A system of three sinusoidal currents that change over time and have a phase shift is called three-phase alternating current. With the help of this system, convenient and economical electric motors are created, electricity is transmitted over long distances, the material consumption of transformers and power cables is reduced. The operation of all large power plants and electricity consumers is based on three-phase current.
Content
- Historical reference
- Generator device
- Connection of windings of an electrical machine
- Benefits of three-phase systems
Historical reference
Three-phase current is special case of multiphase current. For the first time, a two-phase current was obtained by the famous inventor Nikola Tesla. A great contribution to the formation of three-phase systems was made by the Russian scientist M. O. Dolivo-Dobrovolsky. He used three- and four-wire AC transmission systems and built an asynchronous motor on its basis.
The main feature of his invention was the squirrel-cage squirrel-cage rotor, which is still used in asynchronous electric motors. Another achievement of the inventor was a power line, built by him using a generator and three-phase alternating current transformers. The line was 170 km long, which was a huge step forward for the late 19th century.
Generator device
A three-phase system is considered structure consisting of three electrical circuits, in which electromotive forces (EMF) of the same frequency are generated, shifted relative to each other by 120 °. A high power synchronous electric machine is used as a generator. It converts mechanical energy of rotation into electrical energy. Three windings are placed in the stator slots, in which EMF is induced, equal in amplitude and differing in phase by 1/3 of the period.
Each winding (phase) is an independent source of electrical energy. The rotor, made in the form of a permanent magnet, is driven by an electric motor. The magnetic field of a rotating rotor induces an EMF in the stator windings. If you connect wires to the ends of each winding, you get three independent networks. There will be six wires in the system and there is no gain over three separate generators.
In modern three-phase networks, three or four wires are usually used, depending on the connection diagram.
Connection of windings of an electrical machine
Generator windings and loads connected in star or delta patterns. When connected to a star, a common zero point is formed from the ends of the windings connected to each other, and linear wires are connected to the beginning of the windings. The neutral or zero points of the generator and the load are connected by a neutral wire. The voltage created between the line wire and zero is called phase, and between two line wires - line.
The neutral wire is designed to equalize the voltage on all phases with an unbalanced load. The strength of the current flowing in this wire is less than in linear wires, which makes it possible to choose a conductor with a smaller cross-section. Dependences for linear and phase currents and voltages when connected by a star are of the form: Il = Iph, Ul = √3 Uph ≈ 1.73 Uph.
When the circuit is executed, the triangle end of each winding is connected to the beginning of the next one. This circuit uses three wires leading from the generator to the load. The ratio between currents and voltages, linear and phase, is equal to: Ul = Uph, Il = √3 Iph.
The generator windings are often star-connected. With a delta connection, each phase must be rated for 1.73 times the voltage of a star connection. This entails an increase in the insulation of the windings, an increase in the number of turns and a rise in the cost of machines.
In distribution networks, where there are many single-phase consumers, it becomes impossible to ensure a symmetrical load on the phases. Such networks are executed four-wire with a neutral conductor.
Conductors belonging to different phases and neutral conductors have different colors. This is done in order to ensure safety during electrical work and for convenience during the repair and installation of electrical networks. In Russia, the neutral conductor is usually made blue, the first phase is yellow, the second is green, and the third is red.
Choosing a connection method for the consumer depends on the following characteristics:
- rated voltage of consumers of electrical energy;
- the nature of the load;
- supplied three-phase voltage.
In the practical application of 3-phase networks, it is important to remember that when connected with a star, the loads are affected by phase voltage, and when connected with a delta - line voltage, which is 1.73 times more than phase.
Benefits of three-phase systems
These systems are used in industry, in transport, for power supply of residential premises. Such widespread use due to great advantages, which this system gives in comparison with a single-phase power supply system:
- Fewer wires are required, resulting in greater savings when transmitting power over long distances.
- Three-phase transformers have a smaller magnetic circuit compared to single-phase transformers of the same power.
- During operation, a rotating magnetic field is created, which is necessary for the operation of asynchronous motors.
- Possibility of using two operating voltages.
- Balance of symmetrical three-phase systems.
The proliferation of three-phase systems has helped to solve many problems of power supply, the development of transmission capacities and the improvement of technological processes. The use of three-phase transformers, generators and electric motors has greatly simplified and reduced the cost of the process of generating energy and increased its availability for consumers.
