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:

N_s = 120f/P

s = (N_s − N_r)/N_s

N_r > N_s gives s < 0

Here f is supply frequency, P is the number of poles, N_s is synchronous speed and N_r 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.

Quick Trick

Induction machine above synchronous speed → generator.

Why Other Options Are Wrong

A requires separate field excitation. B is identified as a motor, not the requested generator. C normally needs rotor field excitation. D best matches the stated characteristics.

Common Mistake

Saying an induction generator is completely self-excited in every case. Grid-connected units take magnetizing VARs from the grid; isolated units need suitable capacitors and residual magnetism.

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:

N_s = 120f/P

s = (N_s − N_r)/N_s

N_r > N_s gives s < 0

Here f is supply frequency, P is the number of poles, N_s is synchronous speed and N_r 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.

Quick Trick

Induction machine above synchronous speed → generator.

Why Other Options Are Wrong

A requires separate field excitation. B is identified as a motor, not the requested generator. C normally needs rotor field excitation. D best matches the stated characteristics.

Common Mistake

Saying an induction generator is completely self-excited in every case. Grid-connected units take magnetizing VARs from the grid; isolated units need suitable capacitors and residual magnetism.

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:

N_s = 120f/P

s = (N_s − N_r)/N_s

N_r > N_s gives s < 0

Here f is supply frequency, P is the number of poles, N_s is synchronous speed and N_r 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.

Quick Trick

Induction machine above synchronous speed → generator.

Why Other Options Are Wrong

A requires separate field excitation. B is identified as a motor, not the requested generator. C normally needs rotor field excitation. D best matches the stated characteristics.

Common Mistake

Saying an induction generator is completely self-excited in every case. Grid-connected units take magnetizing VARs from the grid; isolated units need suitable capacitors and residual magnetism.