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Motor Starting Methods: Star-Delta, DOL, Soft Starter, VFD
Compare starting current and torque of an electric motor across DOL, star-delta, soft starter and VFD.
| Method | Start current | Start torque |
|---|---|---|
| DOL | — A | 100% |
| Star-Delta | — A | ~33% |
| Soft Starter | — A | ~25–50% |
| VFD | — A | up to 150% (controlled) |
Star-delta ≈ 1/3 of DOL current and torque. Soft starter ≈ 3×Iₙ (adjustable). VFD draws near rated current with full controllable torque.
Choosing a starting method
Direct-on-line is simplest but draws the highest inrush. Star-delta cuts starting current and torque to ~1/3 (for light starts). Soft starters give a smooth ramp; VFDs add full speed/torque control and energy savings on variable loads. Size protection with our overload/fuse/contactor tool. DRG Motor supplies IE3/IE4/IE5 motors from 0.55 to 355 kW.
Frequently asked questions
How much does star-delta reduce starting current?
To about one third of the direct-on-line value — but starting torque also drops to ~1/3, so it suits light or no-load starts.
When should I use a soft starter or VFD?
A soft starter gives a smooth start and limits current (~3×Iₙ). A VFD adds full speed and torque control plus energy savings on variable-torque loads like pumps and fans.
What is the typical DOL inrush?
Around 6–8 times the full-load current for a few seconds; use the motor data for the exact locked-rotor current.