Sometimes an AC 3-phase motor star connection below 4kW. Why?

The reason for using a star connection for three-phase AC motors below 4 kW.

AC three-phase motors below 4kW typically use a star (Y) connection due to several advantages it offers:

1. Reduce the voltage on the windings: In star connection, each phase winding is subjected to a voltage that is 1/√3 of the line voltage, i.e., 220V instead of 380V. This helps to reduce the voltage on the windings, thereby lowering the insulation level requirements.

2. Reduce Starting Current: Star connection significantly reduces starting current, which is beneficial for protecting the motor and electrical equipment from excessive impact currents. Lower starting current also helps extend the motor's lifespan.

3. Suitable for small power motors: Due to the star connection's ability to effectively reduce power, it is particularly suitable for use in small power motors. Motors below 4 kW usually do not require high power output, making the star connection a suitable choice.

Differences between star connection and delta connection

• Power Size: For motors with delta connection, star-delta starting is used for light load starting to reduce the starting current. Light load is a condition because the torque will be smaller with star connection, and reducing the starting current is the purpose by using star connection to lower the starting current. Delta connection has high power and large starting current, while star connection has low power and small starting current.

• Pros and Cons:

• Delta Connection: This method helps to increase motor power, but the disadvantage is that it has a large starting current and the winding withstands a high voltage (380V).

• Star Connection: This helps to reduce the voltage (220V) on the winding, lowering the insulation grade and decreasing the starting current. However, the drawback is that it reduces the motor power.

Conclusion

In summary, the star connection for three-phase AC motors below 4 kW is mainly used to reduce the voltage and starting current borne by the winding, while adapting to their low power requirements. This connection method helps protect the motor and electrical equipment and extend the service life of the motor.