Korndörfer autotransformer starter

Background

The Korndörfer starter was invented in 1908 by Max Korndörfer of Berlin, who filed a U.S. patent application in May 1908 and was granted patent US 1,096,922 in May 1914, assigned to General Electric Company. The patent claimed four methods of using an autotransformer for reduced voltage motor starting; the fourth method, with a star switch closed and a changeover from reduced voltage tap to direct-on-line, has been in use for over a hundred years. Since 1920, the autotransformer starter has been the most popular device for reducing starting current inrush for induction motors.

The starting sequence involves closing the star switch, then the start switch to energize the autotransformer, connecting the motor at a selected reduced voltage tap. After a predetermined period, the star switch opens, followed by a millisecond delay before the run switch closes, applying full line voltage. The start switch then opens, and the motor reaches operational speed. The circuit uses a triple-pole line contactor, start contactor, running contactor, three single-pole overload relays, an autotransformer with tap-changing links, a timer, and start/stop pushbuttons for automatic operation.

The autotransformer is de-energized after start, and the motor runs directly from the network. The most effective autotransformer ratio is between 65-80%. Manufacturers offer two configurations: 2-coil or 3-coil construction, with reduced voltage taps at 50%, 65%, and 80% based on NEMA recommendations. An improved 1000 kW, 11,000 volt starter autotransformer includes axial cooling fans, cast resin encapsulated coils, and 9 starting voltage tap connections allowing ±5% variation around NEMA taps.