Phase and line voltage in three-phase circuits

Electricity is supplied to cities, businesses and dwellings using a three-phase network. It so happened historically that three-phase AC machines are used to generate electricity and consume it (in electrical installations). This amount was chosen to minimize the cost of creating a rotating magnetic field or using this energy to generate electricity. There are also specific 6-phase generators, in cars for example, but there they are needed for other purposes. In this article, we will talk about what the phase and line voltage in three-phase circuits are, how they are connected and what is the difference.

Alternating voltage and its magnitudes

Voltage is distinguished by the nature of the current: alternating and direct. A variable can be of different shapes, the main point is that its sign and magnitude change over time. A constant has a sign of always the same polarity, and the value can be stabilized or unstabilized.

In our sockets, the voltage is alternating sinusoidal. Its different meanings are distinguished, most often the concepts of instantaneous, amplitude and acting are used. As the name implies, instantaneous voltage is the number of volts at a given moment in time. Amplitude is the swing of the sinusoid relative to zero in volts, the effective is the integral of the function voltage over time, the ratio between them is as follows: acting √2 or 1.41 times less amplitude. This is how it looks on the graph:

Voltage in three-phase circuits

In three-phase circuits, two types of voltage are distinguished - linear and phase. To make out their differences, you need to take a look at the vector diagram and graph. Below you see three vectors Ua, Ub, Uc - these are voltage or phase vectors. The angle between them is 120 °, sometimes they say 120 electrical degrees. This angle corresponds to that in the simplest electrical machines between the windings (poles).

If you reflect the vector Ub so that its inclination angle is preserved, but the beginning and end are reversed, its sign will change to the opposite. Then we will set the beginning of the vector –Ub to the end of the vector Ua, the distance between the beginning of Ua and the end of –Ub will correspond to the vector of the line voltage Ul.

In simple words, we see that the value of the line voltage is greater than the phase voltage. Let's take a look at the voltage graph in a three-phase network.

The red vertical line marks the line voltage between phase 1 and phase 2, and the yellow line marks the phase amplitude of phase 2.

BRIEFLY:Line voltage is measured between phase and phase, and phase voltage is measured between phase and zero.

From the point of view of calculations, the difference between the stresses is determined by the solution of this formula:

The line voltage is √3 or 1.73 times greater than the phase voltage.

The load to a three-phase network can be connected via three or four wires. The fourth conductor is zero (neutral). Depending on the type, the network can be with an insulated neutral and a solidly earthed one. In general, with a uniform load, three phases can be supplied without a neutral wire. It is needed so that voltages and currents are distributed evenly and there is no phase imbalanceand also as a protective one. In solid-grounded networks, in case of a breakdown on the case, the circuit breaker will knock out or the fuse in the switchboard will blow, so you will avoid the danger of electric shock.

The great thing is that in such a network we simultaneously have two voltages that can be used based on the load requirements.

For example: pay attention to the electrical panel at the entrance of your house. Three phases come to you, and one of them and zero is brought into the apartment. Thus, you get 220V (phase) in the sockets, and 380V (linear) between the phases at the entrance.

Diagrams for connecting consumers to three phases

All motors, high-power heaters and other three-phase loads can be connected in a star or delta connection. Moreover, most electric motors in borno have a set of jumpers, which, depending on their position, form a star or triangle from the windings, but more on that later. What is star connection?

Star connection involves connecting the generator windings in such a way when the ends of the windings are connected to one point, and the load is connected to the beginning of the windings. The windings of the motor and powerful heaters are also connected by a star, only instead of windings, heating elements act in them.

Let's think about the example of an electric motor. When connecting its windings with a star, a line voltage of 380 V is applied to two windings, and so on with each pair of phases.

In the figure, A, B, C are the beginning of the windings, and X, Y, Z are the ends connected at one point and this point is grounded. Here you see a network with a solidly grounded neutral (N wire). In practice, it looks like the photo of the electric motor boron:

The ends of the windings are highlighted in a red square, they are interconnected by jumpers, such an arrangement of the jumpers (in a line) indicates that they are connected by a star. In blue - supplying three phases.

In this photo, the beginnings (W1, V1, U1) and ends (W2, V2, U2) are marked, note that they are shifted relative to the origins, this is necessary for a convenient connection into a triangle:

When connected in a delta, line voltage is applied to each winding, this leads to the fact that large currents flow. The winding must be dimensioned for this connection.

Each of the methods of switching on has its own advantages and disadvantages, some motors generally switch from a star to a delta during the start-up process.

Nuances

Continuing the conversation about motors, the question of choosing a switching circuit cannot be ignored. The fact is that usually the engine on its nameplate contains the marking:

On the first line, you see the legend for the triangle and the star, notice that the triangle comes first. Further, 220 / 380V is the voltage across the triangle and the star, which means that when connecting with a triangle, it is necessary that the line voltage be equal to 220V. If your network voltage is 380, then you need to connect the motor to a star. While the phase is always 1.73 less, regardless of the value of the linear.

The following engine is a great example:

Here, the nominal voltages are already 380/660, which means that for the linear 380 it needs to be connected with a triangle, and the star is designed to be powered from three 660V phases.

If in powerful loads they often operate with the values ​​of the interphase voltage, then in lighting circuits in 99% of cases, phase voltage is used (between phase and zero). The exception is electric cranes and the like, where a transformer with secondary windings with a linear 220 V can be used. But these are rather subtleties and specificity of specific devices. It is easier for beginners to remember this: the phase voltage is that which is in the outlet between phase and zero, linear - in the line.

You probably don't know:

• How to make 380 out of 220 volts
• How to assemble a three-phase electrical panel
• How to distribute the load across phases