Fig (3.9) (i) shows the connections of a shunt wound generator. The armature current I_a splits up into two parts; a small fraction I_sh flowing through shunt field winding while the major part I_L 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 result, the internal characteristic (E/I_a) drops down slightly as shown in Fig.(3.9) (ii).

(iii) External characteristic
Curve 2 shows the external characteristic of a shunt generator. It gives the
relation between terminal voltage V and load current I_L.

V = E – IaRa = E -(I_L +I_sh)R_a

Therefore, external characteristic curve will lie below the internal characteristic curve by an amount equal to drop in the armature circuit [i.e., (I_L +I_sh)R_a ] as shown in Fig. (3.9) (ii).

Note. It may be seen from the external characteristic that change in terminal
voltage from no-load to full load is small. The terminal voltage can always be
maintained constant by adjusting the field rheostat R automatically

Critical External Resistance for Shunt Generator

If the load resistance across the terminals of a shunt generator is decreased, then load current increase? However, there is a limit to the increase in load current with the decrease of load resistance. Any decrease of load resistance beyond this point, instead of increasing the current, ultimately results in reduced current. Consequently, the external characteristic turns back (dotted curve) as shown in Fig. (3.10). The tangent OA to the curve represents the minimum external resistance required to excite the shunt generator on load and is called critical external resistance. If the resistance of the external circuit is less than the critical external resistance (represented by tangent OA in Fig. 3.10), the machine will refuse to excite or will de-excite if already running This means that external resistance is so low as virtually to short circuit the machine and so doing away with its excitation.

Note. There are two critical resistances for a shunt generator viz., (i) critical field resistance (ii) critical external resistance. For the shunt generator to build up voltage, the former should not be exceeded and the latter must not be gone below.

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