Starting Motor With Auto-transformer
Starting Motor With Auto-transformer

Circuit and function

An auto-transformer starter makes it possible to start squirrel-cage induction motors with reduced starting current, as the voltage across the motor is reduced during starting.

In contrast to the star-delta connection, only three motor leads and terminals are required. On starting, the motor is connected to the tappings of the auto-transformer; transformer contactor K2M and star contactor K1M are closed.

The motor starts at the voltage reduced by the transformer, with a correspondingly smaller current.

By this means the feeding current in comparison to direct starting would be reduced by the square of the transformer voltage ratio; nevertheless, it is in most cases noticeably higher, as it also covers the relatively high transformer losses.

Depending on the tapping and starting current ratio of the motor, the starting current lies at (1 … 5) · Ie. In contrast, the motor torque falls with the square of the voltage across the windings. Auto-transformers usually have three available taps in each phase (for example 80 %, 65 %, 50 %), so that the motor starting characteristic can be adjusted to the load conditions.

If the motor has reached 80 … 95 % of its rated speed (depending on the desired reduction of the current surge after switching-over), the star contactor K1M on the transformer is opened.

Now the transformer part-windings act as chokes. The motor voltage is only reduced by the chokes below the supply voltage and the motor speed does not fall. The main contactor K3M closes via auxiliary contacts of the star contactor and applies the full supply voltage to the motor.

For its part, the main contactor K3M drops out the transformer contactor K2M.

The entire procedure is thus uninterrupted.

Auto-transformer starter with uninterrupted switching over (Korndörfer starting method)
Figure 1 – Auto-transformer starter with uninterrupted switching over (Korndörfer starting method)

Rating of the starter

The main contactor K3M and the motor protective device F1 are selected according to the motor rated operational current Ie. Transformer contactor and star contactor are only briefly closed during starting.

Their rating is determined by the required contact breaking capacity, as they must reliably cope with any unforeseen disconnection during start up.

The star contactor also operates with every start-up during switching-over. The values of the rated operational currents for the transformer contactor K2M, depending on the start time and starting current, are between (0.3 … 1) · Ie, for the star contactor between (0.45 … 0.55) · Ie.

Testing AC Motors and Working on Westinghouse Generator

Resource: Allen Bradley – Low Voltage Switchgear and Controlgear

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


  1. Semazzi Zaake
    Dec 05, 2016

    Basic calculations show that motor current increases when an under voltage occurs. The auto transformer starter operates by lowering the motor voltage in order to reduce the starting current, how is this possible?

  2. Darrell Bardin
    Jan 29, 2016

    I want to start 3 electric motors, one 150 hp, and two 100 hp. on one auto transformer. I have all the switching for 3 individual systems but only 1 transformer. I know how to wire 1 motor to 1 transformer. How do I go about wiring the timers, small relays and interlocks?

    Thank you,
    Darrell Bardin

  3. Edward
    Jan 12, 2016

    I have a question, the autotransformer method has 3 selectable taps to chooce which are normally 80%, 65% and 50%. How to choose which tap we should change for motor starting?

  4. Helen
    Jun 24, 2015

    In order to progress my Arc Flash study – I require typical kVA and Z values for transformers used in an auto-transformer motor starter application. The motors using this type of starter are 90 kW and 45 kW. When speaking to transformer manufacturers in Australia – these values are not readily, if at all available. Can you suggest where I may find such information?

  5. Darlington
    May 09, 2015

    Very nice thanks for this.

  6. Biniam
    Mar 29, 2015

    very interesting note,,,,,I like it.I need the control circuit for autotransformer starter,and am not clear how to select the star and autotransformer contactor(KM1&KM2)

  7. Elsayed elkholy
    Mar 11, 2015

    Firstly, thanks for this information.
    what is the importance of star contactor K1M?
    i mean why dont we connect Y directly…

  8. LE Tan
    Jul 15, 2014

    How to design a auto-transformer starter control circuit so it can prevent from the auto transformer burning?

    • prem bhojwni
      Feb 05, 2015

      Use the high insulation capacity conductor or use the good quality paper

  9. Huned N Motiwalla
    Mar 20, 2013

    How do I size the transformer, do you have an excel sheet for calculation & also how do I select cable conductor size.

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