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Motor starting current

Compare starting current and torque across DOL, star delta, autotransformer, and soft start, with the voltage dip each method causes.

Enter the motor rating and a supply to compare starting methods.

The locked-rotor current is what the motor draws the instant it is energized, and it does not care how light the load is; it is set by the rotor design, typically 5.5 to 8 times full load for a modern cage motor, which is why NEMA publishes code letters as kVA per horsepower (IEC 60034-12 covers the same duty for IEC motors). Starting kVA is the number that matters to the supply: it sets the voltage dip, and it is what a generator must accept without stalling. Torque falls with the square of the voltage at the motor terminals, so a star-delta starter leaves one third, a 65 percent autotransformer tap leaves 42 percent. One honest caveat: the drive row prices torque with the same voltage-squared rule at line frequency, but a real drive ramps voltage with frequency (constant V/f), so its actual starting torque is far higher than the figure shown; its row is a line-current comparison, not a torque prediction.
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