Three-phase motors above a few horsepower almost always have six leads brought out to the terminal box, not three. The reason: the same windings can be reconnected in two fundamentally different topologies — star (also called wye, or Y) and delta (Δ) — and the choice changes how much voltage each winding sees, how much current it draws, and how much torque the motor produces.

The geometry:

• Star (Y): One end of each winding ties to a common neutral point. Each winding sees the line-to-neutral voltage, which is the line voltage divided by √3 (≈1.732).
• Delta (Δ): The windings form a closed triangle. Each winding sees the full line-to-line voltage.

The consequence: A winding designed for 400 V will be happy in delta on a 400 V supply, or in star on a 690 V supply (since 690/√3 ≈ 400). That's why European motor nameplates often read "400/690 V Δ/Y" — the same motor, two supplies.

Star-Delta starting (the classic trick): Direct-on-line (DOL) starting of a large motor can draw 6–8× full-load current, slamming the supply and tripping breakers. Wire the motor in star during startup, and each winding sees only 1/√3 of the rated voltage. Since current is proportional to voltage, line current drops by a factor of 3, and starting torque drops by the same factor of 3. Once the motor is near speed, a contactor switches it to delta for full power.

Quick calculation: A 30 kW, 400 V delta-connected motor draws roughly 55 A full-load. Starting DOL: ~350 A. Starting in star: ~117 A — a third of DOL inrush, but also only a third of the starting torque. Rule of thumb: star-delta works only if the load can accelerate at ⅓ rated torque (pumps and fans usually can; loaded conveyors and compressors usually can't).

Power relationships you'll need:

• In star: line current = phase current; line voltage = √3 × phase voltage.
• In delta: line voltage = phase voltage; line current = √3 × phase current.
• Total three-phase power either way: P = √3 × V_line × I_line × cos φ.

Failure mode to know: If a star-delta starter's transition timing is wrong, the motor briefly sees an open circuit (or worse, both contactors closed simultaneously, which is a phase-to-phase short through the windings). Modern soft starters and VFDs have largely replaced star-delta in new installs, but you'll find millions of legacy installations still humming along on six-wire terminal boxes.