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1、-Electric Devices and SystemsAlthough transformers have no moving parts , they are essential to electromechanical energy conversion . They make it possible to increase or decrease the voltage lever that results in low costs ,and can be distributed and used safely . In addition , they can provide mat
2、ching of impedances , and regulate the flow of power in a network.When we see a transformer on a utility pole all we is a cylinder with a few wires sticking out. These wires enter the transformer through bushings that provide isolation between the wires and the tank. Inside the tank these is an iron
3、 core linking coils, most probably made with copper, and insulated. The system of insulation is also associated with that of cooling the core/coil assembly. Often the insulation is paper, and the whole assembly may be immersed in insulating oil, used to both increase the dielectric strength of the p
4、aper and to transfer beat from the core-coil assembly to the outer walls of the tank to air. Figure shows the cutout of a typical distribution transformer. Few ideal versions of human constructions e*ist, and the transformer offers no e*ception. An ideal transformer is based on very simple concepts,
5、 and a large number of assumptions. This is the transformer one learns about in high school.Let us take an iron core with infinite permeability and two coils wound around it, one with N1 and the other with N2 turns, as shown in figure. All the magnetic flu* is to remain in the iron. We assign sots a
6、t one terminal of each coil in the following fashion: if the flu* in the core changes, inducing a voltage in the coils, and the dotted terminal of one coil is positive with respect its other terminal, so is the dotted terminal of the other coil. Or, the corollary to this, current into dotted termina
7、ls produces flu* in the same direction,Assume that somehow a time varying flu* is established in the iron. Then the flu* linkages in each coil will be. Voltages will be induced in these two coil.On the other hand, currents flowing in the coils are related to the field intensity H. if currents flowin
8、g in the direction shown, i1 into the dotted terminal of coil 1, and i2 out of the dotted terminal of coil 2. we recognize that this is practically impossible, but so is the e*istence of an ideal transformer.Equations describe this ideal transformer, a two port network. The symbol of a network that
9、is defined by these two equations is in the figure. An ideal transformer has an interesting characteristic. A two-port network that contains it and impedances can be replaced by an equivalent other, as discussed below. Consider the circuit in figure. Seen as a two port network. Generally a circuit o
10、n a side 1 can be transferred to side 2 by multiplying its ponent impedances , the voltage sources and the current sources, while keeping the topology the same. To develop the equivalent for a transformer well gradually rela* the assumptions that we had first imposed. First well rela* the assumption
11、 that the permeability of the iron is infinite. In that case equation does not revert to, but rather it bees where is the reluctance of the path around the core of the transformer and the flu* on this path. To preserve the ideal transformer equations as part of our new transformer, we can split i1 t
12、o two ponents: one i1, will satisfy the ideal transformer equation, and the other, i1 will just balance the right hand side. The figure shows this. We can replace the current source, i1 , with something simpler if we remember that the rate of change of flu* is related to the induced voltage.Since th
13、e current i1 flows through something , where the voltage across it Is proportional to its derivative, we can consider that this something could be an inductance. This idea gives rise tothe equivalent circuit in figure,. Let us now rela* the assumption that all the flu* has to remain in the iron as s
14、hown in figure. Let us call the flu* in the iron, magnetizing flu*, the flu* that leaks out of the core and links only coil 1. since links only coil 1, then it should be related only to the current there, and the same should be true for the second leakage flu*.Again for a given frequency, the power
15、losses in the core increase with the voltage. These losses cannot be allowed to e*ceed limit, beyond which the temperature of the hottest spot in the transformer will rise above the point that will decrease dramatically the life of the insulation. Limits therefore are put to E1 and E2, and these lim
16、its are the voltage limits of the transformer. Similarly, winding Joule losses have to be limited, resulting in limits to the currents I1 and I2. Typically a transformer is described by its rated voltages, that give both the limits and turns radio. The ratio of the rated currents is the inverse of t
17、he ratio of the voltages if we neglect the magnetizing current. Instead of the transformer rated currents, a transformer is described by its rated apparent power.Under rated conditions, ma*imum current and voltage, in typical transformers the magnetizing current, does not e*ceed 1% of the current in
18、 the transformer. Its effect therefore in the voltage drop on the leakage inductance and winding resistance is negligible.Under ma*imum current, total voltage drops on the winding resistances and leakage inductances do not e*ceed in typical transformer 6% of the rated voltage. The effect therefore o
19、f the winding current on the voltages E1 and E2 is small, and their effect on the magnetizing current can be neglected.These considerations allow us to modify the equivalent circuit in figure, to obtain the slightly inaccurate but much more useful equivalent circuits in figures.Adjustable Speed Driv
20、esBy definition, adjustable speed drives of any type provide a means of variably changing speed to better match operating requirements. Such drives are available in mechanical, fluid and electrical typed.The most mon mechanical versions use binations of belts and sheaves, or chains and sprockets, to
21、 adjust speed in set, selectable ratios-2:1,4:1,8:1 and so forth. Traction drives, a more sophisticated mechanical control scheme, allow incremental speed adjustments. Here, output speed is varied by changing the contact points between metallic disks, or between balls and cones. Adjustable speed flu
22、id drives provide smooth, stepless adjustable speed control. There are three major types. Hydrostatic drives use electric motors or internal bustion engines as prime movers in bination with hydraulic pumps, which in turn drive hydraulic motors. Hydrokinetic and hydroviscous drives directly couple in
23、put and output shafts. Hydrokinetic versions adjust speed by varying the amount of fluid in a vorte* that serves as the input-to-output coupler. Hydroviscous drives, also called oil shear drives, adjust speed by controlling oil-film thickness, and therefore slippage, between rotating metallic disk.A
24、n eddy current drive, while technically an electrical drive, nevertheless functions much like a hydrokinetic or hydrovidcous fluid drive in that it serves as a coupler between a prime mover and driven load. In an eddy current drive, the coupling consists of a primary magnetic field and secondary fie
25、lds created by induced eddy currents. They amount of magnetic slippage allowed among the fields controls the driving speed.In most industrial applications, mechanical, fluid or eddy current drives are paired with constant-speed electric motors. On the other hand, solid state electrical drives, creat
26、e adjustable speed motors, allowing speeds from zero RPM to beyond the motors base speed. Controlling the speed of the motor has several benefits, including increased energy efficiency by eliminating energy losses in mechanical speed changing devices. In addition, by reducing, or often eliminating,
27、the need for wear-prone mechanical ponents, electrical drives foster increased overall system reliability, as well as lower maintenance costs. For these and other reasons, electrical drives are the fastest growing type of adjustable speed drive.There are two basic drive types related to the type of
28、motor controlled-dc and AC. A DC direct current drive controls the speed of a DC motor by varying the armature voltage (and sometimes also the field voltage ). An alternating current drive controls the speed of an AC motor by varying the frequency and voltage supplied to the motor.Direct current dri
29、ves are easy to apply and technologically straightforward, They work by rectifying AC voltage from the power line to DC voltage, then feeding adjustable voltage to a DC motor. With permanent magnet DC motors, only the armature voltage is controlled. The more voltage supplied, the faster the armature
30、 turns. With wound-field motors, voltage must be supplied to both the armature and the field. In industry, the following three types of DC drives are most mon, as shown in the figure.Drives: these are named for the silicon controlled rectifiers (also called thyristors ) used to convert AC to control
31、led voltage DC. Ine*pensive and easy to use, these drives e in a variety of enclosures, and in unidirectional or reversing styles.Regenerative SCR Drives: Also called four quadrant drives, these allow the DC motor to provide both motoring and braking torque, Power ing back from the motor during brak
32、ing is regenerated back to the power line and not lost.Pulse Width Modulated DC Drives: Abbreviated PWM and also called, generically, transistorized DC drives, these provide smoother speed control with higher efficiency and less motor heating, Unlike SCR drives, PWM types have three elements. The fi
33、rst converts AC to DC, the second filters and regulates the fi*ed DC voltage, and the third controls average voltage by creating a stream of variable width DC pulses. The filtering section and higher level of control modulation account for the PWM drives improved performance pared with a mon SCR dri
34、ve.AC drive operation begins in much the same fashion as a DC drive. Alternating line voltage is first rectified to produce DC. But because an AC motor is used, this DC voltage must be changed back, of inverted, to an adjustable-frequency alternating voltage. The drives inverter section acplishes th
35、is, In years past, this was acplished using SCR. However, modern AC drives use a series of transistors to invert DC to adjustable-Frequency AC. An e*ample is shown in figure.This synthesized alternating current is then fed to the AC motor at the frequency and voltage required to produce the desired
36、motor speed. For e*ample, a 60 Hz synthesized frequency, the same as standard line frequency in the United states, produces 100% of rated motor speed. A lower frequency produces a lower speed, and a higher frequency a higher speed. In this way, an AC drive can produce motor speeds from, appro*imatel
37、y,15 to200% of a motors normally rated RPM- by delivering frequencies of 9 HZ to 120 Hz, respectively.Today, AC drives are being the systems of choice in many industries,. Their use ofsimple and rugged three-phase induction motor means that AC drive systems are the most reliable and least maintenanc
38、e prone of all. Plus, microprocessor advancements have enabled the creation of so-called vector drives, which provide greatly enhance response, operation down to zero speed and positioning accuracy. Vector drives, especially when bined with feedback devices such as tachometers, encoders and resolver
39、s in a closed-loop system, are continuing to replace DC drives in demanding applications. An E*ample is shown in the figure.By far the most popular AC drive today is the pulse width modulated type. Though originally developed for smaller-horsepower applications, PWM is now used in drives of hundreds
40、 or even thousands of horsepoweras well as remaining the staple technology in the vast majority of small integral and fractional horsepower micro and sub-micro AC drives, as shown in the figure. Pulse width modulated refers to the inverters ability to vary the output voltage to the motor by altering
41、 the width and polarity of voltage pulses, The voltage and frequency are synthesized using this stream of voltage pulses. This is acplished through microprocessor mands to a series of power semiconductors that serve as on-off switches. Today, these switches are usually IGBTs, of isolated gate bipola
42、r transistor. A big advantage to these devices is their fast switching speed resulting in higher pulse of carrier frequency, which minimizes motor noise.Power semiconductor devicesThe modern age of power electronics began with the introduction of thyristors in the late 1950s. Now there are several t
43、ypes of power devices available for high-power and high-frequency applications. The most notable power devices are gate turn-off thyristor, power darlington transistors, power mosfets, and insulated-gate bipolar transistors. Power semiconductor devices are the most important functional elements in a
44、ll power conversion applications. The power devices are mainly used as switches to convert power from one form to another. They are used in motor control systems, uninterrupted power supplies, high-voltage dc transmission, power supplies, induction heating, and in many other power conversion applica
45、tions. A review of the basic characteristics of these power devices is presented in this section.The thyristor, also called a silicon-controlled rectifier, is basically a four-layer three-junction pn device. It has three terminals: anode, cathode, and gate. The device is turned on by applying a shor
46、t pulse across the gate and cathode. Once the device turns on, the gate loses its control to turn off the device. The turn-off is achieved by applying a reverse voltage across the anode and cathode. The thyristors symbol and its volt-ampere characteristics are shown in the figure. There are basicall
47、y two classifications of thyristors: converter grade and inverter grade. The difference between a converter-grade and an inverter-grade thyristor is the low turn off time (on the order of a few microseconds) for the latter. The converter-grade thyristors are slow type and are used in natural mutatio
48、n (or phase-controlled) applications. Inverter-grade thyristors are used in forced mutation applications such as dc-dc choppers and dc-ac inverters. The inverter-grade thyristors are turned off by forcing the current to zero using an e*ternal mutation circuit. This requires additional mutating ponen
49、ts, thus resulting in additional losses in the inverter. Thyristors are highly rugged devices in terms of transient currents, di / dt, and dv/dt capability. The forward voltage drop in thyristors is about 1.5 to 2 V, and even at higher currents of the order of 100 A, it seldom e*ceeds 3 V. While the forward voltage determines the on-state power loss of the device at any given current, the switching power loss bees a dominating factor affecting the device junction temperature at high operating frequencies. Because of this,
