Star-Delta and DOL Motor Current Calculator
Compare line current, winding branch current (IΔ = In / √3) and star contactor current (IY = In / 3) for direct-on-line (DOL) and star-delta motor starters.
Formula used
In = P / (√3 × U × cos φ × η) · IΔ = In / √3 · IY = In / 3
Direct-on-line (DOL) and star-delta current table · 400 V (cos φ 0.85, η 0.90, k = 6)
Formula results for cos φ 0.85, efficiency 0.90 and starting current ratio k = 6. In star-delta starting, the main and delta contactors carry the phase current (In / √3 ≈ 0.58 In) and the star contactor carries In / 3; actual selection uses the motor nameplate current and IEC 60947-4-1 AC-3 ratings.
| Power (kW) | Line current In (A) | Main / delta contactor IΔ (A) | Star contactor IY (A) | DOL starting current (A) | Star starting current (A) |
|---|---|---|---|---|---|
| 1.5 | 2.83 | 1.63 | 0.94 | 16.98 | 5.66 |
| 2.2 | 4.15 | 2.40 | 1.38 | 24.91 | 8.30 |
| 3 | 5.66 | 3.27 | 1.89 | 33.96 | 11.32 |
| 4 | 7.55 | 4.36 | 2.52 | 45.28 | 15.09 |
| 5.5 | 10.38 | 5.99 | 3.46 | 62.26 | 20.75 |
| 7.5 | 14.15 | 8.17 | 4.72 | 84.90 | 28.30 |
| 11 | 20.75 | 11.98 | 6.92 | 124.53 | 41.51 |
| 15 | 28.30 | 16.34 | 9.43 | 169.81 | 56.60 |
| 18.5 | 34.91 | 20.15 | 11.64 | 209.43 | 69.81 |
| 22 | 41.51 | 23.97 | 13.84 | 249.05 | 83.02 |
| 30 | 56.60 | 32.68 | 18.87 | 339.62 | 113.21 |
| 37 | 69.81 | 40.31 | 23.27 | 418.86 | 139.62 |
| 45 | 84.90 | 49.02 | 28.30 | 509.43 | 169.81 |
| 55 | 103.77 | 59.91 | 34.59 | 622.63 | 207.54 |
| 75 | 141.51 | 81.70 | 47.17 | 849.04 | 283.01 |
Example: 15 kW, 400 V three-phase motor (cos φ 0.85, η 0.90, k = 6)
- Line current: In = 15,000 W / (√3 × 400 V × 0.85 × 0.90) = 28.30 A.
- Star-delta branch currents: main and delta contactors IΔ = In / √3 = 16.34 A; star contactor IY = In / 3 = 9.43 A.
- Starting current: 169.81 A in direct-on-line starting and 56.60 A in star starting.
How are contactor and thermal overload relay currents calculated in star-delta starting?
In a star-delta starter, the main contactor (KM) and delta contactor (KΔ) are connected in the winding loop and carry IΔ = In / √3 (about 58% of line current), while the star contactor (KY) carries IY = In / 3 (about 33% of line current). If the thermal overload relay is mounted under the main contactor in the phase loop, it is set to In / √3; if mounted on the supply line, it is set to In.
Why do branch currents differ in a star-delta starter?
When the main and delta contactors are wired in series with the motor windings, they carry 1/√3 of the line current (about 58%), while the star contactor carries 1/3 during starting.
How are the thermal relay and contactors verified?
Final selection must follow the motor nameplate current, AC-3 utilization category, starting time, transition delay and the manufacturer's type-1/type-2 coordination tables.