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AUSELECTRICAL.TOOLS — MOTORS

Motor Starting Voltage Dip Calculator

Bus Voltage Dip During Motor Starting

Estimate the approximate voltage dip on a supply bus when a motor starts, comparing DOL, star-delta, soft starter and VSD starting methods. Large inrush currents cause a temporary voltage sag that can affect other equipment on the same bus — this calculator uses a simplified impedance-divider method incorporating transformer and feeder impedance to estimate retained voltage during starting.

CALCULATOR

Voltage Dip

SUPPLY IMPEDANCE
Enter values above to calculate

Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.

Methodology

This calculator uses a simplified impedance-divider approach: the motor starting impedance is estimated from starting current and system voltage, then compared against total upstream impedance (transformer + feeder) to estimate retained bus voltage:

Retained Voltage
Vretained≈ZmotorZsystem+ZmotorV_{retained} \approx \frac{Z_{motor}}{Z_{system} + Z_{motor}}

Assumptions & Limitations

  • Simplified magnitude-only impedance model — does not account for phase angle interaction between source and motor impedance
  • Assumes utility source is an infinite bus (does not model upstream source impedance beyond the transformer)
  • Does not model background load current concurrent with motor starting
  • A detailed power system study (e.g. ETAP, PowerFactory) is recommended for critical or borderline cases

Related Calculators

AusElectrical.tools
Built with reference to AS/NZS 3000, AS/NZS 3008, AS/NZS 1359
Results for guidance only — not a substitute for engineering judgement
Does not reproduce copyrighted Standards Australia material. Always consult current standards for compliance.