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When load on alternator is suddenly reduced, voltage will be
Increase
Decrease
Remain same
None of above
Increase
When the load on an alternator is suddenly reduced, the armature reaction (which is demagnetizing for lagging power factor loads) decreases. This reduction in the demagnetizing effect leads to an increase in the net magnetic flux per pole, causing the terminal voltage of the alternator to rise.
When the load on an alternator is suddenly reduced, the armature reaction (which is demagnetizing for lagging power factor loads) decreases. This reduction in the demagnetizing effect leads to an increase in the net magnetic flux per pole, causing the terminal voltage of the alternator to rise.
V=EfтАЛтИТIaтАЛ(RaтАЛ+jXsтАЛ) тАФ Terminal voltage equation for a synchronous alternator
╬жnetтАЛ=╬жfтАЛтИТ╬жaтАЛ тАФ Net flux relation considering armature reaction
The alternator terminal voltage is determined by the equation V=EfтАЛтИТIaтАЛ(RaтАЛ+jXsтАЛ). When the load is reduced, the armature current IaтАЛ decreases. For lagging power factor loads, the armature reaction opposes the main field flux. A reduction in IaтАЛ weakens this opposition, resulting in a higher resultant flux, which increases the internal generated EMF (EfтАЛ) and consequently the terminal voltage (V).
The phenomenon is primarily observed in alternators feeding lagging power factor loads.
For leading power factor loads, the armature reaction is magnetizing, and load reduction might cause a decrease in voltage.
Voltage regulation is defined as VE0тАЛтИТVтАЛ├Ч100%.
Voltage rise is limited by the inherent internal impedance of the machine.
Automatic voltage control systems (AVR) are designed to stabilize this rise.
Understanding this helps in setting protective relay thresholds.
Sudden voltage surges can damage sensitive insulation.
Over-voltage may cause secondary breakdown in connected electrical equipment.
Power system stability analysis.
Design of excitation control systems for synchronous generators.
The magnitude of voltage rise depends heavily on the power factor of the load at the instant of rejection.
Option B is incorrect because a decrease in voltage typically occurs when the load is increased, not decreased.
A is correct тАФ The reduction in load decreases the demagnetizing armature reaction, leading to an increase in the net magnetic flux and terminal voltage.
Always check the power factor specified in the question; for leading loads, the voltage regulation can be negative, meaning a load reduction would result in a voltage drop rather than a rise.