Answer MET Question 54
Question : Describe the no-load saturation characteristic of a d.c. generator.
Answer:
The field winding of the generator (whether shunt or series wound) is disconnected from the machine and connected to an external source of direct current. The field or exciting current $\displaystyle \small \mathrm{I_f }$ is varied rheostatically and its value read on the ammeter A. The machine is drived at constant speed by the prime mover and the generated e.m.f. on on-load is measured by the voltmeter connected across the armature.
$\displaystyle \small \mathrm{I_f }$ is increased by suitable steps (starting from zero) and the corresponding values of Eo are measured.
On plotting the relation between $\displaystyle \small \mathrm{I_f }$ and Eo, a curve of this form shown in Figure is obtained.
Due to residual magnetism in the poles, some e.m.f. is generated even when $\displaystyle \small \mathrm{I_f }$ = 0. Hence, the curve starts a little way up. The slight curvature at the lower end is due to magnetic inertia. It is seen that the first part of the curve is practically straight.
This is due to the fact that at low flux densities, reluctance of iron path being negligible (due to high permeability), total reluctance is given by the airgap reluctance which is constant. Hence, the flux and consequently, the generated e.m.f. is directly proportional to the exciting current. However, at high flux densities, – where μ is small, iron path reluctance becomes appreciable and straight relation between E and If no longer holds good.
In other words, after a point on curve, saturation of poles starts. However, the initial slope of the curve is determined by air-gap width.
It should be noted that O.C.C. for a higher speed would lie above this curve and for a lower speed, would lie below it. It should also be noted and the load-saturation curve for a shunt generator is similar to the one shown in figure.
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