脉宽调制技术外文翻译

脉宽调制技术外文翻译内容摘要：

so that the average or fundamental ponent frequency is the same as f and its amplitude is proportional to the mand modulating voltage .The same carrier wave can be used for all three phases, as shown The typical wave shape of line and phase voltages for an isolated neutral load can be plotted graphically as shown to be of the following form: ）（）（ wwf r e q u e n c yh ig hwts cd0a NMV   (533) Where m=modulation index,  =fundamental frequency in r/s（ same as the modulating frequency） and  =phase shift of output, depending on the position of the modulating wave. The modulating index m is defined as TPm VV (534) Figure Principle of sinusoidal PWM for threephase bridge inverter Figure Line and phase voltage waves of PWM inverter Where PV =peck value of the modulating wave and TV = peck value of the carrier wave. Ideally, m can be varied between 0 and 1 to give a linear relation between the modulating and output wave. The inverter basically acts as a linear amplifier. Combining Equations（ ） and（ ） ,the amplifier gain G is given as TVVV VG dPd  (535) At m=1,the maximum value of fundamental peak voltage is dV ,which is percent of the peak voltage（ 4 dV /2  ） of the square wave. In fact, the maximum value in the linear range can be increased to percent of that of the square wave by mixing the appropriate values of triplen harmonics with the modulating wave. At m=0, 0av is a square wave at carrier frequency with symmetrical pulse and notch widths. The PWM output wave contains carrier frequencyrelated harmonics with modulating frequencyrelated sidebands in the formwwc NM  ,which are shown in Equation（ ） ,where M and N are integer and M+N=an odd integer. For a carriertomodulating frequency ratio 15w/w c P ,Table gives a summary of output harmonics. Table Family of Output Harmonics for Sinusoidal PWM with 15w/wc  m Harmonics 1 15w 15w 2w 15w 4w  2 30w 30w 3w 30w 5w  3 45w 45w 2w 45w 4w   It can be shown that the amplitude of the harmonics is independent of P and diminishes with higher values of M and N. With higher carrier frequency ratio P, the inverter line current harmonics will be wellfilter by nominal leakage inductance of the machine and will practically approach a sine wave. The selection of a carrier frequency depends on the tradeoff between the inverter loss and the machine loss. Higher carrier frequency（ same as switching frequency）increases inverter switching loss but decrease machine harmonic loss. An optimal carrier frequency should be selected such that the total system loss in minimal. An important effect of PWM switching frequency is the generation of acoustic noise（ known as magic noise） by the magostriction effect when the inverter supplies power to machine. The effect can be alleviated by randomly varying the switching frequency（ radom SPWM） ,or it can be pletely eliminated by increasing the switching frequency above the audio range. Modern highspeed IGBTs easily permit such acoustically noisefree variablefrequency drives. Lowpass line filter can also eliminate this problem. Overmodulation Region As the modulation index m approaches 1,the notch and pulse widths near the center of positive and negative halfcycles,respectively, tend to vanish. To plete switching operation of device, minimum notch and pulse widths must be maintained. When minimumwidth notches and pulses are dropped, there will be some transient jump of load current. The jump may be small for IGBT inverters, but it is substantial for highpower GTO inverter because of the slow switching of the devices. The value of m can be increased beyond the value of 1 to enter into the quasiPWM region, shown in Figure for positive halfcycle only. The 0av wave indicates that the notches near the center part have disappeared, giving a quasisquarewave output with a higher fundamental ponent. The transfer characteristics in the overmodulation region are nonlinear in Figure ,and the harmonics th5 , th7 ,. Ultimately, with a high m value, that is, a large modulating signal, there will be one switching at the leading edge and anther switching at the trailing edge, giving squarewave output. At this condition, the fundamental phase voltage peak value is 4（ dV ） / ,which is 100 percent, as indicated in Figure . Frequency Relation For variablespeed drive applications, the inverter output voltage and frequency are to be varied in the relation shown in the constant power region, the maximum voltage can be obtained by operating the inverter in squarewave mode, but in the constant torque region, the voltage can be controlled using the PWM principle. It is usually desirable to operate the inverter with an integral ratio P of carriertomodulating frequency, where the modulating wave remains synchronized with the carrier wave in entire region. A fixed value of P cause a low carrier frequency as the fundamental frequency goes down, which is not desirable from the machine harmonic loss point of view. A practical carrierto fundamental frequency relation of a GTO inverter is shown in Figure a low fundamental frequency, the carrier frequency is maintained constant and the inverter operates in the freerunning, or asynchronous, mode. In this region, the ratio P may be nonintegral, and the phase may continually drift. This gives rise to a unharmonic problem with drifting dc offset (beating effect), which tends to be worse as the ffc ratio decreases. It could be mentioned here that the modern IGBT switching frequency is so large pared to the fundament。