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Autotransformer starters


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Any body got any good references on "rising star" autotransformer starters??


Best regards,

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"He's not dead, he's just pinin' for the fjords!"
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What exactly did you want to know?


Rising star auto-transformer starters are seldomly used these days. They provide advantage (use to) in applications where a more gradual application of voltage to the motor was deemed necessary, the driving factors normally being to further extend the starting time of loads that tend to accelerate very quickly or those that could be damaged through high starting torque.


A rising star auto-transformer starter provides multi-stage starting and this is achieved by shifting the start point on the auto-transformer ie rather than to shift through the tappings on a standard auto-transformer starter.


When properly designed, all switching is closed transition.


I have found rising star auto-transformer starters to provide only limited advantage over standard designs in many cases and believe that soft starters can provide better results with far less system complexity and cost.


Not sure if the above helps in any way and/or where you might be able to find more information. I would suggest however that if you wanted to know more your best bet would be to view refence material produced during the 60's & 70's. I doubt very much that you will find any reference to rising star auto-transformer starters in more recent publications.




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I recently did a search for information on this type of auto transformer starter so that I could refer people on to it, but todate I have not found anything of any substance.


As I understand it, one implementation is to have a transformer with a winding similar to say a standard autotransformer starter with a 50% tap, and then add on to the end of that winding, further turns such that as you shift the star point further away from the output tap, the turns ratio changes from 2 : 1 to closer to 1: 1 but never actualy gets there.


For example, lets assume that the transformer has 100 turns to the first tap, plus 100 turns to the second tap plus 100 turns to a third tap and 100 turns to the end of the coil.

We connect phase in to the start of the coil, the motor to tap one and the star point to tap two. The voltage applied to the motor is 50%. Now disconnect the star from tap two and reconnect it to tap three. During the star disconnection, the current can flow into the motor via the coils which act as reactors. (closed transition) In this example, when the star is on tap three, the voltage output is 66%. If we now disconnect the star from tap three and reconnect it to the end of the winding, the output voltage will have risen to 75%. To go to full voltage, disconnect the star point and bridge the motor connection to the incoming phases.

One disadvantage of this system is that the flux in the iron of the transformer is reducing as you increase the output voltage. This will increase the leakage reactance of the transformer and reduce the output voltage.

Perhaps I will have to write my own information sheet.


Best regards,

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