【正文】 field current to flow that is just sufficient to develop a flux required for the generated EMF that causes the field current to flow. The circuit carries only dc current, so that the field current depends only on the field circuit resistance, Rf. This may consist of the field circuit resistance Rf, the field current depends on the generated voltage in accordance with Ohm’s law. It should be evident that on a new machine or one that has lost its residual flux because of a long idle period, some magism must be created. This is usually done by connecting the field winding only to a separate dc source for a few seconds. This procedure is generally known as flashing the field. Series Generators As mentioned previously, the field winding of a series generator is in series with the armature. Since it carries the load current the series field winding consists of only a few turns of thick wire. At no load, the generated voltage is small due to residual field flux only. When a load is added, the flux increases, and so does the generated voltage. shows the load characteristic of a series generator driven at a certain speed. The dashed line indicates the generated EMF of the same machine with the armature opencircuited and the field separately excited. The difference between the two curves is simply the IR drop in the series field and armature winding, such that )( SAAGt RRIEV ??? where RS is the series field winding resistance. 7 Figure 7 Series generator: (a)circuit diagram。 the generator is then said to be flatpounded. If the number of series turns is more than necessary to pensate for the voltage drop, the generator is overe pounded. In that case the fullload voltage is higher than the noload voltage. 8 Figure 9 Terminal characteristics of pound generators pared with that of the shunt generator The overpounded generator may be used in instances where the load is at some distance from the generator. The voltage drops in the feeder lines are the pensated for with increased loading. Reversing the polarity of the series field in relation to the shunt field, the fields will oppose each other more and more as the load current increase. Such a generator is said to be differentially pounded. It is used in applications where feeder lines could occur approaching those of a short circuit. An example would be where feeder lines could break and short circuit the generator. The shortcircuit current, however, is then limited to a “safe” value. The terminal characteristic for this type of generator is also shown in . Compound generators are used more extensively than the other types because they may be designed to have a wide varity of terminal characteristics. As illustrated, the fullload terminal voltage can be maintained at the noload value by the proper degree of pounding. Other methods of voltage control are the use of rheostats, for instance, in the field circuit. However, with changing loads it requires a constant adjustment of the field rheostat to maintain the voltage. A more useful arrangement, which is now mon practice, is to use an automatic voltage regulator with the generator. In essence, the voltage regulator is a feedback control system. The generator output voltage is sensed and pared to a fixed reference voltage deviation from the reference voltage gives an error signal that is fed to a power amplifier. The power amplifier supplies the field excitation current. If the error signal is positive, for example, the output voltage is larger than desired 9 and the amplifier will reduce its current drive. In doing so the error signal will be reduced to zero. TRANSFORMER 1. INTRODUCTION The highvoltage transmission was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this high voltage must be reduced, because ultimately is must supply a load. The transformer makes it possible for various parts of a power system to operate at different voltage levels. In this paper we discuss power transformer principles and applications. 2. TOWWINDING TRANSFORMERS A transformer in its simplest form consists of two stationary coils coupled by a mutual magic flux. The coils are said to be mutually coupled because they link a mon flux. In power applications, laminated steel core transformers (to which this paper is restricted) are used. Transformers are efficient because the rotational losses normally associated with rotating machine are absent, so relatively little power is lost when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher values applying to the larger power transformers. The current flowing in the coil connected to the ac source is called the primary winding or simply the primary. It sets up the flux φ in the core, which varies periodically both in magnitude and direction. The flux links the second coil, called the secondary winding or simply secondary. The flux is changing。 and (2) it provides a ponent to account for the hysteresis and eddy current losses in the core. There bined losses are normally referred to as the core losses. The noload current Iθ is usually few percent of the rated fullload current of the transformer (about 2 to 5%). Since at noload the primary winding acts as a large reactance due to the iron core, the noload current will lag the primary voltage by nearly 90186。 in phase behind the primary voltage VP. It is this ponent that sets up the flux in the core。, therefore, they are 180186。 thus the induced quantities Ep and Es vary as a sine function. The average value of the induced voltage given by 11 Eavg = turns c h a n g e in flu x in a g iv e n tim eg iv e n tim e which is Faraday’s law applied to a finite time interval. It follows that Ea