Soft Starter Sizing Calculator

Work out soft starter sizing instantly with clear inputs, formula shown and shareable results.

About Soft Starter Sizing Calculator

A soft starter is rated on motor full load current, with an allowance for long ramp times because the thyristors dissipate heat throughout the ramp. Limiting starting current to 300% instead of the 650% of a direct-on-line start greatly reduces supply voltage dip and mechanical shock.

Formula

Soft starter

rating >= motor FLC x duty factor; starting current = FLC x current limit setting

Worked example

With the default values below, Soft Starter Sizing Calculator returns Starter current rating required of 54.10. Change any input above and every figure here is recalculated in your browser.

Inputs used

Motor rating: 30 kW
Supply voltage: 400 V
Current limit setting: 300 %
Ramp time: 10 s

Results

Starter current rating required: 54.10 A
Motor full load current: 54.10 A
Limited starting current: 162.30 A
Direct-on-line current avoided: 189.35 A

Step by step

1Values used
2Soft starter
3Starter current rating required= 54.10 A
4Motor full load current= 54.10 A
5Limited starting current= 162.30 A
6Direct-on-line current avoided= 189.35 A

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Does a soft starter reduce starting torque?

Yes, as the square of the current limit. At 300% current the torque is roughly (3/6.5) squared of DOL torque, so high-breakaway loads may not start.

When is a VFD better?

Whenever speed control is useful during running, or when very low starting torque with full current limiting is needed. A soft starter only manages the start.