Open circuit characteristics (O.C.C.)
Table of Contents
Open circuit Characteristics (OCC)
The curve that depicts the relationship between the field current (If) at constant speed and the Generated EMF at no load is known as the open circuit Characteristic (O.C.C.) or Magnetization Characteristic. Another name for it is the no-load Saturation curve. Whether separately excited or self-excited, it has a shape that is essentially the same for all types of DC generators.
Open circuit Characteristics Explanation
The field winding of a DC Generator is disconnected from the machine and excited by an external DC source in order to analyse its open circuit Characteristics. The Generator is being operated at its usual rate. A voltmeter connected across the Armature terminals measures the Generated EMF at no load (E0) as the field current (If) is Gradually Increased from zero.
On plotting the relation between the E0 and If, we obtain the open circuit characteristics as shown in the figure.
Following points can be observed from the open circuit characteristics curve −
- The field current starts out at zero, with some emf OA generated by the residual magnetism.
- The curve is linear from point A to point B. This is so that the air gap’s resistance does not significantly outweigh the iron core’s in this range. Since the air gap’s resistance is constant, it has a linear relationship.
- The resistance of iron also enters the picture after point B. Since the relative permeability (r) of the iron decreases at higher flux densities, the iron’s reluctance is no longer negligible. This causes the curve to deviate from a linear relationship.
- The pole cores start to become magnetically saturated after point C,. And the generated emf (E0) at no load tends to level off.