"Three-phase power" concept is: coil rotation in the magnetic field, the wire cutting the line of magnetic force will produce an induced electromotive force, the law of change can be expressed as a sinusoidal curve. If we take three coils, they will be in the space position of each other 120 degrees angle, three coils are still in the magnetic field at the same speed rotation, must be induced by three frequency of the same induced electromotive force. Because of the three coils in the space position 120 degrees from each other, so the current is also three-phase sinusoidal change, called three-phase sinusoidal alternating current. Industrial equipment in many places using three-phase power supply, such as three-phase AC motors. Power is relatively large, if you use single-phase power, the current will be very large, wires, meters, etc. are not economical, and three-phase power phase voltage is higher, the sum of the three-phase current is 0, the power supply is easy;
The need for a larger torque motor, single-phase motors are generally capacitive phase-shifting start and work, torque is not large, only for small power household appliances and so on, high-power motors have to be used with three-phase motors.