Motor starting voltage-drop equations
Starting method changes the locked-rotor current applied to the source impedance.
IFLC = Pout / (√3 × VLL × pf × η)Istart = IFLC × KLRC × KmethodΔV% ≈ Istart × |Zsource| / Vphase × 100where:
- IFLC
- Motor full-load current[A]
- KLRC
- DOL locked-rotor current multiple
- Kmethod
- Starting-method current factor
- Zsource
- Transformer plus feeder impedance magnitude[Ω]
Critical systems need a complex-impedance dynamic motor-start study.
How to use the Motor Starting Voltage Drop Calculator
Estimate bus voltage dip during motor starting from motor current, starting method, transformer impedance, and feeder impedance. Start by choosing the operating mode, then enter values from nameplates, measurements, or project documents in the units shown. The result updates immediately and keeps intermediate quantities visible for review.
- Confirm the system and calculation mode.
- Enter measured or manufacturer-rated values.
- Review the primary result and intermediate metrics.
- Continue with the related protection or equipment-selection checks.
Calculation method and validation
The equations and symbol definitions above show the complete calculation basis. Inputs are checked before calculation so zero, negative, incompatible, or physically impossible combinations are flagged instead of silently producing a misleading result.
How to interpret the result
Use the main result as a starting value and read every supporting metric. Balanced three-phase motor Transformer and feeder impedance are represented mainly by their magnitudes Utility source upstream of the transformer is stiff A passing or recommended result means the entered mathematical conditions are satisfied; it does not certify the complete installation or a particular product combination.
- Balanced three-phase motor
- Transformer and feeder impedance are represented mainly by their magnitudes
- Utility source upstream of the transformer is stiff
Selection limits and next checks
Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions. A full study should use complex R+jX source, motor locked-rotor data, other running loads, and dynamic acceleration. Compare the output with the current applicable standard, local rules, exact VIOX product datasheet, environmental conditions, protection coordination, and the engineer's project requirements before procurement or installation.
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Assumptions
- Balanced three-phase motor
- Transformer and feeder impedance are represented mainly by their magnitudes
- Utility source upstream of the transformer is stiff
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- A full study should use complex R+jX source, motor locked-rotor data, other running loads, and dynamic acceleration.