【正文】 er the machine will generate a voltage and what determines the voltage. For voltage to “build up” as it is called, there must be some remanent magism in the field poles. Ordinarily, if the generator has been used previously, there will be some remanent magism. We have seen in Section 3 that if the field would be disconnected, there will be small voltage Ef generated due to this remanent magism, provided that the generator is driven at some speed. Connecting the field for selfexcitation, this small voltage will be applied to the shunts field and drive a small current through the field circuit. If this resulting small current in the shunt field is of such a direction that it weakens the residual flux, the voltage remains near zero and the terminal voltage does not build up. In this situation the weak main pole flux opposes the residual flux. Figure 5 Shunt generator:(a)circuit。(b)load characteristic 6 If the connection is such that the weak main pole flux aids the residual flux, the induced voltage increases rapidly to a large, constant value. The buildup process is readily seen to be cumulanve. That is, more voltage increases the field current, which in turn increases the voltage, and so on. The fact that this process terminates at a finite voltage is due to the nonlinear behavior of the magnctic circuit. In steady state the generated voltage is causes a 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。(b)load characteristicsCompound Generators The pound generator has both a shunt and a series field winding, the latter winding wound on top of the shunt winding. shows the circuit diagram. The two windings are usually connected such that their ampereturns act in the same direction. As such the generator is said to be cumulatively pounded. The shunt connection illustrated in is called a long shunt connection. If the shunt field winding is directly connected across the armature terminals, the connection is referred to as a short shunt. In practice the connection used is of little consequence, since the shunt field winding carries a small current pared to the fullload current. Furthermore, the number of turns on the series field winding. This implies it has a low resistance value and the corresponding voltage drop across it at full load is minimal. Curves in represents the terminal characteristic of the shunt field winding alone. By the addition of a small series field winding the drop in terminal voltage with increased loading is reduced as indicated. Such a generator is said to be underpounded. By increasing the number of series turns, the noload and fullload terminal voltage can be made equal。 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