The main current system, universally accepted at present, is three-phase electric circuits, which have several advantages over single-phase ones.
A three-phase current is a system of three single-phase currents created by three electromotive forces having the same amplitudes and frequencies, but shifted one relative to the other in phase by 120⁰ or in time by a third of the period.
Each individual circuit of such a three-phase system in abbreviation is called a phase.
Three-phase current, which conducts three-phase circuits, can be obtained by connecting three identical generators giving alternating single-phase current, the rotors of which are in the same position are rigidly attached to each other and do not change their position during rotation. In this case, the stator windings of the generators are rotated relative to each other by 120⁰ in the direction of rotation of the rotor. Under such conditions, it is quite obvious that the electromotive force of the second generator will linger in its changes with respect to the first generator by 120⁰, i.e. the maximum value of the electromotive force of the same direction in the second generator will come after all the rotors of the generators are rotated by 120⁰. The electromotive force of the third generator will also be 120⁰ late relative to the second generator.
But such a method of producing current, which three-phase circuits have, is economically disadvantageous and technically difficult. It is much easier to connect three such stator windings in one housing. Such a generator is called a three-phase current generator.
Thus, the stator of a three-phase current generator has three windings (called generator phases), offset by 120 relative to each other. The rotor of a three-phase current generator is structurally identical with the rotor of a single-phase current generator.
During the rotation of the rotor, electromotive forces of equal frequency and amplitude will be created in all the windings, but only they will not simultaneously reach their maximums. Considering that the maximum electromotive force is created at the moment of passing the center of the north pole of the rotor under the start of the winding, it is easy to see that the maximum electromotive force of the same direction in the second winding will come after the rotor rotates by 120⁰, and in the third after turning by 240⁰ relative to the first.
Connecting each phase of the generator with an external circuit, we get three single-phase current circuits that do not have any electrical connections between each other, and the currents in each separate circuit with the same resistance will be equal in amplitude, but also phase shifted by 120⁰ relative to each other.
To connect such a generator to an external circuit, six wires are needed. To reduce the number of wires that go to the external circuit, it is necessary to connect the windings of the receivers and the generator to each other, forming an electrically connected three-phase system. This connection can be made in two different ways: a triangle and a star.
Both connections make it possible to save material when transmitting the same power from three autonomous three-phase generators.
Three-phase circuits made it possible to create an electric motor, simple in design and easy to operate, which was called asynchronous. His device is based on the use of a rotating magnetic field. In the simplest case, such a magnetic field can be obtained by rotating a horseshoe-shaped magnet.
If a closed conductor mounted on an axis is placed in a rotating field, then the magnetic field, when rotating, crossing the sides of the conductor circuit, will induce an electromotive force of induction in them, creating an induction current in this closed circuit. This current, when interacting with the magnetic field of a rotating magnet, will turn the coil into rotation. The direction of rotation of the turn is determined using the rule of the left hand.
Three-phase electric motors consist of two parts: a rotating part - a rotor and a fixed part - a stator.
A rotating magnetic field is created in the engine not by mechanical rotation of the magnetic poles, but when the stationary stator windings flow around an alternating three-phase current.
Three-phase circuits were developed by one of the outstanding electrical engineers of the 19th and early 20th centuries. - Russian engineer M.O. Dolivo-Dobrovolsky (1862-1919). This system has opened up the widest possibilities of industrial use of electric energy. The most important of them:
- savings in the wires of the line connecting the station to the consumer;
- the possibility of obtaining a rotating magnetic field used in three-phase motors.