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For the digital logic gate, see Inverter (logic gate). An inverter is a circuit for converting direct current (DC) to alternating current (AC). Inverters are used in a wide range of applications, from small switched power supplies for a computer to large electric utility applications to transport bulk power. Inverter applications The following are examples of inverter applications. DC power source utilization An inverter allows the 12 volt DC power available in an automobile to supply AC power to operate equipment that is normally supplied from a mains power source. Inverters are also used to provide a source of AC power from photovoltaic solar cells and fuel cell power supplies. 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 mains power is not available. When mains power is restored, a rectifier is used to supply DC power to recharge the batteries. Induction heating Inverters are used to convert low frequency mains 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. 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. 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 mains AC power. Since an inverter is the key component, variable frequency drives are sometimes called inverter drives or just inverters. Inverter circuit description Basic inverter designs
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. More advanced inverter designs There are many different power circuit topologies and control strategies used in inverter designs. Different design approaches are used to 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 symmetrical, except for sign, 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 contains mutiple steps an various 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.• Three phase inverters 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. 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. See also Citations | |||||||||
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