Typical cases of star and delta connections of generators, transformers and electrical receivers are discussed in the articles "Star connection diagram" and "Triangle connection diagram". Let us now dwell on the most important question about power when connected in a star and delta, since for the operation of each mechanism driven by an electric motor or powered by a generator or transformer, it is ultimately important exactly the power.
In AC networks, a distinction is made between:
full (apparent) power S = E × I or S = U × I;
active power P = E × I× cos φ
or P = U × I× cos φ
;
reactive power Q = E × I× sin φ
or Q = U × I× sin φ
,
where E- electromotive force (emf); U- voltage at the terminals of the electrical receiver; I- current; φ is the phase angle between current and voltage 1.
When determining the power of generators, the formulas include e. etc. with, when determining the power of electrical receivers - the voltage at their terminals. When determining the power of electric motors, the efficiency is also taken into account, since the power on its shaft is indicated on the nameplate of the electric motor.
If the phase powers S a ( P a, Q a); S b ( P b, Q b); S c ( P c, Q c) are the same and, accordingly, equal S f, P f and Q f, then the power of the three-phase system, expressed in terms of phase quantities, is equal to the sum of the powers of the three phases and is:
complete S= 3 × S f;
active P= 3 × P f;
reactive Q= 3 × Q f.
Power when connected to a star
When connected to a star, linear currents I and phase currents I f are equal, and between phase
and line voltages there is a relation U= √3 × U f, whence U f = U / √3.
Comparing these formulas, we see that the powers expressed in terms of linear quantities when connected to a star are equal to:
complete S= 3 × Sφ = 3 × ( U/ √3) × I= √3 × U × I;
active P= √3 × U × I× cos φ
;
reactive Q= √3 × U × I× sin φ
.
Delta power
When connected in a triangle, linear U and phase U f voltages are equal, and between the phase and linear currents there is a relationship I= √3 × I f, whence I f = I / √3.
Therefore, the powers expressed in terms of linear quantities when connected to a triangle are equal to:
complete S= 3 × Sφ = 3 × U × ( I/ √3) = √3 × U × I;
active P= √3 × U × I× cos φ
;
reactive Q= √3 × U × I× sin φ
.
Important note. The same form of power formulas for star and triangle connections sometimes causes misunderstandings, as it pushes insufficiently experienced people to the wrong conclusion that the type of connections is always indifferent. Let's show with one example how wrong this view is.
The electric motor was connected in a triangle and operated from a 380 V network at a current of 10 A with full power
S= 1.73 × 380 × 10 = 6574 V × A.
Then the electric motor was reconnected to a star. At the same time, a 1.73 times lower voltage fell on each phase winding, although the voltage in the network remained the same. The lower voltage caused the current in the windings to decrease by 1.73 times. But this is not enough. When connected in a triangle, the line current was 1.73 times higher than the phase current, and now the phase and line currents are equal.
Thus, the line current upon reconnection to a star decreased by 1.73 × 1.73 = 3 times.
In other words, although the new cardinality needs to be calculated by the same formula, but it should be substituted other quantities, namely:
S 1 = 1.73 × 380 × (10/3) = 2191 V × A.
It follows from this example that when reconnecting an electric motor from a triangle to a star and feeding it from the same mains, the power developed by the electric motor is decreases 3 times.
What happens when switching from star to triangle and back in the most common cases?
We make a reservation that we are not talking about internal reconnections (which are performed at the factory or in specialized workshops), but about reconnections on the shields of the devices, if the beginnings and ends of the windings are displayed on them.
1. When switching from star to delta of generator windings or secondary windings of transformers the voltage in the network decreases 1.73 times, for example, from 380 to 220 V. The power of the generator and transformer remains the same. Why? Because the voltage of each phase winding remains the same and the current in each phase winding is the same, although the current in the line wires increases 1.73 times.
When switching windings of generators or secondary windings of transformers from delta to star the opposite phenomena occur, that is, the line voltage in the network increases 1.73 times, for example, from 220 to 380 V, the currents in the phase windings remain the same, the currents in the linear wires decrease 1.73 times.
This means that both generators and secondary windings of transformers, if they have all six ends, are suitable for networks with two voltages that differ by 1.73 times.
2. When switching lamps from a star to a triangle(provided they are connected to the same network in which the lamps, switched on by a star, burn at normal incandescence), the lamps will burn out.
When switching lamps from a triangle to a star(provided that the lamps, when connected in a triangle, burn at normal incandescence), the lamps will give a dim light. This means that lamps, for example, for 127 V, in a network with a voltage of 127 V must be connected with a triangle. If they have to be powered from a 220 V network, you need to connect to a star with a neutral wire (for more details, see the article "Star connection diagram"). Only lamps of the same power, evenly distributed between phases, can be connected to a star without a neutral wire. in theater chandeliers.
3. Everything said about lamps applies to resistances, electric ovens and similar electrical receivers.
4. Capacitors from which batteries are assembled to increase the cos φ , have a rated voltage that indicates the mains voltage to which the capacitor is to be connected. If the mains voltage is, for example, 380 V, and the rated voltage of the capacitors is 220 V, they should be connected in a star. If the mains voltage and the rated voltage of the capacitors are the same, the capacitors are connected in a delta.
5. As explained above, when switching electric motor from triangle to star its power is reduced by about three times. Conversely, if the electric motor is switched from the star to the triangle, the power rises sharply, but at the same time the electric motor, if it is not designed to operate at a given voltage and a delta connection, will burn.
Squirrel-cage starting with star-delta switching
used to reduce the starting current, which is 5 to 7 times the operating current of the motor. With motors of relatively high power, the starting current is so high that it can cause blown fuses, switch off the machine and lead to a significant decrease in voltage. Reducing the voltage reduces the incandescence of the lamps, reduces the torque of the electric motors 2, and can cause the contactors and magnetic starters to trip. Therefore, they strive to reduce the starting current, which is achieved in several ways. All of them ultimately boil down to lowering the voltage in the stator circuit for the starting period. To do this, a rheostat, a choke, an autotransformer are introduced into the stator circuit for the start-up period, or the winding is switched from a star to a triangle. Indeed, before starting and in the first period of starting, the windings are connected to a star. Therefore, each of them is supplied with a voltage 1.73 times less than the nominal one, and, therefore, the current will be much less than when the windings are turned on at full mains voltage. During the starting process, the electric motor increases the speed and the current decreases. Then the windings are switched into a triangle.
Warnings:
1. Switching from a star to a delta is permissible only for motors with a light starting mode, since when connected to a star, the starting torque is approximately half the torque that would be with direct starting. This means that this method of reducing the starting current is not always suitable, and if it is necessary to reduce the starting current and at the same time achieve a large starting torque, then an electric motor with a phase rotor is taken, and a starting rheostat is introduced into the rotor circuit.
2. It is possible to switch from star to delta only those electric motors that are designed to operate when connected in delta, that is, they have windings designed for the line voltage of the network.
Switching from triangle to star
Underloaded motors are known to operate at very low power factor cos φ ... Therefore, it is recommended to replace underloaded electric motors with less powerful ones. If, however, the replacement cannot be performed, and the power reserve is large, then an increase in cos φ switching from a triangle to a star. At the same time, it is necessary to measure the current in the stator circuit and make sure that it, when connected to a star, does not exceed the rated current at load; otherwise, the motor will overheat.
1 Active power is measured in watts (W), reactive power - in reactive volt-amperes (var), total - in volt-amperes (V × A). Values 1000 times larger are respectively called kilowatt (kW), kilovar (kvar), kilovolt-ampere (kV × A).
2 The motor torque is proportional to the square of the voltage. Therefore, when the voltage is reduced by 20%, the torque is reduced not by 20, but by 36% (1² - 0.82² = 0.36).
Today, asynchronous electric motors are popular due to their reliability, excellent performance and relatively low cost. Motors of this type are designed to withstand high mechanical stress. In order for the unit to start up successfully, it must be connected correctly. For this, connections of the "star" and "triangle" type, as well as their combination, are used.
Connection types
The design of an electric motor is quite simple and consists of two main elements - a stationary stator and an internally rotating rotor... Each of these parts has its own current-carrying windings. The stator is laid in special grooves with the obligatory observance of a distance of 120 degrees.
The principle of operation of the motor is simple - after turning on the starter and applying voltage to the stator, a magnetic field arises, forcing the rotor to rotate. Both ends of the windings are led out into a junction box and are arranged in two rows. Their conclusions are marked with the letter "C" and receive a digital designation in the range from 1 to 6.
To connect them, you can use one of three ways:
- "Star";
- "Triangle";
- "Star-triangle".
However, the combined circuit cannot be used if it is necessary to reduce the starting current, but at the same time a large torque is required. In this case, an electric motor with a wound rotor, equipped with a rheostat, should be used.
If we talk about the advantages of combining the two connection methods, then two can be noted:
- Thanks to the soft start, the service life is increased.
- It is possible to create two power levels for the unit.
Today, the most widely used electric motors are designed to operate in 220 and 380 volt networks. It is on this that the choice of the connection scheme depends. Thus, it is recommended to use the "triangle" at a voltage of 220 V, and the "star" at 380 V.
Today, asynchronous electric motors are popular due to their reliability, excellent performance and relatively low cost. Motors of this type are designed to withstand high mechanical stress. In order for the unit to start up successfully, it must be connected correctly. For this, connections of the "star" and "triangle" type, as well as their combination, are used.
Connection types
The design of an electric motor is quite simple and consists of two main elements - a stationary stator and an internally rotating rotor... Each of these parts has its own current-carrying windings. The stator is laid in special grooves with the obligatory observance of a distance of 120 degrees.
The principle of operation of the motor is simple - after turning on the starter and applying voltage to the stator, a magnetic field arises, forcing the rotor to rotate. Both ends of the windings are led out into a junction box and are arranged in two rows. Their conclusions are marked with the letter "C" and receive a digital designation in the range from 1 to 6.
To connect them, you can use one of three ways:
- "Star";
- "Triangle";
- "Star-triangle".
However, the combined circuit cannot be used if it is necessary to reduce the starting current, but at the same time a large torque is required. In this case, an electric motor with a wound rotor, equipped with a rheostat, should be used.
If we talk about the advantages of combining the two connection methods, then two can be noted:
- Thanks to the soft start, the service life is increased.
- It is possible to create two power levels for the unit.
Today, the most widely used electric motors are designed to operate in 220 and 380 volt networks. It is on this that the choice of the connection scheme depends. Thus, it is recommended to use the "triangle" at a voltage of 220 V, and the "star" at 380 V.
A three-phase electric motor is an electrical machine designed to operate in alternating current. Such a motor consists of a stator and a rotor. The stator has three windings shifted one hundred and twenty degrees. When a three-phase voltage appears in the winding circuit, magnetic fluxes are formed at the poles, and the rotor rotates. Electric motors are synchronous and asynchronous. Three-phase are widely used in industry and in everyday life. Such motors are single-speed, in which case the motor windings are connected according to the "star" or "delta" scheme, and multi-speed. The last units are switchable, in which case there is a transition from one connection diagram to another.
Three-phase electric motors are divided according to the winding connection schemes. There are two connection schemes - star and delta connection. The star connection of the motor windings is the connection of the ends of the motor windings to one point (zero node): an additional terminal is obtained - zero. The free ends are connected to the phases of the electric current network of 380 V. Outwardly, such a connection resembles a three-pointed star. The photo shows the following diagram: a "star" and "triangle" connection. The connection of the motor windings according to the "triangle" type represents windings: the end of the first is connected to the beginning of the second winding, the end of the second - with the beginning of the third, and the end of the third with the beginning of the first. A three-phase voltage is supplied to the winding connection nodes. With such a connection of the windings, there is no zero terminal. Outwardly, it resembles a triangle.
Star and delta connections are equally common; they do not differ significantly. To connect the windings according to the "star" type (when the motor is operating in nominal mode), the line voltage must be higher than when connected according to the "delta" type. Therefore, in the characteristics of a three-phase motor, they indicate as follows: 220/380 V or 127/220 V. If necessary, it is required to connect with the rated winding in the "star" type, and the rated voltage of the motor will be 380/660 V (in the "delta" type).
It should be noted that a combined star and delta connection is often used. This is done in order to start the electric motor more smoothly. At start-up, a "star" connection is used, and then with the help of a special relay, it switches to "delta", thus reducing the starting current. Such circuits are recommended for starting high-power electric motors that require a large starting current. It is important to remember that in this case, the starting current exceeds the nominal seven times.
There are other combinations when connecting electric motors, for example, a star and delta connection can be replaced by a double, triple star, as well as other connection options. Such methods are used for multi-speed (two-, four-, etc.) electric motors.
Asynchronous three-phase motors are more efficient than single-phase motors and are much more common. Electric devices powered by motor traction are most often equipped with three-phase electric motors.
The electric motor consists of two parts: a rotating rotor and a stationary stator. The rotor is located inside the stator. Both elements have conductive windings. The stator winding is laid in the grooves of the magnetic circuit, observing a distance of 120 electrical degrees. The beginnings and ends of the windings are brought into and fixed in two rows. The contacts are marked with the letter C, each is assigned a digital designation from 1 to 6.
The phases of the stator windings when connected to the mains are connected according to one of the following diagrams:
- "Triangle" (Δ);
- Star (Y);
- combined "star-delta" (Δ / Y) scheme.
Connection by combined scheme used for motors over 5 kW.
« Star"Call the connection of all ends of the stator windings at one point. Nutrition is supplied at the beginning of each of them. When the windings are connected in series into a closed cell, " triangle". The contacts with the terminals are positioned so that the rows are displaced relative to each other, C1 is located opposite the C6 terminal, etc.
The supply of a supply voltage from a three-phase network to the stator windings creates a rotating magnetic field that sets the rotor in motion. The torque generated after the start is insufficient to start. To increase the torque, additional elements are included in the network.
The easiest and most common way to connect to household networks is to connect using a phase-shifting capacitor.
When supplying voltage from both types of power grids, the rotor speed of the induction motor will be almost the same. At the same time, the power in three-phase networks is higher than in similar single-phase ones. Accordingly, connecting a three-phase electric motor to a single-phase network is inevitably accompanied by a noticeable loss of power.
There are electric motors that were not originally designed to be connected to a household network. When purchasing an electric motor for domestic use, it is better to immediately look for models with a squirrel-cage rotor.
Motor connection "star" and "delta" in networks with different rated voltage
In accordance with the rated supply voltage, asynchronous three-phase motors of domestic production are divided into two categories: for operation on 220/127 V and 380/220 V. highly limited.
Electric motors designed for a rated voltage of 380/220 V are ubiquitous.
Regardless of the rated voltage, when installing the motor, the rule is used: lower voltage values are used when connected to the "delta", high - only in the connections of the stator windings in the "star" circuit.
That is, the voltage in 220 V served on " triangle», 380 V- on " a star”, Otherwise the motor will quickly burn out.
The main technical characteristics of the unit, including the recommended connection diagram and the possibility of changing it, are displayed on the motor tag and its technical passport. The presence of a mark of the type Δ / Y indicates the possibility of connecting the windings with both a "star" and a "triangle". In order to minimize power losses, which are inevitable when operating from single-phase household networks, it is better to connect a motor of this type with a "triangle".
Y designates motors where delta connection is not available. In the junction box of such models, instead of 6 contacts, there are only three, the connection of the other three is made under the housing.
Connection of three-phase with a rated supply voltage of 220/127 V to standard single-phase networks is carried out only in the "star" type. Connecting a unit designed for a low supply voltage in the "delta" will quickly render it unusable.
Features of the operation of an electric motor when connected in different ways
Connecting an electric motor with a "triangle" and "star" is characterized by a certain set of its advantages and disadvantages.
Star connection of the motor windings provides a smoother starting. In this case, there is a significant loss of power of the unit. According to this scheme, all electric motors of domestic origin for 380V are also connected.
Delta connection provides output power up to 70% of the nominal, but the starting currents at the same time reach significant values and the motor may be damaged. This circuit is the only correct option for connecting imported European-made electric motors designed for a nominal voltage of 400/690 to Russian power grids.
The start function for star-delta switching circuits is only used for motors marked Δ / Y in which both connection options are possible. The engine is started with a star connection to reduce the starting current.
When the engine accelerates, it shifts to delta to obtain the maximum possible power output.
The use of the combined method is inevitably associated with current surges. At the moment of switching between the circuits, the current supply stops, the rotor rotation speed decreases, in some cases it sharply decreases. After a while, the rotation speed is restored.
Examples of connection with a star and a triangle in the video
