Tähtikytkentä

Tähtikytkentä is a Finnish term referring to a star connection, a method of connecting windings in AC electrical machines such as transformers and three-phase motors. In a star connection, one end of each of the three windings is connected to a common neutral point, while the other ends are connected to the three phases of the power supply. This configuration creates a neutral wire that can be used as a reference point for voltage or for carrying current in unbalanced loads.

The primary advantage of a star connection in a three-phase system is its ability to provide both line voltage and phase voltage. The line voltage, measured between any two phases, is $\sqrt{3}$ times the phase voltage, which is the voltage across a single winding. This means that for a given phase voltage, a star-connected system can deliver a higher line voltage. It also allows for the connection of single-phase loads between a phase and the neutral point.

In transformers, star connection is often used for high-voltage windings due to the lower voltage stress on the insulation, as the voltage across each winding is reduced. In three-phase motors, the star connection determines the motor's operating voltage and current characteristics. It is also a common method for starting smaller three-phase motors, sometimes in conjunction with a delta connection, to reduce starting current. The choice between star and delta connection depends on the specific application requirements, such as voltage levels, load characteristics, and starting methods.