For low-voltage motors, the connection type depends on the power rating.

• Motors up to 3 kW usually use a star (Y) connection,

• Motors above 3 kW typically use a delta (Δ) connection.

For variable-frequency (VFD) motors, the dividing line is higher:

• Motors up to 45 kW are star-connected,

• Those above 45 kW use delta connection to handle higher currents.

In crane and metallurgical motors, star connection is more common due to safety and control stability, though large-capacity lifting motors may adopt delta connection for higher torque performance.

Star Connection (Y-Connection)

• Wiring method: The three-phase power lines (L1, L2, L3) are connected to the three starting points of the motor windings (U1, V1, W1), while the other ends (U2, V2, W2) are connected together at a common point, forming a "star" shape.

• Features:

• Lower starting current: Star connection reduces the voltage applied to the motor during startup, resulting in lower inrush current, which helps protect the motor and the power grid.

• Lower starting torque: It is suitable for applications where the motor doesn’t require a high starting torque.

• Energy-efficient: The lower current consumption leads to more energy-efficient operation, making it ideal for long-duration, low-power applications.

• Balanced load: The current and load are more evenly distributed across the motor windings.

Delta Connection (Δ-Connection)

• Wiring method: In a Delta connection, the three-phase power lines (L1, L2, L3) are directly connected to the starting points of the motor windings (U1, V1, W1). The other ends (U2, V2, W2) form a loop, connecting each winding to the next in a triangle shape.

• Features:

• Higher starting torque: Delta connection allows for higher starting current, providing greater starting torque, which is suitable for high-power applications.

• Higher efficiency: Once the motor reaches full speed, Delta connection allows it to operate at a higher power output, with lower resistance and better efficiency.

• Risk of unbalanced load: If one of the windings fails, it can lead to an unbalanced load, which may damage the motor.

• Ideal for continuous operation: Delta connection is commonly used for motors that need to run for long periods at higher loads, offering better performance and efficiency after reaching operational speed.

Each phase measured separately for phase resistance, voltage, and current—mainly to locate a faulty phase when a problem occurs. During normal operation, we mainly monitor line resistance, line current, and line voltage.

Star (Y) connection

• DC phase resistance: R_phase=R_line/2

• Phase voltage: U_phase=U_line/√3

• Phase current: I_phase=I_line

Delta (Δ) connection

• DC phase resistance: R_phase=3R_line/2

• Phase voltage: U_phase=U_line

• Phase current: I_phase=I_line/√3