Three-phase circuits

The main current system currently adopted is three-phase electrical circuits, which have several advantages over single-phase circuits .

A three-phase current is a system of three single-phase currents created by three electromotive forces having the same amplitudes and frequency, but shifted one in relation to the other in phase by 120 ° or in time by a third of the period.

Each, separately taken, chain of such a three-phase system in the contraction is called the phase.

The three-phase current that conducts three-phase circuits can be obtained by connecting three identical generators giving an alternating single-phase current, the rotors that are in an unchanged position are rigidly tied 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 120 ° in the direction of rotation of the rotor. Under such conditions it is quite obvious that the electromotive force of the second generator will be delayed in its changes with respect to the first generator by 120 °, i.е. The maximum value of the electromotive force of the same direction in the second generator will come after all the rotors of the generators turn by 120 °. The electromotive force of the third generator will also be delayed relative to the second generator by 120 °.

But such a method of obtaining a current that has three-phase circuits turns out to be economically unprofitable and technically complex. It is much easier to combine three such stator windings in one housing. This generator was called the generator of three-phase current.

Thus, the stator of the three-phase current generator has three windings (called generator phases), offset by 120 ° relative to each other. The rotor of the three-phase current generator is structurally identical with the rotor of the single-phase current generator.

During the rotation of the rotor, all windings will produce the same electromotive forces of the same frequency and amplitude, 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 winding, it is easy to see that the maximum of electromotive force of the same direction in the second winding will come after the rotation of the rotor by 120⁰, and in the third - after a rotation of 240⁰ relative to the first.

By connecting each phase of the generator to the external circuit, we obtain three single-phase current circuits that do not have any electrical connections, and the currents in each individual circuit with equal resistance will be equal in amplitude but shifted in phase relative to each other also by 120 °.

To connect such a generator to an external circuit, you need six wires. 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. Such a connection can be made in two different ways: a triangle and a star.

Both connections make it possible to save material when transferring the same power from three autonomous three-phase generators.

Three-phase circuits made it possible to create an easy-to-operate and easy-to-use electric motor, which was called asynchronous. Its 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 fixed on the axis is placed in the rotating field, then the magnetic field, crossing the sides of the conductor contour, will induce in them an electromotive force of induction, creating an induction current in this closed circuit. This current, when interacting with the magnetic field of the rotating magnet, will cause the turn to rotate. The direction of rotation is determined by the left hand rule.

Three-phase electric motors consist of two parts: a rotating part - a rotor and a stationary one - a stator.

A rotating magnetic field is created in the motor not by mechanical rotation of the magnetic poles, but when flowing through the alternating three-phase current of the stationary stator windings.

Three-phase circuits were developed by one of the most outstanding electrical engineers of the 19th and early 20th centuries. - Russian engineer M.O. Dolivo-Dobrovolsky (1862-1919). This system has opened the widest opportunities for the industrial use of electrical energy. The most important of them:

• Saving in the wires of the line connecting the station with the consumer;
• The possibility of obtaining a rotating magnetic field, used in three-phase engines.