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Encyclopedia > Inverter (electrical)

An inverter is an electronic circuit that converts direct current (DC) to alternating current (AC). Inverters are used in a wide range of applications, from small switching power supplies in computers, to large electric utility applications that transport bulk power. In digital logic, an inverter is a logic gate which inverts the digital signal driven on its input. ... An inverter may be: inverter (electrical), which converts direct current (DC) to alternating current (AC) inverter (logic gate), also called a NOT gate. ... An electronic circuit is an electrical circuit that also contains active electronic devices such as transistors or vacuum tubes. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... City lights viewed in a motion blurred exposure. ... A switched-mode power supply, switch-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches power transistors (such as MOSFETs) rapidly on and off in order to stabilize the output voltage or current. ... An electric utility is a company (often a public utility) that engages in the generation, transmission, and distribution of electricity for sale generally in a regulated market. ...


The inverter is so named because it performs the opposite function of a rectifier. AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ...

Contents

Inverter applications

The following are examples of inverter applications.


DC power source utilization

Inverter designed to provide 115 VAC from the 12 VDC source provided in an automobile

An inverter allows the 12 or 24 volt (battery) DC power available in an automobile or from solar panels to supply AC power to operate equipment that is normally supplied from a main power source. Image File history File links Download high resolution version (1024x736, 78 KB) Summary Radio Shack 12 VDC to 115 VAC 140 watt portable power inverter photographed by the contributor. ... Image File history File links Download high resolution version (1024x736, 78 KB) Summary Radio Shack 12 VDC to 115 VAC 140 watt portable power inverter photographed by the contributor. ...


Inverters are also used to provide a source of AC power from photovoltaic solar cells and fuel cell power supplies. Photovoltaic tree in Styria, Austria Photovoltaics, or PV for short, is a solar power technology that uses solar cells or solar photovoltaic arrays to convert light from the sun directly into electricity. ... A solar cell, made from a monocrystalline silicon wafer A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. ... A fuel cell is an electrochemical device similar to a battery, but differing from the latter in that it is designed for continuous replenishment of the reactants consumed; i. ...


Uninterruptible power supplies

One type of uninterruptible power supply uses batteries to store power and an inverter to supply AC power from the batteries when main power is not available. When main power is restored, a rectifier is used to supply DC power to recharge the batteries. An uninterruptible power supply (UPS), uninterruptible power source or sometimes called a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ...


Induction heating

Inverters convert low frequency main AC power to a higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power. A semiconductor induction heater with a small inductor Induction heating is the process of heating a metal object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ...


High-voltage direct current (HVDC) power transmission

With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC. HVDC or high-voltage, direct current electric power transmission systems contrast with the more common alternating current systems as a means for the bulk transmission of electrical power. ... A static inverter station is the terminal equipment for a high voltage direct current transmission line, in which direct current is converted to three-phase alternating current, and, usually, the reverse. ...


Variable-frequency drives

A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters. Small Variable Frequency Drive (VFD) A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ...


Electric vehicle drives

Adjustable speed motor control inverters are currently used in some electric locomotives and diesel-electric locomotives as well as some battery electric vehicles and hybrid electric highway vehicles such as the Toyota Prius. Various improvements in inverter technology are being developed specifically for electric vehicle applications.[1] An Electric Locomotive is a locomotive powered by electric motors which draws current from an overhead wire, a third rail, or an on-board storage device such as a battery or a flywheel energy storage system. ... A number of vehicles use a diesel-electric powerplant for providing locomotion. ... Battery Electric Vehicles or BEVs are electric vehicles whose main energy storage is in the chemical energy of batteries. ... A hybrid vehicle (HV) is a vehicle that uses two or more distinct power or fuel sources such as: An on-board rechargeable energy storage system (RESS) and a fueled power source (internal combustion engine) for vehicle propulsion Human or sail powered bicycle with battery assist A human-powered or... Hybrid Synergy Drive The Toyota Prius is one of the first mass-produced and marketed hybrid electric vehicles. ...


Inverter circuit description

Simple inverter circuit shown with an electromechanical switch and with a transistor switch

Image File history File links Inverter_ckt_01cjc. ... Image File history File links Inverter_ckt_01cjc. ...

Basic inverter designs

In one simple inverter circuit, DC power is connected to a transformer through the centre tap of the primary winding. A switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding and then the other. The alternation of the direction of current in the primary winding of the transformer produces alternating current (AC) in the secondary circuit. Figure 1:Three-phase pole-mounted step-down transformer. ... City lights viewed in a motion blurred exposure. ...


The electromechanical version of the switching device includes two stationary contacts and a spring supported moving contact. The spring holds the movable contact against one of the stationary contacts and an electromagnet pulls the movable contact to the opposite stationary contact. The current in the electromagnet is interrupted by the action of the switch so that the switch continually switches rapidly back and forth. This type of electromechanical inverter switch, called a vibrator or buzzer, was once used in vacuum tube automobile radios. A similar mechanism has been used in door bells, buzzers and tattoo guns. In early electronics vibrators were used in inverter circuits to provide an alternating current (AC) electric power supply from a direct current (DC) source. ... Structure of a vacuum tube diode Structure of a vacuum tube triode In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. ... Traditional 2 coil tattoo machine 4 views A tattoo gun, is a hand held machine for creating a tattoo, i. ...


As they have become available, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs. Photo of transistor types (tape measure marked in centimeters) Transistor in the SMD form factor The transistor is a solid state semiconductor device used for amplification and switching. ... A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ...

Square waveform with fundamental sine wave component, 3rd harmonic and 5th harmonic
Square waveform with fundamental sine wave component, 3rd harmonic and 5th harmonic

Image File history File links Squarewave01CJC.png‎ Square wave with fundamental sine wave component, 3rd harmonic and 5th harmonic (by C J Cowie using MS Excel,MicroGrafx Designer and Adobe Photoshop Elements) I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document... Image File history File links Squarewave01CJC.png‎ Square wave with fundamental sine wave component, 3rd harmonic and 5th harmonic (by C J Cowie using MS Excel,MicroGrafx Designer and Adobe Photoshop Elements) I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document...

Inverter output waveforms

The switch in the simple inverter described above produces a square voltage waveform as opposed to the sinusoidal waveform that is the usual waveform of an AC power supply. Using Fourier analysis, periodic waveforms are represented as the sum of an infinite series of sine waves. The sine wave that has the same frequency as the original waveform is called the fundamental component. The other sine waves, called harmonics, that are included in the series have frequencies that are integral multiples of the fundamental frequency. Waveform quite literally means the shape and form of a signal, such as a wave moving across the surface of water, or the vibration of a plucked string. ... In trigonometry, an ideal sine wave is a waveform whose graph is identical to the generalized sine function y = Asin[ω(x − α)] + C, where A is the amplitude, ω is the angular frequency (2π/P where P is the wavelength), α is the phase shift, and C... Harmonic analysis is the branch of mathematics which studies the representation of functions or signals as the superposition of basic waves. ... In mathematics, a periodic function is a function that repeats its values after some definite period has been added to its independent variable. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ...


The quality of the inverter output waveform can be expressed by using the Fourier analysis data to calculate the total harmonic distortion (THD). The total harmonic distortion is the square root of the sum of the squares of the harmonic voltages divided by the fundamental voltage: The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental. ...


mbox{THD} = {sqrt{V_2^2 + V_3^2 + V_4^2 + cdots + V_n^2} over V_1}


The quality of output waveform that is needed from an inverter depends on the characteristics of the connected load. Some loads need a nearly perfect sine wave voltage supply in order to work properly. Other loads may work quite well with a square wave voltage.


More advanced inverter designs

H-bridge inverter circuit with transistor switches and antiparallel diodes
H-bridge inverter circuit with transistor switches and antiparallel diodes

There are many different power circuit topologies and control strategies used in inverter designs. Different design approaches address various issues that may be more or less important depending on the way that the inverter is intended to be used. Image File history File links H-bridge_inverter_cjc. ... Image File history File links H-bridge_inverter_cjc. ... An H-bridge is an electronic circuit which enables DC electric motors to be run forwards or backwards. ... The topology of an electronic circuit is the basic configuration of the circuit without regard to specific component values or ratings. ... It has been suggested that this article or section be merged with Control theory. ...


The issue of waveform quality can be addressed in many ways. Capacitors and inductors can be used to filter the waveform. If the design includes a transformer, filtering can be applied to the primary or the secondary side of the transformer or to both sides. Low-pass filters are applied to allow the fundamental component of the waveform to pass to the output while limiting the passage of the harmonic components. If the inverter is designed to provide power at a fixed frequency, a resonant filter can be used. For an adjustable frequency inverter, the filter must be tuned to a frequency that is above the maximum fundamental frequency. See Capacitor (component) for a discussion of specific types. ... An inductor is a passive electrical device employed in electrical circuits for its property of inductance. ... Television signal splitter consisting of a hi-pass and a low-pass filter. ... A low-pass filter is a filter that passes low frequencies well, but attenuates (or reduces) frequencies higher than the cutoff frequency. ... This article is about resonance in physics. ...


Since most loads contain inductance, feedback rectifiers or antiparallel diodes are often connected across each semiconductor switch to provide a path for the peak inductive load current when the semiconductor is turned off. The antiparallel diodes are somewhat similar to the freewheeling diodes used in AC/DC converter circuits. A rectifier is one or more diodes arranged for converting alternating current (AC) to direct current (DC). ... Two antiparallel molecules run side-by-side in opposite directions. ... Types of diodes A diode functions as the electronic version of a one-way valve. ... A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ...


Fourier analysis reveals that a waveform, like a square wave, that is antisymmetrical about the 180 degree point contains only odd harmonics, the 3rd, 5th, 7th etc. Waveforms that have steps of certain widths and heights eliminate or “cancel” additional harmonics. For example, by inserting a zero-voltage step between the positive and negative sections of the square-wave, all of the harmonics that are divisible by three can be eliminated. That leaves only the 5th, 7th, 11th, 13th etc. The required width of the steps is one third of the period for each of the positive and negative 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. When voltage control is not required, a fixed pulse width can be selected to reduce or eliminate selected harmonics. Harmonic elimination techniques are generally applied to the lowest harmonics because filtering is more effective at high frequencies than at low frequencies. Multiple pulse-width or carrier based PWM control schemes produce waveforms that are composed of many narrow pulses. The frequency represented by the number of narrow pulses per second is called the switching frequency or carrier frequency. These control schemes are often used in variable-frequency motor control inverters because they allow a wide range of output voltage and frequency adjustment while also improving the quality of the waveform. Pulse-width modulation of a signal or power source involves the modulation of its duty cycle, to either convey information over a communications channel or control the amount of power sent to a load. ...


Multilevel inverters provide another approach to harmonic cancellation. Multilevel inverters provide an output waveform that exhibits multiple steps at several voltage levels. For example, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the inverter output terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped waveform is generated at the inverter output. This is an example of a three level inverter: the two voltages and ground. [2] Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... It has been suggested that Ground conductor be merged into this article or section. ...


Three phase inverters

3-phase inverter with wye connected load
3-phase inverter with wye connected load

Three-phase inverters are used for variable-frequency drive applications and for high power applications such as HVDC power transmission. A basic three-phase inverter consists of three single-phase inverter switches each connected to one of the three load terminals. For the most basic control scheme, the operation of the three switches is coordinated so that one switch operates at each 60 degree point of the fundamental output waveform. This creates a line-to-line output waveform that has six steps. The six-step waveform has a zero-voltage step between the positive and negative sections of the square-wave such that the harmonics that are multiples of three are eliminated as described above. When carrier-based PWM techniques are applied to six-step waveforms, the basic overall shape, or envelope, of the waveform is retained so that the 3rd harmonic and its multiples are cancelled. Image File history File links 3-phase_inverter_cjc. ... Image File history File links 3-phase_inverter_cjc. ... Three-phase power transformer which is the sole transfer point for electricity to a suburban shopping mall in Canada. ... Small Variable Frequency Drive (VFD) A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. ... HVDC or high-voltage, direct current electric power transmission systems contrast with the more common alternating current systems as a means for the bulk transmission of electrical power. ...

3-phase inverter switching circuit showing 6-step switching sequence and waveform of voltage between terminals A and C
3-phase inverter switching circuit showing 6-step switching sequence and waveform of voltage between terminals A and C

To construct inverters with higher power ratings, two six-step three-phase inverters can be connected in parallel for a higher current rating or in series for a higher voltage rating. In either case, the output waveforms are phase shifted to obtain a 12-step waveform. If additional inverters are combined, an 18-step inverter is obtained with three inverters etc. Although inverters are usually combined for the purpose of achieving increased voltage or current ratings, the quality of the waveform is improved as well. Image File history File links 6-step_waveform_CJC.png‎ 3-phase inverter circuit showing six-step switching sequence and voltage between terminals A and C -- drawn by C J Cowie using MicroGrafx Designer I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document... Image File history File links 6-step_waveform_CJC.png‎ 3-phase inverter circuit showing six-step switching sequence and voltage between terminals A and C -- drawn by C J Cowie using MicroGrafx Designer I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document...


History

Early inverters

From the late nineteenth century through the middle of the twentieth century, DC-to-AC power conversion was accomplished using rotary converters or motor-generator sets. In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyratron. Power conversion is the process of converting power from one form into another. ... Radio station motor-generator, converting from low to the high voltage power supply. ... In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ... Gas filled tubes are arrangements of electrodes in a gas within an insulating, temperature-resistant envelope. ... A thyratron is a type of gas filled tube used as a high energy electrical switch. ...


The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator's commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be separately generated from the AC; with a synchronous converter, in a certain sense it can be considered to be "mechanically rectifed AC". Given the right auxiliary and control equipment, an M-G set or rotary converter can be "run backwards", converting DC to AC. Hence an inverter is an inverted converter.[3][4]


Controlled rectifier inverters

Since early transistors were not available with sufficient voltage and current ratings for most inverter applications, it was the 1957 introduction of the thyristor or silicon-controlled rectifier (SCR) that initiated the transition to solid state inverter circuits. Circuit symbol for a thyristor The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. ... SCR schematic symbol A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a 4-layer solid state device that controls current flow. ... In electronics, solid state circuits are those that do not contain vacuum tubes. ...

12-pulse line-commutated inverter circuit
12-pulse line-commutated inverter circuit

The commutation requirements of SCRs are a key consideration in SCR circuit designs. SCRs do not turn off or commutate automatically when the gate control signal is shut off. They only turn off when the forward current is reduced to zero through some external process. For SCRs connected to an AC power source, commutation occurs naturally every time the polarity of the source voltage reverses. SCRs connected to a DC power source usually require a means of forced commutation that forces the current to zero when commutation is required. The least complicated SCR circuits employ natural commutation rather than forced commutation. With the addition of forced commutation circuits, SCRs have been used in the types of inverter circuits described above. Image File history File links 12-pulse_inverter_cjc. ... Image File history File links 12-pulse_inverter_cjc. ...


In applications where inverters transfer power from a DC power source to an AC power source, it is possible to use AC-to-DC controlled rectifier circuits operating in the inversion mode. In the inversion mode, a controlled rectifier circuit operates as a line commutated inverter. This type of operation can be used in HVDC power transmission systems and in regenerative braking operation of motor control systems. A regenerative brake is an apparatus, a device or system which allows a vehicle to recapture and store part of the kinetic energy that would otherwise be lost to heat when braking. ...


Another type of SCR inverter circuit is the current source input (CSI) inverter. A CSI inverter is the dual of a six-step voltage source inverter. With a current source inverter, the DC power supply is configured as a current source rather than a voltage source. The inverter SCRs are switched in a six-step sequence to direct the current to a three-phase AC load as a stepped current waveform. CSI inverter commutation methods include load commutation and parallel capacitor commutation. With both methods, the input current regulation assists the commutation. With load commutation, the load is a synchronous motor operated at a leading power factor. In electronics, the word dual refers to two devices or circuits that have mathematical descriptions that are identical except that voltages in one correspond to currents in the other, and vice versa. ... It has been suggested that this article or section be merged with Voltage source. ... It has been suggested that this article or section be merged with Current source. ...


As they have become available in higher voltage and current ratings, semiconductors such as transistors that can be turned off by means of control signals have become the preferred switching components for use in inverter circuits.


Rectifier and inverter pulse numbers

Rectifier circuits are often classified by the number of current pulses that flow to the DC side of the rectifier per cycle of AC input voltage. A single-phase half-wave rectifier is a one-pulse circuit and a single-phase full-wave rectifier is a two-pulse circuit. A three-phase half-wave rectifier is a three-pulse circuit and a three-phase full-wave rectifier is a six-pulse circuit.[5] AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ...


With three-phase rectifiers, two or more rectifiers are sometimes connected in series or parallel to obtain higher voltage or current ratings. The rectifier inputs are supplied from special transformers that provide phase shifted outputs. This has the effect of phase multiplication. Six phases are obtained from two transformers, twelve phases from three transformers and so on. The associated rectifier circuits are 12-pulse rectifiers, 18-pulse rectifiers and so on.


When controlled rectifier circuits are operated in the inversion mode, they would be classified by pulse number also. Rectifier circuits that have a higher pulse number have reduced harmonic content in the AC input current and reduced ripple in the DC output voltage. In the inversion mode, circuits that have a higher pulse number have lower harmonic content in the AC output voltage waveform.


External links

  • Solar Panel Inverters explained in great detail.
  • Sine Wave Versus Modified Sine Wave Power Inverter Video

See also

A grid-tie inverter is an electrical device that allows solar power users to compliment their grid power with solar power. ... A push-pull converter is a type of DC to DC converter that uses a transformer to step the voltage of a DC power supply. ... Small Variable Frequency Drive (VFD) A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. ... A static inverter station is the terminal equipment for a high voltage direct current transmission line, in which direct current is converted to three-phase alternating current, and, usually, the reverse. ... A switched-mode power supply, switch-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches power transistors (such as MOSFETs) rapidly on and off in order to stabilize the output voltage or current. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ... Map of the route of the Pacific Intertie transmission route and stations The Pacific DC Intertie (also called Path 65) is an electric power transmission line which transmits electricity from the Pacific Northwest to the Los Angeles area using high voltage direct current (HVDC). ...

References

Citations

  1. ^ (2000) "Power Electronics: Energy Manager for Hybrid Electric Vehicles". Oak Ridge National Laboratory Review 33 (3). Retrieved on 2006-11-08. 
  2. ^ Rodriguez, Jose; et al. (August 2002). "Multilevel Inverters: A Survey of Topologies, Controls, and Applications". IEEE Transactions on Industrial Electronics 49 (4): 724-738. 
  3. ^ Inverter FAQ. PowerStream (2006). Retrieved on 2006-11-08.
  4. ^ Owen, Edward L. (January/February 1996). "Origins of the Inverter". IEEE Industry Applications Magazine: History Department 2 (1): 64-66. 
  5. ^ [1972] in D. R. Grafham and J. C. Hey, editors: SCR Manual, Fifth Edition, Syracuse, N.Y. USA: General Electric, 236-239. 

Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 312th day of the year (313th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 312th day of the year (313th in leap years) in the Gregorian calendar. ...

General references

  • Bedford, B. D.; Hoft, R. G. et al (1964). Principles of Inverter Circuits. New York: John Wiley & Sons, Inc.. ISBN 0-471-06134-4. 
  • Mazda, F. F. (1973). Thyristor Control. New York: Halsted Press Div. of John Wiley & Sons. ISBN 0-470-58116-6. 

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Inverter (electrical): Definition and Links by Encyclopedian.com (253 words)
An inverter is a circuit for converting direct current electrical power to alternating current.
Most inverters consist of an oscillator driving a transistor, that is used to interrupt the incoming direct current to create a square wave.
This is then fed through a transformer to smooth the square wave[?] into a sine wave[?] and to produce the required output voltage.
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