Inverter (electrical)
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For other uses, see Inverter (logic gate) and Inverter.
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.
The inverter is so named because it performs the opposite function of a rectifier.
[edit] Inverter applications
The following are examples of inverter applications.
[edit] 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.
Inverters are also used to provide a source of AC power from photovoltaic solar cells and fuel cell power supplies.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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]
[edit] Inverter circuit description
Simple inverter circuit shown with an electromechanical switch and with a transistor switch
[edit] 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.
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.
As they have become available, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs.
Square waveform with fundamental sine wave component, 3rd harmonic and 5th harmonic
[edit] 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.
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 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.
[edit] More advanced inverter designs
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.
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.
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.
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.
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]
[edit] Three phase inverters
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.
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.
[edit] History
[edit] Early inverters
From the late nineteenth century through the middle of the twentieth century,
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]
[edit] 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.
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.
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.
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.
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.
[edit] 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]
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.
Talk:Inverter (electrical)
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[edit] Diagrams
This page would really benefit from some diagrams. Alaric 14:55 May 8, 2003 (UTC)
I'd like to see a better diagram than the tube oscillator - which isn't even correct, since the plate voltage supply is backward! --Wtshymanski 18:43, 14 March 2006 (UTC)
[edit] Good short article
I think this is one of the better articles explaining electronics on Wikipedia. --Grouse 12:14, 19 Jul 2004 (UTC)
I agree. Explains what was needed. However I think external links should point to sites that contain more info about the topic. Commercial sites should be tagged as such. Does anyone else think the same? I am new here, hence I'm hesitant to change the main page --Padme 21:44, 17 Aug 2005
From the first paragraph of the text, it would seem to me that the opposite would be true for computers. Most PSUs take in alternating current from the wall and convert it to direct.
You really need to understand switched mode power supplies to understand this. Perhaps the link should be more closely coupled to the psu references, rather than just mentioned at the end of the paragraph. StealthFox 18:48, 13 December 2005 (UTC)
I took out he following sentence: An inverter can have one or two switched-mode power supplies (SMPS).. Maybe it's the other way around? --Apoc2400 07:23, 3 March 2006 (UTC)
[edit] PDM/digital power inverters
Do digital power inverters exist? Do there exit any power inverters, analog or digital, that use pulse density modulation (PDM)?Myrtone (the strict Australian wikipedian)(talk)|contributions|Testpage
There are many manufacturers of Variable Frequency Drive inverters. They have generally adopted the latest electronic components and design techniques as soon as they have been able to do so. Today, most of these products use embedded microprocessors to control IGBT transistors. Most products use some form of PWM strategy to provide a simulated sine wave output with controlled voltage and frequency.
I am not familiar with PDM, but it appears to me that the zeros and ones all have the same fixed width and the modulation consists of controlling the number and position of ones vs. zeros. What happens if there are two ones in succession? Does that become a pulse that is twice as wide? I think that PWM power inverters operate like PDM with a lot of instances of two or more ones in succession joined to form wider pulses. Because of the switching losses in the power switching devices, the switching frequency is usually limited to 3 to 9 kHz but the widths and positions of the pulses are adjusted in very small increments. Manufacturers usually publish a product's switching frequency but don't often provide details about the specific scheme that they use for setting the widths and positions of the modulation pulses.
I hope that helps to answer your question. -- C J Cowie 15:36, 14 March 2006 (UTC)
[edit] Ambitguity
"Modified-sine inverters may cause some loads, such as motors, to operate less efficiently."
The above sentence is in the article. I can't understand if it means that modified-sine inverters are worse (cause some loads to be less efficient) than a no inverter or than a simple inverter. In other words, the waveform generated by a modified-sine inverter is being compared to what? To a true , perfect sinewave or to the waveform of a simple inverter (one with only two possible voltages)? Ambiguous.
Since I don't know the answer, I can't correct by myself.
[edit] Disambiguation
I don't think this page should get primary-topic disambiguation. The word 'inverter' is used very often to refer to NOT gates. --Smack (talk) 02:03, 14 June 2006 (UTC)
[edit] What??
"Simple inverters generate harmonics which affect the quality of power obtained using them. But PWM inverters eliminate this by means of a sine wave cancellation using the properties of Fourier Series." — Omegatron 16:04, 28 October 2006 (UTC)
Hmmm this is indeed a strange explanation... What about something like "Simple inverters generate square waveforms which are not suited to some application (especially transformers and motors), because of their high harmonic content. In this case, PWM inverters can be preferable" ? CyrilB 16:40, 28 October 2006 (UTC)
Certainly better. I'm just trying to figure out what the original author was trying to say. PWM inverters generate a more smooth sinusoidal waveform (if built well), while simple switching ones generate a filtered square wave, but what does this have to do with sine wave cancellation or the Fourier series? — Omegatron 18:20, 28 October 2006 (UTC)
The only thing that the Fourier series has to do with this is that the concept of harmonic distortion, based of Fourier analysis, is used to quantify the quality of inverter output waveforms. "Harmonic cancellation" may be a useful way of describing the effect of PWM techniques in improving the waveform, but it doesn't make much sense as presented. -- C J Cowie 20:45, 28 October 2006 (UTC)
[edit] Quality of an inverter
The use of the term pulse may need some clarification in thie paragraph:
The quality of an inverter is described by its pulse-rating: a 3-pulse is a very simple arrangement, utilising only 3 transistors, whereas a more complex 12-pulse system will give an almost exact sine wave. In remote areas where a utility generated power is subject to significant external, distorting influences such as inductive loads or semiconductor-rectifier loads, a 12-pulse inverter may even offer a better, "cleaner" output than the utility-supplied power grid, and are thus often used in these areas. Inverters with greater pulse ratings do exist.
I believe that the term pulse here refers to the number of steps in the inverter waveform. The six-step waveform is described in the article. A square wave would be a two-step waveform. There are also multiples of the six-step waveform such as 12-step, 18-step and 24-step.
The term pulse is often used to describe AC to DC converters. I can't find any references that use the term pulse as it has been used here.
If no one objects, I think I will change this paragraph to describe 12-step, 18-step and 24-step inverters and eliminate 3-pulse etc.C J Cowie 21:31, 8 November 2006 (UTC)
The article Pacific Intertie describes 6-pulse converters. I've long intended to look up what that means and verify correct usage. Are you proposing to eliminate mention of 6 pulse? — EncMstr 00:09, 9 November 2006 (UTC)
The 6-pulse and 12-pulse converters described in the Pacific Intertie article are phase-controlled AC to DC converters and also the same circuits operated as line commutated AC to DC inverters. This article does not cover line commutated inverters. There are also load commutated inverters that are not covered here. These are all controlled rectifier circuits. I think that it might be best to cover the various types of controlled rectifier circuits all in another article and eliminate the terms 6-pulse and 12-pulse from this article.--C J Cowie 00:47, 9 November 2006 (UTC) For more just call Maryaustin on on how 08063496667
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