Bus voltage dip when a large motor starts direct-on-line. The dip matters twice over: other equipment may drop out, and the starting motor's own torque falls with the square of voltage.
Standard method — stated scope limits apply
Motor
Supply
Enter motor and supply data.
About This Calculation
A directly-started induction motor draws 5–8 times its full-load current for the first several seconds of starting — large enough that it can pull the local bus voltage down (voltage dip) far enough to disturb other equipment on the same supply: contactors dropping out, lighting flicker, sensitive electronics resetting, or nearby motors stalling. This calculation checks whether a given motor's starting current, against the supply system's available fault capacity (source impedance), produces an acceptable voltage dip — the key input to deciding whether direct-on-line starting is acceptable or a reduced-voltage starting method (soft starter, VFD, star-delta) is genuinely needed.
Frequently Asked Questions
What voltage dip is generally considered acceptable during motor starting?
A commonly used guideline allows a dip of 10–15% at the point of common coupling for occasional motor starts, though sensitive equipment (VFDs, PLCs, some lighting) may need tighter limits — always confirm against the actual equipment supplier’s tolerance and any site or utility-imposed limit, since there is no single universal number.
Why does source impedance (fault level) matter for voltage dip, not just motor starting current?
Voltage dip is fundamentally a voltage divider between source impedance and the motor’s starting impedance — the same starting current causes a much smaller dip on a "stiff" high-fault-level supply than on a "weak" supply with high source impedance, which is exactly why the same motor can be fine on one site and problematic on another.
How much does a soft starter or VFD actually reduce the dip compared to direct-on-line?
Substantially — a soft starter or VFD can limit starting current to roughly 2–3 times full-load current instead of DOL’s 5–8 times, which correspondingly cuts the voltage dip by a similar fraction, since dip scales roughly linearly with starting current at a given source impedance.
Does star-delta starting always help enough to avoid a reduced-voltage starter?
It reduces starting current to roughly a third of DOL current during the star phase, which often helps meaningfully — but it also reduces starting torque to about a third, so it only works where the driven load’s starting torque requirement is genuinely low; a high-inertia or high-starting-torque load may not accelerate at all in the star connection.