Korndörfer autotransformer starter

Lore & 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.

Reader's Guide

The Korndörfer autotransformer starter's significance lies in its ability to limit inrush current for large induction motors, where direct connection or star-delta starting is not possible, especially under significant load. It provides maximum starting torque with minimum line current, and its output waveform is a pure sine wave without harmonic distortion, requiring no special shielding or cable length limitations. Cooling requirements are similar to those of transformers or switchgear.

However, the circuit is complex and involves an expensive autotransformer, and its physical size may prevent retrofitting into existing machines with limited space. The 'open transition' control scheme can cause the motor to slow and get out of sync, leading to mechanical and electrical transients. Even with 'closed transition,' a transient of less than 5 microseconds occurs at star-point switch contact separation, which is the origin of destructive voltage transients in large medium/high voltage starters above 1000 kW. This is explained by Faraday's law of electromagnetic induction: a very fast dv/dt induces extremely large voltages into redundant windings of the first starting stage.

Medium/high voltage starters have reported random autotransformer failures. Investigations by Dr. S.E.A. Emam and Prof. Dr. A.H. Amer for a petroleum company on a 5,400 kW motor found no clear reason; Siemens recommended R-C suppression/surge arresters. On the Q4000 oil drill vessel, four 2,900 kW thruster motor autotransformer starters suffered repeated failures before 2008, with no clear cause from transformer manufacturers, switchgear manufacturers, or a European design organization; all six 3000 kW starters were replaced by variable speed electronic drives in 2008.

Did You Know?

• The patent claimed four methods; only the fourth method, using a star switch and changeover to direct-on-line, has been used for over a hundred years.
• Repeated autotransformer failures on the Q4000 oil drill vessel led to replacement of all six 3000 kW starters with variable speed electronic drives in 2008.