It is impossible to imagine a modern lifestyle without the 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 comes a real “end of the world”.
The vast majority of modern power plants generate three-phase alternating current. Among its advantages, it should be noted the ease of obtaining and subsequent transformations, the high reliability and simplicity of the design of asynchronous electric motors designed for it . Three-phase current is the most common type of electricity in the world.
A three-phase electric current system is a combination of three single-phase current circuits with the same frequency and amplitude, however, offset from each other by 120 degrees (or, what is the same, 1/3 of the period). Each of these circuits is called a phase, respectively, all three form a three-phase current.
The theoretical basis is quite simple: a metal frame rotates in a magnetic field, crossing the lines of tension. In order to obtain an electric current in accordance with the law of electromagnetic induction , it is enough to connect a load to its terminals and create a circuit. If a three-phase current is needed, then the device becomes more complicated: three identical frames are placed in the mechanism, shifted one relative to the other by 120 degrees. The result is the generation of three electromotive forces (EMF). In standard power plants, the rotation speed is constant.
In practice, the implementation is slightly different from theory. Three-phase current is created by special machines - generators. In them, the windings of the phase circuits are stationary (compare with theory) and are located in a certain way on the poles of the stator (the stationary part of the machine). A rotating magnetic field is created by the rotor. The torque is reported to him by the energy of the falling water in hydroelectric power plants, a steam turbine in nuclear power plants, etc.
One of the features of circuits that use a three-phase current is to use only three or four wires on the consumer side - three phase and zero. This is achieved thanks to the method of connecting the windings of the generator - a star or a triangle.
Star connection implies that the ends of all three windings converge at one zero point. Based on Kirchhoff’s law, it follows that the sum of all currents at this point (node) is zero, therefore, no closure occurs. From the zero point, the neutral wire is output . The voltage measured between this wire and any of the three linear wires is 1.73 times less than the voltage value between the linear wires themselves. In the first case, a phase voltage is obtained, and in the second, a linear voltage.
An important feature of a star connection is the need to avoid phase imbalance, that is, to control that the currents flowing in the branches are approximately equal. That small inevitable difference leads to a small current in the neutral wire, but it is small.
A completely different type of connection of the generator windings - a triangle, allows you to eliminate the neutral wire. With its implementation, each end of the winding is connected to the beginning of the next, in fact, forming a triangle, and the stresses are removed from its vertices. With this method, the phase and linear voltages are equal. It is also necessary to control the equality of currents in the branches, since ignoring this, the total current value in a closed circuit can become excessive, causing the generator to heat up and break down.
Most electric motors designed for a three-phase network provide for the possibility of choosing the method of connecting the windings to a star or triangle. This allows you to select the operating voltage. So, when connecting the load windings with a star, the calculated voltage will be 1.73 times less than with a triangle.