电气工程方面的英汉论文.doc

Electric Devices and Systems
Although 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 matching 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 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 paper 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 exist, and the transformer offers no exception. An ideal transformer is based on very simple concepts， 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 flux is to remain in the iron. We assign sots at one terminal of each coil in the following fashion： if the flux 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 terminals produces flux in the same direction，
Assume that somehow a time varying flux is established in the iron. Then the flux 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 flowing 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 existence of an ideal transformer。
Equations describe this ideal transformer， a two port network。 The symbol of a network that 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 on a side 1 can be transferred to side 2 by multiplying its component impedances , the voltage sources and the current sources， while keeping the topology the same。 To develop the equivalent for a transformer we’ll gradually relax the assumptions that we had first imposed. First we’ll relax the assumption that the permeability of the iron is infinite。 In that case equation does not revert to, but rather it becomes where is the reluctance of the path around the core of the transformer and the flux on this path. To preserve the ideal transformer equations as part of our new transformer we can split i1 to two components: 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 flux is related to the induced voltage。
Since the 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 relax the assumption that all the flux has to remain in the iron as shown in figure。 Let us call the flux in the iron, magnetizing flux, the flux 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 flux.
Again for a given frequency， the power losses in the core increase with the voltage. These losses cannot be allowed to exceed 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 limits 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 the 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, maximum current and voltage, in typical transformers the magnetizing current, does not exceed 1％ of the current in the transformer. Its effect therefore in the voltage drop on the leakage inductance and winding resistance is negligible。
Under maximum current， total voltage drops on the winding resistances and leakage inductances do not exceed in typical transformer 6％ of the rated voltage. The effect therefore of 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 Drives
By 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 common mechanical versions use combinations of belts and sheaves， or chains and sprockets, to 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 fluid drives provide smooth， stepless adjustable speed control. There are three major types. Hydrostatic drives use electric motors or internal combustion engines as prime movers in combination with hydraulic pumps， which in turn drive hydraulic motors。 Hydrokinetic and hydroviscous drives directly couple input and output shafts. Hydrokinetic versions adjust speed by varying the amount of fluid in a vortex 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.
An 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 fields 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， create adjustable speed motors, allowing speeds from zero RPM to beyond the motor’s 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， the need for wear—prone mechanical components, 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 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 drives 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 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 common, as shown in the figure.
Drives: these are named for the silicon controlled rectifiers （also called thyristors ） used to convert AC to controlled voltage DC. Inexpensive and easy to use, these drives come 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 coming back from the motor during braking 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 first converts AC to DC, the second filters and regulates the fixed 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 drive's improved performance compared with a common SCR drive.
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 drive’s inverter section accomplishes this， In years past， this was accomplished using SCR. However， modern AC drives use a series of transistors to invert DC to adjustable—Frequency AC。 An example 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 motor speed。 For example， 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, approximately，15 to200% of a motor’s normally rated RPM—- by delivering frequencies of 9 HZ to 120 Hz， respectively.
Today， AC drives are becoming 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 maintenance 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 combined with feedback devices such as tachometers， encoders and resolvers in a closed—loop system， are continuing to replace DC drives in demanding applications。 An Example 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 or even thousands of horsepower—as 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 inverter’s ability to vary the output voltage to the motor by altering the width and polarity of voltage pulses， The voltage and frequency are synthesized using this stream of voltage pulses。 This is accomplished through microprocessor commands to a series of power semiconductors that serve as on—off switches. Today， these switches are usually IGBTs， of isolated gate bipolar 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 devices
The modern age of power electronics began with the introduction of thyristors in the late 1950s。 Now there are several types 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 all 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 applications. 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 short 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 basically 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 commutation (or phase-controlled) applications。 Inverter-grade thyristors are used in forced commutation 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 external commutation circuit。 This requires additional commutating components， 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 exceeds 3 V。 While the forward voltage determines the on-state power loss of the device at any given current, the switching power loss becomes a dominating factor affecting the dev