Question

Negative phase sequence current in an alternator produces:

• A. Over speed
• B. Over voltage
• C. Rotor heating
• D. Under frequency

Hint:

Negative phase sequence currents in alternators lead to rotor heating and potential damage. Protective schemes like NPS relays are essential to mitigate this effect.

Answer 1

The Correct Option is C

Step 1: Negative phase sequence (NPS) current arises in an alternator when there are unbalanced loads, which result in asymmetrical currents in the stator. 
Step 2: These NPS currents rotate in the opposite direction to the rotor’s rotation, inducing high-frequency currents in the rotor. 
Step 3: The effects of NPS current include: 
- Rotor heating: The opposing rotation of the NPS current induces eddy currents in the rotor, leading to substantial heating and I²R losses. 
- Potential damage to the rotor: Prolonged exposure to NPS current can lead to rotor damage and degradation. 
Step 4: Other options: 
- Over speed (Incorrect): The alternator speed is determined by the prime mover and load, not by NPS current. 
- Over voltage (Incorrect): Voltage changes are controlled by the excitation system, not NPS current. 
- Under frequency (Incorrect): Frequency is regulated by grid conditions, not by NPS currents.