Motor Starting Method

Quick pick (TL;DR)

Small motors / strong supply → DOL (simplest, full torque).
Medium with light start → star-delta (cuts inrush to ~⅓ but only ⅓ torque).
Pumps/fans where you want a smooth ramp & less inrush → soft starter.
Need speed control, energy saving or high inertia → VFD.
Always check the supply can hold voltage dip within ~15% during start.

Step 1 — Choose the method

Method	Start current	Start torque	Use for
DOL	6–7× FLC	Full	Small motors
Star-delta	~⅓ (2–3×)	~⅓	Light-start loads
Soft starter	2.5–4×	Controlled	Pumps, fans
VFD	~1–1.5×	Controlled	Speed control

Step 2 — Voltage-dip check

90 kW: DOL vs soft start

Given FLC = 166 A , source short-circuit = 6000 A Option A — DOL I_start = 6.5 × 166 = 1079 A approx dip ≈ I_start / I_sc = 1079 / 6000 ≈ 18% > 15% limit → NG Option B — soft start I_start = 3.0 × 166 = 498 A approx dip ≈ 498 / 6000 ≈ 8% ≤ 15% → OK Answer → soft starter, start current 3×FLC

Volt-dip ≈ start current ÷ source fault level. DOL dips 18%; soft starter keeps within limits.

Step 3 — Other deciders

Star-delta only for light-start loads.
Frequent starts / speed → VFD.
Soft starter needs bypass contactor.

Starting voltage-dip calculator

Inrush & approximate bus voltage dip

I_start = factor × FLC; dip ≈ I_start / source fault. Keep ≤ ~15%.

FLC (A)

Starting method

Source fault (A)

Dip limit (%)

First-order screening — a real study models impedances & torque curves.

Common mistakes

Star-delta on a loaded conveyor.
DOL on a weak supply.
VFD without harmonic mitigation.

Requisition & datasheet

Power, voltage, FLC, load, inertia.
Method, start current, volt-dip, protection.

Frequently asked questions

What should I know about Motor Starting Method?

Small motors / strong supply → DOL (simplest, full torque). Medium with light start → star-delta (cuts inrush to ~⅓ but only ⅓ torque). Pumps/fans where you want a smooth ramp & less inrush → soft starter. Need speed control, energy saving or high inertia → VFD. Always check the supply can hold voltage dip within ~15% during start.

Which standards or codes apply?

IEC 60947-4-1 (starters & contactors), IEC 60034 (motors), IEC 61800 (VFD drives), NEMA ICS 2.

Standards — IEC 60947-4-1; IEC 60034; IEC 61800; NEMA ICS 2.