Electrical Engg (EEE) - MCQ Practice Questions

The double-field revolving theory represents a single-phase AC winding's pulsating magnetic field as two counter-rotating fields, used to analyze single-phase induction motor behavior.

At constant V and reduced frequency, the air gap flux increases (Φ = V/f), but the rotor reactance decreases. However, the overall effect results in reduced starting and running torque.

In a DC series motor, when unloaded: (1) Load torque decreases, so armature current reduces significantly since field current equals armature current in series connection. (2) With reduced current, voltage drop (Ia×Ra) decreases. (3) Back EMF increases (Eb = V - Ia×Ra). (4) Since N ∝ Eb/Φ and both Eb increases while Φ decreases (less field current), speed increases substantially. This is why series motors are unsuitable for no-load operation.