1、附录 附录 A 外文翻译 原文部分 Alternating Current An alternating current is one which varies in the circuit with time. The value of the current at any given instant of time is the instantaneous value of current, symbolized by i . The instantaneous value of current i is deemed positive for one direction of flow
2、though the cross-section of a conductor, and negative for the opposite direction. The direction of current flow in which instantaneous values are positive is called positive. A current is fully specified, if one knows its instantaneous value as a function of time, i =F(t),and its positive direction.
3、 Current whose value recur for equal increments of time are called periodic, and the least increment of time for which this recurrent takes place is called the period T . For a periodic current i = F(t)= F(t+T) Fig.4.1 shows an example of the relationship i = F(t) for a periodic current. The arrow i
4、n the diagram indicates the positive direction of current flow. The dotted arrows show the actual direction of current flow at the instants of time when i 0 and i 0. The segments of the curve between points a and b or O and c cover a complete cycle of current alternations over one period. The number
5、 of cycle or period per second is the frequency of a periodic current. It is reciprocal of its period. Tf /1 It is usually to specify the frequency of any periodic quantity in cycle per second. Thus the frequency of a periodic current will be 1 cycle per second, if its period is second, or 1cycle/se
6、c.A direct current may be regarded as a special case of a periodic current whose period is infinitely long and the frequency is thus zero. The term alternating current is often used in the narrow sense of a periodic current whose constant(direction-current)component is zero, or 01 0 T idtT The frequ
7、encies of alternating current encountered in practice range over wide values. The mains frequency is 50Hz in the Soviet Union and Europe, and 60Hz in the United States. Some industrial processes use frequencies from 10Hz to 9105.2 Hz .In radio practice, frequencies up to 10103 Hz are employed. The d
8、efinitions for current just introduced(and, indeed, those that will be introduced shortly) fully apply to periodic voltages, e.m.f.s, magnetic fluxes and any other electrical and magnetic quantities. Some additional remarks are only needed with regard to the sign of alternating voltage and e.m.f.s.
9、An alternating voltage between two point A and B, determined along a specified path 1, periodically changes sign, so that if it is assumed to be positive in the direction from A to B. it will be negative in direction from B to A at the same instant of time. This is why it is important to label which
10、 of the two directions is assumed positive. In diagrams, such a direction is labeled either by arrows or subscripts in the symbols for voltage and is regarded to be the positive reference direction of a voltage(or of an e.m.f.s). Electrical engineering uses the simplest and commonest type of alterna
11、ting current, the one which varies sinusoidally with time； hence the term a harmonic or a sinusoidal current. The preference for sinusoidal currents is explained modulated by the signal in amplitude, frequency or phase. Periodic non-sinusoidal current may likewise be treated as composed of sinusoida
12、l current at a variety of frequencies occurring simultaneously. This is why thorough of sinusoidal-current circuits is of primary importance. The A.C Generator An A.C generator consists of a stationary part, the stator, and a revolving part, the rotor. As a rule the rotor carries magnetic poles with
13、 coils around them. These are the filed coils of the generator ,because they establish a magnetic filed in the machine. They are energized with direct current through slip rings and brushes. The slots of the stator staked up from electrical-sheet steel punchings receive the coils of the stator windi
14、ng. The stator coils are connected in series, as shown by the fully and dotted lines in the drawing. The e.m.f. induced in a stator conductor is given by BlvE where B magnetic induction of the field moving relative to the conductor； l active length of the conductor； v speed with which the magnetic f
15、ield moves relative to the conductor. Since l and v are constant, the induced e.m.f. will vary exactly as B varies. If the induced e.m.f is to be sinusoidal(which is usually sought), the distribution of B around the circumference of the stator should be as close to sinusoidal as practicable. With p
16、pole-pairs on the rotor, the e.m.f. will undergo p cycles of changes every revolution. If the speed of the rotor is revolutions per minutes (r/min), this works out to pn cycles per minute, and the frequency of the induced e.m.f. is：60/pnf For f=50Hz, the rotor of a generator with one pair of poles s
17、hould run at 3000r/min.And with two pole-pairs, at 1500r/min. With speeds like this, rotors are usually fabricated with non-salient piles for greater mechanical strength. High-frequency generators operating at 800 to 8000Hz are all of special designs. Their uses are in the heat-treatment and inducti
18、on-heating field. Still higher frequencies are generated by valve and semiconductor oscillators. Sinusoidal Current The instantaneous value of a sinusoidal current is given by )2s in( tTIi m where mI is the peak value or amplitude of the current, and )2( tT is the phase of the current. The quantity
19、is the initial phase of the current (for t=0) and is termed the epoch angle. The phase of a current continuously increases with time. When it has increase by 2 , the whole cycle of changes is repeated exactly all over again. Therefore, s peaking of the phase of a current, it is customary to drop the
20、 integer 2 and to consider the phase within or between zero and 2 . The above change of phase by 2 occurs during the period T. therefore, the rate of change of the phase is given by T/2 . It is symbolized by the Greek letter (omega) and is called the pulsatance. Noting that Tf /1 , we may write fT 2
21、/2 The above expression, relating and f ,has been responsible for the fact that the pulstance is also termed the angular velocity or frequency. It is expressed in radians per second. Thus, for Hzf 50 , srad /314 . Introduce in Eq.(4.1) we obtain )sin( tII m Sinusoidal current of the same frequency, but differing in amplitude and epoch angles, are shown in Fig.4.2. theirinstantaneous values are )s in( 11 1 tIi m )s in ( 22 2 tIi m The time t or the product t ,proportional to time, is laid off as abscissa.
