Three-phase current power: some formulas for calculating and methods of measuring power

Alternating and direct currents differ from one another in many parameters, and especially in the presence of phases in the first type. These differences are associated with more complex formulas and methods for calculating the numerical values ​​of the quantities characterizing the alternating current, including the power of the three-phase current.

Characteristics of three-phase circuits

Electrical systems using three-phase current as a power source have two main types of connection: "star" and "delta". In the diagrams showing the connection of a three-phase power supply, it is customary to designate phases using a set of Latin letters:

• A, B, C;
• or U, V, W.

And the so-called neutral is designated by the letter N.

In practice, quite often you have to deal with the need to calculate the power of an electric current. In the case of direct current, this problem is solved extremely simply - by multiplying the voltage and current. These

parameters are not subject to changes in time, therefore, the power value will be unchanged, since the system is balanced and is constantly in this state.

A completely different situation arises when it is necessary to calculate the power of an electric current that varies in time in magnitude and direction of flow. Performing such calculations requires specialized knowledge of the nature of alternating current and its features.

Three-phase current power is calculated as the sum of the individual values ​​for each phase and expressed by the formula:

Provided that the network is evenly loaded, the power consumed by each of them is defined as follows: . That is, this value in a separate phase is found using the product of the corresponding voltages and currents by the cosine of the phase angle.

And since the load is distributed equally to each phase, then the power characteristics separately will be equal to each other. As a result, the power of a three-phase network in this situation can be found by multiplying this value by 3, calculated for a separate phase: .

Star connection

The use of such a circuit when connecting the phases makes it possible to balance the system and obtain the total voltage at the point of their intersection N equal to zero. In the case of a star connection, the three-phase current is characterized by two types of voltages: phase and line. The phase voltage is measured between one of the phases (A, B or C) and the zero point N, and the linear voltage shows the value of the potential difference between the two phases (A-B, B-C or A-C).

The relationship between line and phase voltages and currents with such a connection scheme is as follows: and .

And consequently, the general power characteristic is found by the formula:.

Connection diagram delta

When connecting loads in a three-phase circuit according to the "triangle" principle, the values ​​of the linear and phase voltages will be the same, and the magnitudes of the current strength (linear and phase) will be related by the ratio:.

The resulting formula for the power of a 3-phase current with a uniform load on each phase in this connection will look like .

Power measurement

To measure the power of three-phase circuits, wattmeters, special devices designed for this purpose, allow. Their number and connection methods depend on the specific electrical circuit: its characteristics and load connection diagrams. Three-phase networks are distinguished by the number of supply wires and the distribution of the load over the phases, namely:

• three-wire system;
• four-wire system;
• uniform load;
• asymmetric load.

Depending on the variant of the combination of the system and the load, the method for measuring the power in the electrical network is determined.

Symmetrical load

If the system consists of four wires (3 phases and "zero"), and the load is evenly distributed between phases, then in order to find out the total power value, it is enough to have one device for measurements. The current winding of the wattmeter is connected in series to one of the linear wires, and the voltage winding of the measuring device is connected between the linear and neutral wires. This type of connection makes it possible to find out the number of watts per phase. And since the load in the system is evenly distributed, the resulting power of the three-phase network is found by multiplying the readings obtained by the number of phases, that is, by 3.

In the case of a three-wire system, the voltage winding of the measuring device is connected to the line voltage of the mains, and its current winding passes the linear electric current through itself. Therefore, the total power of the network will be greater than the readings of the wattmeter in once.

Uneven distribution of consumers

Circuits with unbalanced phase loads require the use of several wattmeters to determine the power characteristic. In a system consisting of four wires, three devices must be connected in such a way that the voltage windings of each are connected between the neutral wire and one of the phases. The overall result is found by summing the individual readings of each wattmeter.

A three-wire system will require a minimum of two wattmeters to determine the power of the entire circuit. The voltage windings of each individual wattmeter are connected to the input current clamp and the remaining free line wire. The readings obtained are added and the value of this quantity is obtained for a three-phase circuit. This wiring diagram for measuring instruments is based on the first Kirchhoff's law.

Such nuances are very important when designing a three-phase network for the private sector. And also they should be taken into account when properly servicing existing power supply systems.