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    Electrical / Motor starting

    Motor Starting Voltage Dip

    The voltage dip when a large motor starts, from the starting kVA against the available fault level. Utilities and plants both have limits, and a dip beyond about 15% makes contactors chatter.

    Voltage dip8.6%Starting kVA1877 kVAFault level20 MVA
    Voltage dip
    8.6%
    Within limits
    Starting kVA
    1877 kVA
    6.0× FLC
    Fault level
    20 MVA
    At the terminals
    Verdict
    Acceptable
    15% is the common limit
    Dip against fault level
    Dip ≈ kVA_start / (kVA_fault + kVA_start).
    Dip against fault level
    Fault level (MVA)Dip
    527.3%
    1015.8%
    208.6%
    503.6%
    1001.8%
    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Dip against fault level
    A stiff supply hides a big motor; a weak one does not.
    Dip against fault level0.0006.1212.2318.3524.4630.585 MVA5 MVA10 MVA10 MVA20 MVA20 MVA50 MVA50 MVA100 MVA100 MVAMotor Starting Voltage Dip
    Canonical equations
    The exact relationships implemented by this engine.

    ΔV ≈ S_start/(S_fault + S_start) · Per-unit voltage drop.

    √3·V·I_start · At the motor terminals.

    engineering methodology

    A clear path from inputs to results.

    The voltage dip when a large motor starts, from the starting kVA against the available fault level. Utilities and plants both have limits, and a dip beyond about 15% makes contactors chatter.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set motor rating (kw).
    2. Set starting current (× flc).
    3. Set fault level at the terminals (mva).
    4. Set motor efficiency (%).
    5. Read the headline metrics, then check the table for the detail.

    The formula

    ΔV≈Sstart/(Sfault+Sstart)ΔV ≈ S_start/(S_fault + S_start)

    Reference: IEEE 399 (Brown Book) motor-starting voltage dip: dip ≈ kVA_start / (kVA_fault + kVA_start), with NEMA MG 1 locked-rotor current.

    Worked example

    At the default inputs

    Inputs
    • Motor rating (kW): 250; Starting current (× FLC): 6; Fault level at the terminals (MVA): 20; Motor efficiency (%): 94; Power factor at start: 0.35
    Calculation path
    1. 1

      Voltage dip: 8.6% · Starting kVA: 1877 kVA · Fault level: 20 MVA · Verdict: Acceptable

    Result

    Voltage dip: 8.6% · Starting kVA: 1877 kVA · Fault level: 20 MVA

    Interpreting the result

    A 250 kW motor drawing 6.0× FLC dips the terminals 8.6% on a 20 MVA supply.

    Assumptions & limitations

    • Radial supply, single motor starting.
    • No soft starter or VFD.

    Frequently asked questions

    What dip is acceptable?

    About 15% at the motor terminals; less on a sensitive bus.

    Does a VFD help?

    Yes — it limits the starting current to about the running current.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.