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 resistance RC for the shunt generator. It may be defined as under: The maximum field circuit resistance (for a given speed) with which the shunt generator would just excite is known as its critical field resistance. It should be noted that shunt generator will build up voltage only if field circuit resistance is less than critical field resistance.

Critical Resistance for a Series Generator

Fig. (3.6) shows the voltage build up in a series generator. Here R₁,R₂ etc. represent the total circuit resistance (load resistance and field winding resistance). If the total circuit resistance is R₁, then series generator will build up a voltage OL. The line OC is tangent to O.C.C. and represents the critical resistance RC for a series generator. If the total resistance of the circuit is more than R_c (say line OD), the generator will fail to build up voltage. Note that Fig. (3.6) is similar to Fig. (3.5) with the following differences:

(i) In Fig. (3.5), R₁,R₂ etc. represent the total field circuit resistance. However, R₁,R₂ etc. in Fig. (3.6) represent the total circuit resistance (load resistance and series field winding resistance etc.).

(ii) In Fig (3.5), field current alone is represented along X-axis. However, in Fig. (3.6) load current I_Lis represented along Y-axis. Note that in a series generator, field current = load current I_L.

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