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The last edit to this post was made by It will be more tomorrow_bZzMU on 2020-2-2 at 15:48. A star-delta starter is actually a step-down starter; during startup, it connects the motor’s stator windings in a star configuration, and once the speed approaches the rated value, it switches them to a delta configuration in order to reduce the starting current. Principle of star-delta starting: 1. The star-delta starting method can be used when the load does not have strict requirements regarding the starting torque of the motor, it is necessary to limit the starting current for motor protection, the motor meets the 380V/Δ wiring conditions, and the stator windings are connected in a triangular configuration during normal operation of the motor. 2. The method is as follows: the motor is connected in star configuration at the time of startup, and once it starts successfully, it is switched to delta configuration (via a double-pole switch for quick switching). 3. Since the motor’s starting current is proportional to the supply voltage, and the starting current provided by the power grid at this time is only 1/3 of the starting current at full voltage, its starting torque is also only 1/3 of the starting torque at full voltage. 4. Star-delta starting is a type of reduced-voltage starting method; it achieves a reduction in starting current at the cost of increased power consumption. Therefore, it cannot be determined solely based on the motor’s power rating to decide whether star-delta starting should be used – the type of load also plays a role in this decision. Star-delta starting is generally applicable in situations where the load is light during startup but heavy during operation. Typically, the starting current of a squirrel-cage motor is 5-7 times its operating current, and the voltage requirement for the power grid is usually within plus or minus 10%. To prevent the motor’s starting current from causing excessive stress on the grid voltage, star-delta starting can be used. It is generally required to use star-delta starting when the power of the squirrel-cage motor exceeds 10% of the transformer’s rated power. 5. In actual use, sometimes when the motor’s power is 11 KW, a star-delta starting method is required. For example, a fan with a rated power of 11 KW generates a current that is 7–9 times higher during startup (around 100 A). A thermal relay with a normal setting cannot enable the motor to start (even closing the air valve doesn’t help). If the thermal relay is sized too large, it fails to provide proper protection for the motor; therefore, it is recommended to use the star-delta starting method. Conditions for using star-delta starting: 1. The star-delta starting method can be employed when the load does not have strict requirements regarding the starting torque of the motor, it is necessary to limit the motor’s starting current, and the motor meets the requirements for 380V/Δ wiring ； 2. The method is as follows: when the motor starts, it is connected in star configuration; once the motor has started successfully, it is then connected in delta configuration (via a double-throw switch for quick switching) ； 3. Since the motor’s starting current is proportional to the supply voltage, the starting current provided by the power grid at this time is only 1/3 of the starting current at full voltage. Star-delta starting is a type of reduced-voltage starting method; it achieves a reduction in starting current at the cost of increased power consumption. Therefore, it cannot be generalized; the decision regarding whether to use star-delta starting depends not only on the motor’s power but also on the type of load. Generally, when the load is light during startup and heavy during operation, star-delta starting can be used. Typically, the starting current of a squirrel-cage motor is 5–7 times its operating current, and the voltage required from the power grid is usually within a range of plus or minus 10%. To avoid excessive stress on the grid voltage, star-delta starting is employed; generally, it is required to use this method when the power of the squirrel-cage motor exceeds 10% of the transformer’s rated capacity. Star-delta starting is only used for squirrel-cage motors. Method of implementing star-delta starting: During motor startup, the motor is connected in a star configuration; once the motor starts successfully, it is then connected in a delta configuration (via a double-pole switch for quick switching) ； Precautions for star-delta starting 1. The wiring of the starter must be correct ； The motor stator windings should be connected in a triangular configuration when operating normally. 2. For manually operated star-delta starters, the switch should be changed when the motor speed approaches the operating speed ； The automatically switching starter should have its delay mechanism properly adjusted according to the motor load requirements. 3. The pressure of the starter contacts shall meet the requirements specified in the product’s technical documentation, and the operation shall be smooth. 4. The starter should be installed vertically, and the installation must be secure. The installation location should be convenient for operation and maintenance. 5. The newly installed oil-immersed star-delta starter must be filled with qualified insulating oil. Before filling with oil, the starter and the oil tank must be cleaned thoroughly, and the oil tank should be dry with no moisture present. The coil insulation should be good; its insulation resistance, measured using a 500V megohmmeter, should be no less than 0.5 MΩ. The oil level must not be below the specified level. 6. The starter housing must be reliably grounded (connected to the neutral). 7. The thermal relay is set correctly according to the motor it controls. 8. After installation, testing must be carried out to check whether the voltage loss trip function operates properly. 9. After operating under load, conduct another thorough inspection of the star-delta starting control cabinet to check for any of the aforementioned noises and vibrations, any smell of burning, whether the instrument readings are correct, and whether the contacts and cores of the AC contactors, relays, etc., are engaging properly. Use a clamp meter to check whether the operating current is normal. It can be put into formal use only after 24 hours of normal operation without any issues.