Reduced-Voltage Starter Current and Torque

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.

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.

The reduced-voltage current and torque relations are published in NEMA ICS 2 and standard motor references; the across-the-line LRA and torque come from the motor data.

Estimate. AHJ and licensed professional govern.

Field names used by the API: across_line_lra_a, across_line_lrt_pct, starter_type, tap_fraction, motor_current_a, line_current_a, torque_pct

• Torque ~ V^2 starting torque falls with the square of the applied voltageNEMA ICS 2
• Autotransformer line current line current = tap^2 x LRA, not tap x LRA, because it trades voltage for currentautotransformer action
• Wye-delta fixed one-third of current and torquewye-delta connection