Three-phase circuit

The Main system current, to the currently accepted universally, are three-phase electric circuits, which have several advantages over single-phase.

Three-phase current is a system of three single-phase currents generated by three electromotive forces have the same amplitude and frequency but shifted one relative to another in phase by 120⁰ or in time for third period.

Each individual chain, this three phase system is called the reduction phase.

Three-phase current, which is carried out three-phase circuit, can be obtained by combining three identical generator giving an alternating single-phase current, the rotors of which are in the same position, tightly linked to each other and do not change position during rotation. Thus the stator windings of generators deployed relative to each other at 120⁰ in the direction of rotation of the rotor. Under these conditions it is obvious that the electromotive force of the second generator will linger in their changes relative to the first generator on 120⁰, i.e. the maximum value of the electromotive force in the same direction in the second generator will occur once all the rotors of the machines turn at 120⁰. Electromotive force of the third generator will also fall behind the second generator at 120⁰.

But such a method of generating electric current, which have a three-phase circuit, it is economically unprofitable and technically difficult. Much easier three stator windings to combine in one housing. Such a generator is called alternator.

Thus, the stator of the alternator has three windings (called phases of the generator), offset by 120⁰ relative to each other. The rotor of the alternator is structurally identical with the rotor of the generator single-phase current.

During the rotation of the rotor, all windings will produce the same frequency and the amplitude of the electromotive forces, but they will not simultaneously reach their maxima. Considering that the maximum electromotive force is created at the moment of passing the centre of the North pole of the rotor under the top winding, it is easy to see that the maximum electromotive force in the same direction in the second coil will come after the rotation of the rotor at 120⁰, and the third – after turning to 240⁰ relative to the first.

Connecting with the external circuit per phase of the generator, we get a three circuit single-phase current, not having any electrical connections, and the currents in each circuit with the same resistance will be equal in amplitude but phase-shifted relative to each other also at 120⁰.

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To connect this generator with the external circuit, you need six wires. To reduce the number of wires that go into the external circuit, it is necessary to connect the windings of the receivers and generator with each other, forming an electrically connected, three-phase system. Such a link can be accomplished in two different ways: a triangle and a star.

Both connections give you the opportunity to save some material while transferring the same power from three Autonomous three phase generators.

Three-phase circuit given the opportunity to create simple in design and easy to operate the electric motor, called asynchronous. The device is based on the application of the rotating magnetic field. In the simplest case this magnetic field can be obtained by rotating horseshoe magnet.

If in a rotating field to put a closed conductor, attached to the axis, the magnetic field during its rotation intersecting sides of the contour of the conductor, will be inducting in them an electromotive force of induction, creates an induction current in this closed circuit. This current, when interacting with the magnetic field of the rotating magnet will cause the coil to rotate. The direction of rotation of the coil is determined by using left-hand rule.

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

Rotating magnetic field is generated in the engine by mechanical rotation of magnetic poles, and in the flow of alternating three-phase current stationary windings of the stator.

Three-phase circuit was designed by one of the foremost electrical engineers of the XIX and beginning of XX century – Russian engineer M. O. Dolivo-Dobrovolsky (1862-1919). This system has opened up tremendous opportunities for industrial usage of electrical energy. The most important of them are:

• Savings in the wires of the line connecting the station with the consumer;
• The possibility of obtaining a rotating magnetic field applied in three-phase motors.