Motor Starting Voltage Dip Calculator
A motor starting across the line draws locked-rotor current — commonly six times full load — and that inrush pulls the bus voltage down for as long as it takes to accelerate. If the dip is deep enough, contactors drop out, drives fault on undervoltage, and lighting visibly flickers. The screening question is simple: how stiff is the source relative to the motor's starting demand?
Run the numbers
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The formula
| LRkVA | Locked-rotor kVA of the motor being started |
| SCkVA | Short-circuit kVA available at the bus, a measure of source stiffness |
Worked example
A motor with 1,200 kVA locked-rotor demand on a bus with 20,000 kVA short-circuit capacity:
Dip = 1200 / (1200 + 20000) × 100 = 5.7%
Acceptable for most equipment. Halve the source stiffness to 10,000 kVA and the dip becomes 10.7%, which is where contactor dropout and drive undervoltage trips start to appear.
Which standard governs this
The screening method is documented in IEEE Std 399 (the Brown Book). Motor locked-rotor codes are defined by NEMA MG 1, and the code letter on the nameplate is what gives you locked-rotor kVA per horsepower.
What this calculation does not account for
A steady-state screening estimate. It does not model the acceleration curve, the load torque the motor is starting against, or how long the dip persists — and duration matters as much as depth for whether a contactor actually drops out. Reduced-voltage starting and soft starters change the picture entirely.
Common mistakes
Using full-load kVA instead of locked-rotor kVA, which understates the dip by a factor of about six. Ignoring dip duration. Assuming a generator-backed bus behaves like a utility source, when a generator is far softer and dips much further.