Correct answer: D — Induction generator
How an induction machine generates
An induction generator is an induction machine driven mechanically in its generating region. For a conventional grid-connected machine, generation occurs when the rotor turns faster than the rotating stator field. Its slip becomes negative:
Ns = 120f/P
s = (Ns − Nr)/Ns
Nr > Ns gives s < 0
Here f is supply frequency, P is the number of poles, Ns is synchronous speed and Nr is rotor speed in rpm. In this region, electromagnetic torque opposes the mechanical drive. Mechanical power supplied by the prime mover is converted into electrical power delivered through the stator.
What “no separate excitation source” means
The rotor does not need the separate DC field supply used by a conventional wound-field synchronous generator. However, the air-gap magnetic field still requires magnetizing reactive power. A grid-connected induction generator draws it from the grid or local compensation. A suitable standalone self-excited arrangement obtains it from capacitors, subject to residual magnetism, speed and load conditions.
A directly grid-connected induction generator operates over a limited speed range above synchronous speed while grid frequency remains fixed. A wide variable-speed range generally requires power-electronic conversion. Thus, the answer does not mean that excitation energy is unnecessary or that every induction generator supports unrestricted speed variation.
