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Chapter 3Power Electronic Technology Chapter 31Section 3 DC-AC Convertersn Textn New Words and Expressionsn Exercisesn Endn Transition of part of speechSection 3 DC-AC Converters T2Section 3 DC-AC ConvertersIntroduction DC-AC converters are conventionally called the inverters.Such converters are very popular in the battery-operated power 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 will be on the inverter operation for low-frequency outputSection 3 DC-AC ConvertersI3Section 3 DC-AC Convertersvoltages 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,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 withSection 3 DC-AC Convertersv4Section 3 DC-AC Converterszero DC component,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 and 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 carryingSection 3 DC-AC Convertersz5Section 3 DC-AC Converterscapabilities.Switching devices in a practical design 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 Convertersc6Section 3 DC-AC Converters Electrical loads can be of three types:impedance load,AC 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 of AC current sink are constant torque drives for the induction and synchronous motors.Section 3 DC-AC Converters 7Section 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-sinusoidal 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 8Section 3 DC-AC Converters Output current generally 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.A unidirectional switch in the inverters,therefore,has an inverse connected diode in parallel to allow bidirectional current flow.Section 3 DC-AC Converters 9Section 3 DC-AC ConvertersFig.3-6 Output current and output voltage waveform in an inverter Section 3 DC-AC ConvertersF10Section 3 DC-AC Converters Inverters derived from voltage source(VSI)are 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 classified 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 11Section 3 DC-AC Converters Fundamental frequency is either equal to the switching frequency of the inverter(square-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 Voltage-Mode control is the adjustable speed drives of induction and synchronous motors.An example of Current-ModeSection 3 DC-AC Converters 12Section 3 DC-AC Converterscontrol 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 the input side.The inverter is square-wave controlled.This arrangement is expensive but exhibits a faster response time than PWM inverters.The control circuitry is also simpler.Section 3 DC-AC Convertersc13Section 3 DC-AC Converters Harmonic profiling is an extra feature added over the amplitude control.The objective of harmonic profiling is to bolster the amplitude of the fundamental component of the output voltage and the elimination or reduction of high order harmonics.One of the techniques involves the notching or adding step pulses to a base pulse.The other technique involves multiple pulses per cycle of the fundamental,each pulse-width modulated to achieve desired wave-shaping of the output waveform(Sine-PWM).Section 3 DC-AC Converters 14Section 3 DC-AC ConvertersInverter topologies The DC-AC inverter can be thought of as a three-position switch as shown in Fig.3-7(a).The load is connected to the pole of the switch and the first two positions are connected to dc voltage or dc current sources of opposite polarities.The third position is connected to the zero terminal of the source.The pole of the switch stays in the first and second positions for equal amounts of time in a switching period.The output waveform for zero amount of time on the third position is a square wave.The output waveform for nonzero time is pulse wave.Section 3 DC-AC ConvertersI15Section 3 DC-AC Converters Fig.3-7(a)The DC-AC inverter implemented by a 3-position switch;(b)the 3-position switch synthesized from two 2-position switches;(c)full-bridge inverter topology Section 3 DC-AC Converters 16Section 3 DC-AC Converters The three-position switch may be synthesized by two unidirectional electronic switches as shown in Fig.3-7(b).The third position of the switch is simulated by turning both switches off in VSI topologies and on in CSI topologies.This is the basic bridge inverter topology.The two diodes provide the path for current flow from the load to the source as this may be required in an inverter operation involving more than one quadrant.Inverter supplying load to an R-L load operates in all four quadrants because the output voltage andSection 3 DC-AC Converters 17Section 3 DC-AC Converterscurrent waveforms,for some segments in a switching period,have opposing polarities.A MOSFET has a built-in body diode that serves as the path for the current from the source to the drain.An IGBT,however,requires an external diode.The 3-half-bridge inverter is made of three half-bridge inverters connected in parallel each switched 120 apart.This topology is capable of handling high power with minimum component stresses.The half-bridge topology of Fig.3-7(b)requires two separate voltage sources US/2.Fig.3-7(c)shows a full-bridge topology that requires a single voltage source.ReturnSection 3 DC-AC Convertersc18New Words and Expressionsinvertern.逆变器uninterruptible power supplies(UPS)不间断电源single-phasea.单相的three-phasea.三相的parametern.参数unidirectionala.单极性的，单向的electromagnetica.电磁的relayv.中继，转播；n.继电器 New Words and Expressionsinver19New Words and Expressionssynchronous motor同步电动机torquen.转矩non-sinusoidala.非正弦的low-pass 低通smoother n.滤波器，整平器polarity n.极性bidirectional a.双向的harmonics n.谐波 New Words and Expressionssynch20New Words and Expressionshigh order harmonics 高次谐波inductionn.感应square-wave方波response time响应时间notchingn.;a.阶梯式，下凹的，切口，开槽step pulse 阶跃脉冲 ReturnNew Words and Expressionshigh 21Transition of Part of Speech三、转换为汉语形容词三、转换为汉语形容词 1、副词转换成形容词、副词转换成形容词(1)Robotics is so closely associated with cybernetics that it is sometimes mistakenly considered to be synonymous.机器人技术与控制论的联系如此密密切切，以致两者有时被错误地认为是同一回事。(2)Miniature gas detector is chiefly featured by small size,light weight,complete functions and long continuous working time.袖珍式瓦斯检测器的主主要要特点是体积小、重量轻、功能齐全、连续工作时间长。Transition of Part of Speech三、22Transition of Part of Speech(3)Gases and liquids are perfectly elastic.气体和液体都是完全的完全的弹性体。Transition of Part of Speech(323Transition of Part of Speech2、名词转换成汉语形容词、名词转换成汉语形容词(1)The nuclear power system designed in China is of great precision.中国设计的核动力系统十分精确精确。(2)Electronic computers and microprocessors are of great importance to us.电子计算机和微处理器对我们来说十分重要重要。(3)Single crystals of high perfection are an absolute necessity for the fabrication of integrated circuits.高度完整的单晶对于制造集成电路来说是绝对必必要要的的。Transition of Part of Speech2、24Transition of Part of Speech四、转换成汉语副词四、转换成汉语副词 1、形容词转换成汉语副词、形容词转换成汉语副词 1.(1)The engine has given a constantly good performance.2.这台发动机一直运转良好良好。(2)The same principles of low internal resistance also apply to milliammeters.低内阻的原理同样同样适用于毫安表。(3)A graph gives a visual representation of the relationship.图表可以直观地直观地显示要说明的关系。Transition of Part of Speech四、25Transition of Part of Speech2、其他词转换成副词、其他词转换成副词(1)Rapid evaporation at the heating-surface tends to make the steam wet.加热面上的迅速蒸发，往往往往使蒸汽的湿度加大。(2)The added device will ensure accessibility for part loading and unloading.增添这种装置将保证工件装卸方便方便。(3)We shall develop the electric power industry in a way.我们将大规模大规模发展电力工业。ReturnTransition of Part of Speech2、26Exercises I.Choose the best answer into the blank 1.High power inverters are usually .A.single-phase typeB.two-phase type C.three-phase typeD.DC-DC converters2.The output of a inverter is desired to be a .A.dc voltageB.sinusoidal voltage C.dc currentD.time-varying voltage Exercises I.Choose the best a27Exercises I.Choose the best answer into the blank 3.The constant speed drive for synchronous motors is the example of .A.impedance load B.ac voltage sink load C.AC current sink load D.DC load 4.Voltage source inverters are theoretically suited for supplying power to .A.capacitive loads B.AC voltage buses C.inductive loads D.dc motors Exercises I.Choose the best a28Exercises I.Choose the best answer into the blank 5.Output amplitude in Voltage-Mode control and Current-Mode control modes can be con-trolled by varying the DC source voltage or by varying the .A.dc source currentB.pulse-width C.pulse-amplitudeD.harmonic Exercises I.Choose the best a29Exercises II.Answer the following questions according to the text 1.What parameters may be used as control parameters in the DC-AC inverter system?2.What does the DC-AC inverter consist of?3.What characteristics do the ideal switches with the diode have?4.What functions do low-pass filters have?5.What aspects are concerned for inverter control in practical applications?Exercises II.Answer the follo30Exercises III.Translate the following into Chinese 1.Reducing the harmonic content of the output voltage or current is one of the important tasks of the designers.Harmonics appear as the undesirable noise.Harmonics reduce the power factor.They may excite mechanical resonance at detrimental frequencies or generate acoustical noise in ac motor drive systems.The ripple content(harmonics)is easily reduced by post-filtering after the converter.The post converter low-pass filter may be designed for cutoff frequencies that are several times lowerExercises III.Translate the f31Exercises III.Translate the following into Chinese than the switching frequency(usually in kilohertz).In the DC-AC inverters,however,the fundamental component of the output voltage or current is either the same as or a simple fraction of the switching frequency.Designing an effective low-pass filter with cutoff frequency between the fundamental frequency and the first undesired harmonic frequency is a difficult task.Hence,an active harmonic reduction strategy is often desirable.Exercises III.Translate the f32Exercises III.Translate the following into Chinese 2.Converter control refers to specifying the desired nominal operating condition and then regulating the converter so that it stays close to the nominal performance in the face of disturbances,noise and modeling errors.We can analyze the dynamic behavior of the power electronic converter(or local)of the steady-state operating point.The converter operation involves switching modes,therefore an overall converter model is a nonlinear model.The averaged model,localized to the steady-stateExercises III.Translate the f33Exercises III.Translate the following into Chinese operating point,is a linear model.The analytical techniques for linear systems are very well known.ReturnExercises III.Translate the f34Chapter 3Section 3 EndChapter 335
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