Motor Starting Current & Voltage Dip Estimator

Starting an induction motor direct-on-line (DOL) draws locked-rotor current — typically 5 to 7 times full-load current — for the fraction of a second it takes to accelerate. That surge pulls down the bus voltage in proportion to how large the starting load is relative to the source's short-circuit strength. This calculator uses the widely used MVA method: VD% = S_start / (S_start + S_sc) × 100, where S_start is the motor's starting apparent power (√3 · V · I_LR) and S_sc is the available fault MVA at the bus — a fast approximation that avoids a full impedance model while remaining accurate enough for feasibility checks.

Use it to check whether a motor can be started direct-on-line without tripping undervoltage protection elsewhere on the bus, sagging lighting, or dropping out contactors — a common problem when a large motor shares a service with a weak utility connection or a small on-site generator. It flags the result against a voltage dip limit you set; 10% is a common starting point for general equipment, though sensitive control or lighting circuits may need a tighter limit.

This is a magnitude-only approximation: it assumes the starting current is dominated by reactive current (a reasonable assumption at locked rotor, where power factor is typically 0.2-0.4) and doesn't model the source's actual X/R ratio or downstream cable impedance between the source and motor. For a marginal result, or when the dip must be verified precisely, use a reduced-voltage starter (autotransformer, soft starter, or VFD) analysis or a full dynamic simulation instead.