Electric motor connection diagram with triangle. What is the difference between asynchronous motor connections: star and delta? Pros and cons of "star"

Content:

Asynchronous electric motors have proven themselves in operation with such indicators as operational reliability, the ability to obtain high torque power, and excellent performance. An important indicator of the operation of these motors is the ability to switch between star and delta connections - and this means stability during operation. Each connection has its own advantages, which must be understood when using asynchronous electric motors correctly.

Optimal choice of motor connection

The conversion of a “star” into a “delta” in an asynchronous electric motor, as well as the ability to repair the motor windings, and, compared to other motors, low cost, combined with resistance to mechanical stress, have made this type of motor the most popular. The main parameter that characterizes the advantage of asynchronous motors is simplicity in design. With all the advantages of this type of electric motor, it also has negative aspects during operation.

In practice, three-phase asynchronous electric motors can be connected to the network in a star and delta configuration. A “star” connection is when the ends of the stator winding are wrapped around one point, and a 380-volt network voltage is applied to the beginning of each winding; schematically this type of connection is indicated by the sign (Y).

If the “triangle” option is selected in the switching box for connecting the electric motor, the stator windings must be connected in series:

• the end of the first winding - with the beginning of the second;
• connecting the end of the “second” - with the beginning of the third;
• the end of the third - with the beginning of the first.

Electric motor connection diagrams

Experts, without going into the basics of electrical engineering, cite the fact that electric motors connected in a star circuit operate softer than those connected in a triangle (Δ) circuit. This is a good circuit for low power engines. They also focus on the fact that during soft operation, when the “star” (Y) circuit is used, the electric motor does not gain rated power.

When choosing the optimal option for connecting an electric motor, you should consider the fact that a delta connection (Δ) allows the motor to gain maximum power, but the value of the starting current increases significantly.

Comparing power indicators, this is the main difference between star and delta connections (Y, Δ), experts note that electric motors with a star connection (Y) have a power 1.5 times lower than those connected with a delta connection. (Δ).

To reduce the current parameters at the moment of starting in different switching circuits (Δ) - (Y), it is recommended to use a “star and delta” motor connection, a combined switching circuit. A combined, or also called mixed, type of connection is recommended for electric motors with high rated power.

When the star (Y) and (Δ) connection circuit is switched on, the star (Y) connection works from the beginning of the start; after the electric motor reaches sufficient speed, it switches to the delta connection (Δ). There are devices for automatically switching motor connections. Let's look at the differences between electric motor starting schemes and what the difference is between them.

How to control motor switching

Often, to start a high-power electric motor, switching the delta connection to a star connection is used; this is necessary to reduce the current parameters at start-up. In other words, the engine starts in star mode, and all work is carried out on a delta connection. For this purpose, a three-phase contactor is used.

When automatically switching, the following prerequisites must be met:

• block contacts from simultaneous activation;
• mandatory performance of work, with a time delay.

A time delay is necessary for 100% disconnection of the star connection, otherwise, when the delta connection is turned on, a short circuit will occur between the phases. A time relay (RT) is used, which delays the switching by an interval of 50 to 100 milliseconds.

How can you delay switching times?

When a “star and delta” circuit is used, it is necessary to delay the connection turn-on time (Δ) until the connection (Y) is turned off; experts prefer three methods:

• using a normally open contact in the time relay, which blocks the delta circuit when the electric motor starts, and the switching moment is controlled by the current relay (RT);
• using a timer in a modern time relay, which has the ability to switch modes with an interval of 6 to 10 seconds.

• by external control of starter contactors from automatic units or manual switching.

Standard switching scheme

The classic option of switching from “star” to “triangle” is considered by experts to be a reliable method, it does not require large expenses, is easy to implement, but, like any other method, it has a drawback - these are the overall dimensions of the time relay. This type of RF is guaranteed to perform a time delay by magnetizing the core, and it takes time to demagnetize it.

The mixed (combined) switching circuit works as follows. When the operator turns on the three-phase circuit breaker (AB), the motor starter is ready for action. Through the contacts of the “Stop” button, the normally closed position and through the normally open contacts of the “Start” button, which is pressed by the operator, electric current passes into the contactor coil (CM). Contacts (BKM) provide self-picking of power contacts and keep them in the on position.

The relay in the circuit (KM) provides the ability for the operator to turn off the electric motor with the “Stop” button. When the “control phase” passes through the start button, it also passes through closed normally located contacts (BKM1) and contacts (RV) - the contactor (KM2) starts, its power contacts supply voltage to the connection (Y), and the rotation of the electric motor rotor begins.

When the operator starts the engine, the contacts (BKM2) in the contactor (KM2) open, this creates an inoperative state of the power contacts (KM1), which provide power to the motor connection Δ.

The current relay (RT) operates almost immediately due to high current values, which are included in the circuit of current transformers (CT1) and (CT2). The control circuit of the contactor coil (KM2) is shunted by the contacts of the current relay (RT), which prevents the (RV) from operating.

In the contactor circuit (KM1), the contact block (BKM2) opens when starting (KM2), which prevents the coil (KM1) from operating.

With the set of the desired motor rotor speed parameter, the contacts of the current relay open, since the starting current decreases in the control of the contactor (KM2), simultaneously with the opening of the contacts supplying voltage to the winding connection (Y), BKM2 are connected, which brings the contactor (KM1) into the operating position ), and in its circuit the block of contacts BKM2 opens, and, as a result, the RV is de-energized. The transformation of the "triangle" into a "star" occurs after the engine is stopped.

Important! The temporary relay does not turn off immediately, but with a delay, which allows the relay contacts in the circuit (KM1) to be closed for some time, this ensures the start (KM1) and operation of the engine in a delta pattern.

Disadvantages of the standard scheme

Despite the reliability of the classical circuit for switching from one connection to another connection of a high-power electric motor, it has its disadvantages:

• it is necessary to correctly calculate the load on the electric motor shaft, otherwise it will take a long time to gain speed, which will not allow the current relay to quickly operate and then switch to operation via the Δ connection, and it is also extremely undesirable to operate the motor for a long time in this mode;

• to avoid overheating of the motor windings, experts recommend including a thermal relay in the circuit;
• when a modern type of RV is used in a classical scheme, it is necessary to comply with the passport requirements for the load on the shaft;

Conclusion

An important condition when using a star-delta connection diagram is the correct calculation of the load on the motor shaft. In addition, it cannot be denied that when the contactor of one connection Y is turned off, and the engine has not yet reached the required speed, the self-induction factor is triggered, and increased voltage enters the network, which can disable other nearby equipment and devices.

Experts recommend starting electric motors with average power according to the Y scheme, this gives soft operation and a smooth start. The methods for selecting switching differ according to the available voltage at the facility and the load.

Almost any production these days cannot do without a powerful asynchronous electric motor. When starting such an engine, the starting current is 3-8 times higher than the rated current required for operation in normal stable mode.

A large starting current is required in order to spin the rotor from rest. This requires much more effort than to further maintain a constant number of revolutions in a given period of time.

Significant starting currents for asynchronous motors are a very undesirable phenomenon, since this can lead to a short-term lack of energy for other equipment connected to the same network (voltage drop). There are many examples of such influence found both in production and in everyday life. The first thing that comes to mind is the “blinking” of a light bulb when the welding machine is operating, but there are more serious cases: a voltage sag can cause a defective batch of goods in production, which leads to large financial and labor costs. High inrush current can also cause significant thermal overload on the motor winding, resulting in insulation aging, damage, and ultimately motor burnout.

All this motivated us to find a solution to minimize starting currents. One such solution is the star-delta engine starting method. First, let’s figure out what a “star” is and what a “triangle” is, and how they differ from each other. Star and delta are the most common and practically used connection diagrams for three-phase electric motors. When turning on a three-phase electric motor with a star (see. Picture 1) the ends of the stator windings are connected together, the connection occurs at one point called the zero point or neutral. Three-phase voltage is supplied to the beginning of the windings.

Figure 1 - Star connection diagram

When the stator windings are connected in a star, the relationship between linear and phase voltages is expressed by the formula:

Where U l- voltage between two phases, U f- voltage between phase and neutral wire

The values ​​of linear and phase currents coincide, i.e. I l = I f.

When a three-phase electric motor is switched on in a delta pattern (see. Figure 2) the stator windings of the electric motor are connected in series. Thus, the end of one winding is connected to the beginning of the next, voltage in this case is applied to the connection points of the windings. When connecting the stator windings with a triangle, the phase voltage is equal to the linear voltage between the two wires: U l = U f.
Figure 2 - Triangle connection diagram

However, the current in the line (network) is greater than the current in the phase, which is described by the formula:

Where I l— linear current, I f— phase current

It turns out that by connecting the windings with a “star”, we reduce the linear current, which is what we initially sought. But there is also a downside to this scheme: as we see from the formula, the starting torque of the motor is directly proportional to the phase voltage:

Where U— phase voltage of the stator winding, r 1— active resistance of the stator winding phase, r 2— reduced value of the active resistance of the rotor winding phase,
x 1— inductive reactance of the stator winding phase, x 2- reduced value of the inductive reactance of the stationary rotor winding phase,
m- number of phases, p- number of pole pairs

To make it more clear, let's look at an example: suppose that the working circuit of the winding of an asynchronous electric motor is a “triangle”, and the linear voltage of the supply network is 380 V, the resistance of the stator winding Z = 10 Ohm. If the windings are connected as a star during start-up, the voltage and current in the phases will decrease:

The phase current is equal to the line current and is equal to:

After the engine has gained the required speed, i.e. has accelerated, we switch the windings from “star” to “delta”, in this case we get completely different current and voltage values:

Accordingly, when starting the engine according to the “star” circuit, the phase voltage is √3 times less than the linear voltage, and when starting the “delta” circuit they are equal. It follows that the torque when starting according to the “star” scheme is 3 times less, which means that by starting the engine according to this scheme, we will not be able to achieve the engine’s rated power. Solving one problem, a second one arises, no less acute than increased inrush currents. But there is still a single solution: it is necessary to combine the motor connection circuits so that when starting a powerful motor there is no large current in the network, and after the engine reaches the speed required for its operation, it switches to a “triangle” circuit, which allows you to work with 100% load without any problems.

Does the job perfectly time relay Finder 80.82. When power is applied to the relay, the contact is instantly closed, which is responsible for the star connection. After a given period of time, at which the engine speed reaches the operating frequency, the star contact opens and the contact responsible for the delta connection closes. The contacts will remain in this position until the power is removed from the relay. A visual diagram of the operation of this relay is presented in Figure 3.

Figure 3 - Timing diagram of time relay 80.82

Let us consider in more detail the implementation of this scheme in practice. It is only applicable to motors whose nameplate indicates “Δ/Y 380/660V”. On Figure 4 the power part of the star-delta circuit is presented, which uses three electromagnetic starters.

Figure 4 - Power part of the star-delta circuit

As described earlier, to control the switching from a star to a delta circuit, you must use a Finder 80.82 relay. On Figure 5 A control diagram using this relay is presented.

Figure 5 - Star-delta control

Let's look at the algorithm for how this scheme works:

After pressing the S1.1 button, the coil of the KM1 starter is energized, as a result of which the power contacts of KM1 are closed and, with the help of an additional contact KM1.1, self-retaining of the starter is realized. At the same time, voltage is applied to time relay U1. The contacts of the time relay 17-18 are closed and the KM2 starter is turned on. Thus, the engine starts according to the “star” scheme. After time T (see Figure 3), time relay contact 17-18 will instantly open, time delay Tu will pass, and contact 17-28 will close. As a result, the KM3 starter will operate, which switches to the “triangle” circuit. Normally closed contacts of starters KM2.2 and KM3.2 are used to prevent the simultaneous activation of starters KM2 and KM3. To protect the motor from overload, a thermal relay KK1 is installed in the power circuit. In case of overload, the thermal relay will open the power circuit and the control circuit through contact KK1.1. The engine stops when the S1.2 button is pressed, which breaks the self-retaining circuit and de-energizes the KM1 starter coil.

Summarizing what has been written, we can conclude that to make it easier to start a powerful electric motor, it is recommended to initially start it according to the “star” circuit, which can significantly reduce the starting currents, reduce the voltage drop in the network, but does not allow the motor to reach its nominal operating mode. To reach the rated mode of the motor, it is necessary to switch the stator windings to a delta circuit. The circuit for switching windings from “star” to “delta” is implemented using time relay Finder 80.82, in which the acceleration time of the electric motor is set.

Bibliography:
1. GOST 11828-86 “Determination of torques and starting currents.”
2. Veshenevsky S.N. Characteristics of motors in electric drives. // 6th edition, revised - Moscow, Publishing House "Energia", 1977
3. Voinarovsky P. D. Electric motors // Encyclopedic Dictionary of Brockhaus and Efron: in 86 volumes (82 volumes and 4 additional) - St. Petersburg, 1890-1907

Content:

The operation of three-phase electric motors is considered to be much more efficient and productive than single-phase motors designed for 220 V. Therefore, if there are three phases, it is recommended to connect the appropriate three-phase equipment. As a result, connecting a three-phase motor to a three-phase network ensures not only economical, but also stable operation of the device. The connection diagram does not require the addition of any starting devices, since immediately after starting the engine, a magnetic field is formed in its stator windings. The main condition for the normal operation of such devices is the correct connection and compliance with all recommendations.

Connection diagrams

The magnetic field created by the three windings ensures the rotation of the electric motor rotor. Thus, electrical energy is converted into mechanical energy.

The connection can be made in two main ways - star or triangle. Each of them has its own advantages and disadvantages. The star circuit provides a smoother start of the unit, however, the engine power drops by about 30% of the rated value. In this case, the delta connection has certain advantages, since there is no loss of power. However, this also has its own peculiarity associated with the current load, which increases sharply during startup. This condition has a negative impact on the insulation of wires. The insulation may be broken and the motor may fail completely.

Particular attention should be paid to European equipment equipped with electric motors designed for voltages of 400/690 V. They are recommended for connection to our 380 volt networks only using the delta method. If connected with a star, such motors immediately burn out under load. This method is applicable only to domestic three-phase electric motors.

Modern units have a connection box into which the ends of the windings are led out. Their number can be three or six. In the first case, the connection diagram is initially assumed to be a star method. In the second case, the electric motor can be connected to a three-phase network in both ways. That is, with a star circuit, the three ends located at the beginning of the windings are connected into a common twist. The opposite ends are connected to the phases of the 380 V network from which power is supplied. With the triangle option, all ends of the windings are connected in series to each other. The phases are connected to three points at which the ends of the windings are connected to each other.

Using a star-delta circuit

A combined connection diagram known as “star-delta” is used relatively rarely. It allows for a smooth start with a star circuit, and during the main operation a triangle is turned on, providing maximum power to the unit.

This connection diagram is quite complex, requiring the use of three windings installed in the connections at once. The first MP is connected to the network and with the ends of the windings. MP-2 and MP-3 are connected to opposite ends of the windings. The delta connection is made to the second starter, and the star connection is made to the third. Simultaneous activation of the second and third starters is strictly prohibited. This will cause a short circuit between the phases connected to them. To prevent such situations, an interlock is installed between these starters. When one MP turns on, the contacts of the other open.

The entire system operates according to the following principle: simultaneously with MP-1 being turned on, MP-3, connected by a star, is turned on. After a smooth start of the engine, after a certain period of time set by the relay, the transition to normal operating mode occurs. Next, MP-3 is turned off and MP-2 is turned on according to a triangle diagram.

Three-phase motor with magnetic starter

Connecting a three-phase motor using a magnetic starter is carried out in the same way as through a circuit breaker. This circuit is simply supplemented with an on/off block with corresponding START and STOP buttons.

One normally closed phase connected to the motor is connected to the START button. When pressed, the contacts close, after which current flows to the motor. However, it should be noted that if the START button is released, the contacts will be open and no power will be supplied. To prevent this, the magnetic starter is equipped with another additional contact connector, the so-called self-retaining contact. It functions as a locking element and prevents the circuit from breaking when the START button is turned off. The circuit can only be completely disconnected using the STOP button.

Thus, connecting a three-phase motor to a three-phase network can be done in various ways. Each of them is selected in accordance with the unit model and specific operating conditions.

Three-phase asynchronous motors are deservedly the most popular in the world, due to the fact that they are very reliable, require minimal maintenance, are easy to manufacture and do not require any complex and expensive devices when connecting, unless adjustment of the rotation speed is required. Most of the machines in the world are driven by three-phase asynchronous motors; they also drive pumps and electric drives of various useful and necessary mechanisms.

But what about those who do not have a three-phase power supply in their personal household, and in most cases this is exactly the case. What to do if you want to install a stationary circular saw, electric jointer or lathe in your home workshop? I would like to please the readers of our portal that there is a way out of this predicament, and one that is quite simple to implement. In this article we intend to tell you how to connect a three-phase motor to a 220 V network.

Operating principles of three-phase asynchronous motors

Let us briefly consider the principle of operation of an asynchronous motor in its “native” three-phase 380 V networks. This will greatly help in later adapting the motor for operation in other, “non-native” conditions - single-phase 220 V networks.

Asynchronous motor device

Most of the three-phase motors produced in the world are squirrel-cage induction motors (SCMC), which do not have any electrical contact between the stator and the rotor. This is their main advantage, since brushes and commutators are the weakest point of any electric motor; they are subject to intense wear and require maintenance and periodic replacement.

Let's consider the ADKZ device. The engine is shown in cross-section in the figure.

The cast housing (7) houses the entire electric motor mechanism, which includes two main parts - a stationary stator and a movable rotor. The stator has a core (3), which is made of sheets of special electrical steel (an alloy of iron and silicon), which has good magnetic properties. The core is made of sheets due to the fact that under conditions of an alternating magnetic field, Foucault eddy currents can arise in the conductors, which we absolutely do not need in the stator. Additionally, each core sheet is coated on both sides with a special varnish to completely eliminate the flow of currents. We only need from the core its magnetic properties, and not the properties of an electric current conductor.

A winding (2) made of enameled copper wire is laid in the grooves of the core. To be precise, there are at least three windings in a three-phase asynchronous motor - one for each phase. Moreover, these windings are laid in the grooves of the core with a certain order - each is located so that it is at an angular distance of 120° to the other. The ends of the windings are brought out into the terminal box (in the figure it is located at the bottom of the engine).

The rotor is placed inside the stator core and rotates freely on the shaft (1). To increase efficiency, they try to make the gap between the stator and the rotor minimal - from half a millimeter to 3 mm. The rotor core (5) is also made of electrical steel and it also has grooves, but they are not intended for wire winding, but for short-circuited conductors, which are located in space so that they resemble a squirrel wheel (4), for which they received their Name.

The squirrel wheel consists of longitudinal conductors that are connected both mechanically and electrically to the end rings. Typically, the squirrel wheel is made by pouring molten aluminum into the grooves of the core, and at the same time, both rings and fan impellers (6) are molded as a monolith. In high-power ADKZ, copper rods welded with end copper rings are used as cell conductors.

What is three-phase current

In order to understand what forces make the ADKZ rotor rotate, we need to consider what a three-phase power supply system is, then everything will fall into place. We are all accustomed to the usual single-phase system, when the socket has only two or three contacts, one of which is (L), the second is a working zero (N), and the third is a protective zero (PE). The rms phase voltage in a single-phase system (the voltage between phase and zero) is 220 V. The voltage (and when a load is connected, the current) in single-phase networks varies according to a sinusoidal law.

From the above graph of the amplitude-time characteristic it is clear that the amplitude value of the voltage is not 220 V, but 310 V. So that readers do not have any “misunderstandings” and doubts, the authors consider it their duty to inform that 220 V is not the amplitude value, but the root mean square or current. It is equal to U=U max /√2=310/1.414≈220 V. Why is this done? For convenience of calculations only. Constant voltage is taken as the standard, based on its ability to produce some work. We can say that a sinusoidal voltage with an amplitude value of 310 V in a certain period of time will produce the same work that a constant voltage of 220 V would do in the same period of time.

It must be said right away that almost all generated electrical energy in the world is three-phase. It’s just that single-phase energy is easier to manage in everyday life; most electricity consumers only need one phase to operate, and single-phase wiring is much cheaper. Therefore, one phase and neutral conductor are “pulled out” from a three-phase system and sent to consumers - apartments or houses. This is clearly visible in the entrance panels, where you can see how the wire goes from one phase to one apartment, from another to a second, from a third to a third. This is also clearly visible on the poles from which the lines go to private households.

Three-phase voltage, unlike single-phase, has not one phase wire, but three: phase A, phase B and phase C. Phases can also be designated L1, L2, L3. In addition to the phase wires, of course, there is also a working zero (N) and a protective zero (PE) common to all phases. Let's consider the amplitude-time characteristic of three-phase voltage.

It is clear from the graphs that three-phase voltage is a combination of three single-phase ones, with an amplitude of 310 V and an rms value of the phase (between phase and working zero) voltage of 220 V, and the phases are shifted relative to each other with an angular distance of 2 * π / 3 or 120 ° . The potential difference between the two phases is called linear voltage and is equal to 380 V, since the vector sum of the two voltages will be U l =2*U f *sin(60°)=2*220*√3/2=220* √3=220*1.73=380.6 V, Where U l– linear voltage between two phases, and U f– phase voltage between phase and zero.

Three-phase current is easy to generate, transmit to its destination and subsequently convert it into any desired type of energy. Including the mechanical energy of rotation of the ADKZ.

How does a three-phase asynchronous motor work?

If you apply an alternating three-phase voltage to the stator windings, currents will begin to flow through them. They, in turn, will cause magnetic fluxes, also varying according to a sinusoidal law and also shifted in phase by 2*π/3=120°. Considering that the stator windings are located in space at the same angular distance - 120°, a rotating magnetic field is formed inside the stator core.

three phase electric motor

This constantly changing field crosses the “squirrel wheel” of the rotor and causes an EMF (electromotive force) in it, which will also be proportional to the rate of change of the magnetic flux, which in mathematical language means the time derivative of the magnetic flux. Since the magnetic flux changes according to a sinusoidal law, this means that the EMF will change according to the cosine law, because (sin x)’= cos x. From the school mathematics course it is known that the cosine “leads” the sine by π/2=90°, that is, when the cosine reaches its maximum, the sine will reach it after π/2 - after a quarter of the period.

Under the influence of EMF, large currents will arise in the rotor, or more precisely, in the squirrel wheel, given that the conductors are short-circuited and have low electrical resistance. These currents form their own magnetic field, which spreads along the rotor core and begins to interact with the stator field. Opposite poles, as is known, attract, and like poles repel each other. The resulting forces create a torque causing the rotor to rotate.

The stator's magnetic field rotates at a certain frequency, which depends on the supply network and the number of pole pairs of the windings. The frequency is calculated using the following formula:

n 1 =f 1 *60/p, Where

• f 1 – alternating current frequency.
• p – number of pole pairs of stator windings.

Everything is clear with the frequency of alternating current - in our power supply networks it is 50 Hz. The number of pole pairs reflects how many pairs of poles there are on the winding or windings belonging to the same phase. If one winding is connected to each phase, spaced 120° from the others, then the number of pole pairs will be equal to one. If two windings are connected to one phase, then the number of pole pairs will be equal to two, and so on. Accordingly, the angular distance between the windings changes. For example, when the number of pole pairs is two, the stator contains a winding of phase A, which occupies a sector of not 120°, but 60°. Then it is followed by the winding of phase B, occupying the same sector, and then phase C. Then the alternation is repeated. As the pole pairs increase, the sectors of the windings decrease accordingly. Such measures make it possible to reduce the rotation frequency of the magnetic field of the stator and, accordingly, the rotor.

Let's give an example. Let's say a three-phase motor has one pair of poles and is connected to a three-phase network with a frequency of 50 Hz. Then the stator magnetic field will rotate with a frequency n 1 =50*60/1=3000 rpm. If you increase the number of pole pairs, the rotation speed will decrease by the same amount. To increase the engine speed, you need to increase the frequency supplying the windings. To change the direction of rotation of the rotor, you need to swap two phases on the windings

It should be noted that the rotor speed always lags behind the rotation speed of the stator magnetic field, which is why the motor is called asynchronous. Why is this happening? Let's imagine that the rotor rotates at the same speed as the stator's magnetic field. Then the squirrel wheel will not “pierce” the alternating magnetic field, but it will be constant for the rotor. Accordingly, no EMF will be induced and currents will stop flowing, there will be no interaction of magnetic fluxes and the moment driving the rotor in motion will disappear. That is why the rotor is “in constant striving” to catch up with the stator, but it will never catch up, since the energy causing the motor shaft to rotate will disappear.

The difference in the rotation frequencies of the magnetic field of the stator and the rotor shaft is called the slip frequency, and it is calculated by the formula:

∆ n=n 1 -n 2, Where

• n1 – rotation frequency of the stator magnetic field.
• n2 – rotor speed.

Slip is the ratio of the sliding frequency to the rotation frequency of the stator magnetic field, it is calculated by the formula: S=∆n/n 1 =(n 1 —n 2)/n 1.

Methods for connecting windings of asynchronous motors

Most ADKZ has three windings, each of which corresponds to its own phase and has a beginning and an end. Winding designation systems may vary. In modern electric motors, a system has been adopted for designating windings U, V and W, and their terminals are designated by number 1 as the beginning of the winding and by number 2 as its end, that is, winding U has two terminals U1 and U2, winding V–V1 and V2, and winding W - W1 and W2.

However, asynchronous motors made during the Soviet era and having the old marking system are still in use. In them, the beginnings of the windings are designated C1, C2, C3, and the ends are C4, C5, C6. This means that the first winding has terminals C1 and C4, the second winding C2 and C5, and the third winding C3 and C6. The correspondence between the old and new notation systems is presented in the figure.

Let's consider how windings can be connected in an ADKZ.

Star connection

With this connection, all ends of the windings are combined at one point, and phases are connected to their beginnings. In the circuit diagram, this connection method really resembles a star, which is why it got its name.

When connected by a star, a phase voltage of 220 V is applied to each winding individually, and a linear voltage of 380 V is applied to two windings connected in series. The main advantage of this connection method is small starting currents, since the linear voltage is applied to two windings, and not to one. This allows the engine to start “softly,” but its power will be limited, since the currents flowing in the windings will be less than with another connection method.

Delta connection

With this connection, the windings are combined into a triangle, when the beginning of one winding is connected to the end of the next - and so on in a circle. If the linear voltage in a three-phase network is 380 V, then much larger currents will flow through the windings than with a star connection. Therefore, the power of the electric motor will be higher.

When connected by a delta at the moment of starting, the ADKZ consumes large starting currents, which can be 7-8 times higher than the rated ones and can cause network overload, so in practice, engineers have found a compromise - the engine starts and spins up to rated speed using a star circuit, and then automatic switching to triangle.

How to determine which circuit the motor windings are connected to?

Before connecting a three-phase motor to a single-phase 220 V network, it is necessary to find out what circuit the windings are connected to and at what operating voltage the ADKZ can operate. To do this, you need to study the plate with technical characteristics - the “nameplate”, which should be on each engine.

You can find out a lot of useful information on such a “nameplate”

The plate contains all the necessary information that will help connect the motor to a single-phase network. The presented nameplate shows that the engine has a power of 0.25 kW and a speed of 1370 rpm, which indicates the presence of two pairs of winding poles. The ∆/Y symbol means that the windings can be connected either by a triangle or a star, and the following indicator 220/380 V indicates that when connected by a triangle, the supply voltage should be 220 V, and when connected by a star - 380 V. If such Connect the motor to a 380 V network in a triangle, then its windings will burn out.

On the next nameplate you can see that such a motor can only be connected with a star and only to a 380 V network. Most likely, such an ADKZ will have only three terminals in the terminal box. Experienced electricians will be able to connect such a motor to a 220 V network, but to do this they will need to open the back cover to get to the winding terminals, then find the beginning and end of each winding and make the necessary switching. The task becomes much more complicated, so the authors do not recommend connecting such motors to a 220 V network, especially since most modern ADKZ can be connected in different ways.

Each motor has a terminal box, most often located on the top. This box has inputs for power cables, and on top it is closed with a lid that must be removed with a screwdriver.

As electricians and pathologists say: “An autopsy will tell.”

Under the cover you can see six terminals, each of which corresponds to either the beginning or the end of the winding. In addition, the terminals are connected by jumpers, and by their location you can determine by what scheme the windings are connected.

Opening the terminal box showed that the “patient” had obvious “star fever”

The photo of the “opened” box shows that the wires leading to the windings are labeled and the ends of all windings – V2, U2, W2 – are connected to one point by jumpers. This indicates that a star connection is taking place. At first glance, it may seem that the ends of the windings are located in the logical order V2, U2, W2, and the beginnings are “confused” - W1, V1, U1. However, this is done for a specific purpose. To do this, consider the ADKZ terminal box with connected windings according to a triangle diagram.

The figure shows that the position of the jumpers changes - the beginnings and ends of the windings are connected, and the terminals are located so that the same jumpers are used for reconnection. Then it becomes clear why the terminals are “mixed up” - it’s easier to transfer jumpers. The photo shows that terminals W2 and U1 are connected by a piece of wire, but in the basic configuration of new engines there are always exactly three jumpers.

If, after “opening” the terminal box, a picture like the one in the photograph is revealed, this means that the motor is intended for a star and a three-phase 380 V network.

It is better for such an engine to return to its “native element” - in a three-phase alternating current circuit

Video: An excellent film about three-phase synchronous motors, which has not yet been painted

It is possible to connect a three-phase motor to a single-phase 220 V network, but you must be prepared to sacrifice a significant reduction in its power - in the best case, it will be 70% of the nameplate, but for most purposes this is quite acceptable.

The main connection problem is the creation of a rotating magnetic field, which induces an emf in the squirrel-cage rotor. This is easy to implement in three-phase networks. When generating three-phase electricity, an EMF is induced in the stator windings due to the fact that a magnetized rotor rotates inside the core, which is driven by the energy of falling water at a hydroelectric power station or a steam turbine at hydroelectric power stations and nuclear power plants. It creates a rotating magnetic field. In engines, the reverse transformation occurs - a changing magnetic field causes the rotor to rotate.

In single-phase networks, it is more difficult to obtain a rotating magnetic field - you need to resort to some “tricks”. To do this, you need to shift the phases in the windings relative to each other. Ideally, you need to make sure that the phases are shifted relative to each other by 120°, but in practice this is difficult to implement, since such devices have complex circuits, are quite expensive, and their manufacture and configuration require certain qualifications. Therefore, in most cases, simple circuits are used, while somewhat sacrificing power.

Phase shift using capacitors

An electric capacitor is known for its unique property of not passing direct current, but passing alternating current. The dependence of the currents flowing through the capacitor on the applied voltage is shown in the graph.

The current in the capacitor will always “lead” for a quarter of the period

As soon as a voltage increasing along a sinusoid is applied to the capacitor, it immediately “pounces” on it and begins to charge, since it was initially discharged. The current will be maximum at this moment, but as it charges, it will decrease and reach a minimum at the moment when the voltage reaches its peak.

As soon as the voltage decreases, the capacitor will react to this and will begin to discharge, but the current will flow in the opposite direction, as it discharges it will increase (with a minus sign) as long as the voltage decreases. By the time the voltage is zero, the current reaches its maximum.

When the voltage begins to increase with a minus sign, the capacitor is recharged and the current gradually approaches zero from its negative maximum. As the negative voltage decreases and it approaches zero, the capacitor discharges with an increase in the current through it. Next, the cycle repeats again.

The graph shows that during one period of alternating sinusoidal voltage, the capacitor is charged twice and discharged twice. The current flowing through the capacitor leads the voltage by a quarter of a period, that is - 2* π/4=π/2=90°. In this simple way you can obtain a phase shift in the windings of an asynchronous motor. A phase shift of 90° is not ideal at 120°, but it is quite sufficient for the necessary torque to appear on the rotor.

Phase shift can also be obtained by using an inductor. In this case, everything will happen the other way around - the voltage will lead the current by 90°. But in practice, more capacitive phase shift is used due to simpler implementation and lower losses.

Schemes for connecting three-phase motors to a single-phase network

There are many options for connecting ADKZ, but we will consider only the most commonly used and easiest to implement. As discussed earlier, to shift the phase, it is enough to connect a capacitor in parallel with any of the windings. The designation C p indicates that this is a working capacitor.

It should be noted that connecting the windings in a triangle is preferable, since more useful power can be “removed” from such an ADKZ than from a star. But there are motors designed to operate in networks with a voltage of 127/220 V. There must be information about this on the nameplate.

If readers come across such an engine, then this can be considered good luck, since it can be connected to a 220 V network using a star circuit, and this will ensure a smooth start and up to 90% of the nameplate rated power. The industry produces ADKZs specially designed for operation in 220 V networks, which can be called capacitor motors.

Whatever you call the engine, it’s still asynchronous with a squirrel-cage rotor

It should be noted that the nameplate indicates an operating voltage of 220 V and the parameters of the operating capacitor 90 μF (microfarad, 1 μF = 10 -6 F) and a voltage of 250 V. It is safe to say that this motor is actually three-phase, but adapted for single-phase voltage.

To facilitate the start-up of powerful ADSCs in 220 V networks, in addition to the working capacitor, they also use a starting capacitor, which is turned on for a short time. After the start and a set of rated speeds, the starting capacitor is turned off, and only the working capacitor supports rotor rotation.

The starting capacitor “gives a kick” when the engine starts

The starting capacitor is C p, connected in parallel to the working capacitor C p. It is known from electrical engineering that when connected in parallel, the capacitances of the capacitors add up. To “activate” it, use the SB push-button switch, held down for several seconds. The capacity of the starting capacitor is usually at least two and a half times higher than that of the working capacitor, and it can retain its charge for quite a long time. If you accidentally touch its terminals, you can get a fairly noticeable discharge through the body. In order to discharge C p, a resistor connected in parallel is used. Then, after disconnecting the starting capacitor from the network, it will be discharged through a resistor. It is selected with a sufficiently high resistance of 300 kOhm-1 mOhm and a power dissipation of at least 2 W.

Calculation of the capacity of the working and starting capacitor

For reliable start-up and stable operation of the ADKZ in 220 V networks, you should most accurately select the capacitances of the working and starting capacitors. If the capacitance C p is insufficient, insufficient torque will be created on the rotor to connect any mechanical load, and excess capacitance can lead to the flow of too high currents, which can result in an interturn short circuit of the windings, which can only be “treated” by very expensive rewinding.

Scheme	What is calculated	Formula	What is needed for calculations
	Capacitance of the working capacitor for connecting star windings – Cp, µF	Cр=2800*I/U; I=P/(√3*U*η*cosϕ); Cр=(2800/√3)*P/(U^2*n* cosϕ)=1616.6*P/(U^2*n* cosϕ)	For all: I – current in amperes, A; U – network voltage, V; P – electric motor power; η – engine efficiency expressed in values ​​from 0 to 1 (if it is indicated on the engine nameplate as a percentage, then this indicator must be divided by 100); cosϕ – power factor (cosine of the angle between the voltage and current vector), it is always indicated in the passport and on the nameplate.
	Capacity of the starting capacitor for connecting star windings – Cp, µF	Cп=(2-3)*Cр≈2.5*Ср
	Capacitance of the working capacitor for connecting the windings in a triangle – Cp, µF	Cр=4800*I/U; I=P/(√3*U*η*cosϕ); Cр=(4800/√3)*P/(U^2*n* cosϕ)=2771.3*P/(U^2*n* cosϕ)
	Capacity of the starting capacitor for connecting the windings in a triangle – Cn, µF	Cп=(2-3)*Cр≈2.5*Ср

The formulas given in the table are quite sufficient to calculate the required capacitor capacity. Passports and nameplates may indicate efficiency or operating current. Depending on this, you can calculate the necessary parameters. In any case, that data will be enough. For the convenience of our readers, you can use a calculator that will quickly calculate the required working and starting capacity.

The current-carrying windings of the electric motor are led out into the distribution box. The terminals of the windings form two parallel rows, each marked with the letter C and numbers from 1 to 6. This is done in order to mark the beginning and end of all three windings.
The connections are quite complicated. This can be found out using a simple tester. Calling the terminals of the windings, we will find that only two of them are connected along a large diagonal. The rest are connected along small diagonals.
Ringing the windings is necessary when using an old electric motor; in a new one, such work is unlikely to be required. After checking, the motor can be connected either in a star or delta configuration.
Note: a combined star-delta circuit is also used to connect electric motors with a power of more than 5 kW.

Turning on the windings with a star

The "star" circuit involves connecting the ends of the windings at one point, which is called the neutral, and applying supply voltage to the beginning of each of the windings. The "triangle" circuit provides for a series connection of windings.

For a star connection, two jumpers (three jumpers are included with the electric motor) are installed on the terminals in the same row. Then the jumpers are fixed with nuts. Wires from a three-phase network are connected to the three terminals of the second row.

Switching on the motor windings with a triangle

The “triangle” circuit is used to connect the electric motor to a single-phase 220 V network. Three jumpers connect the terminals located opposite each other. On one side, the jumpers are fixed with nuts, on the opposite side we connect wires from the network to two terminals, and to the third - a wire from the working capacitor (the capacitance must be calculated correctly).

Tip: when purchasing an electric motor, it is advisable to check the number of wires in the junction box. The presence of 6 wires to the contacts indicates the possibility of connecting the motor according to any scheme. Three wires mean that the winding contacts are already connected in a star configuration and connection to a single-phase network in a delta configuration is impossible. In this case, you will have to open the engine and remove the missing ends. This will be quite difficult to do.
Each connection scheme has its own characteristics. The electric motor, when connected in a star configuration, operates smoothly, but cannot develop the power that is indicated in the product data sheet.
The "triangle" circuit allows the electric motor to achieve maximum power, but to reduce the value of the resulting starting currents, it is necessary to use a starting rheostat.