Question

(a) Describe the no-load saturation characteristic of a d.c. generator.

(6) A 4-pole machine running at 1500 r.p.m. has an armature with 80 slots and 6 conductors per pole. The flux per pole is 6 × 10^6 lines. Determine the terminal e.m.f. of d.c. generator if the coils are lap connected. If the current per conductor is 100 Amps, determine the electrical power.

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.