Detailed explanation of star-delta starting circuit diagram and wiring diagram

1. When the load does not have strict requirements on the starting torque of the motor but the starting current of the motor must be limited and the motor meets the 380V/Δ wiring conditions, the star-delta starting method can be used;

2. The method is: connect the motor to star wiring when the motor starts, and then change the motor to delta wiring after the motor starts successfully (quickly switch through a double-throw switch);

3. Since the motor starting current is proportional to the power supply voltage, the starting current provided by the power grid is only 1/3 of the full voltage starting current, but the starting torque is also only 1/3 of the full voltage starting torque. Star-delta starting is a step-down starting, which is achieved by sacrificing power in exchange for reducing starting current. Therefore, the size of the motor power cannot be used to determine whether star-delta starting is needed. It also depends on what kind of load it is. Generally, star-delta starting can be used when the load is light when starting is required and when the load is heavy. Generally speaking, squirrel cage The starting current of a type motor is 5-7 times the operating current, and the voltage requirement for the power grid is generally plus or minus 10% (from my memory). In order to avoid excessive impact on the grid voltage, star-delta starting is required. Generally, the requirements are When the power of the squirrel-cage motor exceeds 10% of the rated power of the transformer, star-delta starting is required. Only squirrel cage motors use star-delta starting. Let's just listen to what one person says. In actual use, I found that motors that require star-delta step-down starting are required starting from 11KW, such as fans. When starting, the current of 11KW is about 7-9 times (100) A. Press The normally configured thermal relay cannot be started at all (closing the damper is useless). The thermal relay is too large and cannot protect the motor, so it is recommended to use a reduced voltage to start. On some motors with smaller starting loads, due to the short time it takes for the motor to reach constant speed, the impact of current impact during starting is small. Therefore, for motors around 30KW, use a circuit breaker with 1.5 times the rated current to start directly. There will be no problem at all in long-term operation. No.

The three-phase winding of the motor with star-delta step-down starting has six external terminals: AX, BY, CZ (the motor with a rated voltage of 380V is used as an example below) Star-shaped starting: XYZ is connected, and the three terminals A, B, and C are connected to the three-phase AC The voltage is 380V. At this time, the voltage of each phase winding is 220, which is much lower than directly adding 380V starting current, avoiding the impact of excessive starting current on the power grid. The torque at this time is relatively small, but the motor can reach a certain speed.

Angular operation: After the motor has been started in star shape and reaches a certain speed for a period of time (about tens of seconds), the electrical switch converts the six terminals into a delta connection and connects it to the 380V power supply again. The voltage of each phase winding is 380V. The torque and speed are greatly increased, and the motor enters the operation process under rated conditions.

The voltage reduction startup here means that the voltage drops from 380 to 220 at the beginning, which is the star connection method. One end of the motor is connected separately, and the three wires at the other end are connected together. After a certain time of starting (usually 30 seconds to one minute), the voltage will drop to 220. When you disconnect the star-shaped ones and connect them to the triangle-shaped ones, they must be interlocked, otherwise they may burst accidentally. The triangle is running all-in.

L1/L2/L3 respectively represent three phase lines;

QS means air switch;

Fu1 represents the insurance on the main circuit;

Fu2 represents the insurance on the control loop;

SP means stop button;

ST means start button;

KT represents the coil of the time relay, and the suffix numbers represent its different contacts;

KMy represents the coil of the star contactor, and the suffix numbers represent its different contacts;

KM△ represents the coil of the delta contactor, and the suffix numbers represent its different contacts;

KM represents the coil of the main contactor, and the suffix numbers represent its different contacts;

U1/V1/W1 respectively represent the three terminals with the same name of the motor winding;

U2/V2/W2 respectively represent the other three ends of the motor winding with the same name;

For the convenience of description, the drawings have been organized and the numbers of the contacts have been added. The compiled drawings are shown in the attached figure. Close QS, press St, KT and KMy will get electric action.

KMY-1 is closed and KM gets electric action;

KMY-2 is closed, the motor coil is in star connection,

KMY-3 is disconnected to avoid KM△ malfunction;

KM-1 is closed, self-protection start button;

kM-2 closure prepares the triangle for work;

KM-3 is closed, the motor is powered and running, and is in star starting state.

After the time relay delay arrives, the delay contact KT-1 is disconnected.

KMy coil is powered off, KMY-1 is disconnected, and KM is still powered through KM-2;

KMY-2 is disconnected to prepare for the motor coil to be in delta connection;

KMY-3 is closed, causing KM△ to be electrically attracted;

KM△-1 is disconnected and stops supplying power to the time relay coil;

KM△-2 is disconnected to ensure that KMY cannot malfunction after receiving power:

KM△-3 is closed when the motor coil is in a triangle operating state.

In the triangle running state of the motor, SP must be pressed to cause all contactor coils to lose power and trip, and then the operation can be stopped.