Motor Starting Current Calculator
Calculate the starting (inrush) current of an electric motor based on its full-load current, starting method, and motor parameters.
Typical range: 5–8× FLA for DOL. Check motor nameplate.
Formulas
DOL Starting Current:
Istart = LRC × IFLA
Star-Delta Starting Current (supply side):
Istart(Y) = (LRC × IFLA) / 3
Autotransformer Starting Current (supply side):
Istart(AT) = tap² × LRC × IFLA
VFD / Soft Starter Starting Current:
Istart = Current Limit Setting × IFLA
Apparent Starting Power (3-phase):
Sstart = √3 × VL × Istart
Apparent Starting Power (1-phase):
Sstart = V × Istart
Where: LRC = Locked Rotor Current multiplier (typically 5–8 for squirrel-cage induction motors), IFLA = Full-Load Amperes, tap = autotransformer voltage ratio (0.50, 0.65, or 0.80).
Assumptions & References
- Applies to squirrel-cage induction motors (the most common type). Wound-rotor and synchronous motors have different starting characteristics.
- The Locked Rotor Current (LRC) multiplier is typically 5–8× FLA per NEMA MG 1 and IEC 60034-12. Always verify against the motor nameplate or datasheet.
- Star-Delta (Y-Δ): Reduces supply-side starting current to 1/3 of DOL value. Motor must be designed for delta operation at rated voltage. Torque is also reduced to 1/3 of DOL starting torque.
- Autotransformer: Supply-side current is reduced by tap² (e.g., 80% tap → 64% of DOL current). Motor-side current is reduced by tap only. Per IEEE Std 141 (Red Book).
- VFD: Limits starting current to the programmed current limit (typically 100–150% of FLA). Provides smooth acceleration with minimal inrush. Per IEC 61800-2.
- Soft Starter: Reduces starting current by ramping up voltage. Typical current limit is 200–400% of FLA. Per IEC 60947-4-2.
- Apparent power calculation assumes balanced load and sinusoidal supply.
- Power factor during starting is typically 0.1–0.3 (highly inductive); this calculator reports apparent power (kVA), not real power (kW).
- References: NEMA MG 1, IEC 60034-12, IEEE Std 141-1993, IEC 60947-4-2, IEC 61800-2.