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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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When exciter failed, alternator works as
Induction motor
Induction generator
Transformer
None of above
Induction generator
Quick Summary: When an alternator loses its DC excitation, the magnetic field produced by the rotor vanishes. Consequently, the machine begins to consume reactive power from the grid to sustain a rotating magnetic field, effectively operating as an induction generator drawing excitation current from the stator terminals.
When an alternator loses its DC excitation, the magnetic field produced by the rotor vanishes. Consequently, the machine begins to consume reactive power from the grid to sustain a rotating magnetic field, effectively operating as an induction generator drawing excitation current from the stator terminals.
Ns=P120f — Synchronous speed relationship
s=NsNr−Ns — Slip calculation when operating as an induction machine
In the absence of DC field excitation, the synchronous motor or alternator rotor acts as a squirrel cage or a salient pole mass in which currents are induced by the stator's rotating magnetic field. The machine then runs at a speed slightly above synchronous speed (Ns) if driven by a prime mover, drawing magnetizing reactive power (Q) from the system to maintain the air-gap flux, which defines the mode of an induction generator.
Loss of excitation causes the machine to lose its synchronous tie with the grid.
The machine starts drawing magnetizing current from the power system.
The prime mover continues to provide mechanical torque, forcing the machine into induction generator mode.
Induction generators require external reactive power for excitation.
Simple construction (no field winding requirement in this fault state)
Self-protecting against over-speed in some limited configurations
High reactive power demand from the grid can cause voltage instability.
Increased stator heating due to magnetizing current flow.
Analysis of fault conditions in power system stability studies.
Protection relay coordination (Loss of field protection).
This phenomenon is a common scenario in synchronous machine stability analysis.
Protection relays (e.g., ANSI 40) are used to detect 'Loss of Field' and trip the breaker to prevent grid voltage collapse.
B is correct — When the DC excitation of an alternator fails, it loses its ability to generate its own excitation and functions as an induction generator by drawing magnetizing current from the stator-connected grid.
Always remember that synchronous machines require external reactive power to act as induction generators; they cannot produce it themselves without DC field excitation.