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Stardelta Vs Autotrasnformer


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Hello to all,


Would any one tell me, since both Star/delta & Autotrasnformer are reduced voltage type starters, where exactly the both type of starters finds their application in low voltage motors or either of one is suitable for any specific application and for what reasons/advantages.


With best regards,



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Hello Prassana


First, I suggest you have a look at our page on reduced voltage starters : http://www.lmphotonics.com/m_start.htm This may provide some of the answers for you.

The auto transformer and star/delta starters are similar in that they are both constant voltage starters (as opposed to constant impedance or constant current). This means that the voltage applied to the motor winding remains constant as the motor speed and impedance changes. The constant impedance and constant current starters have a rising voltage as the motor impedance rises.

The star/delta starter is the equivilent of an auto transformer starter on a 57% voltage tap except that the auto transformer starter would normally be a closed transition starter and the star delta starter is almost always an open transition starter.


The major advantage of the star/delta starter is price price price. The disadvantage is that:

  • You can not alter the start voltage. it is either 57% or 100%. If there is insufficient torque to start to full speed on 57% then you apply full voltage (DOL).

  • It is OPEN TRANSITION. A closed transition varient (Wanchope) is possible but is a lot more expensive to do properly. Generally the circuit is correct but the components are much too small to work correctly.

  • Open Transition leads to severe torque and voltage transients causing high mechanical and electrical damage.

The auto transformer has a number of start voltage options and it is possible to tune the starter to give some advantage over DOL, although, due to it's constant voltage nature, the performance is usually not as good as a constant impedance or constant current starter on anything other than an inertial load.


In My opinion, the star delta starter should never be used as it does more damage than DOL and generally operates as DOL anyway. Correctly designed and built closed transition starters are useful only for inertial loads.

The auto transformer starter operates best with inertial loads, but is expensive if extended or multiple starts are required. For loads other than inertial loads, the auto transformer starter needs to be multi stage to give real benefit.


Best regards,

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