Lower consumption of conductor material, lower cost and higher efficiency of the transmission line with the same power and voltage transmission lines.
The possibility of obtaining two operating voltages (linear and phase) in one three-phase four-wire system.
The ability to simply get rotating magnetic field (VMP), on the use of which the work of the most common consumers is based electric power - three-phase asynchronous and synchronous electric motors.
Three-phase power
Three-phase power - this is the sum of the respective powers of all three phases (power losses in the neutral wire are usually neglected):
As in single phase circuit active, reactive and full power of a three-phase circuit are connected by the relation: 
.
The power of any phase is expressed by the usual formula:
In the case of a symmetrical load, the power of all three phases respectively equal to:
and for the power of a three-phase circuit can be written:.
In a three-phase circuit with a symmetrical load:
therefore, for the power of a three-phase circuit can be written:
In addition, with a symmetrical load, the relations between linear and phase voltages and currents are known: I L = I F, U L 
U F - when connecting according to the “star” scheme, I L I F, U L = U F - when connecting according to the “triangle” scheme.
After substituting these expressions into the three-phase circuit power formula in the general case with a symmetrical load, we obtain:.
In the case of an unbalanced load, the power of a three-phase circuit should be found as the sum of the corresponding powers of all three phases (i.e. as the sum of the corresponding phase powers):
Three-phase active power measurement
Active power in the circuit alternating current P = I U cos φ is measured using an electrodynamic power meter, the measuring mechanism of which consists of two coils, one of which can rotate.
Fixed coil winding - consistent or current winding - has low resistance and is included in the measured circuit consistently and the winding of the moving coil - voltage winding - has great resistance and turns on parallel on load clamps (consumer). where k is the construction factor, I is the current in the series winding of the power meter.
When the wattmeter is switched on in the circuit, one should pay attention to the correct connection of the wattmeter windings, the beginnings of which (generator clamps) are indicated by asterisks (*). Both generator terminals must be connected to the same wire from the source of electrical energy (generator).
To measure the active power of a three-phase circuit, a single-phase wattmeter of active power is often used, switched on according to different schemes.
Measurement of active power using one watt meter
The method of one wattmeter is used in three-phase circuits only with a symmetrical phase load. With a symmetrical load, the power consumed by each of the three phases is the same; therefore, it suffices to measure the power of one phase and, multiplying the measurement result by the number of phases, obtain the power of the three-phase circuit:.
Consequently, to measure power at a symmetrical load, one wattmeter is enough, the current winding of which is connected in series with the phase load, and the voltage winding is switched on to the phase voltage.
If the neutral point of the load is not available, then the measurement of the phase power in the star connection is carried out according to a circuit with an artificial neutral point created by the voltage winding of a wattmeter connected to the star Z V and two equal in resistance additional resistors Z 2
and
Z
3
: 
.
Active power - the sum of the active powers of the phases of the load of the active power in the neutral wire, if its active resistance is not equal to zero:.
Reactive power - the sum of the reactive powers of the phases of the load and the reactive power in the neutral wire if its reactance is not zero, that is.
Net power is determined by the formula:.
If the load is symmetrical and uniform, then the active and reactive powers of the neutral wire are zero, the active powers of the load phases are equal, and are determined using the values of the phase current and phase voltage, that is, the reactive powers of the load phases are also equal, and are determined using the values of the phase current and phase voltage: where angle is the angle between phase voltages or voltages at the phase of the load and phase current or current flowing through the phase of the load. Then the active load power can be determined by the formula, and reactive power load can be determined by the formula :.
With a uniform load of phases, regardless of the method of connection, the following equality holds:, then, therefore, the total power of the load can be determined by the formula :.
Measurement of the active power of a three-phase circuit.
In the general case, when the load is uneven and the neutral wire is present, it is necessary to connect three watt meters to the circuit, while the active power of the circuit will be equal to the sum of the readings of these three wattmeters.
With a uniform load, it suffices to measure the power of one phase and triple the result.
If the zero wire is missing the power can be measured using two wattmeters. The sum of the readings of two wattmeters determines the active power of the entire circuit, regardless of how the load is connected.
The first wattmeter shows the value of the magnitude, the second - the value of the value.
Summing up the wattmeter readings, we get:.
36. Transformer - e / m apparatus designed to convert, by a magnetic field, the electrical energy of alternating current of one voltage into electrical energy of alternating current of another voltage, provided the frequency is maintained. In a transformer, the transmission of electricity from the primary to the secondary circuit is carried out by means of an alternating magnetic field in the core.
Transformer - static electromagnetic device having two or more inductively connected coils, designed to convert an alternating current of one voltage into an alternating current of another voltage of the same frequency by means of electromagnetic induction without significant loss of power.
37. Transformer - a device that converts alternating current of one voltage into alternating current of another voltage of the same frequency.
Classification:
by appointment:
power (in electricity distribution networks);
measuring (as elements of measuring devices):
welding (in electric welding);
furnaces (as elements of electrothermal devices);
by design:
single phase
three phase
multiwinding
according to the cooling method:
aerial
oil
Measuring transformers are divided into current transformers and voltage transformers.
Symmetrical three-phase mode
In fig. 7 shows a topographic diagram and a vector diagram of currents in the symmetric mode for the circuit on rice four and the inductive nature of the load (j\u003e 0).
Neutral current is missing:
therefore, with a symmetric receiver neutral wire is not used. Line voltages are defined as phase voltage differences:
From the isosceles triangle ANB we have: 
In fig. 8 are given vector diagrams voltages and currents in symmetrical mode andj \u003e 0 for the circuit. Line currents are defined as phase current differences:
Active power balanced three-phase receiver
Whereas when connecting the branches of the receiver with a star
and when connecting the receiver branches with a triangle
regardless of the type of compound
It should be remembered that in this expression j - phase shift between the phase voltage and phase current.
Similarly, for the reactive and total powers of a symmetric three-phase receiver, we have
We determine the total instantaneous power of a three-phase receiver in a symmetric mode. We write the instantaneous values of the phase voltages and currents, taking the initial phase voltageu A is equal to zero:
and expressions for the instantaneous power values of each phase of the receiver:
When summing up the instantaneous values of the powers of the individual phases, the second terms in the sum will give zero. Therefore, the total instantaneous power
does not depend on time and is equal to active power.
Multiphase circuits in which the instantaneous value of power is constantly called balanced.
Note that in a two-phase symmetric circuit (Fig. 9) with an asymmetric system, the EMF of the power source ( see fig. 3, b) the system of currents is also asymmetric, however the circuit is balanced, since the sum of the instantaneous values of the powers in the phases is constant. This can be shown in the same way that the symmetry of a three-phase circuit was shown. 
The constancy of the instantaneous values of power creates favorable conditions for the operation of generators and engines in terms of their mechanical load, since there are no torque pulsations observed in single phase generators and engines.
Considering the symmetric regimes of coupled three-phase circuits, it is easy to show the advantages of the latter in economic terms as compared with unrelated three-phase systems of circuits. An unrelated three-phase circuit system has six wires with currents.I l = I f. Three-phase circuit without a neutral wire that feeds the same receivers connected by a star, there are only three wires with the same currentsI l = I φ and linear voltages, to the root of three times the large linear voltages in the unbound three-phase system of circuits, for whichU l = U f. In the case of connecting the receivers with a triangle, it also produces half as many wires as in an unrelated three-phase system of circuits (three instead of six), while the currents in the linear wires are not 2 times the phase currents, but only the root of three times. This reduces material costs for wires.