Step 1: Understanding the Problem:
This question asks for the device or component used to eliminate or reduce the phenomenon of "hunting" in synchronous machines.
Step 2: Key Formula or Approach:
The solution requires understanding the mechanical dynamics of the rotor.
When the load on a synchronous machine changes suddenly, the rotor oscillates around its synchronous speed, which is a state of transient instability called hunting.
Step 3: Detailed Explanation:
• Under stable, steady-state conditions, the rotor of a synchronous machine rotates at exactly the synchronous speed ($N_s$).
• If a sudden load change or system disturbance occurs, the rotor speed momentarily deviates from $N_s$, causing the rotor to swing back and forth around its equilibrium position.
• These persistent mechanical oscillations of the rotor are known as hunting, which can lead to high current fluctuations and potential loss of synchronism.
• To mitigate hunting, damper windings are employed.
• Damper windings consist of heavy, short-circuited copper bars embedded in slots on the pole faces of the rotor.
• When hunting occurs, the relative speed between the rotating stator magnetic field and the oscillating rotor is no longer zero.
• This relative motion induces currents in the short-circuited damper bars, producing a dampening electromagnetic torque (similar to an induction motor) that opposes and quickly dampens the rotor's oscillations.
• Once the rotor regains synchronous speed, the relative speed becomes zero, and no further current is induced in the damper windings.
Step 4: Final Answer
Thus, damper windings are used to prevent hunting, corresponding to option (B).