Electrical wiring diagram for a delta double star motor. The main advantages of using the star scheme. Non-standard terminal blocks Brno

Windings of generators, transformers, electric motors and other electrical receivers when connected to three-phase network connected in two ways: star or triangle. These connection diagrams are very different from each other and carry different current loads. Therefore, there is a need to understand the question of how star and delta connections are made - what is the difference?

What are schemes?

Connecting the windings with a star is their connection at one point, which is called the zero point or neutral. It is designated by the letter "O".

The triangle connection diagram is serial connection the ends of the working windings, in which the beginning of one winding is connected to the end of another.

The difference is obvious. But what is the purpose of these types of connections, why are star-delta connections used in different electrical installations, what is the effectiveness of both. Many questions arise on this topic, and we need to sort them out.

Let's start with the fact that when starting the same electric motor, the current, which is called starting, has a high value, which exceeds its rated value by six or eight times. If it is a low-power unit, then the protection can withstand such a current, and if it is an electric motor high power, then no protective blocks will withstand. And this will definitely cause a “sag” in voltage and failure of fuses or circuit breakers. The engine itself will begin to rotate with low speed, different from the passport one. That is, there are many problems with starting current.

Therefore, it just needs to be reduced. There are several ways to do this:

install one of the following devices in the electric motor connection system: transformer, inductor, rheostat;
the connection diagram of the rotor windings changes.

It is the second option that is used in production as the simplest and most effective. The star-delta circuit is simply converted. That is, when starting the engine, its windings are connected in a star pattern, then as soon as the motor picks up speed, it switches to a triangle. The process of switching star to delta is automatic.

It is recommended that in electric motors where two connection options are used simultaneously - star-delta - to connect the windings according to the star circuit, that is, to their common connection point, connect the neutral from the power supply. Why is this necessary? The thing is that while working on this option connection, there is a high probability of asymmetry of the amplitudes of different phases. It is the neutral that will compensate for this asymmetry, which usually appears due to the fact that the stator windings may have different inductive reactance.

Advantages of the two schemes

The star scheme has quite serious advantages:

smooth start of the electric motor;
its rated power will correspond to the passport data;
the engine will operate normally both under short-term high loads and long-term low overloads;
During operation, the motor housing will not overheat.

As for the triangle circuit, its main advantage is that the electric motor achieves maximum power during its operation. But at the same time, it is recommended to strictly adhere to the operating modes that are described in the motor passport. Testing of electric motors connected in a delta configuration has shown that its power is three times greater than those connected in a star configuration.

If we talk about generators that supply current to the power supply network, then the star and delta connection schemes have their own technical parameters exactly the same. That is, the voltage output by the triangle will be greater, although not three times, but not less than 1.73 times. In fact, it turns out that the generator voltage at star, equal to 220 volts, is converted into 380 volts if you switch from one option to another. But it should be noted that the power of the unit itself remains unchanged, because everything obeys Ohm’s law, in which voltage and current are in inverse proportion. That is, an increase in voltage by 1.73 times reduces the current by exactly the same amount.

Hence the conclusion: if all six ends of the windings are located in the terminal box of the generator, then it will be possible to obtain voltages of two ratings that differ from each other by a factor of 1.73.

Drawing conclusions

Why are delta and star connections found in all modern high-power electric motors today? From all of the above, it becomes clear that the main requirement of the situation is to reduce the current load that occurs during the startup of the unit itself.

If you write down the formulas for such a connection, they will look like this:

Uph=Ul/1.73=380/1.73=220, where Uph is the voltage on the phases, Ul is on the supply line. This is a star connection.

After the electric unit accelerates, that is, its rotation speed corresponds to the passport data, a transition to a triangle from a star will occur. From here the phase voltage will become equal to the linear voltage.

Asynchronous motors have a whole range of undoubted advantages. Among the advantages asynchronous motors First of all, I would like to mention the high performance and reliability of their operation, absolutely low cost and unpretentiousness of engine repair and maintenance, as well as the ability to withstand fairly high mechanical overloads. All these advantages that asynchronous motors have are due to the fact that this type The engines have a very simple design. But, despite big number advantages, asynchronous motors also have certain negative aspects.

IN practical work It is customary to use two main methods of connecting three-phase electric motors to the electrical network. These connection methods are called: “star connection” and “delta connection”.

When a three-phase electric motor is connected using the star connection type, then the ends of the stator windings of the electric motor are connected at one point. In this case, three-phase voltage is supplied to the beginning of the windings. Below, in Figure 1, the connection diagram for a star asynchronous motor is clearly illustrated.

When a three-phase electric motor is connected using the “delta” connection type, then the stator windings of the electric motor are connected in series one after another. In this case, the beginning of the subsequent winding is connected to the end of the previous winding, and so on. Below, in Figure 2, the connection diagram of a delta asynchronous motor is clearly illustrated.

If you do not go into the theoretical and technical foundations of electrical engineering, then you can take for granted the fact that the operation of those electric motors whose windings are connected in a star configuration is softer and smoother than that of electric motors whose windings are connected in a delta configuration ". But here it is worth paying attention to the fact that electric motors, the windings of which are connected in a star configuration, are not capable of developing the full power declared in the passport characteristics. In the event that the connection of the windings is made according to the "triangle" scheme, then the electric motor operates at the maximum power, which is stated in the technical data sheet, but at the same time there are very high values starting currents. If we make a comparison in terms of power, then electric motors whose windings are connected in a delta configuration are capable of delivering power one and a half times higher than those electric motors whose windings are connected in a star configuration.

Based on all of the above, in order to reduce currents during startup, it is advisable to use a combined delta-star connection of the windings. This type of connection is especially relevant for electric motors with more power. Thus, in connection with the connection according to the " triangle-star"Initially the start is carried out according to the “star” scheme, and after the electric motor has “gained speed”, it switches to automatic mode according to the "triangle" pattern.

The electric motor control circuit is shown in Figure 3.

Rice. 3 Control circuit

Another option for the electric motor control circuit is as follows (Fig. 4).

Rice. 4 Engine control circuit

The NC (normally closed) contact of time relay K1, as well as the NC contact of relay K2, in the circuit of the short-circuit starter coil, is supplied with supply voltage.

After will turn on starter short circuit, normal closed contacts Short circuits disengage the circuits of the starter coil K2 (prohibition of accidental activation). The short circuit contact in the power supply circuit of the starter coil K1 closes.

When the magnetic starter K1 starts, the K1 contacts close in the power circuit of its coil. The time relay turns on at the same time, the contact of this relay K1 in the short circuit starter coil circuit opens. And in the starter coil circuit K2 it closes.

When the short circuit starter winding is disconnected, the short circuit contact in the starter coil circuit K2 will close. After the K2 starter turns on, it opens the power circuit of the short-circuit starter coil with its K2 contacts.

Three-phase supply voltage is supplied to the beginning of each of the windings W1, U1 and V1 using the power contacts of the starter K1. When the short-circuit magnetic starter is triggered, then with the help of its short-circuit contacts a short circuit is made, through which the ends of each of the electric motor windings W2, V2 and U2 are connected to each other. Thus, the motor windings are connected using a star connection.

The time relay combined with the magnetic starter K1 will operate after certain time,. This causes a shutdown magnetic starter short circuit and simultaneous activation magnetic starter K2. Thus, the power contacts of the starter K2 will be closed and the supply voltage will be supplied to the ends of each of the windings U2, W2 and V2 of the electric motor. In other words, the electric motor is switched on according to the “delta” connection diagram.

In order to start the electric motor using a delta-star connection, various manufacturers produce special starting relays. These relays can have various names, for example, “start-delta” relay or “start time relay”, as well as some others. But the purpose of all these relays is the same.

A typical circuit made with a time relay designed for starting, that is, a delta-star relay, to control the starting of a three-phase asynchronous type electric motor is shown in Figure 5.

Fig. 5 Typical circuit with a starting time relay (star/delta relay) for controlling the start of a three-phase asynchronous motor.

So, let's summarize all of the above. In order to reduce starting currents, starting the electric motor is required in a certain sequence, namely:

first, the electric motor is started at low speeds connected in a star configuration;
then the electric motor is connected in a delta pattern.

The initial start according to the "triangle" circuit will create the maximum torque, and the subsequent connection according to the "star" circuit (for which the starting torque is 2 times less) with continued operation in the nominal mode, when the engine has "picked up speed", there will be a switch to the "delta" connection circuit "in automatic mode. But do not forget about the load that is created on the shaft before starting, since the torque is weakened when connecting in a star configuration. For this reason it is unlikely that this method starting will be acceptable for electric motors with high load, since in this case they may lose their performance.

To increase the transmission power without increasing the network voltage, to reduce voltage ripple in power supplies, to reduce the number of wires when connecting the load to the power supply, they are used various schemes connections of windings of power sources and consumers (star and delta).

Scheme

The windings of generators and receivers when working with 3-phase networks can be connected using two circuits: star and triangle. Such circuits have several differences among themselves; they also differ in current load. Therefore, before connecting electrical machines, it is necessary to find out the difference in these two circuits - star and triangle.

Star diagram

Connecting various windings according to a star circuit involves connecting them at one point, which is called zero (neutral), and is designated in the diagrams “O”, or x, y, z. The neutral point may have a connection to the power supply neutral point, but not all cases have such a connection. If there is such a connection, then such a system is considered 4-wire, and if there is no such connection, then it is considered 3-wire.

Triangle diagram

With this scheme, the ends of the windings are not combined into one point, but are connected to another winding. That is, the result is a circuit similar in appearance to a triangle, and the windings in it are connected in series with each other. It should be noted that the difference from the star circuit is that in the delta circuit the system is only 3-wire, since there is no common point.

In a triangle circuit, with the load off and symmetrical, the EMF is 0.

Phase and linear quantities

In 3-phase power networks there are two types of current and voltage - phase and linear. Phase voltage is its value between the end and beginning of the receiver phase. Phase current flows in one phase of the receiver.

When using a star circuit, the phase voltages are U a , U b , U c, and the phase currents are I a, I b, I c. When using a delta circuit for load or generator windings, the phase voltages are - U aв, U bс, U са, phase currents – I ac, I bс, I са.

Linear voltage values are measured between the beginnings of phases or between linear conductors. Line current flows in the conductors between the power source and the load.

In the case of a star circuit, line currents are equal to phase currents, and line voltages are equal U ab, U bc, U ca. In a triangle circuit, everything turns out the other way around - phase and linear voltages are equal, and linear currents are equal I a, I b, I c.

Great importance is given to direction EMF voltage and currents in the analysis and calculation of 3-phase circuits, since its direction affects the relationship between the vectors in the diagram.

Features of the circuits

There is a significant difference between these schemes. Let's figure out why various electrical installations use different schemes, and what are their features.

When starting an electric motor, the starting current has an increased value, which is several times greater than its rated value. If this is a low-power mechanism, then the protection may not work. When you turn on a powerful electric motor, the protection will definitely work, turning off the power, which will cause a voltage drop for some time and blown fuses, or turn off the electrical circuit breakers. The electric motor will operate at a low speed, which is less than the rated speed.

It can be seen that there are many problems arising due to the high inrush current. It is necessary to somehow reduce its value.

To do this, you can use some methods:

Connect to start the electric motor, throttle, or.
Change the type of connection of the electric motor rotor windings.

In industry, the second method is mainly used, since it is the simplest and gives high efficiency. The principle of switching the windings of an electric motor to circuits such as star and triangle works here. That is, when the motor is started, its windings have a “star” connection, after a set of operating speeds, the connection diagram changes to “triangle”. They have learned to automate this switching process in industrial settings.

It is advisable to use two schemes at once - star and triangle. It is necessary to connect the neutral of the power source to the zero point, since when using such circuits there is an increased likelihood of phase amplitude distortion. The source neutral compensates for this asymmetry, which arises due to different inductive reactances stator windings.

Advantages of the schemes

Star connection has important advantages:

Smooth start of the electric motor.
Allows the electric motor to operate with the stated rated power, corresponding to the passport.
The electric motor will have normal operating mode when different situations: at high short-term overloads, at long-term minor overloads.
During operation, the motor housing will not overheat.

The main advantage of the triangle circuit is to obtain the highest possible operating power from the electric motor. It is advisable to maintain operating modes according to the engine passport. When studying electric motors with a triangle circuit, it turned out that its power increases 3 times compared to a star circuit.

When considering generators, the star and delta circuits are similar in parameters to the operation of electric motors. Output voltage There will be more generator in a delta circuit than in a star circuit. However, as the voltage increases, the current decreases, since according to Ohm's law these parameters are inversely proportional to each other.

Therefore, we can conclude that when different connections At the ends of the generator windings, two different voltage ratings can be obtained. In modern powerful electric motors, when starting the circuit, star and delta switch automatically, as this makes it possible to reduce the current load that occurs when starting the motor.

Processes that occur when the star and delta circuits change in different cases

Here, a change in the circuit - this means switching on switchboards and in terminal boxes electrical devices, provided that there are winding leads.

Generator and transformer windings

When moving from star to triangle, the voltage decreases from 380 to 220 volts, the power remains the same, since the phase voltage does not change, although the line current increases by 1.73 times.

When switching back, the opposite phenomena occur: the linear voltage increases from 220 to 380 volts, and the phase currents do not change, but the linear currents decrease by 1.73 times. Therefore, we can conclude that if there is an output of all ends of the windings, then secondary windings Transformers and generators can be used for two types of voltage, which differ by 1.73 times.

Lighting lamps

When moving from a star to a triangle, the lamps will burn out. If the switch is reversed, provided that the lamps at the triangle were lit normally, then the lamps will glow dimly. Without a neutral wire, lamps can be connected in a star, provided that their power is the same and is distributed evenly between the phases. This connection is used in theater chandeliers.

A three-phase electric motor is electric car, designed to work in alternating current. Such a motor consists of a stator and a rotor. The stator has three windings shifted by one hundred and twenty degrees. When three-phase voltage appears in the winding circuit, magnetic fluxes are formed at the poles and the rotor rotates. Electric motors are either synchronous or asynchronous. Three-phase ones are widely used in industry and in everyday life. Such motors can be single-speed, in which case the motor windings are connected in a star or delta pattern, and multi-speed. The latter units are switchable, in which case there is a transition from one connection scheme to another.

Three-phase electric motors are divided according to winding connection diagrams. There are two connection schemes - star and delta connections. Connecting the motor windings according to the “star” type is a connection of the ends of the motor windings at one point (zero node): an additional terminal is obtained - zero. The free ends are connected to the network phases electric current 380 V. Externally, this connection resembles a three-pointed star. The photo shows the following diagram: a “star” and “delta” connection. Connecting the windings of an electric motor according to the “delta” type is a winding: the end of the first is connected to the beginning of the second winding, the end of the second to the beginning of the third, and the end of the third to the beginning of the first. Three-phase voltage is supplied to the winding connection nodes. With this connection of the windings, there is no zero terminal. Outwardly, it resembles a triangle.

Star and delta connections are equally common and do not differ significantly. To connect the windings in a star type (when the engine is operating in rated mode), the line voltage must be greater than when connected in a delta type. Therefore, in the characteristics three phase motor indicate as follows: 220/380 V or 127/220 V. If necessary, the rated windings must be connected in a star type, and the rated voltage of the motor will be 380/660 V (delta type).

It should be noted that a combined star and delta connection is often used. This is done in order to start the electric motor more smoothly. When starting, a star connection is used, and then a special relay is used to switch to delta, thus reducing the starting current. Such circuits are recommended for starting high-power electric motors that require large starting current. It is important to remember that in this case the starting current exceeds the rated current by seven times.

There are other combinations when connecting electric motors, for example, a star and delta connection can be replaced by a double, triple star, as well as other connection options. Such methods are used for multi-speed (two-, four-, etc.) electric motors.

Today, high-power asynchronous electric motors are characterized by reliable operation and high performance, ease of operation and maintenance, as well as reasonable price. The design of this type of engine allows it to withstand strong mechanical overloads.

As is known from the basics of electrical engineering, the main parts of any engine are a static stator and a rotor rotating inside it.

Both of these elements consist of conductive windings, while the stator winding is located in the grooves of the magnetic core maintaining a distance of 120 degrees. The beginning and end of each winding are brought out into an electrical distribution box and installed in two rows.

When voltage is applied from a three-phase power supply to the stator windings, a magnetic field is created. This is what makes the rotor rotate.

An experienced electrician knows how to connect an electric motor correctly.

Connecting an asynchronous motor to electrical network is carried out only according to the following schemes: “star”, “triangle” and their combinations.

The choice of one connection or another depends on:

reliability of the power grid;
rated power;
technical characteristics of the engine itself.

Each connection has its own pros and cons in operation. The engine passport from the manufacturer, as well as on the metal label on the device itself, must indicate its connection diagram.

With a “Star” connection, all ends of the stator windings converge at a water point, and voltage is supplied to the beginning of each of them. Connecting the motor with a star guarantees a smooth, safe start of the unit, but at the initial stage there is a significant loss of load.

The “triangle” connection implies a serial connection of the windings in a closed structure, i.e. the beginning of the first phase is connected to the end of the second and. etc.

This connection gives output power up to 70% of the rated value, but in this case the starting currents increase significantly, which can cause breakdown of the electric motor.

There is also a combined star-delta connection (this Y/Δ symbol must appear on the motor housing). The presented circuit causes current surges at the moment of switching, which lead to the rotor speed quickly decreasing and then gradually returning to normal.

Combined circuits are relevant for electric motors with a power of over 5 kW.

Selection dependent on voltage

Nowadays, asynchronous systems are more applicable in industry. three-phase electric motors domestically produced, designed for a rated voltage of 220/380 V mains (127/220 V units are rarely used anymore).

The “triangle” connection diagram is the only correct one for connecting foreign electric motors with a rated voltage of 400-690 V to Russian power grids.

Connecting a three-phase motor of any power is carried out according to a certain rule: low-power units are connected in a “delta” configuration, and high-power units are connected only in a “star” configuration.

This way the electric motor will last a long time and work without failures.

The “star” method is used when connecting three-phase asynchronous motors with a rated voltage of 127/220 V to single-phase networks.

How to reduce starting currents of an electric motor?

The phenomenon of a significant increase in inrush currents when starting high-power devices connected according to the Δ circuit leads in networks with overload to a short-term voltage drop below permissible value. All this is explained by the special design asynchronous electric motor, in which the rotor with a large mass has high inertia. Therefore, at the initial stage of operation, the motor is overloaded, this is especially true for the rotors of centrifugal pumps, turbine compressors, fans, and machine tools.

To reduce the influence of all these electrical processes, they use a “star” and “delta” connection to the electric motor. When the engine picks up speed, the knives of a special switch (a starter with several three-phase contactors) transfers the stator windings from the Y circuit to the Δ circuit.

To implement mode changes, in addition to the starter, you need a special time relay, thanks to which there is a time delay of 50-100 ms when switching and protection against three-phase short circuits.

The procedure itself combined scheme Y/Δ effectively helps reduce the starting currents of powerful three-phase units. This happens as follows:

When a voltage of 660 V is applied according to the “triangle” circuit, each stator winding receives 380 V (√3 times less), and, therefore, according to Ohm’s law, the current strength decreases by 3 times. Therefore, when starting, the power in turn decreases by 3 times.

But such switching is possible only for motors with a rated voltage of 660/380 V when they are connected to a network with the same voltage values.

It is dangerous to connect an electric motor with a rated voltage of 380/220 V to a 660/380 V network; its windings can quickly burn out.

And also remember that the switching described above cannot be used for electric motors that have a load without inertia on the shaft, for example, the weight of a winch or the resistance of a piston compressor.

For such equipment, special three-phase electric motors with a wound rotor, where rheostats reduce the value of currents during startup.

To change the direction of rotation of the electric motor, it is necessary to swap any two phases of the network for any type of connection.

For these purposes, when operating an asynchronous electric motor, special manual control electrical devices are used, which include reversing switches and packet switches or more modernized devices remote control— reversible electromagnetic starters (switches).