Asynchronous motors offer many operational advantages. This is reliability, high power, good performance. Connecting the electric motor with star and delta ensures its stable operation.
There are two main parts to an electric motor: a rotating rotor and a static stator. Both have a set of conductive windings in their structure. The electrical windings of the stationary element are located in the grooves of the magnetic wire at a distance of 120 degrees. All ends of the windings are output to the electrical distribution block and are fixed there. Contacts are numbered.
Motor connections can be star, delta, as well as all kinds of switching. Each connection has its own advantages and disadvantages. Motors connected in a star configuration have smooth, soft operation; the action of the electric motor is limited by power compared to a triangle, since its value is one and a half times greater.
An association V one general point: star connection
The ends of the stator windings are connected together at one point. Three-phase voltage is supplied to the beginning of the windings. The value of inrush currents when connecting a triangle is more powerful. A star connection means a connection between the ends of the stator winding. Voltage is supplied to the beginning of each winding.
The windings are connected in series with a closed cell and form a triangular connection. The rows of contacts with terminals are located parallel to each other. For example, the beginning of pin 1 is opposite the end of 1. Network power is supplied to the stator windings, creating rotation of the magnetic field, leading to the movement of the rotor. The torque generated after connecting a three-phase electric motor is insufficient for starting. An increase in the rotating element is achieved by using an additional element. For example, a three-phase frequency converter connected to an asynchronous motor in the figure below.
Connection drawing of a classic frequency converter with a star
According to this scheme, domestic 380 volt motors are connected.
Mixed way
The combined connection type is applicable for electric motors with a power of 5 kW or more. The star-delta circuit is used when it is necessary to reduce the starting currents of the unit. The principle of operation begins with a star, and after the engine reaches the required speed, it automatically switches to a triangle.
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This scheme is not suitable for devices with overloads, as a weak torque occurs, which can lead to breakdown.
Principle work
The power supply starts using the second and relay contact. Then the third starter is triggered on the stator, thereby opening the circuit formed by the coil of the third element, and a short circuit occurs in it. Next, the first stator winding begins to work. Then a short circuit occurs in the magnetic starter, a temporary thermal relay is triggered, which closes at the third point. Next, the contact of the temporary thermal relay in the electrical circuit of the second stator winding is observed to close. After disconnecting the windings of the third element, the contacts in the chain of the third element are closed.
To the beginning of the windings, current passes in three phases. It enters through the power contacts of the magnet of the first element. The contacts of the third starter turn it on and close the ends of the windings, which are connected by a star.
Then the time relay of the first starter is turned on, the third is turned off, and the second is turned on. Contacts K2 are closed, voltage is supplied to the ends of the windings. This is the inclusion of a triangle.
Various manufacturers make the start relay needed to start an electric motor. They differ in appearance and name, but perform the same function.
Typically, the connection to network 220 occurs with a phase-shifting capacitor. Power comes from any electrical network and rotates the rotor at the same frequency. Of course, the power from a three-phase network will be greater than from a single-phase one. If a three-phase motor operates from a single-phase network, power is lost.
Some types of motors are not designed to operate from a household network. Therefore, when choosing a device for your home, preference should be given to motors with squirrel-cage rotors.
Based on rated power, domestic electric motors are divided into two types: 220 - 127 volts and 380 - 220 volts. The first type of low-power electric motors is used infrequently. The second devices are widespread.
When installing an electric motor of any power, a certain principle applies: devices with low power are connected in a triangle, and devices with high power are connected in a star. The 220 power supply goes to the delta connection, the 380 voltage goes to the star connection. This will ensure long and high-quality operation of the mechanism.
The recommended diagram for connecting the motor is listed in the technical document. The △ icon means a connection in the same form. The letter Y indicates the recommended star connection. The characteristics of numerous elements are indicated by colors, due to their small dimensions. For example, the denomination or resistance can be read by color. If both signs are present, then the connection is possible by switching △ and Y. When there is one specific marking, for example, Y, then the connection available will only be in a star configuration.
Circuit △ provides output power up to 70 percent, the value of inrush currents reaches the maximum value. And this can ruin the engine. This circuit is the only option for operating foreign asynchronous motors with a power of 400 - 690 volts from Russian electrical networks.
Therefore, choosing the correct connection or switching must take into account the characteristics of the electrical network and the power of the electric motor. In each case, you should familiarize yourself with the technical characteristics of the motor and the equipment for which it is intended.
In this article I would like to tell you how the engine power changes with the winding connection diagram star – triangle and vice versa.
Due to the specifics of my work, I am faced with repairs of various asynchronous motors, and in most cases, motor failure occurs when the motor windings are switched incorrectly, since people do not understand how the motor power changes when switching from delta to star and back, and how this may affect the performance of the engine itself.
It is known [L1. With. 34], that when connected into a star, the linear currents Il and phase currents Iph are equal to each other, while there is a relationship between the phase Uph and line voltage Ul, where Ul = √3*Uph, as a result Uph = Ul/√3.
Based on this, the total power is determined through linear quantities:
In a delta connection scheme, the phase and line voltages are equal to each other Ul = Uph, while there is a relationship between the currents: Il = √3*Iph, as a result Iph = Il/√3.
Based on this, the total power is determined as:
To determine active and reactive power, the following formulas are used:
Due to the fact that the formulas for the star and triangle connection diagrams have the same form, few experienced engineers have misunderstandings that the type of connection is indifferent and does not affect anything.
Let's look at an example to see how erroneous these statements are. In this example, we will consider an electric motor of type AIR90L2, which has two connection diagrams ∆/Y, technical characteristics of the motor:
- power factor cosφ = 0.84;
- efficiency factor, η = 78.5%;
We determine the motor current at a voltage of 380 V and a triangle connection diagram; the power with this connection is 3 kW:
Now let's connect the motor windings in a star. As a result, the phase winding had a 1.73 times lower voltage Uph = Ul/√3, respectively, and the current decreased by 1.73 times, but since when connected into a triangle, Ul = Uph, and the linear current was 1.73 times more than phase Il = √3*Iph, it turns out that when connected to a star, the power will decrease by √3*√3 = 3 times, and accordingly the current will decrease by 3 times.
From all of the above, the following conclusions can be drawn:
1. When switching the engine from star to delta, engine power increases 3 times and vice versa. You can use these switchings if the motor wiring diagram allows for ∆/Y switching, otherwise the motor may burn out when you switch from star to delta.
2. As you already understood, using a circuit for switching the motor windings from star to delta, we reduce the starting currents when starting the motor at a reduced voltage, and then increase it to the rated voltage. When the motor windings are connected in a star, each of them is supplied with a voltage less than the rated voltage by 1.73 times. During the starting process, the motor increases rotation speed and the current decreases. At this time, the switch to triangle occurs.
Please note that engines that are underloaded operate with very low cosφ. Therefore, it is recommended to replace the underloaded engine with an engine of lower power. If an underloaded engine has a large power reserve, then cosφ can be increased by switching windings from delta to star without the risk of overheating the engine.
As we see, there is nothing complicated in determining the power in a star-delta circuit.
Literature:
1. Star and triangle. E.A. Kaminsky, 1961
Electric motor connection diagrams. Star, triangle, star - triangle.
Asynchronous motors, having a number of undeniable advantages such as reliability in operation, high performance, the ability to withstand large mechanical overloads, unpretentiousness and low cost of maintenance and repair due to the simplicity of the design, have, of course, their own certain disadvantages.
In practice, the main methods of connecting three-phase electric motors to the network are used: “star connection” and “delta connection”.
When connecting a three-phase electric motor with a star, the ends of its stator windings are connected together, the connection occurs at one point, and three-phase voltage is supplied to the beginning of the windings (Figure 1).
When connecting a three-phase electric motor according to the “triangle” connection diagram, the stator windings of the electric motor are connected in series in such a way that the end of one winding is connected to the beginning of the next and so on (Figure 2).
Without going into the technical and theoretical foundations of electrical engineering, it is known that electric motors with windings connected by a star operate smoother and softer than electric motors with windings connected by a triangle, it should be noted that when the windings are connected by a star, the electric motor cannot develop full power. When the windings are connected according to a delta circuit, the electric motor operates at full rated power (which is 1.5 times more power than when connected by a star), but at the same time it has very high starting currents.
In this regard, to reduce starting currents, it is advisable (especially for electric motors with higher power) to connect according to the star-delta circuit; Initially, the start-up is carried out according to the “star” circuit, after which (when the electric motor has “gained speed”), automatic switching occurs according to the “triangle” circuit.
Control circuit:
Another version of the engine control circuit
Connecting the supply voltage through the NC (normally closed) contact of the time relay K1 and the NC contact K2, in the starter coil circuit K3.
After turning on the starter K3, with its normally closed contacts it opens the circuits of the coil of the starter K2 with contacts K3 (blocking accidental switching) and closes contact K3 in the power circuit of the coil of the magnetic starter K1, which is combined with the contacts of the time relay.
When the starter K1 is turned on, the contacts K1 close in the coil circuit of the magnetic starter K1 and at the same time the time relay turns on, the contact of the time relay K1 opens in the coil circuit of the starter K3, and the time relay contact K1 closes in the coil circuit of the starter K2.
Switching off the winding of the starter K3, contact K3 closes in the coil circuit of the magnetic starter K2. After turning on the starter K2, it opens its contacts K2 in the circuit of the power coil of the starter K3.
(Start of stator windings: U1; V1; W1. Winding ends: U2; V2; W2. On the terminal board, the studs of the beginning and ends of the windings are located in strict sequence: W2; U2; V2; below them are located: U1; V1; W1. When connecting the motor in a “triangle”, the studs are connected by jumpers: W2-U1; U2-V1; V2-W1.)
Three-phase voltage is supplied to the beginning of the windings U1, V1 and W1 through the power contacts of the magnetic starter K1. When the magnetic starter K3 is triggered using its contacts K3, a short circuit occurs, connecting the ends of the windings U2, V2 and W2 to each other; the motor windings are connected by a star.
After some time, the time relay, combined with the starter K1, is activated, turning off the starter K3 and simultaneously turning on K2, the power contacts of K2 are closed and voltage is supplied to the ends of the motor windings U2, V2 and W2. Thus, the electric motor is switched on in a triangle pattern.
To start engines according to the star-delta circuit, different manufacturers produce so-called starting relays, they may have different names: “Start time relays”, “start-delta” relays, etc., but their purpose is the same:
Typical circuit with a starting time relay (star/delta relay) to control the start of a three-phase asynchronous motor:
Conclusion: To reduce starting currents, the engine must be started in the following sequence: first switched on in a star configuration at low speeds, then switch to delta.
Starting first with a triangle creates the maximum torque, and then switching to a star (the starting torque is 2 times less) with further operation in the nominal mode, when the electric motor has “gained speed”, an automatic switchover to a triangle occurs, it is worth taking into account the load on the shaft before starting, After all, the torque at the star is weakened, so this starting method is unlikely to be suitable for very loaded engines and may fail.
It happens that a three-phase electric motor falls into your hands. It is from such engines that homemade circular saws, emery machines and various types of shredders are made. In general, a good owner knows what can be done with it. But the trouble is, a three-phase network in private homes is very rare, and it is not always possible to install it. But there are several ways to connect such a motor to a 220V network.
It should be understood that the engine power with such a connection, no matter how hard you try, will drop noticeably. Thus, a delta connection uses only 70% of the engine power, and a star connection uses even less - only 50%.
In this regard, it is desirable to have a more powerful engine.
Important! When connecting the motor, be extremely careful. Take your time. When changing the circuit, turn off the power supply and discharge the capacitor with an electric lamp. Work with at least two people.
So, in any connection scheme, capacitors are used. In essence, they act as the third phase. Thanks to it, the phase to which one terminal of the capacitor is connected shifts exactly as much as necessary to simulate the third phase. Moreover, to operate the engine, one capacity is used (working), and for starting, another (starting) is used in parallel with the working one. Although this is not always necessary.
For example, for a lawn mower with a blade in the form of a sharpened blade, a 1 kW unit and only working capacitors will be sufficient, without the need for containers for starting. This is due to the fact that the engine is idling when starting and it has enough energy to spin the shaft.
If you take a circular saw, a hood or another device that puts an initial load on the shaft, then you cannot do without additional banks of capacitors for starting. Someone may say: “why not connect the maximum capacity so that there is not enough?” But it's not that simple. With such a connection, the motor will overheat and may fail. Don't risk your equipment.
Important! Whatever the capacitance of the capacitors, their operating voltage must be at least 400V, otherwise they will not work for a long time and may explode.
Let's first consider how a three-phase motor is connected to a 380V network.
Three-phase motors come with either three terminals - for connection to a star only - or with six connections, with the ability to select a circuit - star or delta. The classic scheme can be seen in the figure. Here in the figure on the left is a star connection. The photo on the right shows how it looks on a real engine frame.
It can be seen that for this it is necessary to install special jumpers on the required pins. These jumpers come with the motor. In the case where there are only 3 terminals, the star connection is already made inside the motor housing. In this case, it is simply impossible to change the winding connection diagram.
Some say that they did this to prevent workers from stealing units from home for their own needs. Be that as it may, such engine options can be successfully used for garage purposes, but their power will be noticeably lower than those connected by a triangle.
Connection diagram for a 3-phase motor in a 220V network connected by a star.
As you can see, the 220V voltage is distributed over two series-connected windings, where each is designed for such a voltage. Therefore, the power is lost almost twice, but such an engine can be used in many low-power devices.
The maximum power of a 380V motor in a 220V network can only be achieved using a delta connection. In addition to minimal power losses, the engine speed also remains unchanged. Here, each winding is used for its own operating voltage, hence the power. The connection diagram for such an electric motor is shown in Figure 1.
Fig. 2 shows a terminal with a 6-pin terminal for delta connection. The three resulting outputs are supplied with: phase, zero and one terminal of the capacitor. The direction of rotation of the electric motor depends on where the second terminal of the capacitor is connected - phase or zero.
In the photo: an electric motor only with working capacitors without starting capacitors.
If there is an initial load on the shaft, it is necessary to use capacitors for starting. They are connected in parallel with the workers using a button or switch at the time of switching on. As soon as the engine reaches maximum speed, the starting tanks should be disconnected from the workers. If it is a button, we simply release it, and if it is a switch, then we turn it off. Then the engine uses only working capacitors. Such a connection is shown in the photo.
How to select capacitors for a three-phase motor using it in a 220V network.
The first thing you need to know is that the capacitors must be non-polar, that is, not electrolytic. It is best to use containers of the brand ― MBGO. They were successfully used in the USSR and in our time. They perfectly withstand voltage, current surges and the damaging effects of the environment.
They also have mounting eyes that help you easily place them at any point on the device’s body. Unfortunately, getting them now is problematic, but there are many other modern capacitors that are no worse than the first ones. The main thing is that, as mentioned above, their operating voltage is not less than 400V.
Calculation of capacitors. Capacity of the working capacitor.
In order not to resort to long formulas and torture your brain, there is a simple way to calculate a capacitor for a 380V motor. For every 100 W (0.1 kW) 7 µF is taken. For example, if the motor is 1 kW, then we calculate it like this: 7 * 10 = 70 µF. It is extremely difficult to find such a capacity in one jar, and it is also expensive. Therefore, most often the containers are connected in parallel, gaining the required capacity.
Starting capacitor capacity.
This value is taken at the rate of 2-3 times greater than the capacity of the working capacitor. It should be taken into account that this capacity is taken in total with the working capacity, that is, for a 1 kW motor, the working capacity is equal to 70 μF, multiply it by 2 or 3, and get the required value. This is 70-140 µF of additional capacitance - starting. At the moment of switching on, it is connected to the working one and the total is 140-210 µF.
Features of the selection of capacitors.
Capacitors, both working and starting, can be selected using the method from smallest to largest. Having thus selected the average capacity, you can gradually add and monitor the operating mode of the engine so that it does not overheat and has enough power on the shaft. Also, the starting capacitor is selected by adding until it starts smoothly without delays.
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 by one hundred and twenty degrees. When three-phase voltage appears in the winding circuit, magnetic fluxes are formed at the poles and the rotor rotates. Electric motors are either synchronous or asynchronous. Three-phase ones are widely used in industry and in everyday life. Such motors can be single-speed, in which case the motor windings are connected in a star or delta pattern, and multi-speed. The latter units are switchable, in which case there is a transition from one connection scheme to another.
Three-phase electric motors are divided according to winding connection diagrams. There are two connection schemes - star and delta connections. Connecting the motor windings according to the “star” type is a connection of the ends of the motor windings at one point (zero node): an additional terminal is obtained - zero. The free ends are connected to the phases of the 380 V electric current network. Externally, this connection resembles a three-pointed star. The photo shows the following diagram: a “star” and “delta” connection. Connecting the windings of an electric motor according to the “delta” type is a winding: the end of the first is connected to the beginning of the second winding, the end of the second to the beginning of the third, and the end of the third to the beginning of the first. Three-phase voltage is supplied to the winding connection nodes. With this connection of the windings, there is no zero terminal. Outwardly, it resembles a triangle.
Star and delta connections are equally common and do not differ significantly. To connect the windings in a star type (when the engine is operating in rated mode), the line voltage must be greater than when connected in a delta type. Therefore, the characteristics of a three-phase motor are indicated as follows: 220/380 V or 127/220 V. If necessary, the rated windings must be connected as a star, and the rated voltage of the motor will be 380/660 V (delta type).
It should be noted that a combined star and delta connection is often used. This is done in order to start the electric motor more smoothly. When starting, a star connection is used, and then a special relay is used to switch to delta, thus reducing the starting current. Such circuits are recommended for starting high-power electric motors that require large starting current. It is important to remember that in this case the starting current exceeds the rated current by seven times.
There are other combinations when connecting electric motors, for example, a star and delta connection can be replaced by a double, triple star, as well as other connection options. Such methods are used for multi-speed (two-, four-, etc.) electric motors.
