Three-Phase Systems Questions and Answers for Viva

Answer-1: A three-phase system is a method of generating, transmitting, and distributing electrical power using three alternating currents that are 120 degrees out of phase with each other.

Answer-2: The phases are typically labeled as phase A, phase B, and phase C.

Answer-3: The phase sequence refers to the order in which the three phases (A, B, C) reach their peak values. It can be either clockwise (ABC) or counter-clockwise (ACB).

Answer-4: The line voltage (√3 times the phase voltage) is higher than the phase voltage in a three-phase system. Mathematically, line voltage (VL) = √3 * phase voltage (VP).

Answer-5: In a star (Y) connection, one end of each phase winding is connected together to form the neutral point, while the other ends are connected to the power supply lines.

Answer-6: In a delta (Δ) connection, each phase winding is connected in series to form a closed loop, resembling the Greek letter delta (Δ).

Answer-7: A star (Y) connection provides a neutral point, allowing the connection of single-phase loads, while a delta (Δ) connection does not require a neutral conductor and is suitable for balanced loads.

Answer-8: In a balanced three-phase system, the currents and voltages are equal in magnitude and phase angle, ensuring equal power distribution among the phases.

Answer-9: Power is transmitted in a three-phase system through three conductors, with each conductor carrying one phase of the system. This allows for efficient power transmission and distribution.

Answer-10: Power factor in a three-phase system is the ratio of the real power (kW) to the apparent power (kVA). It indicates the efficiency of power utilization and is influenced by the phase difference between voltage and current.

Answer-11: A three-phase transformer has three primary and three secondary windings, allowing it to handle three-phase power transmission. It is more efficient and compact compared to multiple single-phase transformers.

Answer-12: Phase rotation refers to the direction in which the magnetic fields of the three phases rotate. It is important for ensuring proper operation of three-phase equipment.

Answer-13: To convert a star connection to a delta connection, each phase is connected to the next phase in series.

Answer-14: Synchronous speed is the speed at which the rotating magnetic field in the motor rotates. It is directly proportional to the frequency of the power supply and inversely proportional to the number of poles in the motor.

Answer-15: The phase sequence can be determined using a phase sequence indicator or by observing the rotation direction of a three-phase motor.

Answer-16: Phase imbalance occurs when the loads or impedances on the three phases are not equal, leading to differences in phase currents or voltages.