Open Circuit Characteristics of a DC Generator

The open circuit characteristics (O.C.C) or magnetization characteristics is the curve that shows the relationship between the generated EMF at no-load (E₀) and the field current (I_f) at constant speed. It is also known as no-load saturation curve. Its shape practically the same for all types of DC generator whether separately excited or self-excited.

Explanation

In order to determine the open circuit characteristics of a DC generator, the field winding is disconnected from the machine and is excited by an external DC source. The generator is run at its normal speed. The field current (I_f) is increased gradually from zero and the corresponding values of generated EMF at no-load (E₀) is noted from a voltmeter connected across the armature terminals.

On plotting the relation between the E₀ and I_f, we obtain the open circuit characteristics as shown in the figure.

Following points can be observed from the open circuit characteristics curve −

• Initially the field current is zero, there being some generated emf OA, which is due to the residual magnetism.
• From point A to B, the curve is linear. It is because in this range, the reluctance of iron core is negligible as compared to the air gap. The reluctance of air gap is constant and hence it has linear relationship.
• After point B, the reluctance of iron also comes into picture. Since, at higher flux densities, the relative permeability (μ_r) of the iron decreases and hence the reluctance of iron is no longer negligible. This results in the deviation of the curve from the linear relationship.
• After point C, the pole cores begin to saturate magnetically and the generated emf (E₀) at no-load tends to level off.