main current system adopted by the now ubiquitous, are three-phase circuits, which have a number of advantages over single-phase.
called three-phase current system of three-phase currents created by the three electromotive forces having the same amplitude and frequency, but shifted with respect to each other in phase by 120⁰ or time in the third period.
each separate circuit, a three-phase system in the reduction phase is called.
phase current, which is carried out three-phase circuit can be obtained by combining three of the same generator, giving a single-phase alternating current, the rotors which are in a fixed position, are rigidly attached to each other and do not change their position during rotation.Wherein stator windings of the deployed relative to one another at 120⁰ in the direction of rotation of the rotor.Under such circumstances, it is quite obvious that the electromotive force of the second generator will linger in their changes with respect to the first generator at 120⁰, iethe maximum value of the electromotive force of the same direction in the second generator occurs after all the generator rotor rotated by 120⁰.The electromotive force of the third generator will also be delayed relative to the second generator 120⁰.
But such a method of producing current, which are three-phase circuit is uneconomical and technically complicated.Much easier to three stator windings are connected in a single package.Such a generator is called three-phase generator.
Thus, the three-phase AC generator stator has three windings (called phase generator) to 120⁰ displaced relative to each other.The rotor of the generator is three-phase structurally identical to the generator rotor single-phase current.
During rotation of the rotor in all windings are created at the same frequency and amplitude of the electromotive force, but they do not simultaneously reach their maxima.Assuming that the maximum electromotive force generated at the time of the passage of the center of the north pole of the rotor under the winding, it is easy to see that the maximum electromotive force in the same direction in the second winding occurs after the turn of the rotor 120⁰, and the third - after turning on 240⁰ relative to the first.
connecting to an external circuit each phase generator, we obtain the three chain-phase current, with no interconnected no electrical connections, the current in each circuit at the same their resistance will be equal in amplitude but phase shifted relative to each other also at 120⁰.
To connect a generator to an external circuit, need six wires.To reduce the number of wires that go to the external circuit, it is necessary to connect the winding sets and generator interconnected to form electrically coupled to the three-phase system.This connection can be done in two different ways: a triangle and a star.
Both compounds provide an opportunity to save material in the transmission of the same capacity from three independent three-phase generators.
three-phase circuit enabled on the device to create a simple and easy-to-use electric motor, called asynchronous.His device based on application of a rotating magnetic field.In the simplest case, such a magnetic field can be obtained by rotating the horseshoe magnet.
If the rotating field to put the closed conductor, mounted on an axis, the magnetic field in its rotation crossing the side of the circuit conductor will induce in them an electromotive force of induction, which creates an induced current in the closed circuit.This current interacting with the magnetic field of the rotating magnet coil will rotate.The direction of rotation is determined by the turn of the rules of his left hand.
Three-phase motors consist of two parts: a rotating part - the rotor and fixed - stator.
rotating magnetic field is generated in the engine, not by mechanical rotation of the magnetic poles, and at a flow of three-phase alternating current of fixed stator windings.
three-phase circuits were designed by one of the greatest electrical engineers XIX and early XX century.- Russian engineer MO Dolivo-Dobrovolsky (1862-1919).This system has opened tremendous opportunities for industrial use of electricity.The most important are:
- savings in wiring line connecting the station with the consumer;
- possibility of rotating magnetic field is applied to the three-phase motors.