Definition of three-phase current
A three-phase current system consists of three single-phase alternating currents of the same frequency and amplitude (and therefore, effective value) that present a certain phase difference between them, around 120 °, and are given in a certain order.
Characteristics of the three-phase current
Each of the single-phase currents that make up the system is designated with the phase name.
A three-phase voltage system is said to be balanced when its currents are equal and are symmetrically out of phase. When any of the above conditions is not met, as would be the case of different voltages in the different phases or different phase shifts between them, the stress system is an unbalanced system or unbalanced system.
Received the name of system of imbalanced loads the set of different impedances that cause that by the receiver circulate currents of different amplitudes or with differences of phase between them different from 120 °, although the tensions of the system or of the line are balanced or balanced .
There are two types of connection, in triangle and star. In star, the neutral is the point of union of the phases.
Advantages of the three-phase current
The three-phase current system has a series of advantages such as the economy of its energy transport lines (thinner wires than in a single-phase equivalent line) and the transformers used.
The three-phase system also has a high performance of the receivers, especially electric motors, to which the three-phase line feeds with constant and unpulsed power, as in the case of the single-phase line.
In summary, we could list the following advantages of three-phase current systems.
- The production of three-phase machines is always greater than that of single-phase machines of the same size, approximately 1.5 more. Thus for a given size and voltage a three-phase alternator occupies less space and is less expensive also than single-phase alternators of the same size.
- For transmission and distribution, three-phase systems need less copper or less conductive material than a simple single-phase system given in volt amps and voltage so the transmission is much more economical.
- It is possible to produce rotating magnetic fields with stationary windings using the three-phase system. Therefore the three-phase motors are self-starting.
- In a single-phase system, the instantaneous power is a function of time and fluctuates w.r.t. Sometimes This power fluctuation causes considerable vibrations in single-phase motors. Therefore, the performance of single-phase systems is poor. However, the instantaneous power in three-phase systems is constant.
- Three-phase systems give stable output.
- A single-phase power can be obtained from three-phase circuits but three-phase can not be obtained from a single-phase motor.
- The power factor of the single-phase motors is poor in relation to the equivalent three-phase motors.
- For converting machines such as rectifiers, the DC output voltage is more uniform if the number of phases increases.
Application of three-phase systems
The generators used in power plants are three-phase, since the connection to the electrical network must be three-phase (except for low power plants). Three-phase is widely used in industries, where machines operate with motors for this voltage.