Star-Delta Starting Principle

In Y-Δ starting, the motor windings are initially connected in star (Y), reducing the voltage across each winding to V_L/√3. After the motor reaches ~80% speed, it switches to delta (Δ) for full voltage operation.

Both current and torque are reduced by a factor of 3 (not √3). This is because the winding voltage is reduced by √3, and both current and torque scale with voltage².

Contactor Sizing

• KM1 (Main/Line) — Carries full line current; size for rated current
• KM2 (Delta) — Carries phase current = line current / √3 ≈ 58% of rated
• KM3 (Star) — Carries star current = line current / 3 ≈ 33% of rated

Changeover Time Guidelines

The timer for star-to-delta transition should be set so the motor reaches approximately 75–85% of rated speed before switching. Too early = high transition current spike. Too late = motor overheats in star.

Motor Power	Recommended Timer	Notes
Up to 7.5 kW	3–5 seconds	Small motors accelerate quickly
7.5–30 kW	5–10 seconds	Medium motors, typical industrial
30–90 kW	8–15 seconds	Large motors, higher inertia
>90 kW	10–20 seconds	Very large motors, adjust by monitoring current

Comparison: Starting Methods

Method	Starting Current	Starting Torque	Cost	Best For
DOL	6–8× In	1.5–2.5× Tn	Low	Small motors <7.5 kW
Star-Delta	2–3× In	0.5–0.8× Tn	Medium	Medium motors, light starting load
Soft Starter	2–4× In	0.5–1.5× Tn	Medium-High	Smooth start, pumps, conveyors
VFD	1–1.5× In	1.5× Tn	High	Variable speed, energy savings
Autotransformer	1.5–4× In	Adjustable	Medium-High	High-inertia loads, adjustable taps

Practical Example