SERIES GENERATOR

The circuit diagram of a series generator is shown in Fig.1. The series field winding provides the flux in the machine when the armature current flows through it. Note that the field circuit is not complete unless a load is connected to the machine. The equations governing the steady-state operation are

E_a = V_t + I_a (R_a + R_sr)                                     

                                                                        I_t = I_a

Fig. 1: Equivalent circuit of a dc series generator.

Fig. 2: Magnetization curve (E_a versus I_a) and I_a(R_a + R_sr) versus I_a.

The magnetization curve E_a versus I_a (fig-2) for the series machine can be obtained by separately exciting the series field. To obtain the terminal voltage-current characteristic (i.e., V_t versus I_t), draw a straight line (Fig-2) having the slope R_a + R_sr. This straight line represents the voltage drop across R_a and R_sr. As shown in Fig. 2, the vertical distance between the magnetization curve and this straight line is the terminal voltage for a particular value of I_a.(If the effect of armature reaction is considered, the terminal voltage will be less, as shown in Fig.2. by dashed lines, where ab represents armature reaction in equivalent armature current). The terminal voltages for various values of the terminal current can thus be obtained from Fig.3. These voltages are plotted in Fig. 3. If the load is a resistance of value R_L, the load characteristic, V_t (= R_LI_t) versus I_t, is a straight line with slope R_L. The operating point for this load is the point of intersection (point p in Fig. 3) of the magnetization curve and the load characteristic. Note that if R_L is too large, the terminal voltage will be very small; that is, the series generator will not build up any appreciable voltage.

 

Fig. 3: External characteristic of a series generator.