjustment while also improving the quality of
the waveform. Multilevel inverters provide another approach to
harmonic cancellation. Multilevel invertersprovide an output waveform that exhibits
multiple steps at several voltage levels.
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For example, it is possible to produce a
more sinusoidal wave by having split-raildirect current inputs at two voltages, orpositive and negative inputs with a centralground. By connecting the inverter output
terminals in sequence between thepositive rail and ground, the positive railand the negative rail, the ground rail and
the negative rail, then both to the groundrail, a stepped waveform is generated atthe inverter output. This is an example of a three level inverter: the two voltages
and ground. [2]
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Three phase inverters
Three-phase inverters are used for variable-frequency drive applications and for high powerapplications such as HVDC power transmission.A basic three-phase inverter consists of threesingle-phase inverter switches each connected
to one of the three load terminals. For the mostbasic control scheme, the operation of the threeswitches is coordinated so that one switchoperates at each 60 degree point of thefundamental output waveform. This creates a
line-to-line output waveform that has six steps.The six-step waveform has a zero-voltage stepbetween the positive and negative sections of the square-wave such that the harmonics thatare multiples of three are eliminated asdescribed above.
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When carrier-based
PWM techniques areapplied to six-stepwaveforms, the basic
overall shape, orenvelope, of thewaveform is retainedso that the 3rd
harmonic and itsmultiples arecancelled. 3- phase inverter with wye
connected load
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To construct inverters with higher power ratings,two six-step three-phase inverters can beconnected in parallel for a higher current rating orin series for a higher voltage rating. In eithercase, the output waveforms are phase shifted to
obtain a 12-step waveform. If additional invertersare combined, an 18-step inverter is obtainedwith three inverters etc. Although inverters areusually combined for the purpose of achievingincreased voltage or current ratings, the quality of
the waveform is improved as well.
3- phase inverter switching circuit showing 6-step
switching sequence and waveform of voltage between
terminals A and C