石家庄铁道大学四方学院毕业设计
致 谢
本设计是在老师的悉心指导下完成的，从选题、课题的研究到论文的撰写和修改都得到了导师的精心指导，老师的严谨治学的态度以及渊博的专业知识都给我留下了极其深刻的印象，让我很钦佩。在整个过程中，最重要的是老师让我学会了自学、独立思考以及解决困难的方法，感谢在百忙之中抽出时间评阅论文的各位老师，由于知识水平有限，错误在所难免，恳请各位老师批评指正。
在本次论文设计过程中，感谢我的学校，给了我学习的机会，在学习中，老师从选题指导、论文框架到细节修改，都给予了细致的指导，提出了很多宝贵的意见与建议，老师以其严谨求实的治学态度、高度的敬业精神、兢兢业业、孜孜以求的工作作风和大胆创新的进取精神对我产生重要影响。老师渊博的知识、开阔的视野和敏锐的思维给了我深深的启迪。这篇论文是在老师的精心指导和大力支持下才完成的。
谨以此致谢最后，我要向百忙之中抽时间对本文进行审阅的老师表示衷心的感谢。
24
石家庄铁道大学四方学院毕业设计
附 录
外文资料
Inverter
1 Introduction
An inverter is an electrical device that converts direct current(DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers,switching,and control circuits.Solid-state inverters have no moving parts and are used in a wide range of applications,from small switching power supplies in computers,to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.There are two main types of inverter. The output of a modified sine wave inverter is similar to a square wave output except that the output goes to zero volts for a time before switching positive or negative. It is simple and low cost and is compatible with most electronic devices,except for sensitive or specialized equipment,for example certain laser printers. A pure sine wave inverter produces a nearly perfect sine wave output (<3% total harmonic distortion) that is essentially the same as utility-supplied grid power. Thus it is compatible with all AC electronic devices. This is the type used in grid-tie inverters. Its design is more complex,and costs 5 or 10 times more per unit power The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse,and thus were \convert DC to AC.The inverter performs the opposite function of a rectifier. 2 Applications
2.1 DC power source utilization
An inverter converts the DC electricity from sources such as batteries,solar panels,or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation,or rectified to produce DC at any desired voltageGrid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as
25
石家庄铁道大学四方学院毕业设计
supplied by the distribution system. They can also switch off automatically in the event of a blackout.Micro-inverters convert direct current from individual solar panels into alternating current for the electric grid. They are grid tie designs by default. 2.2 Uninterruptible power supplies
An uninterruptible power supply (UPS) uses batteries and an inverter to supply AC power when main power is not available. When main power is restored,a rectifier supplies DC power to recharge the batteries. 2.3 Induction heating
Inverters convert low frequency main 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. 2.4 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. 2.5 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 main AC power. Since an inverter is the key component,variable-frequency drives are sometimes called inverter drives or just inverters. 2.6 Electric vehicle drives
Adjustable speed motor control inverters are currently used to power the traction motors in some electric and diesel-electric rail vehicles as well as some battery electric vehicles and hybrid electric highway vehicles such as the Toyota Prius and Fisker Karma. Various improvements in inverter technology are being developed specifically for electric vehicle applications.In vehicles with regenerative braking,the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries. 2.7 The general case
A transformer allows AC power to be converted to any desired voltage,but at the same frequency. Inverters,plus rectifiers for DC,can be designed to convert from any voltage,AC or DC,to any other voltage,also AC or DC,at any desired frequency. The
26
石家庄铁道大学四方学院毕业设计
output power can never exceed the input power,but efficiencies can be high,with a small proportion of the power dissipated as waste heat. 3 Circuit description 3.1 Basic designs
In one simple inverter circuit,DC power is connected to a transformer through the centre tap of the primary winding. A switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding and then the other. The alternation of the direction of current in the primary winding of the transformer produces alternating current (AC) in the secondary circuit.
The electromechanical 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 electromagnet pulls the movable contact to the opposite stationary contact. The current in the electromagnet 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. 3.2 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 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 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.
27
石家庄铁道大学四方学院毕业设计
3.3 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. 4 History 4.1 Early inverters
From the late nineteenth century through the middle of the twentieth century,DC-to-AC power conversion was accomplished using rotary converters or motor-generator sets (M-G 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 AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator's commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter,in which the motor and generator windings are combined into one armature,with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in,DC-out. With an M-G set,the DC can be considered to be separately generated from the AC; with a synchronous converter,in a certain sense it can be considered to be \rectified AC\Given the right auxiliary and control equipment,an M-G set or rotary converter can be \Hence an inverter is an inverted converter. 4.2 Controlled rectifier inverters
Since early transistors were not available with sufficient voltage and current ratings for most inverter applications,it was the 1957 introductionof the thyristor or
28