Stardelta

Stardelta, or star-delta starting, is a method used to start three-phase induction motors by initially connecting the windings in star (Y) configuration and then switching to delta (Δ) operation for normal running. The technique reduces the starting current and torque compared with direct-on-line starting, which helps to limit voltage dips and mechanical stress on the power system and the motor.

In operation, the motor begins with the windings connected in star, which lowers the phase voltage to approximately one-third of the line voltage. After a short interval, a control circuit reconfigures the connections to delta, applying full line voltage to each winding. The sequence is typically controlled by contactors and a timer or control logic, and interlocks ensure that the motor cannot be connected to both configurations at once.

Performance-wise, the starting current is reduced to about one third of the direct-on-line start, and the starting torque is reduced accordingly. The exact values depend on motor design and load conditions. Star-delta starting is commonly used for belt-driven fans, pumps, and compressors where the load is light at start and the system can tolerate slower acceleration.

Limitations include only modest reductions in starting torque and current, making it less suitable for high-inertia or high-torque loads. It also relies on the motor and drive being designed to tolerate the star configuration. Modern alternatives such as autotransformer starting, soft start devices, or variable-frequency drives offer greater control over starting current and torque and are increasingly favored in new installations.

Stardelta remains in use in older or cost-sensitive installations where a simple, low-cost starting method is adequate.