Open Circuit Test of a 3-Phase Alternator

Experiment Title: Open Circuit Test of a 3-Phase Alternator

1. Objective

• Understand the behavior of an alternator under no-load conditions.
• Calculate the synchronous reactance and field current required for excitation.

2. Apparatus

The following equipment was used during the experiment:

• 3-phase alternator (synchronous generator)
• AC power supply
• Ammeter and voltmeter
• Rheostat (for field current control)
• Tachometer (to measure rotor speed)
• Circuit breaker and safety switches

3. Theory

3.1 Open Circuit Test

The open circuit test (also known as the no-load test) is conducted to determine the following parameters:

• Synchronous reactance ((X_s)): The impedance seen by the alternator when no load is connected.
• Field current ((I_f)): The current required to establish the magnetic field in the rotor.

A three-phase alternator, also known as a synchronous generator, converts mechanical energy into electrical energy. The open circuit test, also called the no-load test, involves running the alternator at its rated speed without connecting any external load. During this test, the field current is varied, and the corresponding terminal voltage is measured to plot the open circuit characteristics (OCC).

4. Circuit Diagram

5. Procedure

1. Connect the alternator to the AC power supply.
2. Open the circuit breaker to disconnect the load.
3. Gradually increase the field current using the rheostat.
4. Measure the line voltage ((V_{\text{line}})) and field current ((I_f)).
5. Record the rotor speed using the tachometer.
6. Calculate the synchronous reactance: [ X_s = \frac{{V_{\text{line}}}}{{I_f}} ]
7. Determine the synchronous speed ((N_s)) based on the number of poles: [ N_s = \frac{{120 \cdot f}}{{P}} ]
8. Compare the measured rotor speed with the synchronous speed.

6. Discussion

Discuss the results obtained during the experiment:

• Plot the synchronous reactance curve (reactance vs. field current).
• Analyze any deviations from ideal behavior.
• Consider factors affecting the alternator’s performance.

7. Conclusion

In conclusion, the open circuit test provides valuable information about the alternator’s characteristics. We learned about synchronous reactance, field current, and the importance of no-load testing. Understanding these parameters helps in designing and operating synchronous generators effectively.