Motor starting method calculator — DOL / star-delta / soft start / VFD
Compare how much current and torque each starting method demands from the supply: direct-on-line, star-delta, soft starter and VFD, from motor FLA and start current.
Motor & supply parameters
Locked-rotor current (LRC) presets — typical NEMA Design B inrush is 6× FLA.
Starting method comparison
| Method | Start current | Start torque | Note |
|---|---|---|---|
| DOL | 162 A | 150% | Across-the-line: full inrush for the whole starting duration. |
| Star-delta | 54 A | 50% | Winding switched from star to delta once the motor is up to speed. |
| Soft starter | 81 A | 60% | Adjustable 2–4× FLA; torque roughly proportional (≈60% at default). |
| VFD | 29.7 A | 100% | Full rated torque available from zero speed, no line inrush. |
DOL: start current = locked-rotor current, start torque = full start torque. Star-delta: both ≈ ×0.33. Soft starter: current adjustable 2–4× FLA, torque roughly proportional. VFD: current ≈ 1.1× FLA, full rated torque from zero speed.
About
NEC 430 motor circuit — the starting method trades inrush for reduced starting torque. DOL pulls the full locked-rotor current, star-delta cuts it to about a third by closing the winding in star first, a soft starter ramps the applied voltage, and a VFD accelerates the motor at near-rated current. The VFD also avoids the mechanical shock of DOL across-the-line starting, which can strain couplings and gearboxes on large or high-inertia loads. If your supply capacity is below the DOL current, step down to star-delta, a soft starter or a VFD — AO Ctrl stocks all three control options.
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Frequently asked questions
How much current does a motor draw during DOL starting?
A direct-on-line start draws the full locked-rotor current — with the default 27 A FLA motor and 600% locked-rotor preset, that is 162 A for the whole starting duration. DOL also delivers 100% start torque, which is why it needs the largest supply capacity of all four methods.
How does star-delta starting reduce inrush current?
Star-delta connects the winding in star first, cutting both start current and start torque to about one third. With the default 600% locked-rotor current, start current drops from 162 A to about 54 A while torque falls to about 33% of the DOL value — acceptable only where the load does not need high breakaway torque.
Should I use a soft starter or a VFD to start my motor?
A soft starter ramps the applied voltage and draws an adjustable 2–4× FLA (the calculator uses 3×, about 81 A at the 27 A default), with torque roughly proportional at about 60% of rated. A VFD draws only about 1.1× FLA (about 30 A) while delivering full rated torque from zero speed, and additionally avoids the mechanical shock of across-the-line starting that strains couplings and gearboxes.
AO Ctrl provides this tool for reference only.