The advantages of the three-phase system over the single-phase system are that for the same size, three-phase generators and motors have higher power than single-phase motors, 10 horsepower 215T frame that three-phase transmission lines require less copper than single-phase motors to charge the same power. Single-phase motors (except collector motors) do not have their own starting, and three-phase motors can start without external assistance.
The basic feature of the three-phase induction motor (MIT) is to operate at a somewhat lower speed than the synchronous speed, and this speed is defined by the number of poles and the frequency of power.
MIT has a major disadvantage, the fact that in its direct starting the current value is very high, almost up to ten times its rated current. The agent for this high current is the low value of its impedance, which is the quotient of the voltage by the intensity of an equivalent alternating current at startup, which is sometimes less than at its rated speed. When you just want to slow the start without changing the speed, the electronic soft-starter must be used in order to keep the current as low as possible.