Question

(a) Describe the no-load saturation characteristic of a D.C. generator. (6)

(b) A D.C. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.2 . The machine has six poles and the armature is lap-connected with 864 conductors. The flux per pole is 0.05 Wb. Calculate (10)

(i) The speed

(ii) The gross torque developed by the armature

Answer 1

Part (a)

The no-load saturation characteristic of a DC generator, also known as the magnetic or open-circuit characteristic, is a curve illustrating the relationship between the field current (If) and the generated voltage (Eo) in the armature under no-load conditions.

To obtain this characteristic, the generator is operated at a constant speed without any load connected. The field current is gradually increased, and the corresponding terminal voltage is recorded. This data is then plotted, with field current on the x-axis and generated voltage on the y-axis.

The generated EMF (Eg) is directly proportional to the flux (Φ), expressed by the equation Eg = KΦ, where K is a constant. Initially, as the field current increases, the generated voltage increases proportionally due to the increasing magnetic flux. However, once the magnetic field reaches saturation, the flux (Φ) essentially plateaus, regardless of further increases in field current. As a result, the generated voltage also levels off, resulting in a nearly straight-line portion on the saturation curve.

Even when the field current is zero, a small amount of EMF is generated due to residual magnetism in the field poles. This is represented by a non-zero intercept on the voltage axis in the graph of the no-load saturation characteristic.

✓ Verified Model Answer (Text Solution)

The no-load saturation characteristic of a DC generator, also known as the magnetic or open-circuit characteristic, is a curve illustrating the relationship between the field current (If) and the generated voltage (Eo) in the armature under no-load conditions.