What are the advantages of a three-phase system compared to a single-phase variable frequency power system?

Three-phase power generation, transmission and distribution systems are very common around the world due to their distinct advantages over single-phase and other polyphase systems.

Single-phase variable frequency power supply system

The sinusoidal AC voltage with a specific time period and frequency produced by a single-winding alternator as the supply voltage is called a single-phase power supply system. Circuits powered by these voltages are called single-phase AC circuits. In other words, A circuit containing a single AC current and voltage is identified as a single-phase circuit.

Three-phase variable frequency power supply system

A system that contains more than one phase is called a polyphase or multiphase system. The three-phase variable frequency power system consists of three phases with uniform frequency where there is a fixed angle of 120° between the source voltages generated by the alternator with three windings. Likewise, in a two-phase power supply system, there is a 90° angular difference between the two phases.

The main advantages of three-phase variable frequency power supply system compared with single-phase variable frequency power supply system

1. In order to transmit a specific power over a specific distance at a given rated voltage, a three-phase system requires less conductor material than a single-phase system.

2. The size of a machine operating on a three-phase system is smaller than that of a machine operating on a single-phase voltage with the same output rating.

3. In a three-phase power supply system, the smaller the voltage drop from the power supply to the load point, the smaller the voltage drop.

4. The three-phase power supply produces a uniform rotating magnetic field. Therefore, the three-phase motor has a simple structure, small size, can be automatically started, and runs smoothly.

5. A polyphase system produces power at a constant rate in the load.

6. Three-phase systems can transmit more power than single-phase systems.

7. Equipment and appliances that operate three-phase are more efficient than machines that operate single-phase.

8. Three-phase motors have lower cost and higher efficiency.

9. The three-phase system provides constant power, while the single-phase system provides pulsating power, which results in smooth and vibration-free operation of the three-phase variable frequency power supply compared to the single-phase power supply with noise and vibration.

10. The output rating of the machine can be increased by increasing the number of phases in the system.

11. A three-phase machine with the same rating takes up less space than a single-phase machine.

12. Single-phase power can be obtained from a three-phase power source to run a 1-camera machine. Three-phase machines cannot operate on single-phase mains voltage.

13. Three-phase power can be easily converted to single-phase power, while converting single-phase power to three-phase power system requires a complex system

14. If a single-phase line fails, the entire system will have to be shut down. In the event of a three-phase single-wire fault, the other two wires provide power to the other single-phase load points connected to it.

15. Polyphase or three-phase motors provide uniform torque, while single-phase motors (except commutator motors) provide pulsating torque.

16. The three-phase motor starts automatically, but the single-phase motor cannot start automatically.

17. If the frame size of a three-phase alternator, motor or transformer is the same as that of a single-phase motor, alternator or transformer, the output of a three-phase machine will be higher than that of a single-phase machine.

18. Compared to single-phase alternators with a pulsating armature reaction, multi-phase alternators can be easily paired and run in parallel.

19. The ripple factor of the DC rectified voltage from the three-phase variable frequency power supply is 4%, while the ripple factor of the DC rectified voltage from the single-phase power supply is 48%. Therefore, the cost of a converter for DC rectification from a three-phase variable frequency power source is lower than that of a converter for rectification. Due to the smaller number of filters used in the rectification of 3-Φ power systems, it is used for DC voltages from single-phase power supplies.

20. Three-phase motors have better power factor than single-phase motors.