Korndorfer starter is a technique used for soft start

Soft start

Soft start is a term describing any electrical circuit which reduces excess current during initial power-up of an electric motor.-Introduction:...

ing of induction motor

Induction motor

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...

s. The circuit uses a three-phase autotransformer

Autotransformer

An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where...

 and three three-phase switches.

This motor starting method has been updated and improved by Hilton Raymond Bacon under the title Korndorfer Autotransformer Starter

Korndörfer autotransformer starter

The reduced voltage autotransformer starter or Korndorfer starter was invented in 1908, by Max Korndörfer of Berlin. He filed the application with the U.S. Patent office in May 1908 and was granted the patent US 1,096,922 in May 1914...

Operation

The Korndorfer starter can be used manually. Newer devices provide full automatic operation, which in addition would have: triple-pole line contactor

Contactor

A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit....

 (switch), start contactor, running contactor, three single-pole overload relay

Relay

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...

s, autotransformer with a set tap-changing links, a suitable timer

Timer

A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...

, and start and stop pushbutton

Pushbutton

Pushbutton is a UK-based digital agency specialising in designing, developing, and delivering interactive television.As one of the industry forerunners creating rich interactive television experiences, Pushbutton have graduated from traditional 'red button' services for Sky, ITV, Disney, National...

s.

Motor stationary

If all switches are open the motor is completely disconnected from the three-phase network.

Soft start (phase 1)

In order to start the motor first the switches 1 and 2 are closed. This the motor is supplied from a voltage lowered by the autotransformer. The lower voltage limits the input current to the initially stationary motor, which starts to gain rotational speed. The torque of the motor is also lowered accordingly.

Soft start (phase 2)

The motor continues to increase its speed until the motor torque and the load torque balance each other and a steady speed it achieved. At this stage switch 2 is opened and momentarily the motor is supplied by even lower voltage, because the windings of the autotransformer act as inductors connected in series with motor. This time is short - just enough to disconnect the switch 2 and engage switch 3, which connects the full voltage to the motor. Further increase in speed begins and motor reaches its full rated speed.

At this point the "soft start" is ended and motor can work under full load.

Full load

When motor is started up, there is no more need for the autotransformer to be energised. At this stage switch 1 can be safely open to break the supply to the autotransformer, whilst the motor is supplied directly from the three-phase network.

Switch off

In order to de-energise the full started motor it is enough to open the only closed switch 3. The motor returns to the stationary position.

Advantages

The Korndorfer starter limits significantly the inrush current

Inrush current

Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases...

. It is used for large motors, in which start by direct connection to the network is not possible. For large motors also the star-delta starter cannot be used, especially if they are started with a significant load.

The circuit has advantage over starting with a regular autotransformer, which needs to be at some point completely disconnected during the start inducing high voltage

High voltage

The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

impulses, which can damage the electrical insulation of the stator.

The most effective ratio of the autotransformer is between 65-80%.

Drawbacks

The circuit is quite complex and involves relatively expensive autotransformer. Due to the physical size of the whole device it might not be possible to add the Korndorfer starter to an existing machine if space is scarce.