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Star-Delta Starter Calculator
Compare DOL vs Star-Delta (Y-Δ) motor starting. Calculate starting currents, torques, contactor sizing, and changeover timing.
Comparison Results
Star-Delta Starting Principle
In Y-Δ starting, the motor windings are initially connected in star (Y), reducing the voltage across each winding to VL/√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 |
⚠️ Important: Star-delta starting only works for motors designed for delta operation at line voltage (e.g., 400V Δ / 690V Y). The motor nameplate must show the dual voltage rating. Y-Δ reduces starting torque by ÷3, which may be insufficient for high-inertia loads.
Practical Example
Given: 22 kW, 400V, In = 42A, starting multiplier = 7×
DOL starting current = 42 × 7 = 294 A
Y-Δ starting current = 294 / 3 = 98 A
DOL starting torque = 1.5× Tn
Y-Δ starting torque = 0.5× Tn — sufficient for centrifugal pumps/fans, may stall on loaded conveyors
In star, the winding voltage is V/√3. Since starting current is proportional to voltage, the winding current is reduced by √3. But the line current in star equals the phase current, while in delta the line current is √3× the phase current. Combined effect: line current is reduced by √3 × √3 = 3.
During changeover, the motor is momentarily disconnected from the supply. This causes a current transient that can be 10-15× rated current for a few milliseconds. A properly timed changeover (at ~80% speed) minimizes this transient. Closed-transition starters avoid the open-circuit period entirely.
No. The motor must be designed for delta operation at the line voltage. Check the nameplate: it should show 400V Δ / 690V Y (for 400V supply). If the motor is rated 400V Y only, Y-Δ starting is not possible.
When the load requires more than 33% of DOL starting torque to accelerate. High-inertia loads (flywheels, large fans) or loads with high breakaway torque (loaded conveyors) may stall in star connection. In such cases, use a soft starter or VFD instead.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Diagnose motor starting issues and vibration problems with professional instruments.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
