Calculators
Motor Starting Voltage Drop
Bus voltage dip during an across-the-line motor start, per IEEE 399 / IEEE 141 screening method. Uses lumped-impedance model (source + transformer + cable) with motor locked-rotor impedance.
References: IEEE Std 399-1997 (Brown Book — Industrial and Commercial Power Systems Analysis), IEEE Std 141-1993 (Red Book — Electric Power Distribution for Industrial Plants). Typical minimum bus voltage during start: 80% nominal (10% drop is preferred; 20% drop is marginal). Torque varies as (V/Vrated)², so a 20% voltage drop gives only 64% starting torque.
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