What is a Start Delta Starter?
Star delta starter is a starting device of asynchronous motors. The starting current of induction motors is relatively large during the starting process, so motors with large capacity can use the wye delta starter. The star delta starter is actually a step-down starter. When it starts, the stator winding of the motor is connected into a star shape, and when the speed reaches close to the rated speed, it is changed into a △ to reduce the starting current.
When using the star delta starter, the starting current is only 1/3 of the original direct starting according to the delta connection. If the starting current in direct starting is calculated as 6 ~ 7Ie, in star delta starter, the starting current is only 2~2.3 times. At the same time, the starting voltage is only 1/3 of the root sign of the original delta connection method when starting directly. When the load has no strict requirements on the starting torque of the motor, and the starting current of the motor must be limited, and the motor meets the 380V/Δ wiring conditions, the star delta starter can be used.
The star delta starter can only be used when the load does not have strict requirements on the starting torque of the induction motor but also limits the starting current of the motor, the motor meets the 380V/Δ wiring conditions, and the stator windings are connected in a delta during normal operation of the motor. When the motor starts, connect the motor to a star connection, and then change the motor to a delta connection when the motor starts successfully (quickly switch through the double throw switch). Since the starting current of the motor is proportional to the power supply voltage, the starting current provided by the grid is only 1/3 of the starting current of the full voltage, so its starting torque is only 1/3 of the starting torque of the full voltage.
Star delta starter is a step-down starter, which is realized by sacrificing power in exchange for lowering the starting current. Therefore, it is impossible to generalize the size of the motor power to determine whether to use star delta starter, but also depends on what kind of load. Generally, star delta starter can be used when the load is light at startup and the load is heavy during operation. Usually, the starting current of squirrel-cage motor is 5 - 7 times of the running current, and the voltage requirement of the power grid is generally plus or minus 10%. So that the starting current of the motor does not have an excessive impact on the grid voltage, star delta starter can be used. It is generally required that the star-delta start is used when the power of the squirrel-cage motor exceeds 10% of the rated power of the transformer.
In the actual application, the motor power is 11kW, and the star delta starter is required. For example, the current of a fan motor with a rated power of 11KW is 7 - 9 times (about 100A) at start-up, and the thermal relay according to the normal configuration cannot be started at all. The thermal relay is too large to protect the motor, so it is recommended to use a star delta starter.