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1、Chapter 3 Power Electronic Technology,Section 3 DC-AC Converters,Text,New Words and Expressions,Exercises,End,Transition of part of speech,Section 3 DC-AC Converters,Introduction DC-AC converters are conventionally called the inverters. Such converters are very popular in the battery-operated power
2、systems such as the uninterruptible power supplies (UPS) for hospitals, and AC motor drives. Low power level inverters are usually single-phase type and medium and high power inverters are three-phase type. We will look into the performance of DC-AC converters of several useful topologies. The focus
3、 will be on the inverter operation for low-frequency output,Section 3 DC-AC Converters,voltages such as 5060 Hz or 400 Hz in aircraft systems. Analyses of DC-AC converter circuits at low and high frequencies are very similar except that at high frequencies the parasitic capacitances and inductances,
4、 charge storage, and heat localizing problems in switching devices must be taken into account. Controlled inversion The DC to AC inverter system is shown in Fig.3-5(a). Input is from a DC source and the output is desired to be a sinusoidal voltage or current with,Section 3 DC-AC Converters,zero DC c
5、omponent, single-phase or three-phase signal. The load is a R-L-C load, an AC voltage or an AC current sink. Control parameter may be, as shall be seen later, an angle, a pulse width, a voltage or a current signal. The converter consists of switches, reactive components such as L, C, transformers an
6、d resistances. Switches including the diodes are assumed to be ideal and unidirectional, that is, they have zero on-state losses, zero off-state losses, zero switching losses and unrestricted voltage or current carrying,Section 3 DC-AC Converters,capabilities. Switching devices in a practical design
7、 example must be selected with these desired characteristics in mind. Inductors, capacitors and transformers are assumed to be ideal.,Fig.3-5 (a) A DC-AC inverter system; (b) operational quadrants on the Uo-Io plane,Section 3 DC-AC Converters,Electrical loads can be of three types: impedance load, A
8、C voltage sink load or AC current sink load. Examples of impedance load are electromagnetic ac relays, electromagnetic pick up and lift equipments. The load comprises of a large value inductor. Examples of ac voltage sink are the constant speed drives for induction and synchronous motors. Examples o
9、f AC current sink are constant torque drives for the induction and synchronous motors.,Section 3 DC-AC Converters,Generally speaking, the input current is non-sinusoidal if the source is a DC voltage source and vice versa. The output voltage and the output current on the other hand, can be non-sinus
10、oidal simultaneously. Hence, input and output low-pass filters are generally employed to reduce high frequency components of the non-sinusoidal signals on either side. A series inductor is a current smoother and a parallel capacitor is a voltage smoother.,Section 3 DC-AC Converters,Output current ge
11、nerally differs in phase with the output voltage. Hence, in a switching period, the polarities of current and voltage vary over all four quadrants, see Fig.3-6. Power flows from the converter to the load in the first and third quadrants and in the reverse direction in the second and fourth quadrants
12、. A unidirectional switch in the inverters, therefore, has an inverse connected diode in parallel to allow bidirectional current flow.,Section 3 DC-AC Converters,Fig.3-6 Output current and output voltage waveform in an inverter,Section 3 DC-AC Converters,Inverters derived from voltage source (VSI) a
13、re ideally suited for supplying power to a resistive and inductive load and the applications that require constant torque operation of AC motors. Current Source Inverters (CSI) are ideally suited for supplying power to largely capacitive loads and AC voltage buses. Inverters can be further classifie
14、d on the basis of the mode of control of the output variable: voltage or current. Inverter control in practice concerns three aspects: fundamental frequency, amplitude, harmonic profile.,Section 3 DC-AC Converters,Fundamental frequency is either equal to the switching frequency of the inverter (squa
15、re-wave and PWM inverters) or its simple fraction (1/3, 1/5, and so on) such as in Multiple PWM or sine PWM inverters. Power electronic applications require the amplitude control of output voltage or output current, called Voltage-Mode control and Current-Mode control respectively. An example of Vol
16、tage-Mode control is the adjustable speed drives of induction and synchronous motors. An example of Current-Mode,Section 3 DC-AC Converters,control is a constant torque drive for induction motors. Output amplitude in both modes can be controlled by varying the DC source voltage or by varying the pulse-width (pulse-width modulation, PWM). The inverters based on the control of DC source voltage are also termed DC-Link Inverters. These inverters use an AC to controlled DC converter on t
