Advantages and disadvantages of three phase system over single phase system   

1. In a single phase circuit the power distributed is pulsating. In three-phase system the total power delivered is constant if the loads are balanced by the power of any one phase or circuit might be negative. So three-phase system is highly desirable mainly for power loads.  

2. The rating of a given machine enhances with the increase in number of phases. For example the output of a three-phase motor is 1.5 times the output of a single-phase motor of the similar size.

3.  Single-phase induction motors, have no initial torque and so its is essential to give these motors with an auxiliary means of starting, but in case of three phase motors except synchronous motors there is no requirement of providing an auxiliary means for starting.  

4. Power factor of single- phase motor is lower than that of three phase motor of the similar rating (output and speed). The efficiency of a three- phase motor is also higher than that of a single- phase motor.