What are the connections for the Korndorfer system?

The simple auto-transformer starter has the disadvantage that at the instant of transition from ‘start’ to ‘run’ the supply to the motor is interrupted. This means that the insulation may be stressed by high transient voltages.

The Korndorfer method keeps the motor connected to the supply continuously by means of the connections shown in the diagram. On the first step (a), switches 1 and 2 close and the motor accelerates at a reduced voltage determined by the transformer tapping. On the second step (b), the star point of the transformer (switch 2) is opened so that the motor continues to run with part of the transformer winding in circuit. Next, this part is short-circuited by the ‘run’ contactor or switch (switch 3 closes) and finally the ‘start’ contactor or switch (1) is opened, as shown at ©.

A fully automatic starter would comprise a triple-pole line contactor, start contactor, running contactor, three single-pole overload relays, auto-transformer with a set of links for tap-changing, a suitable timer, and ‘start’ and ‘stop’ pushbuttons.

Switching sequence for auto-transformer starting by the Korndorfer method.

  1. Motor at reduced voltage from transformer.
  2. Motor with part of transformer winding in series.
  3. Motor at full voltage.