Three-phase network: power calculation, wiring diagram

Not every man in the street understands what electrical circuits are. In apartments, they are 99% single-phase, where current flows to the consumer by one wire, and returns on the other (zero). The three-phase network is a transmission system of an electric current that flows through three wires with a return one at a time. Here, the return wire is not overloaded due to the phase current shift. Electricity is generated by a generator driven by an external drive.

An increase in the load in the circuit leads to an increase in the current strength passing through the windings of the generator. As a result, the magnetic field is more resistant to the rotation of the drive shaft. The number of revolutions starts to decrease, and the speed control regulator commands to increase the drive power, for example by feeding more fuel to the internal combustion engine. The number of turns is restored, and more electricity is generated.

The three-phase system is 3 circuits with an EMF of the same frequency and a phase shift of 120 °.

Features of connecting power to a private house

Many believe that the three-phase network in the house increases the power consumption. In fact, the limit is set by the power supply organization and is determined by the factors:

• The capabilities of the supplier;
• Number of consumers;
• Condition of the line and equipment.

To prevent voltage spikes and phase distortions, they should be loaded evenly. The calculation of the three-phase system is approximate, since it is impossible to determine exactly which devices will be connected at the moment. The presence of pulse devices now leads to increased power consumption during their start-up.

Distribution switchboard with three-phase connection is taken larger than for single-phase power. Variants are possible with the installation of a small opening flap, and the rest - from plastic to each phase and to outbuildings.

Connection to the main line is realized by underground method and by an overhead line. Preference is given to the latter due to the small amount of work, low cost of connection and convenience of repair.

Now the air connection is convenient to do with a self-supporting insulated wire (SIP). The minimum cross-section of the aluminum core is 16 mm ² , which is enough for a private house with a large margin.

SIP is fixed on the supports and the wall of the house with the help of anchor brackets with clamps. The connection with the main air line and the cable of the input into the electrical board of the house is made by tapping piercing clamps. The cable is taken with non-flammable insulation (ВВГнг) and is conducted through a metal pipe inserted into the wall.

Air connection of three-phase power supply at home

At a distance from the nearest support more than 15 m, one more post is needed. This is necessary to reduce the load resulting in sagging or wire breakage.

The height of the connection point is 2.75 m and higher.

Electricity distribution cabinet

Connection to a three-phase network is carried out under the project, where the house is divided into groups of consumers:

• lighting;
• Sockets;
• Separate powerful devices.

Some loads can be turned off for repair while others are working.

The power of consumers is calculated for each group, where the wire of the required cross-section is selected: 1.5 mm ² - for illumination, 2.5 mm ² - for sockets and up to 4 mm ² - for powerful devices.

Wiring is protected against short circuit and overload by circuit breakers.

Electric meter

In any connection scheme, a power consumption meter is required. The 3-phase meter can be connected directly to the mains (direct connection) or via a voltage transformer (semi-indirect), where the instrument reading is multiplied by a factor.

It is important to follow the connection procedure, where odd numbers are power, and even loads. The color of the wires is indicated in the description, and the circuit is placed on the back cover of the device. The input and the corresponding output of the 3-phase counter are indicated in one color. The most common order of joining, when the phases first go, and the last wire - zero.

A 3-phase direct switch for a home is usually designed for power up to 60 kW.

Before choosing a multi-tariff model, you should agree on the issue with the power supply company. Modern devices with charging devices make it possible to calculate the fee for electricity depending on the time of day, to register and record the power values in time.

The temperature characteristics of the instruments are chosen as widely as possible. On average, they range from -20 to +50 ° C. The service life of devices reaches 40 years with an inter-calibration interval of 5-10 years.

The meter is connected after an input three- or four-pole circuit breaker.

Three-phase load

Consumers include electric boilers, asynchronous electric motors and other electrical appliances. The advantage of using them is the uniform distribution of the load in each phase. If a three-phase network contains unevenly connected single-phase high-power loads, this can lead to phase distortion. At the same time, electronic devices start to malfunction, and the illumination lamps glow dimly.

Scheme of connecting a three-phase motor to a three-phase network

Operation of three-phase electric motors is characterized by high efficiency and efficiency. There is no need for additional launchers. For normal operation, it is important to properly connect the device and follow all recommendations.

The connection of a three-phase motor to a three-phase network creates a rotating magnetic field with three windings connected by a star or a triangle.

Each method has its advantages and disadvantages. The star circuit allows smooth start of the engine, but its power is reduced to 30%. This loss is not present in the triangle circuit, but at start-up the current load is much larger.

The engines have a connection box where the winding leads are located. If there are three, then the circuit is connected only by a star. If there are six pins, the motor can be connected in any way.

Power Consumption

It is important for a landlord to know how much energy is consumed. It is easy to calculate by all electrical appliances. Having combined all the capacities and dividing the result by 1000, we get the total consumption, for example 10 kW. For household electrical appliances, only one phase is sufficient. However, current consumption significantly increases in a private house, where there is a powerful technique. One device can have 4-5 kW.

It is important to plan the power consumption of the three-phase network at the design stage to ensure symmetry in voltages and currents.

In the house comes a four-wire wire for three phases and neutral. The voltage of the electrical network is 380/220 V. Between the phases and the neutral wire, electrical appliances are connected to 220 V. In addition, there can be three-phase load.

Calculation of the power of the three-phase network is carried out in parts. First, it is advisable to calculate purely three-phase loads, for example an electric boiler of 15 kW and an induction motor of 3 kW. The total power will be P = 15 + 3 = 18 kW. In the phase conductor, the current I = Px1000 / (√3xUxcosφ) flows. For household electrical systems cosφ = 0,95. Substituting the numerical values in the formula, we obtain the value of the current I = 28.79 A.

Now single-phase loads should be determined. Suppose that for phases they are P _A = 1.9 kW, P _B = 1.8 kW, P _C = 2.2 kW. The mixed load is determined by summation and is 23.9 kW. The maximum current will be I = 10.53 A (phase C). Adding it to the current from the three-phase load, we get I _C = 39.32 A. The currents in the remaining phases will be I _B = 37.4 kW, I _A = 37.88 A.

In calculating the power of a three-phase network, it is convenient to use power tables with regard to the type of connection.

It is convenient for them to select protective devices and determine the cross-section of the wiring.

Conclusion

With proper design and maintenance, the three-phase network is ideal for a private home. It allows you to evenly distribute the load over the phases and connect additional power to the electrical consumers, if the cross section of the wiring permits.