Motor Starting Comparison

Compare locked-rotor current, voltage drop and starting torque across DOL, Y-Δ, soft-start, VFD and auto-transformer.

Inputs

Motor power P kW

Line voltage V V

Power factor —

Efficiency —

Locked-rotor current multiple k ×

Source short-circuit capacity Sₖ kVA

Soft-start initial current %

Auto-transformer tap %

Results

Note — Starting-voltage drop uses a Thevenin approximation (S_start / (S_start + Sₖ)). Torque scales with the square of the terminal voltage. VFD and soft-start are simplified representative models; verify against the drive/manufacturer data and a certified engineer.