What will be the effect of increasing load of a synchronous motor while running with normal excitation?

Explanation:

Effect of Increasing Load on Synchronous Motor with Normal Excitation

Introduction: A synchronous motor is an AC motor that runs at synchronous speed, which means its rotor speed is synchronized with the frequency of the supply current. Synchronous motors are widely used in applications where precise and constant speed is required. The performance of a synchronous motor can be significantly influenced by changes in load while the motor is running with normal excitation.

Understanding the Correct Option (Option 2):

The correct option is:

Option 2: Increase in armature current and decrease in power factor.

When the load on a synchronous motor is increased while running with normal excitation, the following sequence of events occurs:

1. Increased Load Demand: As the load on the synchronous motor increases, the mechanical demand for torque also increases. This requires the motor to draw more electrical power to meet the higher mechanical load.
2. Increase in Armature Current: To supply the additional power required by the increased load, the motor draws more current from the supply. This results in an increase in the armature current.
3. Change in Power Angle: The power angle (the angle between the rotor magnetic field and the stator magnetic field) increases as the load increases. This leads to a shift in the phase relationship between the voltage and current.
4. Decrease in Power Factor: The power factor of a synchronous motor is determined by the phase difference between the voltage and the current. As the load increases, the power angle increases, causing the current to lag more behind the voltage. This results in a decrease in the power factor. The power factor becomes more lagging as the load increases.

Conclusion: Therefore, when the load on a synchronous motor running with normal excitation is increased, the armature current increases, and the power factor decreases. This is accurately described by Option 2.

Additional Information

Analysis of Other Options:

To further understand why the other options are incorrect, let’s evaluate them:

Option 1: Increase in armature current and increase in power factor.

This option is incorrect because while the armature current does increase with an increased load, the power factor does not increase. Instead, the power factor decreases due to the increased phase lag between voltage and current.

Option 3: Decrease in armature current and decrease in power factor.

This option is incorrect because the armature current does not decrease with an increased load. The armature current actually increases to meet the higher power demand. The power factor does decrease, but the armature current increase contradicts this option.

Option 4: Decrease in armature current and increase in power factor.

This option is incorrect for similar reasons as Option 3. The armature current increases with increased load, not decreases. Additionally, the power factor decreases rather than increases.

Conclusion:

Understanding the behavior of synchronous motors under varying loads is crucial for their effective application and operation. When the load on a synchronous motor increases, the armature current increases and the power factor decreases. This knowledge helps in designing and operating synchronous motors efficiently in various industrial and commercial applications.