Soft Starter / Motor Starting Calculator

What this calculator does

This page is a planning tool, not a complete motor-starting study. It helps you screen the likely magnitude of starting current and the effect of reduced-voltage starting methods before you check the actual motor nameplate, starter instructions, feeder impedance, generator support, or project study requirements.

How the locked-rotor current screen is built

The calculator uses the NEMA locked-rotor code midpoint:

LRA estimate = (HP x kVA/HP x 1000) / (sqrt(3) x line voltage)

That gives an estimated direct-on-line locked-rotor current. Real motors can fall anywhere inside the nameplate code band, so treat the result as a planning number rather than a guaranteed factory test result.

How the reduced-voltage estimate works

• DOL: full estimated locked-rotor current.
• Soft starter: current is screened from the selected initial voltage, and torque is screened with the square-of-voltage relationship.
• Wye-delta and autotransformer: current and torque are shown as rough planning factors only.
• VFD: the page uses a current-limit assumption instead of a fixed percentage of locked-rotor current, because actual drive current depends on programming and control mode.

Soft-starter torque reminder

Reduced-voltage starting lowers torque faster than it lowers current. A common planning relationship is:

Starting torque proportional to voltage squared

At 50% initial voltage, the page screens approximately 50% of locked-rotor current and about 25% of full-load torque. That can work for a lightly loaded pump or fan, but it may not start a high-breakaway conveyor, crusher, or compressor.

Worked example

A 50 HP, 480 V motor with a Code G locked-rotor letter is being screened with a soft starter at 50% initial voltage.

• Estimated running FLA: about 48 A from the planning assumptions
• Estimated DOL locked-rotor current: about 350 A
• Estimated soft-starter initial current: about 175 A
• Estimated soft-starter initial torque: about 25% of full-load torque

That result is useful for early comparison, but the final starter frame, bypass arrangement, thermal duty, and acceleration acceptability still need manufacturer and project review.

When this page is not enough

Use a full starting study when you need to prove feeder voltage drop, generator performance, utility flicker, mechanical acceleration time, or guaranteed starting torque. This page does not model the one-line impedance, driven-load inertia, or detailed drive control behavior.