SHUNT (SELF-EXCITED) GENERATOR

In the shunt or self-excited generator, the field is connected across the armature so that the armature voltage can supply the field current. Under certain conditions, to be discussed here, this generator will build up a desired terminal voltage.

The circuit for the shunt generator under no-load conditions is shown in Fig. 1. If the machine is to operate as a self-excited generator, some residual magnetism must exits in the magnetic circuit of the generator. Figure 2. shows the magnetization curve of the dc machine. Also shown in this figure is the field resistance line, which is a plot of R_fI_f versus I_f. A simplistic explanation of the voltage buildup process in the self-excited dc generator is as follows: 

Fig. 1: Schematic of a shunt (self-excited) dc machine. 

 

Fig. 2: Voltage build-up in a self-excited dc generator. 

Assume that the field circuit is initially disconnected from the armature circuit and the armature is driven at a certain speed. A small voltage, E_ar, will appear across the armature terminals because of the residual magnetism in the machine. If the switch SW is now closed (Fig. 1), the field circuit is connected to the armature circuit and hence a current will flow in the field winding. If the mmf of this field current aids the residual magnetism, eventually a current I_fl will flow in the field circuit. The buildup of this current will depend on the time constant of the field circuit. With I_fl flowing in the field circuit, the generated voltage is E_al – from the magnetization curve – but the terminal voltage, V_t = I_flR_f < E_al. The increased armature voltage E_al will eventually increase the field current to the value I_f2, which in turn will build up the armature voltage to E_a2. This process of voltage buildup continues. If the voltage drop across R_a is neglected (i.e., R_a << R_f), the voltage builds up to the value given by the crossing point (P in Fig. 2) of the magnetization curve and the field resistance line. At this point, E_a = I_fR_f = V_t (assume R_a is neglected), and no excess voltage is available to further increase the field current. In the actual case, the changes in I_f and E_a take place simultaneously and the voltage buildup follows approximately the magnetization curve, instead of climbing the flight of stairs. 

Fig. 3: Effect of field resistance.

 

Figure 3 shows the voltage buildup in the self-excited dc generator for various field circuit resistances. At some resistance value R_f3, the resistance line is almost coincident with the linear portion of the magnetization curve. This coincidence condition results in an unstable voltage situation. This resistance is known as the critical field circuit resistance. If the resistance is greater than this value, such as R_f4, buildup (V_t4) will be insignificant. On the other hand, if the resistance is smaller than this value, such as R_f1 or R_f2, the generator will build up higher voltages (V_t1, V_t2). To sum up, three conditions are to be satisfied for voltage buildup in a self-excited dc generator:

1.      Residual magnetism must be present in the magnetic system.

2.      Field winding mmf should aid the residual magnetism.

3.      Field circuit resistance should be less than the critical field circuit resistance.

Voltage – Current Characteristics

The circuit of the dc shunt generator on load is shown in Fig. 4. The equations that describe the steady-state operation on load are

E_a = V_t + I_aR_a       …(1)

E_a = K_a F w_m = function of I_f    …(2)

è magnetization curve (or open-circuit saturation curve)

V_t = I_fR_f = I_f (R_fw + R_fc)     …(3)

V_t = I_tR_L     …(4)

I_a = I_f + I_t    …(5)

The terminal voltage (V_t) will change as the load draws current from the machine. This change in the terminal voltage with current (also known as voltage regulation) is due to the internal voltage drop I_aR_a (Eq. 1.25a) and the change in the generated voltage caused by armature reaction (Eq. 1.25b). The typical terminal characteristic is shown in Fig. 6. It is apparent that the terminal voltage drops faster with the armature current in the self- excited generator. The reason is that, as the terminal voltage decreases with load in the self-excited generator, the field current also decreases, resulting in less generating voltage, whereas in the separately excited generator, the field current and hence the generated voltage remain unaffected.

 

Fig. 5: Dc shunt generator with load. 

 

Fig .6: Terminal characteristic of a dc shunt generator.