Reduce a motor’s terminal voltage to a fraction a of nominal and it draws a times the current but produces a² times the torque.
Every method of taming inrush works by reducing the voltage the motor sees, and that exchange rate is fixed by physics rather than by the equipment. It is why a solid-state soft starter set to 300% of full-load current draws more current than star-delta (3.0× against 2.17×) and still delivers less torque (21% of direct-on-line against 33%). It is also why a supply that sags 10% during the start loses 19% of the starting torque, lengthening the start and deepening the sag.
Star-delta is the exception everyone generalises from and should not. The delta reconnection divides line current and torque by exactly three, so the two look symmetric — a property of the connection, not of reduced-voltage starting. A variable-frequency drive breaks the rule properly: by lowering frequency along with voltage it keeps the magnetic flux at its design value, so it produces full torque or more at little above running current. The square law only applies when you reduce voltage alone.
Estimate with the rule, then check it against the calculator that models it properly.
Open Motor Starting & Inrush →Reduce a motor’s terminal voltage to a fraction a of nominal and it draws a times the current but produces a² times the torque. Every method of taming inrush works by reducing the voltage the motor sees, and that exchange rate is fixed by physics rather than by the equipment.