Motor Starting Voltage Drop Calculator
Estimate the momentary voltage dip when a large motor starts direct-on-line. Enter the motor's starting kVA (locked-rotor apparent power) and the system fault level at the connection point, and the tool returns the percentage voltage drop, the voltage remaining during start, and a quick DOL suitability check.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the motor's starting apparent power in kVA (locked-rotor kVA, roughly 6–8× the running kVA for DOL).
- Enter the system fault level in kVA at the point the motor connects — a stiffer supply has a higher fault level.
- Read the starting voltage drop and the DOL assessment; a drop above about 10–15% usually calls for a reduced-voltage starter.
How it works
The dip is set by how the motor's starting kVA compares with the supply's short-circuit (fault) kVA: drop % = starting kVA ÷ (fault kVA + starting kVA) × 100. A stiff supply (high fault level) barely dips; a weak supply near the motor's own starting demand dips heavily. Remaining voltage is simply 100% minus the drop.
Worked example
Worked example. A 500 kVA starting demand on a bus with a 10,000 kVA fault level: drop = 500 ÷ (10,000 + 500) × 100 = 4.76%, leaving 95.24% of nominal voltage — comfortably within limits for direct-on-line starting.
Common mistakes
- Using running kVA instead of starting (locked-rotor) kVA — DOL inrush is several times the run value, so the dip is much larger.
- Confusing fault level in kVA with fault current in kA; convert consistently before entering.
- Assuming a passing dip means DOL is fine — also check flicker limits, contactor drop-out and the effect on other loads on the same bus.
Frequently asked questions
What voltage drop is acceptable at motor start?
A momentary dip of about 10–15% at the motor terminals is a common rule of thumb, with tighter limits (often ~3%) where lighting flicker or sensitive equipment share the supply. Check the actual limits for your network and load.
How do I reduce the starting dip?
Use a reduced-voltage starter — star-delta, autotransformer, or a soft starter/VSD. These cut the inrush current (and therefore the dip) at the cost of reduced starting torque.
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