Explanation:

In a synchronous alternator, the reactive power generated can be controlled by changing the alternator speed. This is because the reactive power generated by an alternator is directly proportional to the rotor speed.

Reason:
The reactive power generated by a synchronous alternator is a result of the interaction between the rotor magnetic field and the stator winding. The rotor magnetic field is generated by the excitation current passing through the rotor winding, and the strength of this magnetic field is directly related to the speed of the rotor.

Effect of Alternator Speed:
1. Increase in Alternator Speed: When the alternator speed is increased, the rotor magnetic field also increases. This leads to an increase in the reactive power generated by the alternator.
2. Decrease in Alternator Speed: Conversely, when the alternator speed is decreased, the rotor magnetic field decreases, resulting in a decrease in the reactive power generated.

Control of Reactive Power:
By controlling the alternator speed, the reactive power generated can be adjusted according to the requirements of the system. This is particularly important in power systems where it is necessary to maintain a balance between the active and reactive power.

Importance of Reactive Power Control:
1. Voltage Regulation: Reactive power control helps in maintaining the voltage stability of the system. By adjusting the reactive power generation of the alternator, the voltage at the terminals can be regulated within acceptable limits.
2. Power Factor Correction: Reactive power control also enables power factor correction in the system. By adjusting the reactive power output, the power factor can be improved, reducing the burden on the transmission and distribution systems.

Conclusion:
In a synchronous alternator, the reactive power generated can be controlled by changing the alternator speed. By adjusting the rotor speed, the strength of the rotor magnetic field is altered, resulting in a change in the reactive power output. This control is essential for maintaining voltage stability and power factor correction in the system.