must逆变器

must逆变器

       港口3 ：港口三是一个8位双向的IO连接埠与内部pullups 。端口3输出缓冲器可驱动康乐事务的TTL的投入。港口3引脚，有1的书面向他们撤出高由内部pullups ，并在该国可以被用来作为投入。作为投入，港口3引脚是外部器件拉低时将电源电流（ I对数据表此拿锋） ，因为该pullups 。港口三也服务的职能，各种特殊功能的MCS - 51的家庭，由于上市如下：

       港口三还收到一些控制信号为eprom编程和程序验证。

       零售税：复位输入。高在此引脚为2个机器周期，而振荡器运行重置设备。该端口管脚将推动其重置条件时，最低vih1电压应用于是否振荡器正在运行或没有。一内部的下拉电阻许可证上电复位，只电容器连接到vcc 。

       aleprog ：地址锁存使能输出信号闭锁低字节的地址在访问外部存储器。此PIN ，也是脉冲输入的程序（程式）在eprom编程为87c51 。如果想要的ALE运作，可以禁用通过设置位0的SFR的位置8eh 。与此位设置PIN是弱拉高。不过，禁用的ALE功能将暂停使用期间， movx或movc指示，闲置模式掉电模式和冰模式的ALE禁用功能将被终止复位。

       当的ALE禁用的特点是暂停或终止，的ALE引脚将森晌不再停弱。设置的ALE -禁用位没有任何影响，如果微控制器是在外部的执行模式。

       在正常运作的ALE是排放在一个常数为16振荡器的频率，并可能被

       用于外部定时或时钟的目的。不过，请注意，一的ALE脉冲是跳在每个访问外部数据存储器。

       psen ：程序存储使是阅读频闪，以外部程序存储器。当87c51bh是执行从内部程序存储器， psen是无效的（高） 。当装置执行敏枯的代码是从外部程序存储器， psen是激活的两倍，每个机器周期，除2 psen激活被跳过，在每个访问外部数据存储器。

       环境/ vpp ：外部访问使。环境必须物品的VSS ，以使87c51bh ，以

       市值代码从外部程序存储器的地点，开始在0000h最多ffffh 。不过，请注意，如果任何一方的锁定位是编程，逻辑水平，在环境评估是内部锁存在上电复位。环境必须物品vcc内部程序执行。这个引脚还收到编程供应电压年龄（ vpp ）在eprom编程。

       xtal1 ：投入逆变振荡放大器。

       xtal2 ：输出由逆变振荡放大器。

       累死我了..

power factor是什么意思

       power factor

       un.功率因数；功率系数；功率因素

       功率因子；功因；电源功率因素

       例句

       1.weaving factory because of the lower power factor, high cost, lack of confidence, with procurement.

       下游织造工厂因纤腔限电因素、成本高企，信心不足，随需采购。

       2.Power factor is calculated using the actual bullet weight and the average velocity of the three rounds fired.

       利用实际的弹头重量以及射击的三发平均枪口初速来塌派计算威力系数。

       3.Relation between Dielectric Power Factor and Temperature in Current Transformer

       电流互感器介质损耗因数与温度的关系

       4.Ammunition used by a competitor in a match must satisfy the requirements the power factor declared.

       选手在比赛中所团竖贺使用的弹药必须符合所公布的威力系数的限制。

       5.Research on constant output power factor angle control of parallel inverter

       并联型逆变器的定角控制

ups不间断电源哪家有卖？什么品牌好？

       1、科士达KSTAR(中国UPS研发生产企业之一,国家级高新技术企业,行业著名品牌,深陵简圳科士达科技股份有限公司)

       2、伊顿EATON(创立于1911年美国,百年品牌,全球行业领先品牌,全球最大的UPS供应商之一,伊顿(中国)投资有限公司)，被施耐德收购。

       3、APC(创于1981年美国,全球行业领先品牌,全球最大的UPS供应商之一,十大UPS不间断电源品牌,美国电力转换集团)，被施耐德收购。

       4、艾默生Emerson(于1980年,全球领先的工程技术解决方案提供商,世界500强企业,大型跨国集团,艾默生电气(中国)投资有限公司)

       5、梅兰日兰MGE(世界上领先的配电品牌隶属于施耐德电气集团,配电领域的国际标准主要开拓者,施耐德电气(中国)投资有限档汪敏公司)

       6、易事特EAST(于1989年,中国最大规模ups电源研发与制造商，中国驰名商标,广东省名牌,全球领先的整行枝体电源解决方案供应商,广东易事特电源股份有限公司)

       7、科华KELONG(中国驰名商标,中国本土最大的高端UPS电源制造商之一,国家级重点高新技术企业,科华恒盛股份有限公司)

       8、山特Santak(高新技术企业,广东名牌,专业从事不间断电源UPS开发、生产及经营的厂商,山特电子(深圳)有限公司)

       9、台达Delta(交换式电源供应器产品处于世界领先品牌,全球直流稳压电源的最大制造商之一,台达电子工业股份有限公司)

       10、MUST美世乐(专业从事逆变器、ups电源的研发、生产、销售为一体的综合性制造商，深圳市美克能源科技股份有限公司)

求电气工程及其自动化英文论文一篇 内容不限 题目要翻译好 谢谢

       用于分布式在线UPS中空慧的并联逆变器的兆亏顷一种无线族陆控制器

       A Wireless Controller for Parallel Inverters in Distributed Online UPS Systems

       Josep M. Guerrero', Luis Garcia de Vicufia", Jose Matas'*, Jaume Miret", and Miguel Castilla"

       . Departament #Enginyeria de Sistemes, Automatica i Informhtica Industrial. Universitat Polithica de Catalunya

       C. Comte d'Urgell, 187.08036 -Barcelona. Spain. Email: josep.m.guerrero@upc.es .. Departament #Enginyeria Electrbnica. Universitat Polit6cnica de Catalunya

       AV. Victor BaLguer s/n. 08800I - Vilanova i la Geltrh. Spain

       Absiract - In this paper, a novel controller for parallelconnected

       online-UPS inverters without control wire

       interconnections is presented. The wireless control technique is

       based on the well-known droop method, which consists in

       introducing P-oand Q-V schemes into the inverters, in order to

       share properly the power drawn to the loads. The droop method

       has been widely used in applications of load sharing between

       different parallel-connected inverters. However, this method

       has several drawbacks that limited its application, such as a

       trade-off between output-voltage regulation and power sharing

       accuracy, slow transient response, and frequency and phase

       deviation. This last disadvantage makes impracticable the

       method in online-UPS systems, since in this case every module

       must be in phase with the utility ac mains. To overcome these

       limitations, we propose a novel control scheme, endowing to the

       paralleled-UPS system a proper transient response, strictly

       frequency and phase synchronization with the ac mains, and

       excellent power sharing. Simulation and experimental results

       are reported confirming the validity of the proposed approach.

       1. INTRODUCTION

       The parallel operation of distributed Uninterruptible Power

       Supplies (UPS) is presented as a suitable solution to supply

       critical and sensitive loads, when high reliability and power

       availability are required. In the last years, many control

       schemes for parallel-connected inverters has been raised,

       which are derived from parallel-schemes of dc-dc converters

       [I], such as the master-slave control [2], or the democratic

       control [3]. In contrast, novel control schemes have been

       appeared recently, such as the chain-structure control [4], or

       the distributed control [ 5 ] . However, all these schemes need

       control interconnections between modules and, hence, the

       reliability of the system is reduced since they can be a source

       of noise and failures. Moreover, these communication wires

       limited the physical situation ofthe modules [6].

       In this sense, several control techniques has been proposed

       without control interconnections, such as the droop method.

       In this method, the control loop achieves good power sharing

       making tight adjustments over the output voltage frequency

       and amplitude of the inverter, with the objective to

       compensate the active and reactive power unbalances [7].

       This concept is derived from the power system theory, in

       which the frequency of a generator drops when the power

       drawn to the utility line increases [8].

       0-7803-7906-3/03/$17.00 02003 IEEE. 1637

       However, this control approach has an inherent trade-off

       between voltage regulation and power sharing. In addition,

       this method exhibits slow dynamic-response, since it requires

       low-pass filters to calculate the average value of the active

       and reactive power. Hence, the stability and the dynamics of

       the whole system are hardly influenced by the characteristics

       of these filters and by the value of the droop coefficients,

       which are bounded by the maximum allowed deviations of

       the output voltage amplitude and frequency.

       Besides, when active power increases, the droop

       characteristic causes a frequency deviation from the nominal

       value and, consequently, it results in a variable phase

       difference between the mains and the inverter output voltage.

       This fact can be a problem when the bypass switch must

       connect the utility line directly to the critical bus in stead of

       its phase difference. In [9], two possibilities are presented in

       order to achieve phase synchronization for parallel lineinteractive

       UPS systems. The first one is to locate a particular

       module near the bypass switch, which must to synchronize

       the output voltage to the mains while supporting overload

       condition before switch on. The second possibility is to wait

       for the instant when phase matching is produced to connect

       the bypass.

       However, the mentioned two folds cannot be applied to a

       parallel online-UPS system, since maximum transfer time

       ought to be less than a % of line period, and all the modules

       must be always synchronized with the mains when it is

       present. Hence, the modules should be prepared to transfer

       directly the energy from the mains to the critical bus in case

       of overload or failure [lo].

       In our previous works [11][12], we proposed different

       control schemes to overcome several limitations of the

       conventional droop method. However, these controllers by

       themselves are inappropriate to apply to a parallel online-

       UPS system. In this paper, a novel wireless control scheme is

       proposed to parallel different online UPS modules with high

       performance and restricted requirements. The controller

       provides: 1) proper transient response; 2) power sharing

       accuracy; 3) stable frequency operation; and 4) good phase

       matching between the output-voltage and the utility line.

       Thus, this new approach is especially suitable for paralleled-

       UPS systems with true redundancy, high reliability and

       power availability. Simulation and experimental results are

       reported, confirming the validity of this control scheme.

       Fig. 1. Equivalenl cimuif ofan invener connecled 10 a bus

       t"

       Fig. 2. P-odraop function.

       11. REVlEW OF THE CONVENTIONAL DROOP METHOD

       Fig. 1 shows the equivalent circuit of an inverter connected

       to a common bus through coupled impedance. When this

       impedance is inductive, the active and reactive powers drawn

       to the load can be expressed as

       EVcosQ - V2 Q=

       where Xis the output reactance of an inverter; Q is the phase

       angle between the output voltage of the inverter and the

       voltage of the common bus; E and V are the amplitude of the

       output voltage of the inverter and the bus voltage,

       respectively.

       From the above equations it can be derived that the active

       power P is predominately dependent on the power angle Q,

       while the reactive power Q mostly depends on the outputvoltage

       amplitude. Consequently, most of wireless-control of

       paralleled-inverters uses the conventional droop method,

       which introduces the following droops in the amplitude E

       and the frequency U of the inverter output voltage

       u = w -mP (3)

       E = E ' - n Q , (4)

       being W* and E' the output voltage frequency and amplitude

       at no load, respectively; m and n are the droop coefficients

       for the frequency and amplitude, respectively.

       Furthermore, a coupled inductance is needed between the

       inverter output and the critical bus that fixes the output

       impedance, in order to ensure a proper power flow. However,

       it is bulky and increase:; the size and the cost of the UPS

       modules. In addition, tho output voltage is highly distorted

       when supplying nonlinezr loads since the output impedance

       is a pure inductance.

       It is well known that if droop coefficients are increased,

       then good power sharing is achieved at the expense of

       degrading the voltage regulation (see Fig. 2).

       The inherent trade-off of this scheme restricts the

       mentioned coefficients, which can be a serious limitation in

       terms of transient response, power sharing accuracy, and

       system stability.

       On the other hand, lo carry out the droop functions,

       expressed by (3) and (4), it is necessary to calculate the

       average value over one line-cycle of the output active and

       reactive instantaneous power. This can be implemented by

       means of low pass filters with a smaller bandwidth than that

       of the closed-loop inverter. Consequently, the power

       calculation filters and droop coefficients determine, to a large

       extent, the dynamics and the stability of the paralleledinverter

       system [ 131.

       In conclusion, the droop method has several intrinsic

       problems to be applied 1.0 a wireless paralleled-system of

       online UPS, which can he summed-up as follows:

       Static trade-off between the output-voltage regulation

       (frequency and amplitude) and the power-sharing

       accuracy (active an4d reactive).

       2) Limited transient response. The system dynamics

       depends on the power-calculation filter characteristics,

       the droop coefficients, and the output impedances.

       Lost of ac mains synchronization. The frequency and

       phase deviations, due to the frequency droop, make

       impracticable this method to a parallel-connected

       online UPS system, in which every UPS should be

       continuously synchronized to the public ac supply.

       1)

       3)

       111. PROPOSED CONTROL FOR PARALLEL ONLINE UPS

       INVERTERS

       In this work, we will try to overcome the above limitations

       and to synthesize a novel control strategy without

       communication wires that could be appropriate to highperformance

       paralleled industrial UPS. The objective is to

       connect online UPS inverters in parallel without using

       control interconnections. This kind of systems, also named

       inverter-preferred, should be continuously synchronized to

       the utility line. When an overload or an inverter failure

       occurs, a static bypass switch may connect the input line to

       the load, bypassing the inve:rter [14][15].

       Fig. 3 shows the general diagram of a distributed online

       UPS system. This system consists of two buses: the utility

       bus, which is connected lo the public ac mains; and the

       secure bus, connected to the distributed critical loads. The

       interface between these buses is based on a number of online

       UPS modules connected in parallel, which provides

       continuously power to the: loads [16]. The UPS modules

       include a rectifier, a set of batteries, an inverter, and a static

       bypass switch.

       1

       1638

       Q ac mains

       utility bus

       I I I

       j distributed loads !

       Fig. 3. Online distributed UPS system.

       syposr /

       I 4

       (4

       Fig. 4. Operation modes of an online UPS.

       (a) Normal operation. (b) Bypass operation. (c) Mains failure

       The main operation modes of a distributed online UPS

       1) Normal operation: The power flows to the load, from

       the utility through the distributed UPS units.

       2) Mains failure: When the public ac mains fails, the

       UPS inverters supply the power to the loads, from the

       batteries, without disruption.

       Bypass operation: When an overload situation occurs,

       the bypass switch must connect the critical bus

       directly to the ac mains, in order to guarantee the

       continuous supply of the loads, avoiding the damage

       of the UPS modules.

       For this reason, the output-voltage waveform should be

       synchronized to the mains, when this last is present.

       system are listed below (see Fig. 5):

       3)

       Nevertheless, as we state before, the conventional droop

       method can not satisfy the need for synchronization with the

       utility, due to the frequency variation of the inverters, which

       provokes a phase deviation.

       To obtain the required performance, we present a transient

       P-w droop without frequency-deviation in steady-state,

       proposed previously by OUT in [ 111

       w=o -mP (5)

       where is the active power signal without the dccomponent,

       which is done by

       . -

       I t -1s

       P= p ,

       ( s + t - ' ) ( s + o , )

       being zthe time constant of the transient droop action.

       The transient droop function ensures a stable frequency

       regulation under steady-state conditions, and 'at the same

       time, achieves active power balance by adjusting the

       frequency of the modules during a load transient. Besides, to

       adjust the phase of the modules we propose an additional

       synchronizing loop, yielding

       o=w'-m%k,A$, (7)

       where A$ is the phase difference between the inverter and the

       mains; and k, is the proportional constant of the frequency

       adjust. The steady-state frequency reference w* can be

       obtained by measuring the utility line frequency.

       The second term of the previous equality trends to zero in

       steady state, leading to

       w = w' - k4($ -@'), (8)

       being $and $* the phase angles of the output voltage inverter

       and the utility mains, respectively.

       Taking into account that w = d $ / d t , we can obtain the

       next differential equation, which is stable fork, positive

       d$ *

       dt dt

       - + km$ = - + k,$' . (9)

       Thus, when phase difference increases, frequency will

       decrease slightly and, hence, all :he UPS modules will be

       synchronized with the utility, while sharing the power drawn

       to the loads.

       IV. CONTROLLIEMRP LEMENTATION

       Fig. 5 depicts the block diagram of the proposed

       controller. The average active power P , without the dc

       component, can be obtained by means of multiplying the

       output voltage by the output current, and filtering the product

       ........................................................................................

       io

       ",.

       L

       Sj'nchronirorion loop

       .......................................................................................

       Fig. 5. Block diagram of the proposed controller.

       using a band-pass filter. In a similar way, the average

       reactive power is obtained, hut in this case the output-voltage

       must be delayed 90 degrees, and using a low-pass filter.

       In order to adjust the output voltage frequency, equation

       (7) is implemented, which corresponds to the frequency

       mains drooped by two transient-terms: the transient active

       power signal term; and the phase difference term, which

       is added in order to synchronize the output voltage with the

       ac mains, in a phase-locked loop (PLL) fashion. The outputvoltage

       amplitude is regulated by using the conventional

       droop method (4).

       Finally, the physical coupled inductance can be avoided by

       using a virtual inductor [17]. This concept consists in

       emulated an inductance behavior, by drooping the output

       voltage proportionally to the time derivative of the output

       current. However, when supplying nonlinear loads, the highorder

       current-harmonics can increase too much the outputvoltage

       THD. This can be easily solved by using a high-pass

       filter instead of a pure-derivative term of the output current,

       which is useful to share linear and nonlinear loads [I 1][12].

       Furthermore, the proper design of this output inductance can

       reduce, to a large extent, the unbalance line-impedance

       impact over the power sharing accuracy.

       v. SIMULATION AND EXPERIMENTARELS ULTS

       The proposed control scheme, (4) and (7), was simulated

       with the parameters listed in Table 1 and the scheme shown

       in Fig. 6, for a two paralleled inverters system. The

       coefficients m, n, T, and kv were chosen to ensure stability,

       proper transient response and good phase matching. Fig. 7

       shows the waveforms of the frequency, circulating currents,

       phase difference between the modules and the utility line,

       and the evolution of the active and reactive powers. Note the

       excellent synchronization between the modules and the

       ACmiiinr 4 j. ...L...I.P...S...1... ..........................B...u...n...r.r..r..e..s... ................................... i

       Fig. 6. Parallel operation oftwa online UPS modules,

       mains, and, at the same time, the good power sharing

       obtained. This characteristik let us to apply the controller to

       the online UPS paralleled systems.

       Two I-kVA UPS modules were built and tested in order to

       show the validity of the proposed approach. Each UPS

       inverter consisted of a single-phase IGBT full-bridge with a

       switching frequency of 20 kHz and an LC output filter, with

       the following parameters: 1. = 1 mH, C = 20 WF, Vi" = 400V,

       v, = 220 V, I50 Hz. The controllers of these inverters were

       based on three loops: an inner current-loop, an outer PI

       controller that ensures voltage regulation, and the loadsharing

       controller, based on (4) and (7). The last controller

       was implemented by means of a TMS320LF2407A, fixedpoint

       40 MHz digital sigrial processor (DSP) from Texas

       Instruments (see Fig. 8), using the parameters listed in Table

       I. The DSP-controller also includes a PLL block in order to

       synchronize the inverter with the common bus. When this

       occurs, the static bypass switch is tumed on, and the droopbased

       control is initiated.

       1640

       big 7 Wa\cfc)rms for twu.invencr, ;mnectcd in parallel. rpchrontred io Ihc ac mdnl.

       (a) Frequencics ufhoth UPS (b) Clrculattng currcni among modulcs. (CJ Phmc d!Nercn;: betucen ihc UPS a#>dth e ai mum

       (d) Ikiril uf the phze diNmncc (e) md (0 Activc and rcactlw pouerr "I ooih UPS

       Note that the iimc-acs arc deliheratcly JiNercni due in thc disiinct timuion*uni) ofthe \ inrblrr

       1641

       TABLEI.

       PARAMETEROSF THE PARALLELESDYS TEM.

       Filter Order I I

       Filter Cut-off Frequency I 0, I 10 I rags

       Fig. 8 shows the output-current transient response of the

       UPS inverters. First, the two UPS are operating in parallel

       without load. Notice that a small reactive current is circling

       between the modules, due to the measurement mismatches.

       Then, a nonlinear load, with a crest factor of 3, is connected

       suddenly. This result shows the good dynamics and loadsharing

       of the paralleled system when sharing a nonlinear

       load.

       Fig. 8. Output current for the two paralleled UPS, during the connection of B

       common nonlinear load with a crest factor of 3. (Axis-x: 20 mddiv. Axis-y:

       5 Mdiv.).

       VI. CONCLUSIONS

       In this paper, a novel load-sharing controller for parallelconnected

       online UPS systems, was proposed. The controller

       is based on the droop method, which avoids the use of

       control interconnections. In a sharp contrast with the

       conventional droop method, the controller presented is able

       to keep the output-voltage frequency and phase strictly

       synchronized with the utility ac mains, while maintaining

       good load sharing for linear and nonlinear loads. This fact let

       us to extend the droop method to paralleled online UPS.

       On the other hand, the proposed controller emulates a

       special kind of impedance, avoiding the use of a physical

       coupled inductance. Th.e results reported here show the

       effectiveness of the proposed approach.

与逆变器相关的英文文章

       Inverter may refer to

       Inverter (electrical), a device that converts direct current to alternating current

       Inverter (air conditioning), an air conditioner that can continuously regulate its output by altering the compressor speed in response to cooling demand

       Uninterruptible power supply, which often are based on an electrical inverter

       Inverter (logic gate), a logic gate also called a NOT gate

       Inverter (electrical)

       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.

       Static 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.

       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 "inverted", to convert DC to AC.

       The inverter performs the opposite function of a rectifier.

       Applications

       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 voltage.

       Grid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as 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.

       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 is used to supply DC power to recharge the batteries.

       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.

       [edit] 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.

       [edit] Variable-frequency drives

       Main article: variable-frequency drive

       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.

       [edit] 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. Various improvements in inverter technology are being developed specifically for electric vehicle applications.[2] In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries.

       [edit] Air conditioning

       Main article: Inverter (air conditioning)

       An air conditioner bearing the inverter tag uses a variable-frequency drive to control the speed of the motor and thus the compressor.

       [edit] 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 output power can never exceed the input power, but efficiencies can be high, with a small proportion of the power dissipated as waste heat.

       Warnings

       Some low power inverters have a warning not to use conventional fluorescent lighting. This is due to the power correction capacitor connected in parallel with the lamp. Removing the capacitor will fix the problem. What may not be known is that in dual lamp fittings the capacitor may be connected in series with the second lamp, thus removing the problem as well as the stroboscopic effect caused by the mains frequency.

       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.

       [edit] 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 the inverter output waveform can be expressed by using the Fourier analysis data to calculate the total harmonic distortion (THD). The total harmonic distortion is the square root of the sum of the squares of the harmonic voltages divided by the fundamental voltage:

       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.

       [edit] Advanced designs

       H-bridge inverter circuit with transistor switches and antiparallel diodesThere are many different power circuit topologies and control strategies used in inverter designs. Different design approaches address various issues that may be more or less important depending on the way that the inverter is intended to be used.

       The issue of waveform quality can be addressed in many ways. Capacitors and inductors can be used to filter the waveform. If the design includes a transformer, filtering can be applied to the primary or the secondary side of the transformer or to both sides. Low-pass filters are applied to allow the fundamental component of the waveform to pass to the output while limiting the passage of the harmonic components. If the inverter is designed to provide power at a fixed frequency, a resonant filter can be used. For an adjustable frequency inverter, the filter must be tuned to a frequency that is above the maximum fundamental frequency.

       Since most loads contain inductance, feedback rectifiers or antiparallel diodes are often connected across each semiconductor switch to provide a path for the peak inductive load current when the switch is turned off. The antiparallel diodes are somewhat similar to the freewheeling diodes used in AC/DC converter circuits.

       Fourier analysis reveals that a waveform, like a square wave, that is antisymmetrical about the 180 degree point contains only odd harmonics, the 3rd, 5th, 7th etc. Waveforms that have steps of certain widths and heights eliminate or “cancel” additional harmonics. For example, by inserting a zero-voltage step between the positive and negative sections of the square-wave, all of the harmonics that are divisible by three can be eliminated. That leaves only the 5th, 7th, 11th, 13th etc. The required width of the steps is one third of the period for each of the positive and negative steps and one sixth of the period for each of the zero-voltage steps.

       Changing the square wave as described above is an example of pulse-width modulation (PWM). Modulating, or regulating the width of a square-wave pulse is often used as a method of regulating or adjusting an inverter's output voltage. When voltage control is not required, a fixed pulse width can be selected to reduce or eliminate selected harmonics. Harmonic elimination techniques are generally applied to the lowest harmonics because filtering is more effective at high frequencies than at low frequencies. Multiple pulse-width or carrier based PWM control schemes produce waveforms that are composed of many narrow pulses. The frequency represented by the number of narrow pulses per second is called the switching frequency or carrier frequency. These control schemes are often used in variable-frequency motor control inverters because they allow a wide range of output voltage and frequency adjustment while also improving the quality of the waveform.

       Multilevel inverters provide another approach to harmonic cancellation. Multilevel inverters provide an output waveform that exhibits multiple steps at several voltage levels. For example, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the inverter output terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped waveform is generated at the inverter output. This is an example of a three level inverter: the two voltages and ground.[3]

       [edit] Three phase inverters

       3-phase inverter with wye connected loadThree-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.

       3-phase inverter switching circuit showing 6-step switching sequence and waveform of voltage between terminals A and CTo construct inverters with higher power ratings, two six-step three-phase inverters can be connected in parallel for a higher current rating or in series for a higher voltage rating. In either case, the output waveforms are phase shifted to obtain a 12-step waveform. If additional inverters are combined, an 18-step inverter is obtained with three inverters etc. Although inverters are usually combined for the purpose of achieving increased voltage or current ratings, the quality of the waveform is improved as well.

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