JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year ELECTRICAL MACHINES-1 Previous Question Paper (March - 2021 - R18)

A DC generator operates on the principle of Faraday's Law of Electromagnetic Induction, which states that whenever a conductor cuts magnetic flux, an electromotive force (EMF) is induced in it. The magnitude of this induced EMF is given by e = Blv sin(θ).

The construction consists of a stationary magnetic field (stator) and a rotating armature (rotor). When the armature is rotated by a prime mover within the magnetic field, the conductors on the armature cut the magnetic lines of force. This induces an alternating EMF in the armature conductors. To convert this alternating EMF into a unidirectional (DC) voltage, a commutator and brushes are used. The commutator acts as a mechanical rectifier, reversing the connections of the armature coils at the exact moment the current reverses, ensuring that the current collected by the brushes always flows in the same direction.

To find the open circuit voltage and exciting current for a shunt generator, we must identify the intersection of the Open Circuit Characteristic (OCC) curve and the Field Resistance Line (FRL). The FRL is defined by the equation V = I_f × R_f, where R_f = 100 Ω.

We calculate the voltage values for the field resistance line:

• For I_f = 0.4 A, V = 0.4 × 100 = 40 V. (OCC value is 31 V)
• For I_f = 0.6 A, V = 0.6 × 100 = 60 V. (OCC value is 40 V)
• For I_f = 0.5 A, V = 0.5 × 100 = 50 V.

By plotting the OCC and the field resistance line V = 100 × I_f, the intersection point represents the steady-state operating point. Based on the provided data, the intersection occurs where the field resistance line crosses the OCC curve. By linear interpolation between I_f = 0.4 A and 0.6 A, the intersection point is approximately at I_f ≈ 0.52 A and V ≈ 52 V. Thus, the open circuit voltage is approximately 52 V and the exciting current is 0.52 A.