In three-phase circuits, two types of connection of the windings of transformers, electrical receivers and generators are usually used. One of these connections is called a star, the other is called a triangle. Let's take a closer look at what these compounds are and how they differ from each other.
Definition
Star connection means a connection in which all the working ends of the phase windings are combined into one node, called the zero or neutral point and denoted by the letter O.
Triangle connection is a circuit in which the phase windings of a generator are connected in such a way that the beginning of one of them is connected to the end of the other.
Comparison
The difference in these schemes is the connection of the ends of the windings of the electric motor generator. IN star pattern, all ends of the windings are connected together, whereas in triangle pattern the end of one phase winding is mounted with the beginning of the next.
In addition to the basic assembly diagram, electric motors with star-connected phase windings operate much smoother than motors with delta-connected phase windings. But when connected by a star, the electric motor is not able to develop its full rated power. Whereas, when the phase windings are connected in a triangle, the engine always operates at its full declared power, which is almost one and a half times higher than when connected in a star. The big disadvantage of delta connection is the very large inrush currents.
Conclusions website
- In a star connection scheme, the ends of the windings are mounted in one unit.
- In a delta connection diagram, the end of one winding is mounted with the beginning of the next winding.
- An electric motor with star-connected windings runs smoother than a delta-connected motor.
- When connected by a star, the engine power is always lower than the rated value.
- When connected in a triangle, the engine power is almost one and a half times higher than when connected in a star.
The design of a three-phase electric motor is an electrical machine that requires three-phase alternating current networks for normal operation. The main parts of such a device are the stator and the rotor. The stator is equipped with three windings shifted between each other by 120 degrees. When three-phase voltage appears in the windings, magnetic fluxes are formed at their poles. Due to these flows, the engine rotor begins to rotate.
In industrial production and in everyday life, three-phase asynchronous motors are widely used. They can be single-speed, when the motor windings are connected by star and delta, or multi-speed, with the ability to switch from one circuit to another.
Star and delta connection of windings
All three-phase electric motors have windings connected in a star or delta configuration.
When connecting the windings in a star circuit, their ends are connected at one point in the zero node. Therefore, we get one more additional zero output. The other ends of the windings are connected to the phases of the 380 V network.
The delta connection consists of a series connection of windings. The end of the first winding is connected to the starting end of the second winding and so on. Ultimately, the end of the third winding will connect to the beginning of the first winding. Three-phase voltage is supplied to each connection node. The triangle connection is distinguished by the absence of a neutral wire.
Both types of compounds have received approximately the same distribution and do not have significant distinctive features.
There is also a combined connection when both options are used. This method is used quite often; its goal is a smooth start of the electric motor, which cannot always be achieved with conventional connections. At the moment of direct start-up, the windings are in the star position. Next, a relay is used that provides switching to the triangle position. Due to this, the starting current decreases. The combined circuit is most often used when starting high-power electric motors. Such motors also require a significantly higher starting current, approximately seven times the rated value.
Electric motors can be connected in other ways when a double or triple star is used. These connections are used for motors with two or more variable speeds.
Starting a three-phase electric motor with star-delta switching
This method is used to reduce the starting current, which can be approximately 5-7 times the rated current of the electric motor. Units with too much power have a starting current such that fuses easily blow, circuit breakers turn off and, in general, the voltage drops significantly. With such a decrease in voltage, the incandescence of the lamps decreases, the torque of other electric motors decreases, and the contactors spontaneously turn off. Therefore, different methods are used to reduce the inrush current.
Common to all methods is the need to reduce the voltage in the stator windings during direct start-up. To reduce the starting current, the stator circuit can be supplemented with a choke, rheostat or automatic transformer during starting.
The most widespread is switching the winding from a star to a triangle position. In the star position, the voltage becomes 1.73 times less than the rated value, therefore the current will be less than at full voltage. During start-up, the motor speed increases, the current decreases and the windings switch to the delta position.
Such switching is allowed in electric motors that have a lightweight starting mode, since the starting torque is reduced by approximately two times. This method is used to switch those engines that can be structurally connected into a triangle. They must have windings capable of operating at .
When to switch from triangle to star
When it is necessary to make a star and delta connection of the electric motor windings, you should remember that it is possible to switch from one type to another. The main option is the star-delta switching circuit. However, if necessary, the reverse option is also possible.
Everyone knows that electric motors that are not fully loaded have a decrease in power factor. Therefore, it is advisable to replace such engines with devices with lower power. However, if replacement is impossible and there is a large power reserve, a delta-star switch is performed. The current in the stator circuit should not exceed the nominal value, otherwise the electric motor will overheat.
There are situations in life when you need to start a 3-phase asynchronous electric motor from a household network. The problem is that you only have one phase and a “zero” at your disposal.
What to do in such a situation? Is it possible to connect a three-phase motor to a single-phase network?
If you approach your work wisely, everything is possible. The main thing is to know the basic schemes and their features.
Design features
Before starting work, understand the design of the IM (induction motor).
The device consists of two elements - a rotor (moving part) and a stator (fixed unit).
The stator has special grooves (recesses) into which the winding is placed, distributed in such a way that the angular distance is 120 degrees.
The windings of the device create one or more pairs of poles, the number of which determines the frequency with which the rotor can rotate, as well as other parameters of the electric motor - efficiency, power and other parameters.
When an asynchronous motor is connected to a three-phase network, current flows through the windings at different time intervals.
A magnetic field is created that interacts with the rotor winding and causes it to rotate.
In other words, a force appears that turns the rotor at different time intervals.
If you connect the IM to a network with one phase (without performing preparatory work), the current will appear in only one winding.
The torque generated will not be enough to move the rotor and keep it spinning.
That is why, in most cases, the use of starting and operating capacitors is required to ensure the operation of a three-phase motor. But there are other options.
How to connect an electric motor from 380 to 220V without a capacitor?
As noted above, to start an electric motor with a squirrel-cage rotor from a single-phase network, a capacitor is most often used.
It is this that ensures the device starts at the first moment after the single-phase current is supplied. In this case, the capacity of the starting device should be three times higher than the same parameter for the working capacity.
For motors with a power of up to 3 kilowatts and used at home, the price of starting capacitors is high and sometimes comparable to the cost of the motor itself.
Consequently, many are increasingly avoiding containers used only at the moment of start-up.
The situation is different with working capacitors, the use of which allows you to load the motor at 80-85 percent of its power. If they are absent, the power indicator may drop to 50 percent.
However, capacitorless starting of a 3-phase motor from a single-phase network is possible thanks to the use of bidirectional switches that operate for short periods of time.
The required torque is provided by the displacement of phase currents in the windings of the IM.
Today, two schemes are popular, suitable for motors with power up to 2.2 kW.
It is interesting that the start-up time of the IM from a single-phase network is not much lower than in the usual mode.
The main elements of the circuit are triacs and symmetrical dinistors. The first are controlled by multi-polar pulses, and the second by signals coming from the half-cycle of the supply voltage.
Scheme No. 1.
Suitable for 380 Volt electric motors up to 1,500 rpm with delta windings.
The RC circuit acts as a phase-shifting device. By changing the resistance R2, it is possible to achieve a voltage across the capacitor that is shifted by a certain angle (relative to the household network voltage).
The main task is performed by the symmetrical dinistor VS2, which at a certain point in time connects a charged capacitance to the triac and activates this switch.
Scheme No. 2.
Suitable for electric motors with a rotation speed of up to 3000 rpm and for motors with increased resistance at start-up.
Such motors require more starting current, so an open star circuit is more relevant.
A special feature is the use of two electronic switches that replace phase-shifting capacitors. During the adjustment process, it is important to ensure the required shift angle in the phase windings.
This is done as follows:
- Voltage is supplied to the electric motor through a manual starter (it must be connected in advance).
- After pressing the button, you need to select the starting moment using resistor R
When implementing the considered schemes, it is worth considering a number of features:
- For the experiment, radiatorless triacs (types TS-2-25 and TS-2-10) were used, which showed excellent results. If you use triacs on a plastic case (imported), you cannot do without radiators.
- A symmetrical DB3 type dinistor can be replaced with a KP. Despite the fact that the KP1125 is made in Russia, it is reliable and has a lower switching voltage. The main drawback is the scarcity of this dinistor.
How to connect via capacitors
First, decide which circuit is assembled on the ED. To do this, open the bar cover where the blood pressure terminals are output, and see how many wires come out of the device (most often there are six).
The designations are as follows: C1-C3 are the beginnings of the winding, and C4-C6 are its ends. If the beginnings or ends of the windings are combined with each other, this is a “star”.
The most difficult situation is if six wires simply come out of the case. In this case, you need to look for the corresponding designations on them (C1-C6).
To implement a scheme for connecting a three-phase electric motor to a single-phase network, two types of capacitors are required - starting and working.
The first ones are used to start the electric motor at the first moment. As soon as the rotor spins to the required number of revolutions, the starting capacitance is excluded from the circuit.
If this does not happen, there may be serious consequences, including engine damage.
The main function is performed by working capacitors. Here it is worth considering the following points:
- Working capacitors are connected in parallel;
- The rated voltage must be at least 300 Volts;
- The capacity of the working capacitors is selected taking into account 7 µF per 100 W;
- It is desirable that the type of working and starting capacitor be identical. Popular options are MBGP, MPGO, KBP and others.
If you take these rules into account, you can extend the life of the capacitors and the electric motor as a whole.
Capacity calculations must be made taking into account the rated power of the electric motor. If the motor is underloaded, overheating is inevitable, and then the capacity of the working capacitor will have to be reduced.
If you choose a capacitor with a capacitance less than acceptable, the efficiency of the electric motor will be low.
Remember that even after the circuit is turned off, the voltage remains on the capacitors, so it is worth discharging the device before starting work.
Also note that connecting an electric motor with a power of 3 kW or more to conventional wiring is prohibited, as this can lead to disconnection or burnout of the plugs. In addition, there is a high risk of insulation melting.
To connect ED 380 to 220V using capacitors, proceed as follows:
- Connect the containers to each other (as mentioned above, the connection should be parallel).
- Connect the parts with two wires to the electric motor and a single-phase alternating voltage source.
- Turn on the engine. This is done in order to check the direction of rotation of the device. If the rotor moves in the desired direction, no additional manipulations are necessary. Otherwise, the wires connected to the winding should be swapped.
With a capacitor, an additional simplified one is for a star circuit.
With a capacitor, an additional simplified one is for a triangle circuit.
How to connect with reverse
There are situations in life when you need to change the direction of rotation of the motor. This is also possible for three-phase electric motors used in a household network with one phase and zero.
To solve the problem, it is necessary to connect one terminal of the capacitor to a separate winding without the possibility of breaking, and the second - with the possibility of transferring from the “zero” to the “phase” winding.
To implement the circuit, you can use a switch with two positions.
The wires from “zero” and “phase” are soldered to the outer terminals, and the wire from the capacitor is soldered to the central terminal.
How to connect in a star-delta connection (with three wires)
For the most part, domestically produced EDs already have a star circuit assembled. All that is required is to reassemble the triangle.
The main advantage of the star/delta connection is the fact that the engine produces maximum power.
Despite this, such a scheme is rarely used in production due to the complexity of implementation.
To connect the motor and make the circuit operational, three starters are required.
The current is connected to the first (K1), and the stator winding is connected to the other. The remaining ends are connected to starters K3 and K2.
When the K3 starter is connected to the phase, the remaining ends are shortened and the circuit is converted into a “star”.
Please note that simultaneous activation of K2 and K3 is prohibited due to the risk of a short circuit or knocking out of the AV supplying the ED.
To avoid problems, a special interlock is provided, which means turning off one starter when turning on the other.
The operating principle of the circuit is simple:
- When the first starter is connected to the network, the time relay starts and supplies voltage to the third starter.
- The engine starts working in a star configuration and starts working with more power.
- After some time, the relay opens contacts K3 and connects K2. In this case, the electric motor operates in a “triangle” pattern with reduced power. When it is necessary to turn off the power, K1 turns on.
Results
As can be seen from the article, it is possible to connect a three-phase electric motor to a single-phase network without loss of power. At the same time, for home use, the simplest and most affordable option is using a starting capacitor.
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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 triangle turns on, otherwise a three-phase short circuit will occur.
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 speed, 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 of 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, 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.
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 do not 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 mains.
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 might want to 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 one and a half times increase in power 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.