COMPUND DC GENERATOR

Many practical applications require that the terminal voltage remains constant when load changes. But when dc machines deliver current, the terminal voltage drops because of I_aR_a voltage drop and a decrease in pole fluxes caused by armature reaction.

To overcome the effects of I_aR_a drop and decrease of pole fluxes with armature current, a winding can be mounted on the field poles along with the shunt field winding. This additional winding, known as a series winding, is connected in series with the armature winding and carries the armature current. This series winding may provide additional ampere-turns to increase or decrease pole fluxes, as desired. A dc machine that has both shunt and series windings is known as a compound dc machine. Note that in a compound machine the shunt field winding is the main field winding, providing the major portion of the mmf in the machine. It has many turns of smaller cross-sectional area and carries a lower value of current compared to the armature current. The series winding has fewer turns, larger cross-sectional area, and carries the armature current. It provides mmf primarily to compensate the voltage drops caused by I_aR_a and armature reaction. 

Fig. 1: Equivalent circuits of compound dc machines.

(a) Short shunt. (b) Long shunt.

Figure 1. shows the two connections for the compound dc machine. In the short-shunt connection, the shunt field winding is connected across the armature, whereas in the long-shunt connection, the shunt field winding is connected across the series combination of armature and series winding. The equations that govern the steady-state performance of a compound generator are as follows:

Short Shunt

V_t = E_a – I_aR_a – I_tR_sr                                        …1

I_t = I_a – I_f                                                         …2

where R_sr is the resistance of the series field windings.

Long Shunt

V_t = E_a – I_a (R_a + R_sr)                                      …3

I_t = I_a – I_f                                                         …4

I_f = (V_t / (R_fw + R_fc))                                        …5

For either connection, assuming magnetic linearity, the generated voltage is

E_a = K_a (F_sh ± F_sr) w_m                                    …6

where F_sh is the flux per pole produced by the mmf of the shunt field winding, F_sr is the flux per pole produced by the mmf of the series field winding.

When these two fluxes aid each other the machine is called a cumulative compound machine, and when they oppose each other the machine is called a differential compound machine.

Note that both shunt field mmf and series field mmf act on the same magnetic circuit. Therefore, the total effective mmf per pole is

F_eff = F_sh ± F_sr – F_AR                                         …7

N_fI_f(eff) = N_fI_f ± N_srI_sr – N_fI_f(AR)                         …8

Where N_f is the number of turns per pole of the shunt field winding, N_sr is the number of turns per pole of the series field winding, and F_AR is the mmf of the armature reaction.

From Eq. 8,

                                                                                                                        …9

The voltage-current characteristics of the compound dc generators are shown in Fig. 2 With increasing armature current for cumulative compounding, the terminal voltage may rise (over compounding), decrease (under compounding), or remain essentially flat (flat compounding). This depends on the degree of compounding, that is, the number of turns of the series filed winding. For differential compounding (i.e., mmf of the series field winding opposed to that of the shunt field winding) the terminal voltage drops very quickly with increasing armature current. In fact, the armature current remains essentially constant. This current-limiting feature of the differentially compounded dc generator makes it useful as a welding generator.

The voltage-current characteristics of the compound dc generators are shown in Fig. 2. With increasing armature current for cumulative compounding, the terminal voltage may rise (overcompounding), decrease (undercompounding), or remain essentially flat (flat compounding). This depends on the degree of compounding, that is, the number of turns of the series filed winding. For differential compounding (i.e., mmf of the series field winding opposed to that of the shunt field winding) the terminal voltage drops very quickly with increasing armature current. In fact, the armature current remains essentially constant. This current-limiting feature of the differentially compounded dc generator makes it useful as a welding generator. 

Fig. 2: V–I characteristics of compound dc generators.