In a compound generator, both series and shunt excitation are combined as shown in Fig. (3.13). The shunt winding can be connected either across the armature only (short-shunt connection S) or across armature plus series field (long-shunt connection G). The compound generator can be cumulatively compounded or differentially compounded generator. The latter is rarely used in practice. Therefore, we shall discuss the characteristics of cumulatively compounded generator. It may be noted that external characteristics of long and short shunt compound generators are almost identical. External characteristic Fig. (3.14) shows the external characteristics of a cumulatively compounded generator. The series excitation aids the shunt excitation. The degree of compounding depends upon the increase read more

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If we are given O.C.C. of a generator at a constant speed N1 then we can easily draw the O.C.C. at any other constant speed N2.Fig (3.11) illustrates the procedure. Here we are given O.C.C. at a constant speed N1.It is desired to find the O.C.C. at constant speed N2 (it is assumed that n1 < N2)For constant excitation,E α N E2/E1=N2/N1 As shown in Fig. (3.11), for If = OH, E1 = HC. Therefore, the new value of e.m.f. (E2) for the same If but at N2i. E2=HC ×( N2/N1) =HD This locates the point D on the new O.C.C. at N2. Similarly, other points can be located taking different values of If . The locus of these points will be the O.C.C. at N2. Critical Speed (NC ) The critical speed of a shunt generator is the minimum speed below which it fails to excite. Clearly, it is the speed for read more

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Fig (3.9) (i) shows the connections of a shunt wound generator. The armature current Ia splits up into two parts; a small fraction Ish flowing through shunt field winding while the major part IL goes to the external load. (i) O.C.C. The O.C.C. of a shunt generator is similar in shape to that of a series generator as shown in Fig. (3.9) (ii). The line OA represents the shunt field circuit resistance. When the generator is run at normal speed, it will build up a voltage OM. At no-load, the terminal voltage of the generator will be constant (= OM) represented by the horizontal dotted line MC. (ii) Internal characteristic When the generator is loaded, flux per pole is reduced due to armature reaction. Therefore, e.m.f. E generated on load is less than the e.m.f. generated at no load.As a read more

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Fig. (3.7) (i) shows the connections of a series wound generator. Since there is only one current (that which flows through the whole machine), the load currentis the same as the exciting current. (i) O.C.C. Curve 1 shows the open circuit characteristic (O.C.C.) of a series generator. It can be obtained experimentally by disconnecting the field winding from the machine and exciting it from a separate d.c. source as discussed in Sec. (3.2). (ii) Internal characteristic Curve 2 shows the total or internal characteristic of a series generator. It gives the relation between the generated e.m.f. E. on load and armature current. Due to armature reaction, the flux in the machine will be less than the flux at no load. Hence, e.m.f. E generated under load conditions will be less than the read more

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Critical Field Resistance for a Shunt Generator We have seen above that voltage build up in a shunt generator depends upon field circuit resistance. If the field circuit resistance is R1 (line OA), then generator will build up a voltage OM as shown in Fig. (3.5). If the field circuit resistance is increased to R2 (tine OB), the generator will build up a voltage OL, slightly less than OM. As the field circuit resistance is increased, the slope of resistance line also increases. When the field resistance line becomes tangent (line OC) to O.C.C., the generator would just excite. If the field circuit resistance is increased beyond this point (say line OD), the generator will fail to excite. The field circuit resistance represented by line OC (tangent to O.C.C.) is called critical field read more

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Let us see how voltage builds up in a self-excited generator. (i) Shunt generator Consider a shunt generator. If the generator is run at a constant speed, some e.m.f. will be generated due to residual magnetism in the main poles. This small e.m.f. circulates a field current which in turn produces additional flux to reinforce the original residual flux (provided field winding connections are correct). This process continues and the generator builds up the normal generated voltage following the O.C.C. shown in Fig. (3.4) (i). The field resistance Rf can be represented by a straight line passing through the origin as shown in Fig. (3.4) (ii). The two curves can be shown on the same diagram as they have the same ordinate [See Fig. 3.4 (iii)]. Since the field circuit is inductive, there is read more

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The obvious disadvantage of a separately excited d.c. generator is that we require an external d.c. source for excitation. But since the output voltage may be controlled more easily and over a wide range (from zero to a maximum), this type of excitation finds many applications. (i) Open circuit characteristic. The O.C.C. of a separately excited generator is determined in a manner described in Sec. (3.2). Fig. (3.2) shows the variation of generated e.m f. on no load with field current for various fixed speeds. Note that if the value of constant speed is increased, the steepness of the curve also increases. When the field current is zero, the residual magnetism in the poles will give rise to the small initial e.m.f. as shown. (ii) Internal and External Characteristics The external read more

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The O.C.C. for a d.c. generator is determined as follows. The field winding of the d.c. generator (series or shunt) is disconnected from the machine and is separately excited from an external d.c. source as shown in Fig. (3.1) (ii). The generator is run at fixed speed (i.e., normal speed). The field current ( If) is increased from zero in steps and the corresponding values of generated e.m.f.(E o) read off on a voltmeter connected across the armature terminals. On plotting the relation between Eo and If, we get the open circuit characteristic as shown in Fig. (3.1) (i) The following points may be noted from O.C.C.: (i) When the field current is zero, there is some generated e.m.f. OA. This is due to the residual magnetism in the field poles. (ii) Over a fairly wide range of field read more

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