逆變器外文文獻(xiàn)及翻譯-其他專業(yè)-全文預(yù)覽
【正文】 cal 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 electromag pulls the movable contact to the opposite stationary contact. The current in the electromag 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 became available with adequate power ratings, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs Output waveforms The switch in the simple inverter described above, when not coupled to an output transformer, produces a square voltage waveform due to its simple off and on nature 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 ponent. 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 output waveform that is needed from an inverter depends on the characteristics 3 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. Three phase inverters Threephase inverters are used for variablefrequency drive applications and for high power applications such as HVDC power transmission. A basic threephase inverter consists of three singlephase 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 linetoline output waveform that has six steps. The sixstep waveform has a zerovoltage step between the positive and negative sections of the squarewave such that the harmonics that are multiples of three are eliminated as described above. When carrierbased PWM techniques are applied to sixstep waveforms, the basic overall shape, or envelope, of the waveform is retained so that the 3rd harmonic and its multiples are cancelled 4 History Early inverters From the late nieenth century through the middle of the twentieth century, DCtoAC power conversion was acplished using rotary converters or motorgenerator sets (MG 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. The origins of electromechanical inverters explain the source of the term inverter. Early ACtoDC converters used an induction or synchronous AC motor directconnected to a generator (dynamo) so that the generator39。 the converted AC can be at any required voltage and frequency with the use of appropriate tran
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