Star-delta step-down starting control circuit for squirrel cage asynchronous motor

For squirrel cage asynchronous motors above 10kW, their large starting current (5 to 7 times the rated current) will have a huge impact on the power supply system, so they are generally not started directly at full voltage, and are usually started by reduced voltage. Since squirrel cage asynchronous motors above 4kW are all connected in a triangle during normal operation, the use of star-triangle reduced voltage starting can effectively limit the starting current. The star-triangle reduced voltage starting control circuit is shown in the figure below.

When starting, the motor stator winding is connected in star shape, so that the voltage applied to each phase of the motor winding is the rated value, and the current is only 1/3 of the rated value, thereby significantly reducing the starting current. When the motor speed gradually increases and approaches the rated value, the stator winding is switched to a triangle connection and switched to normal operation at the rated voltage.

In order to realize the automatic switching of the starting process, a time relay KT is used in the control circuit. After pressing the start button SB2, the contactor KM is energized and self-locked, KMY is also energized, and the motor starts to run in star connection. At the same time, the coil of the time relay KT is also energized and starts timing. When the set time is reached, its contacts act, the delayed disconnection contact of KT disconnects KMY, and the delayed closing contact connects KM△ and self-locks, so that the stator winding of the motor switches to the triangle connection and turns to normal operation under the rated voltage.