Star Delta Starter Sizing & Selection

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  1. Star delta starter generally find application in Industrial & Commercial loads .
  2. Star delta starter reduces starting current by (3 to 5) times than that DOL starter.
  3. It is important  to know ratings of starter components for different  HP motors .
  4. Improper selection & Sizing of components  in starter may lead to damage to motor  windings , or motor may be in standstill .

Operation of star Delta starter :

At starting / standstill condition motor draws high current ,to reduce the high starting current drawn by motor,  motor is connected in star mode ,where phase voltage is applied across motor causes motor to draw only low starting current untill it reaches (75 to 80 %) of rated speed.  After attaining rated speed motor gets connected in delta connection with full line voltage gets applied on motor winding . Motor continous to run in delta connection because Torque obtained in delta connection is more than in star connection .
Note:- Generally in all industries & Manufacturing units Induction motors run in delta connection only to get Maximum Torque Output .


The Sizing & Selection of star delta starter components are given in below Table .

The above table depicts for different ratings of motor what should the rating of 
  • star/delta contactor .
  • Thermal overload relay setting 
  • Fuse rating 
  • Cable size .


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