Comparison Between Single-Phase and Three-Phase System.

During the earliest day, the alternating current is mainly used for resistive load such as electric lamp and electric heater. That electrical load is resistive type whereby the use of single-phase system works well. However, when ac motor is developed, the single-phase system may not function well since the ac motor require higher starting torque for self-starting.

So what did they do? In order to make the ac motor to self-start, it is fitted with auxiliary winding but the problem is, there will always an alternating component with frequency that is double the frequency of power supply. This scenario is okay for small motor but it will cause a noisy and vibration during operation for a large motor.

Single-phase voltage source are originated from three-phase system. In a balance three-phase system, there are three sinusoidal voltage sources of equal amplitude and equal frequency of variation but out of phase with each other by 2π/3 or 120° but for single-phase system is only having one.

An important advantage of three-phase power supply is that three-phase motors have a non-zero starting torque as compared with the single-phase system.

The characteristic of three-phase system is the currents and voltages are balanced everywhere in the circuit. The most important property of such a circuit is that though the power in an individual phase is pulsating and varies at twice the supply frequency, the sum of powers delivered by all the phases is constant at every instant of time.