What is a three-phase current

The modern way of life can not be imagined without electricity and the benefits that are associated with it. The lack of natural gas is easily compensated by solid fuel sources of heat, water is also available, but without electricity there is a real "end of the world".

The vast majority of modern power plants generate a three-phase alternating current. Among its advantages, one should especially note the ease of obtaining and subsequent transformations, high reliability and simplicity of the design of asynchronous electric motors intended for it . Three-phase current is the most common type of electric power in the world.

The three-phase current system is a set of three single-phase current circuits with the same frequency and amplitude, however, displaced 120 degrees relative to each other (or, which is the same, 1/3 period). Each of these circuits is called a phase, respectively, all three form a three-phase current.

The theoretical foundations are quite simple: the metal frame rotates in a magnetic field, crossing the tension lines. In order to obtain an electric current in accordance with the law of electromagnetic induction , it is sufficient to connect the load to its terminals and create a chain. If three-phase current is required, the device becomes more complicated: in the mechanism there are three identical frames that are shifted relative to each other by 120 degrees. The result is the generation of three electromotive forces (EMF). In standard power plants, the rotation speed is unchanged.

In practice, the implementation is slightly different from the theory. Three-phase current is created by special machines - generators. In them, the windings of the phase circuits are immovable (compare with the theory) and are located at the poles of the stator (fixed part of the machine) in a certain way. A rotating magnetic field is created by the rotor. The moment of rotation it is reported by the energy of falling water in hydroelectric power plants, a steam turbine in a nuclear power plant, etc.

One of the features of circuits using a three-phase current is to use only three or four wires on the consumer side - three phase and zero. This can be achieved due to the way of connecting the generator windings - a star or a triangle.

A star connection means that the ends of all three windings converge at one zero point. Proceeding from Kirchhoff's law, it follows that the sum of all the currents at this point (node) is zero, so there is no closure. A zero wire is output from the zero point. The voltage measured between this wire and any of the three linear ones is 1.73 times less than the voltage value between the linear wires themselves. In the first case, the phase voltage is obtained, and in the second case, it is linear.

An important feature of the star connection is the need to avoid phase misalignment, that is, to control that the currents flowing in the branches are approximately equal. That small inevitable difference leads to the appearance of a small current in the zero wire, but it is small.

A completely different type of winding of the generator - a triangle, allows you to abolish the zero wire. With its implementation, each end of the winding is connected to the beginning of the next, in fact, forming a triangle, and the voltage is removed from its vertices. With this method, the phase and linear voltages are equal. It is also necessary to monitor the equality of currents in the branches, because if ignoring this, the total value of the current in the closed circuit can become excessive, causing the generator to heat up and out of order.

Most electric motors designed for a three-phase network provide for the choice of how the windings are connected to a star or a triangle. This allows you to choose the operating voltage. So, when connecting the windings of a star load, the calculated voltage will be 1.73 times less than with a triangle.