Three-phase alternating current. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electricity for beginners
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At present, three-phase alternating current is widely used in world industrial practice, which has many advantages over single-phase current.
A three-phase system is called such a system that has three electrical circuits with their own variable EMFs with the same amplitudes and frequencies, but shifted in phase relative to each other by 120 ° or 1/3 of the period. Each such circuit is called a phase.
To obtain a three-phase system, you need to take three identical single-phase alternating current generators, connect their rotors to each other so that they do not change their position during rotation. The stator windings of these generators must be rotated relative to each other by 120 ° in the direction of rotation of the rotor. An example of such a system is shown in Fig. 10.
Rice. 10. Example of a three-phase AC system with three generators
According to the above conditions, it turns out that the EMF that occurs in the second generator will not have time to change, compared with the EMF of the first generator, i.e., it will be 120 ° late. The EMF of the third generator will also be late with respect to the second one by 120°.
However, this method of obtaining an alternating three-phase current is very cumbersome and economically unprofitable. To simplify the task, you need to combine all the stator windings of the generators in one housing. Such a generator is called a three-phase current generator.
When the rotor begins to rotate, a changing induction emf occurs in each winding. Due to the fact that the windings are shifted in space, the phases of the oscillations in them also shift relative to each other by 120 °.
In order to connect a three-phase alternator to a circuit, you need to have 6 wires. To reduce the number of wires, the winding of the generator and receivers must be interconnected, forming a three-phase system. These connections are two: star и triangle. When using both methods, you can save wiring.
Author: Smirnova L.N.
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