An alternator is feeding an infinite bus bar. Its prime mover is suddenly shut down. The alternator will: (A) Continue to work as alternator but the direction of rotation will reverse.  

An Alternator Feeding an Infinite Bus Bar:

Definition: An alternator is a device that converts mechanical energy into electrical energy in the form of alternating current. When an alternator is connected to an infinite bus bar, it means that the alternator is connected to a very large electrical system or grid. An infinite bus bar has constant voltage and frequency, irrespective of the power drawn or supplied to it. The alternator operates in synchronization with the bus bar, maintaining the same frequency and phase.

Scenario: The problem describes a situation where an alternator is feeding an infinite bus bar, and its prime mover (which provides the mechanical input to the alternator) is suddenly shut down. When the prime mover is shut down, the mechanical input ceases, and the alternator no longer receives the mechanical energy required to generate electrical energy. However, the alternator remains connected to the infinite bus bar.

Correct Option Analysis:

Option 3: The alternator will continue to work as a synchronous motor, and the direction of rotation will also be the same.

When the prime mover of the alternator is suddenly shut down, the alternator cannot continue operating as a generator because there is no mechanical input to convert into electrical energy. However, because the alternator is still connected to the infinite bus bar, it continues to receive electrical energy from the grid. As a result, the alternator starts operating as a synchronous motor.

In a synchronous motor, electrical energy from the grid is converted into mechanical energy. The alternator (now functioning as a synchronous motor) will maintain the same direction of rotation because the magnetic field interaction within the machine does not reverse the rotational direction. This phenomenon occurs because the alternator remains synchronized with the infinite bus bar, and the direction of rotation is determined by the phase sequence of the supply.

Key Points:

• The alternator transitions from generating electrical energy to consuming electrical energy from the grid.
• It operates as a synchronous motor because it remains connected to the infinite bus bar and synchronized with it.
• The direction of rotation remains the same because the phase sequence of the supply from the infinite bus bar does not change.

Therefore, the correct answer is Option 3: The alternator will continue to work as a synchronous motor, and the direction of rotation will also be the same.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: The alternator will continue to work as an alternator, but the direction of rotation will reverse.

This option is incorrect because once the prime mover is shut down, the alternator cannot continue working as a generator (alternator). Additionally, the direction of rotation does not reverse when the alternator transitions to motor mode because the phase sequence of the supply from the infinite bus bar remains the same.

Option 2: The alternator will come to a standstill.

This option is incorrect because the alternator does not come to a standstill as long as it remains connected to the infinite bus bar. Instead, it transitions to operating as a synchronous motor, driven by the electrical energy supplied by the bus bar.

Option 4: The alternator will work as an induction motor.

This option is incorrect because the alternator, when connected to an infinite bus bar, operates as a synchronous motor, not an induction motor. Induction motors operate based on the principle of slip between the rotor and the synchronous speed, whereas synchronous motors operate at synchronous speed without slip.

Conclusion:

Understanding the behavior of an alternator feeding an infinite bus bar under different scenarios is crucial for analyzing its operation. When the prime mover is suddenly shut down, the alternator transitions to operating as a synchronous motor, continuing to rotate in the same direction as before. This phenomenon is a direct result of the alternator's synchronization with the infinite bus bar and the constant phase sequence of the supply.