The choice of the connection scheme of the phases of the electric motor — connecting the windings with a star and a delta

To connect an asynchronous electric motor to the network, its stator winding must be star or delta connected.

To connect the electric motor to the network according to the "star" scheme, all the ends of the phases (C4, C5, C6) must be electrically connected to one point and all the beginnings of the phases (C1, C2, C3 ) must be connected to the phases of the network. The correct connection of the ends of the phases of the electric motor according to the "star" scheme is shown in fig. 1, a.

To turn on the electric motor according to the «triangle» scheme, the beginning of the first phase is connected to the horse of the second, and the beginning of the second - to the end of the third, and the beginning of the third - to the end of the first. The windings are connected to three phases of the network. The correct connection of the ends of the phases of the electric motor according to the "delta" scheme is shown in fig. 1, b.

Rice. 1.Schemes for connecting a three-phase asynchronous electric motor to the network: a — the phases are connected in a star, b — the phases are connected in a triangle

Star connection of the motor phases

Rice. 2. Connection of the motor phases according to the "delta" scheme

Rice. 3. Star and delta connection of motor windings

Another picture with wiring diagrams of the windings of the loop of the electric motor in the "star" and in the "delta":

To choose the connection scheme of the phases of a three-phase asynchronous electric motor, you can use the data in Table 1.

Table 1. Selection of coil connection scheme

Motor voltage, V Mains voltage, V 380/220 660/380 380/220 star — 660/380 delta star

The table shows that when an asynchronous motor with an operating voltage of 380/220 V is connected to a network with a network voltage of 380 V, its windings can only be star-connected! It is impossible to connect the ends of the phases of such an electric motor according to the "triangle" scheme. Incorrect selection of the connection scheme of the windings of the electric motor can lead to damage during operation.

The delta winding option is designed to connect 660/380 V motors to the mains with mains voltage 660V and phase 380V… In this case, the windings of the motor can be connected according to the scheme, both «star» and «delta».

These motors can be connected to the mains via a star-delta switch (Fig. 4). This technical solution allows to reduce the inrush current of a three-phase squirrel-cage asynchronous motor with high power.In this case, first the windings of the electric motor are connected according to the «star» scheme (with the lower position of the switching knives), then, when the motor rotor reaches the rated speed, its windings are switched to the «delta» circuit (upper position of the switching knives switching).

Rice. 4. The switching scheme of a three-phase electric motor uses a star-to-delta phase switch

Rice. 5. Star-delta connection

A reduction in starting current when switching its windings from star to delta occurs because instead of the «delta» circuit (660V) designed for a given mains voltage, each winding of the motor is switched on at a voltage of 1.73 times less (380V) . In this case, current consumption is reduced by 3 times. The power developed by the electric motor at start-up is also reduced by 3 times.

But in connection with all of the above, such schematic solutions can only be used for motors with a nominal voltage of 660/380 V and connect them to a network with the same voltage. If you try to connect an electric motor with a nominal voltage of 380/220 V according to this scheme, it will fail, since its phases cannot be connected to the «delta» network.

The nominal voltage of an electric motor can be seen on its box, where its technical passport is placed in the form of a metal plate.

To change the direction of rotation of the electric motor, it is enough to change any two phases of the network, regardless of the scheme of its inclusion (Fig. 6).To change the direction of rotation of the asynchronous electric motor, electrical manual control devices (reversing switches, package switches) or remote control devices (reversing electromagnetic starters) are used. The diagram for connecting a three-phase asynchronous electric motor to the network with a reversing switch is shown in Fig. 7.

Rice. 6. Inverted three-phase asynchronous motor

Rice. 7. Scheme for connecting a three-phase electric motor to the network with a reversing switch

See also: Connection diagram of the electric motor with a reversible magnetic starter