A star-delta starter is a type of electrical starter that is used to start a three-phase induction motor. It is called a "star-delta" starter because it uses a "delta" connection for starting and a "star" connection for running.
The main purpose of a star-delta starter is to reduce the starting current of a three-phase induction motor. When a three-phase motor is started directly online, it can draw a very high starting current, which can cause problems such as voltage dips, circuit breaker tripping, and torque pulsations. By using a star-delta starter, the starting current can be reduced to approximately one-third of the direct-on-line starting current. This allows the motor to be started smoothly and reduces the stress on the electrical system.
The star-delta starter works by connecting the motor to the supply voltage in a "delta" configuration during the starting phase. This reduces the starting torque and current of the motor. After the motor reaches a certain speed, the starter switches the motor to a "star" configuration, which increases the motor torque and allows it to run at its full speed.