Load Characteristics of D.C. Compound Generator

Load Characteristics of D.C. Compound Generator

AU May-07, 12, 15, Dec.-03, 09, 10, 15

 • The characteristics depends on whether generator is cumulatively compound or differentially compound generator. In cumulatively compound, ϕ_T = ϕ_sh + ϕ_se. As load current increases, I_a increases hence I_se also increases producing more flux. Thus induced e.m.f. increases and terminal voltage also increases. But as I_a increases, the various voltage drops and armature reaction drop also increases. Hence there is drop in the terminal voltage.

If drop in V_t, due to increasing I_L is more dominating than increase in V_t due to increase in flux then generator is called under compounded and its characteristics is dropping in nature, as shown in the Fig. 3.27.1.

• If drop in V_t due to armature reaction and other drops is much less than increase in V_t due to increase in flux then generator is called over compounded and its characteristics is rising in nature, as shown in the Fig. 3.27.1. If the effects of the two are such that on full load current V_t is same as no load induced e.m.f. i.e. the effects are neutralising each other on full load then generator is called flat compounded or level compounded. Its characteristics is shown in the Fig. 3.27.1.

• In differentially compound, ϕ_T =  ϕ_sh ~ ϕ_se. The net flux is difference between the two. As I_L increases, ϕ_sh is almost constant but ϕ_se increases rapidly. Hence the resultant flux ϕ_T reduces. Hence the induced e.m.f. E and hence the terminal voltage also decreases drastically. There is drop due to armature resistance, series field resistance, armature reaction due to which terminal voltage drops further. Thus we get the characteristics of such differentially compound generator as shown in the Fig. 3.27.1.

Ex. 3.27.1 A long shunt compound generator has a hunt field winding of 1,000 turns per pole and series eld winding of 4 turns per pole and a resistance of .05 Ω. In order to obtain the same voltage both at load nd full load for operating as shunt generator, it is ecessary to increase the field current by 0.2 A. The full ad armature current of the compound generator is 0 A. Calculate the diverter resistance connected in arallel of series field to obtain flat compound operation?

AU: Dec.-15, Marks 8

The generator is shown in the Fig. 3.27.2.

I_a = 80 A

Addiotional ampere-turns required to maintain rated voltage on full load for operation as a shunt generator.

(number or truns om shunt fileld) × (Additional shunt field current)

= 1000 × 0.2 = 200 AT

Number of series turns per pole, N_se = 4

.. Current required by series field to produce 200 AT is,

Review Questions

1. Draw and explain load characteristics of d.c. compound generator.

AU May-07, 15, Dec.-10, 15, Marks 8

2. Draw the performance characteristics of different types of different types of d.c. generators and explain them.

AU: Dec.-03, 09, May-12, Marks 8