The star-delta starting method can only be adopted when the load does not have strict requirements on the starting torque of the motor, there is a need to limit the starting current of the motor, and the motor satisfies the 380V/Δ wiring condition.

This method involves wiring the motor in a star configuration during startup. Once the motor has successfully started, it is then switched to a delta configuration (by quickly toggling a double-throw switch).

Since the motor's starting current is directly proportional to the supply voltage, the starting current provided by the grid at this time is only one-third of the full voltage starting current. However, the starting torque is also reduced to one-third of the full voltage starting torque. The star-delta start is a reduced voltage start method that sacrifices power to reduce the starting current.

Therefore, whether to use the star-delta start cannot be determined solely based on the motor's power. It also depends on the type of load. Generally, for situations where the load is light during startup and heavy during operation, the star-delta start can be used. Normally, the starting current of a squirrel-cage motor is 5 to 7 times its operating current. To avoid a significant impact on the grid voltage, which generally has a tolerance of ±10% from what I remember, the star-delta starting method is employed. It is generally required for squirrel-cage motors with power exceeding 10% of the transformer's rated power.

Only squirrel-cage motors use the star-delta starting method. Take this as an opinion; it's worth listening to. In practice, I've found that motors requiring star-delta reduced voltage starting begin at 11KW, like fans, where the starting current can reach 7-9 times (around 100A), making it impossible to start with the normally configured thermal relay (closing the air damper is useless). Oversizing the thermal relay fails to protect the motor, hence the recommendation for reduced voltage starting.

▲ Squirrel-Cage Motor

For some motors with smaller starting loads, since the motor reaches constant speed quickly and the current surge during startup has a minor impact, motors around 30KW can be directly started with a circuit breaker rated at 1.5 times the nominal current, and they work fine over the long term.

The motor's three-phase winding has six external terminals for star-delta reduced voltage starting: A-X, B-Y, C-Z (taking a motor with a rated voltage of 380V as an example). Star starting: Connect X-Y-Z, and apply a three-phase AC voltage of 380V to terminals A, B, and C. In this case, the voltage across each winding is 220V, significantly reducing the starting current compared to direct 380V starting, thus avoiding a large current impact on the grid. The torque at this point is relatively small, but the motor can reach a certain speed.

Delta operation: After the motor continues to run for a period (about several tens of seconds) and reaches a certain speed in star mode, the electrical switch switches the six wiring terminals to a delta connection and applies 380V again, the voltage across each winding becomes 380V, greatly increasing both torque and speed, allowing the motor to operate under rated conditions.

The reduced voltage starting initially reduces the voltage from 380V to 220V, employing the star configuration, where one end of the motor is wired separately and the three wires on the other end are connected together. After starting for a certain period (generally 30 seconds to a minute), the star connection is disconnected and then connected in a delta configuration. It's crucial to have an interlock to prevent accidents. The delta configuration signifies full voltage operation.

• L1/L2/L3 represent the three phase lines;

• QS represents the air switch;

• Fu1 represents the fuse on the main circuit;

• Fu2 represents the fuse on the control circuit;

• SP represents the stop button;

• ST represents the start button;

• KT represents the coil of the time relay, with the suffix number indicating its different contacts;

• KMy represents the coil of the star contactor, with the suffix number indicating its different contacts;

• KM△ represents the coil of the delta contactor, with the suffix number indicating its different contacts;

• KM represents the coil of the main contactor, with the suffix number indicating its different contacts;

• U1/V1/W1 represent the three identical ends of the motor winding;

• U2/V2/W2 represent the other three identical ends of the motor winding;

For ease of explanation, the diagram has been organized and contact numbers have been added. The organized diagram is attached. When QS is closed and St is pressed, KT and KMy are energized.

• KMY-1 closes, energizing KM;

• KMY-2 closes, placing the motor coil in a star configuration;

• KMY-3 opens to prevent KM△ from operating by mistake;

• KM-1 closes for self-retaining of the start button;

• kM-2 closes preparing for delta operation;

• kM-3 closes, the motor operates in star starting mode.

• After a delay, the delayed contact KT-1 opens,

• KMy coil de-energizes, KMY-1 opens, but KM remains energized through KM-2;

• KMY-2 opens, preparing the motor coil for delta configuration;

• KMY-3 closes, energizing KM△;

• KM△-1 opens, stopping power to the time relay coil;

• KM△-2 opens to ensure KMY cannot be energized by mistake;

• KM△-3 closes, putting the motor coil in delta operation mode.

To stop the motor in delta operation mode, SP must be pressed to de-energize all contactor coils and stop the motor.

▲ Wiring Diagram