Turbine compressor rotor

As is known, three-phase asynchronous electric (el) motors with a squirrel-cage rotor are connected in a star or delta circuit, depending on the line voltage for which each winding is designed.

When starting especially powerful electric power. motors connected in a delta circuit exhibit increased starting currents, which in overloaded networks create a temporary voltage drop below the permissible limit.

This phenomenon is due to the design features of asynchronous electrical systems. engines in which the massive rotor has a fairly large inertia, and when it spins up, the motor operates in overload mode. Starting an electric motor becomes more difficult if there is a load with a large mass on the shaft - rotors of turbine compressors, centrifugal pumps or mechanisms of various machine tools.

Method for reducing starting currents of an electric motor

To reduce current overloads and voltage drops in the network, a special method is used to connect three-phase electricity. engine, which switches from star to delta as the speed increases.

Connection of motor windings: star (left) and triangle (right)

When connecting star-connected windings of a motor designed for triangle connection to a three-phase network, the voltage applied to each winding is 70% less than the nominal value. Accordingly, the current when starting the electric the engine will be smaller, but it should be remembered that the starting torque will also be smaller.

Therefore, star-delta switching cannot be used for electric motors that initially have a non-inertial load on the shaft, such as the weight of a winch load or the resistance of a piston compressor.

Switching modes on an electric motor mounted on a piston compressor is unacceptable.

To operate as part of such units, which have a large load at the time of start-up, special three-phase electric power is used. motors with wound rotor, in which starting currents are regulated using rheostats.

Star-delta switching can only be used for electric motors that have a freely rotating load on the shaft - fans, centrifugal pumps, shafts of machine tools, centrifuges and other similar equipment.

Centrifugal pump with asynchronous electric motor

Implementation of changing motor winding connection modes

It is obvious that in order to start a three-phase electric motor in star mode with subsequent switching to a delta connection of the windings, it is necessary to use several three-phase contactors in the starter.

Set of contactors in the starter for star-delta switching

In this case, it is necessary to ensure that the simultaneous operation of these contactors is blocked, and a short-term switching delay must be provided so that the star connection is guaranteed to turn off before the delta switches on, otherwise a three-phase fault will occur. short circuit.

Therefore, the time relay (RT), which is used in the circuit to set the switching interval, must also provide a delay of 50-100 ms so that a short circuit does not occur.

Methods for implementing switching delay

Mode switching time diagram

There are several principles for implementing delay using:

Manual mode switch

Classic scheme

This system is quite simple, unpretentious and reliable, but has a significant drawback, which will be described below and requires the use of a bulky and obsolete time relay.

This RF provides a delayed shutdown due to the magnetized core, which takes some time to demagnetize.

Electromagnetic time delay relay

It is necessary to mentally walk through the current flow paths to understand the operation of this circuit.

Classic mode switching scheme with current and time relays

After turning on the three-phase automatic switch AV, the starter is ready for operation. Through the normally closed contacts of the “Stop” button and the contact of the “Start” button closed by the operator, current flows through the coil of the KM contactor. The power contacts of the KM are kept in the on state by “self-retaining”, thanks to the BKM contact.

In the fragment of the above diagram, the red arrow indicates the bypass contact

The KM relay is necessary to enable the engine to be turned off with the “Stop” button. The pulse from the “Start” button also passes through the normally closed BKM1 and RV, starting the contactor KM2, the main contacts of which supply voltage to the star-type connection of the windings - the rotor is spun up.

Since at the moment of starting KM2 the contact BKM2 opens, then KM1, which ensures that the delta connection of the windings is turned on, cannot work in any way.

Contactors providing star (KM2) and delta (KM1) connection

Starting current overloads. the engine is forced to almost instantly operate the RT connected to the circuits of current transformers TT1, TT2. In this case, the control circuit of the KM2 coil is shunted by the RT contact, blocking the operation of the RF.

Simultaneously with the launch of KM2, with the help of its additional normally open contact BKM2, a time relay is started, the contacts of which are switched, but KM1 does not operate, since BKM2 is open in the circuit of the KM1 coil.

Turning on the time relay - green arrow, switching contacts - red arrows

As the speed increases, the starting currents decrease and the PT contact in the KM2 control circuit opens. Simultaneously with the disconnection of the power contacts that provide power to the star connection of the windings, BKM2 is closed in the KM1 control circuit and BKM2 is opened in the RV power circuit.

But, since the PB turns off with a delay, this time is enough for its normally open contact in the KM1 circuit to remain closed, due to which KM1 self-pickup occurs, connecting the windings in a triangle.

Normally open self-retaining contact KM1

Disadvantage of the classical scheme

If, due to incorrect calculation of the load on the shaft, it cannot gain momentum, then the current relay in this case will not allow the circuit to switch to delta mode. Long-term operation of electric An asynchronous motor in this mode of starting overload is extremely undesirable; the windings will overheat.

Overheated motor windings

Therefore, to prevent the consequences of an unexpected increase in load when starting a three-phase electric. engine (worn bearing or foreign objects entering the fan, contamination of the pump impeller), you should also connect a thermal relay to the electrical power supply circuit. engine after the KM contactor (not indicated on the diagram) and install the temperature sensor on the casing.

Appearance and main components of the thermal relay

If a timer (modern RV) is used to switch modes, which occurs within a set time interval, then when the motor windings are turned on in a triangle, the rated speed is set, provided that the load on the shaft meets the technical conditions of the electric motor.

Switching modes using a modern time relay CRM-2T

The operation of the timer itself is quite simple - first the star contactor is turned on, and after the adjustable time has elapsed, this contactor is turned off, and with some also adjustable delay the delta contactor is turned on.

Correct specifications for using switching winding connections.

When starting any three-phase electric. The most important condition must be met - the load resistance torque must always be less than the starting torque, otherwise the electric motor simply will not start, and its windings will overheat and burn out, even if the star starting mode is used, at which the voltage is lower than the nominal one.

Even if there is a freely rotating load on the shaft, the starting torque when connected by a star may not be enough and the electric current may not be sufficient. the engine will not reach the speed at which it should switch to delta mode, since the resistance of the medium in which the mechanisms of the units rotate (fan blades or pump impeller) will increase as the rotation speed increases.

In this case, if the current relay is excluded from the circuit, and mode switching is carried out according to the timer setting, then at the moment of transition to the triangle, the same current surges will be observed of almost the same duration as when starting from a stationary rotor state.

Comparative characteristics direct and transient engine starts with a load on the shaft

Obviously, such a star-delta connection will not give any positive results if the starting torque is incorrectly calculated. But at the moment the contactor providing the star connection is turned off, at insufficient engine speed, due to self-induction, there will be a surge of increased voltage into the network, which can damage other equipment.

Therefore, when using star-delta switching, it is necessary to ensure the feasibility of such a connection of a three-phase asynchronous electric power supply. engine and recheck the load calculations.

Asynchronous motors offer many operational advantages. This is reliability high power, good performance. Connecting the electric motor with star and delta ensures its stable operation.

There are two main parts to an electric motor: a rotating rotor and a static stator. Both have a set of conductive windings in their structure. The electrical windings of the stationary element are located in the grooves of the magnetic wire at a distance of 120 degrees. All ends of the windings are output to the electrical distribution block and are fixed there. Contacts are numbered.

Motor connections can be star, delta, as well as all kinds of switching. Each connection has its own advantages and disadvantages. Motors connected in a star configuration have smooth, soft operation; the action of the electric motor is limited by power compared to a triangle, since its value is one and a half times greater.

An association V one general point: star connection

The ends of the stator windings are connected together at one point. Three phase voltage goes to the beginning of the windings. The value of inrush currents when connecting a triangle is more powerful. A star connection means a connection between the ends of the stator winding. Voltage is supplied to the beginning of each winding.

The windings are connected in a series closed cell, forming a triangular connection. The rows of contacts with terminals are located parallel to each other. For example, the beginning of pin 1 is opposite the end of 1. The network power is supplied to the stator windings, creating rotation magnetic field, leading to rotor movement. The torque generated after connecting a three-phase electric motor is insufficient for starting. An increase in the rotating element is achieved by using an additional element. For example, a three-phase frequency converter connected to an asynchronous motor in the figure below.

Connection drawing of a classic frequency converter with a star

According to this scheme, domestic 380 volt motors are connected.

Mixed way

The combined type of connection is suitable for electric motors with a power of 5 kW or more. The star-delta circuit is used when it is necessary to reduce the starting currents of the unit. The principle of operation begins with a star, and after the engine reaches the required speed, it occurs automatic switching to the triangle.

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This scheme not suitable for devices with overloads, as a weak torque occurs, which can lead to breakdown.

Principle work

The power supply starts using the second and relay contact. Then the third starter is triggered on the stator, thereby opening the circuit formed by the coil of the third element, and a short circuit occurs in it. Next, the first stator winding begins to work. Then a short circuit occurs in the magnetic starter, a temporary thermal relay is triggered, which closes at the third point. Next, the contact of the temporary thermal relay in the electrical circuit of the second stator winding is observed to close. After disconnecting the windings of the third element, the contacts in the chain of the third element are closed.

To the beginning of the windings, current passes in three phases. It enters through the power contacts of the magnet of the first element. The contacts of the third starter turn it on and close the ends of the windings, which are connected by a star.

Then the time relay of the first starter is turned on, the third is turned off, and the second is turned on. Contacts K2 are closed, voltage is supplied to the ends of the windings. This is the inclusion of a triangle.

Various manufacturers make the start relay needed to start an electric motor. They differ in appearance and name, but perform the same function.

Typically, the connection to network 220 occurs with a phase-shifting capacitor. Power comes from any electrical network and rotates the rotor at the same frequency. Of course, the power from a three-phase network will be greater than from a single-phase one. If a three-phase motor operates from a single-phase network, power is lost.

Some types of motors are not designed to operate from a household network. Therefore, when choosing a device for your home, preference should be given to motors with squirrel-cage rotors.

Based on rated power, domestic electric motors are divided into two types: 220 - 127 volts and 380 - 220 volts. The first type of low-power electric motors is used infrequently. The second devices are widespread.

When installing an electric motor of any power, a certain principle applies: devices with low power are connected in a triangle, and devices with high power are connected in a star. The 220 power supply goes to the delta connection, the 380 voltage goes to the star connection. This will ensure a long and quality work mechanism.

The recommended diagram for connecting the motor is listed in the technical document. The △ icon means a connection in the same form. The letter Y indicates the recommended star connection. The characteristics of numerous elements are indicated by colors, due to their small dimensions. For example, the denomination or resistance can be read by color. If both signs are present, then the connection is possible by switching △ and Y. When there is one specific marking, for example, Y, then the connection available will only be in a star configuration.

Circuit △ provides output power up to 70 percent, the value of inrush currents reaches the maximum value. And this can ruin the engine. This circuit is the only option for operating foreign asynchronous motors with a power of 400 - 690 volts from Russian electrical networks.

Therefore, choosing the correct connection or switching must take into account the features electrical network, electric motor power. In each case, you should familiarize yourself with technical characteristics motor and equipment for which it is intended.

There are several types of electric motors - three-phase and single-phase. The main difference between three-phase electric motors and single-phase ones is that they are more efficient. If you have a 380 V outlet at home, then it is best to buy equipment with a three-phase electric motor.

Using this type of engine will allow you to save on electricity and gain more power. Also you won't have to use various devices to start the engine, since thanks to a voltage of 380 V, a rotating magnetic field appears immediately after connecting to the electrical network.

380 volt electric motor wiring diagrams

If you do not have a 380 V network, then you can still connect a three-phase electric motor to a standard 220 V electrical network. To do this, you will need capacitors, which must be connected according to this diagram. But when connected to a regular power grid, you will observe a loss of power. You could read about this.

380 V electric motors are designed in such a way that they have three windings in the stator, which are connected like a triangle or star, and three different phases are connected to their tops.

You need to remember that using a star connection, your electric motor will not work at full power, but it will start smoothly. When using a triangle circuit, you will receive a power increase of one and a half times compared to a star, but with such a connection the chance of damaging the winding during startup increases.

Before using an electric motor, you must first become familiar with its characteristics. All necessary information can be found in the data sheet and on the engine nameplate. Particular attention should be paid to three-phase motors of Western European type, as they are designed to operate at a voltage of 400 or 690 volts. In order to connect such an electric motor to domestic networks, it is necessary to use only a triangle connection.

If you want to make a triangle circuit, then you need to connect the windings in series. You need to connect the end of one winding to the beginning of the next and then you need to connect three phases of the electrical network to the three connection points.
Connecting a star-delta circuit.

Thanks to this circuit, we can get maximum power, but we will not have the opportunity to change the direction of rotation. In order for the circuit to work, three starters will be needed. The first (K1) is connected to power on one side, and the ends of the windings are connected to the other. Their origins are connected to K2 and K3. From the K2 starter, the windings are connected to other phases using a triangle connection. When K3 turns on, all three phases are short-circuited and, as a result, the electric motor operates in a star circuit.

It is important that K2 and K3 are not started at the same time, as this can lead to an emergency shutdown. This scheme works as follows. When K1 starts, the relay temporarily turns on K3 and the engine starts as a star. After starting the engine, K3 is turned off and K2 is started. And the electric motor starts working in a triangle pattern. Stopping work occurs by turning off K1.

Each stator of a three-phase electric motor has three coil groups (windings) - one for each phase, and each coil group has 2 terminals - the beginning and end of the winding, i.e. There are only 6 pins that are signed as follows:

• C1 (U1) is the beginning of the first winding, C4 (U2) is the end of the first winding.
• C2 (V1) is the beginning of the second winding, C5 (V2) is the end of the second winding.
• C3 (W1) is the beginning of the third winding, C6 (W2) is the end of the third winding.

Conventionally, in the diagrams, each winding is depicted as follows:

The beginnings and ends of the windings are brought out into the terminal box of the electric motor in the following order:

The main winding connection diagrams are triangle (denoted by Δ) and star (denoted by Y), which we will analyze in this article.

Note: In the terminal box of some electric motors you can only see three outputs- this means that the motor windings are already connected inside its stator. As a rule, the windings inside the stator are connected when repairing an electric motor (if the factory windings are burned out). In such motors, the windings are usually connected in a star configuration and are designed for connection to a 380 Volt network. To connect such a motor, you simply need to supply three phases to its three outputs.

1. Connection diagram of electric motor windings according to the “triangle” diagram

To connect the windings of an electric motor according to the “triangle” diagram, it is necessary: ​​connect the end of the first winding (C4/U2) to the beginning of the second (C2/V1), the end of the second (C5/V2) to the beginning of the third (C3/W1), and the end of the third windings (C6/W2) - with the beginning of the first (C1/U1).

Voltage is applied to terminals “A”, “B” and “C”.

In the terminal box of the electric motor, the connection of the windings according to the “triangle” diagram has the following form:

A, B, C—connection points for the power cable.

1. Connection diagram of electric motor windings according to the “star” scheme

To connect the windings of an electric motor in a star configuration, it is necessary to connect the ends of the windings (C4/U2, C5/V2 and C6/W2) to a common point, while voltage is applied to the beginning of the windings (C1/U1, C2/V1 and C3/W1 ).

Conventionally, this is depicted in the diagram as follows:

In the terminal box of the electric motor, the star connection of the windings has the following form:

1. Definition of winding terminals

Sometimes situations arise when, after removing the cover from the terminal box of an electric motor, you are horrified to discover the following picture:

In this case, the winding terminals are not labeled, what should I do? Don't panic, this issue can be completely resolved.

The first thing to do is to divide the leads into pairs, each pair should have leads related to one winding, this is very easy to do, we will need a tester or a two-pole voltage indicator.

If using a tester, set its switch to the resistance measurement position (underlined by a red line); when using a bipolar voltage indicator, before use, it is necessary to touch the live parts under voltage for 5-10 seconds to charge it and check its functionality.

Next, you need to take any one terminal of the winding, conditionally take it as the beginning of the first winding and accordingly sign it “U1”, then touch the “U1” terminal we signed with one tester or voltage indicator probe, and touch with the second probe any other terminal from the remaining five unsigned ends. If, having touched the second terminal with the second probe, the tester readings have not changed (the tester shows one) or in the case of the voltage indicator - not a single light comes on - we leave this end and touch the other terminal of the remaining four ends with the second probe, and touch the ends with the second probe to until the tester readings change, or, in the case of a voltage indicator, until the “Test” light comes on. Having found the second terminal of our winding in this way, we accept it conditionally as the end of the first winding and sign it “U2” accordingly.

We proceed in the same way with the remaining four pins, also dividing them into pairs and signing them respectively as V1, V2 and W1, W2. You can see how this is done in the video below.

Now that all the pins are divided into pairs, it is necessary to determine the actual beginnings and ends of the windings. This can be done in two ways:

The first and simplest method is the selection method, which can be used for electric motors with a power of up to 5 kW. To do this, we take our conditional ends of the windings (U2, V2 and W2) and connect them, and briefly, preferably no more than 30 seconds, apply three-phase voltage to the conditional beginnings (U1, V1 and W1):

If the engine starts and runs normally, then the beginnings and ends of the windings are determined correctly; if the engine hums a lot and does not develop the proper speed, then there is an error somewhere. In this case, you just need to swap any two terminals of one winding, for example U1 with U2, and start again.

Since they have high reliability - the simplicity of the design allows you to increase the engine life. With commutator motors, from the point of view of connecting to the network, things are simpler - you don’t need any additional devices for start. Asynchronous machines need a capacitor bank or a frequency converter if they need to be connected to a 220 V network.

How the motor is connected to a three-phase 380 V network

Three-phase asynchronous motors have three identical windings; they are connected according to a certain circuit. There are only two schemes for connecting the windings of electric motors:

1. Star.
2. Triangle.

By connecting the windings in a delta pattern, maximum power can be achieved. But at the start-up stage, large currents arise, which pose a danger to equipment.

If you connect in a star configuration, the engine will start smoothly, since the currents are low. True, with such a connection it will not be possible to achieve high power. If you pay attention to these points, it will become clear why electric motors, when connected to a 220 V household network, are connected only in a star configuration. If you choose a “triangle” circuit, the probability of failure of the electric motor increases.

In some cases, when it is necessary to achieve a high power rating from the drive, a combined connection is used. The start-up is carried out with the windings connected in a “star”, and then a transition to a “triangle” is carried out.

Star and triangle

Regardless of which 380 to 220 V you choose, you need to know the design features of the motor. Please note that:

1. There are three stator windings, which each have two terminals - a beginning and an end. They are led out into the contact box. Using jumpers, the terminals of the windings are connected according to the “star” or “delta” circuits.
2. In a 380 V network there are three phases, which are designated by the letters A, B and C.

In order to make a star connection, you need to short-circuit all the beginnings of the windings together.

And the ends are supplied with 380 V power. You need to know this when connecting a 380 to 220 Volt electric motor. To connect the windings in a “triangle” pattern, it is necessary to close the beginning of the coil with the end of the adjacent one. It turns out that you connect all the windings in series, forming a kind of triangle, to the vertices of which power is connected.

Transitional switching circuit

In order to smoothly start a three-phase electric motor and obtain maximum power, it is necessary to turn it on in a star configuration. As soon as the rotor reaches the rated speed, switching occurs and switches to delta switching. But such a transition circuit has a significant drawback - it cannot be reversed.

When using a transition circuit, three magnetic starters are used:

1. The first makes the connection between the initial ends of the stator windings and the power phases.
2. A second starter is required for delta connection. It is used to connect the ends of the stator windings.
3. Using the third starter, the ends of the windings are connected to the supply network.

In this case, the second and third starters cannot be put into operation at the same time, as a short circuit will appear. Hence, circuit breaker installed in the panel will turn off the power supply. To prevent the simultaneous activation of two starters, an electrical interlock is used. In this case, it is possible to turn on only one starter.

How the transition circuit works

Features of the functioning of the transition circuit:

1. The first magnetic starter is turned on.
2. The time relay is started, which allows the third magnetic starter to be put into operation (the engine with windings connected in a star configuration is started).
3. After the time specified in the relay settings, the third starter is switched off and the second starter is put into operation. In this case, the windings are connected in a “triangle” circuit.

In order to stop operation, you need to open the power contacts of the first starter.

Features of connecting to a single-phase network

When used, it will not be possible to achieve maximum power. In order to connect a 380 to 220 electric motor with a capacitor, you need to follow several rules. And the most important thing is to correctly select the capacitance of the capacitors. True, in this case, the motor power will not exceed 50% of the maximum.

Please note that when the electric motor is connected to a 220 V network, even if the windings are connected in a delta pattern, the currents will not reach a critical value. Therefore, it is permissible to use this scheme, even more so - it is considered optimal when operating in this mode.

Connection diagram for 220 V network

If power is supplied from a 380 network, then a separate phase is connected to each winding. Moreover, the three phases are shifted relative to each other by 120 degrees. But in the case of connecting to a 220 V network, it turns out that there is only one phase. True, the second one is zero. But with the help of a capacitor, the third one is made - a shift is made by 120 degrees relative to the first two.

Please note that the easiest way to connect a motor designed to be connected to a 380 V network is to connect it to 220 V using capacitors only. There are two more methods - using a frequency converter or another one. But these methods increase either the cost of the entire drive or its dimensions.

Working and starting capacitors

When starting an electric motor with a power below 1.5 kW (provided that there is no load on the rotor at the initial stage), only a working capacitor is allowed. Connecting an electric motor 380 to 220 without a start capacitor is possible only under this condition. And if the rotor is exposed to a load and motor power of more than 1.5 kW, it is necessary to use a starting capacitor, which must be turned on for a few seconds.

The working capacitor is connected to the zero terminal and to the third vertex of the triangle. If you need to reverse the rotor, then you just need to connect the capacitor output to the phase, and not to zero. The starting capacitor is turned on using a button without a latch in parallel with the working one. It participates in the work until the electric motor accelerates.

To select a working capacitor when turning on the windings according to the “delta” circuit, you need to use the following formula:

The starting capacitor is selected empirically. Its capacity should be approximately 2-3 times greater than that of a worker.