Question

The phasor diagram of a synchronous machine connected to an infinite bus is shown in the figure below. The machine is acting as, 

• A. generator operating at leading p.f.
• B. generator operating at lagging p.f.
• C. motor operating at leading p.f.
• D. motor operating at lagging p.f.

Hint:

For synchronous machines connected to an infinite bus, remember: {E leading V indicates generator operation}, and {over-excitation corresponds to lagging power factor}.

Answer 1

The Correct Option is B

Step 1: Observe the given phasor diagram.
In the phasor diagram, the terminal voltage \(V\) is taken as the reference and is shown along the horizontal axis. The induced emf \(E\) is shown leading the terminal voltage \(V\) by an angle.
Step 2: Identify the operating mode (generator or motor).
For a synchronous machine connected to an infinite bus:
- If the induced emf \(E\) leads the terminal voltage \(V\), the machine is operating as a synchronous generator.
- If the induced emf \(E\) lags behind the terminal voltage \(V\), the machine operates as a synchronous motor.
Since \(E\) is leading \(V\) in the given diagram, the machine is acting as a generator.
Step 3: Determine the power factor nature.
In a synchronous generator:
- If the generator is over-excited, it supplies reactive power to the system and operates at a lagging power factor.
- If the generator is under-excited, it absorbs reactive power and operates at a leading power factor.
The given phasor diagram shows \(E\) significantly leading \(V\), indicating an over-excited condition.
Step 4: Conclusion.
Hence, the synchronous machine is operating as a generator at lagging power factor.