Reduced-Voltage Starter Current and Torque
Why an autotransformer's line current is the tap squared: torque falls with the SQUARE of voltage, so a 65% start delivers only 42% of locked-rotor torque -- reduce too far and the motor will not break the load away. And an AUTOTRANSFORMER draws a line current of tap^2 x LRA (not tap x LRA), trading voltage for current: at a 65% tap the motor sees 65% current (390 A of a 600 A LRA) but the line sees only 42% (254 A). Wye-delta gives a fixed 1/3 on both (200 A, 33% torque); a solid-state/reactor start at 65% draws 390 A on both (no squared line cut) for the same 42% torque. The squared line-current reduction is the autotransformer's advantage. A design aid; the motor speed-torque curve and the load govern.
Formula and source
autotransformer: motor = tap x LRA, line = tap^2 x LRA, torque = tap^2 x LRT. wye-delta: 0.333x on current and torque. solid-state/reactor: motor = line = tap x LRA, torque = tap^2 x LRT.
Reduced-voltage-starter current and torque relations (NEMA ICS 2; torque proportional to voltage squared), by name; the motor speed-torque curve and the load govern.
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