The design of a three-phase electric motor is an electric machine, for the normal operation of which three-phase AC networks are required. The main parts of such a device are the stator and rotor. The stator is equipped with three windings shifted by 120 degrees. When a three-phase voltage appears in the windings, magnetic fluxes are formed at their poles. Due to these flows, the motor rotor begins to rotate.
In industrial production and in everyday life, the widespread use of three-phase asynchronous motors is practiced. They can be single-speed, when a star and delta connection of the motor windings is made, or multi-speed, with the ability to switch from one circuit to another.
Connection of windings with a star and a delta
For all three-phase electric motors, the windings are connected in a star or delta pattern.
When connecting the windings according to the star scheme, their ends are connected at one point at the zero node. Therefore, one more additional zero output is obtained. The other ends of the windings are connected to the phases of the 380 V network.
The delta connection is the series connection of the windings. The end of the first winding is connected to the start end of the second winding, and so on. Ultimately, the end of the third winding will connect to the beginning of the first winding. Three-phase voltage is supplied to each connection node. The triangle connection is distinguished by the absence of a neutral wire.
Both types of compounds have received approximately the same distribution and do not have significant distinguishing features among themselves.
There is also a combined connection, when both options are used. This method is used quite often, its purpose is a smooth start of the electric motor, which cannot always be achieved with conventional connections. At the moment of direct start, the windings are in the star position. Next, a relay is used that provides switching to the delta position. Due to this, the starting current is reduced. The combined circuit is most often used during the start-up of electric motors with high power. For such motors, a significantly higher starting current is also required, exceeding the nominal value by about seven times.
Electric motors can be connected in other ways when a double or triple star is used. These connections are used for motors with two or more variable speeds.
Starting a three-phase electric motor with switching from star to delta
This method is used in order to reduce the starting current, which can be about 5-7 times the rated current of the motor. Units with too much power have such a starting current at which fuses easily blow out, circuit breakers turn off and, in general, the voltage drops significantly. With such a decrease in voltage, the incandescence of the lamps decreases, the torque of other electric motors decreases, and the contactors also spontaneously turn off. Therefore, various methods are used to reduce the starting current.
Common to all methods is the need to reduce the voltage in the stator windings at the time of direct start. To reduce the starting current, the stator circuit can be supplemented with a choke, rheostat or automatic transformer during the start.
The most widespread is the switching of the winding from a star to a triangle position. In the star position, the voltage becomes 1.73 times less than the nominal voltage, so the current will also be less than at full voltage. During start-up, the motor speed increases, the current decreases and the windings switch to the delta position.
Such switching is allowed in electric motors with an easy start mode, since the starting torque decreases by about half. In this way, those engines are switched that can be structurally connected in a triangle. They must have windings capable of operating at.
When to switch from delta to star
When it is necessary to make a star-delta connection of the motor windings, it should be remembered that it is possible to switch from one type to another. The main option is the star-delta switching circuit. However, if necessary, the reverse is also possible.
Everyone knows that motors that are not fully loaded experience a decrease in power factor. Therefore, it is desirable to replace such motors with devices with lower power. However, if replacement is not possible and there is a large power reserve, delta-star switching is performed. The current in the stator circuit must not exceed the nominal value, otherwise the motor will overheat.