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Examoogle • User • info@examoogle.com • EE-2024-8821
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When load on alternator is suddenly reduced, speed will be
Increase
Decrease
Remain same
None of above
Increase
Quick Summary: When the electrical load on an alternator is suddenly reduced, the power output drops, but the mechanical power input from the prime mover remains constant initially. This creates an excess torque, causing the rotor to accelerate and the speed of the alternator to increase until the governor adjusts the fuel/steam supply.
When the electrical load on an alternator is suddenly reduced, the power output drops, but the mechanical power input from the prime mover remains constant initially. This creates an excess torque, causing the rotor to accelerate and the speed of the alternator to increase until the governor adjusts the fuel/steam supply.
Ta=Jdt2d2θ=Tm−Te — The Swing Equation defining dynamic stability
Pacc=Pin−Pout — Power balance governing speed change
The prime mover (e.g., turbine) provides mechanical torque Tm to the alternator rotor. The alternator supplies an electrical torque Te to the load. According to the swing equation, the accelerating torque Ta=Tm−Te. When Te decreases due to load reduction while Tm remains constant, Ta becomes positive, resulting in an increase in the angular velocity ωr of the rotor.
The governor system acts to restore frequency by reducing the prime mover's input power.
A sudden load rejection leads to 'overspeed', which may trigger overspeed protection relays.
The rate of speed increase depends on the moment of inertia (J) of the rotating mass.
Prevents structural damage by automatic governor response
Maintains grid frequency stability through droop characteristics
High transient stress on turbine blades
Potential for frequency oscillations (hunting) during governor recovery
Synchronous generator control
Grid stability analysis
Load shedding relay coordination
Option B is incorrect because speed only decreases if the electrical load increases relative to mechanical power input.
The governor's speed-load characteristic (droop) is typically set at 3-5% for large alternators.
A is correct — The reduction in electrical load causes an excess of mechanical torque, leading to rotor acceleration and a subsequent increase in speed.
Always remember that in an alternator, frequency is tied to synchronous speed (f=120PN); therefore, any increase in speed directly translates to a transient increase in system frequency.