Star delta starter

This is a starting method that reduces the starting current and starting torque. Star delta starter design normally consists of three contactors, an overload relay and a timer for setting the time in the star-position (starting position).

For the star delta starter, a motor must be in delta connected during a normal run and the main purpose is to be able to use star delta starter.

Star delta starter received the starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start.

Star delta starter only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.

Description of Star Delta Starter Operation


For star delta starter,the basic function is to enable the motor to start and the motor windings are configured in a star formation to the supply voltage.

The voltage applied for star delta starter to the individual motor winding is therefore reduced by a factor of  1√3 = 0.58 this connection amounts to approximately 30% of the delta values. The starting current is reduced to one third of the direct starting current.

Due to the reduced starting torque, the star-delta-connection is suitable for drives with a high inertia mass but a resistance torque which is low or only increases with increased speed. It is preferably used for applications where the drive is only put under a load after run-up.

After motor run-up, in most cases an automatic timing relay controls the switch-over from star to delta. The run-up using star connection should last until the motor has reached the approximate operational speed.

so that after switching to delta, as little post acceleration as possible is required. Post-acceleration in delta connection will instigate high currents as seen with direct on-line starting.

The duration of start in star connection depends on the motor load. During delta connection, the full mains voltage is applied to the motor windings.

To enable a switch-over from star to delta, the six ends of the motor winding are connected onto terminals. The contactors of a star-delta starter switch over the windings accordingly.

How star delta starter works?

Starting in star, the main contactor connects the mains to winding endings U1, V1, W1.The star contactor shorts winding endings U2, V2, W2.

After successful run-up, the star contactor switches itself off and the delta contactor connects terminals U1/V2, V1/W2,W1/U2.

When changing from star to delta, we should pay attention to ensure  it running in the correct phase sequence.Incorrect phase sequence can lead to very high current peaks during the cold switch-over pause, due to the easy torque reduction following re-start.

High peaks current can damage the motor windings and stress the control gear unnecessarily. The rotation of the motor has to be considered as well.If wrong rotation,we should change the rotation by switch the phase.I discuss detail about How to change star delta starter motor rotation in my separate post.

For star delta starter circuit diagram,wiring technique and motor base termination,Please read my post for star delta motor connection.Also for simple star delta control circuit wiring and types of star delta starter,please refer to my post for Electro mechanical star delta.

For more detail,please refer to ELECTRIC MOTOR HANDBOOK :-


  1. sathish says

    Good informations in this site. how to to find out the wrong termination in star delta connection. for example in star delta connected motor star in star mode but after change in delta mode contactor of delta and main are in pick up condition but motor got off not in trip. can u explain this

  2. please give a diagram of star-delta starter with transition resistors(Kondorfer)

  3. vivek chilmul says

    The artical which is above is more enough to understand and knowledge of particular thing.

  4. venkateshwaran says

    very usefull

  5. Nicholas Eno says

    plz send to my email a control circuit of forward reverse connection…A.S.A.P….thank you…

  6. can anyone tell me about the difference in delta and star connections from the generator?

  7. Hmiriam says

    please post me the book to my email,thank you.

  8. Hmiriam says


  9. Avinash says

    in a star delta stater how we can set the timer timing to switch to delta from star according to load characterstics.

    • it hard to setting the best timing for star delta starter.Typical setting for timer is around 8 until 10 second….

  10. Parekh Chandrakant says

    Dear Sir,

    We are manufacturer of animal is in pallate fome.and our requirement of Star-Delta Starter for 40HP,please give the Quotation for the same.

    Kind Regards,

    Chandrakant Parekh.
    +91 95588 13099
    Divine Ani-Food Manufacturing Unit.

  11. firmansyah says

    Dear Sir

    please tell me ,
    what will happen if the motor is running and contactornya only 1 ( Delta contactor off )


  12. Ian McClure says

    Hi I am looking for a way to change the rotaion on only delta as motor runs forward in star but tries to reverse rotaion when changing into delta causing fuses to blow.tried to change L1 L3 at motor but motor runs at very high amps

  13. Dol starter pump connection

  14. Chipo oageng says

    Hi, Can u plis give me control circuit for the compressor of star delta with addition of the solinoid valve and pressure switch

  15. kgsuresh says

    we have supplied dol starter of 10hp motor but it fails because of relay got burnt & after that we once again replaced the relay/contactor it works for 03 days same has repeated & motor winding got burnt.
    then we replace the star delta starter found the same has been repeated after 02 days of working condition
    kindly inform us the reason for the above if any suggestions.
    suresh k g

    • Please check your actual full load and motor sizing

    • Please check current rating for your contactor.It should same or high ampere rating from your motor full load ampere.

    • manish sharma says

      motor in starting come in star connection then after delta. motor burn reason is single phasing check the both contactor continuity that incoming band outgoing connection voltage are ok or not and also check the timer working and thing is o/l relay burn there are three reason
      1) bimatlick strip dusty
      2) lose connection
      3)lower rating

  16. amar kogde says

    best information

  17. I need for star Dalta with timer gigram

  18. atu kar says

    Sir.. can u kindly help me out as i am confused while connecting my 40hp motor crampton greaves with the starter. Here i will give u the sequences..

    Starter.. C1 L1 A1 L2 B1 L3
    AO BO CO
    Motor… U1 (u ) W2 (k)
    V1 (v) U2 (l)
    W1 (w) V2 (m)
    SO kindly help give me the direction.. waiting for ur anticipation.

  19. Suhasraj patel says

    How to set a overload relay setting in star as well as in detla connection ??

  20. Suresh Bareth says

    Very useful

  21. Thanks for your information. Please am a new beginer. Can you please send me a book that contains everything about motor staters method. Again I want to know how the. Star contactor is being connected to the three phase machine. Thanks for your anticipated effort


    I have a Centrifugal Fan of 150 HP motor which we run with star delta starter with timer and thermal relay base contactors. How much time can be set between star and delta starter ? any thumb rule which depends upon motor HP or any ??

    When we are starting the motor it draws 340 amp. it reduce gradually up to 285 amp. and then come in delta in 25 sec. In delta it draws 825 amp. and reduce to 70 amp. with in 2 sec. and again after 2 sec. motor trip. What will be the reason ?? Please help me.

  23. hussain says

    What’s star and delta sent me in email

  24. please my rquest send the one video hindi mediem please star delta motor control with conection

  25. hindi longuage send star delta motor control conection

  26. m k azam says

    please say
    Me how do select 3 magnetic contractor and over load on star delta operation 75 hp motor . please gave me calculation
    3 phase
    75 hp

  27. sir place say
    Me how do the select star delta magnetic connector and over load. please gave me calculation
    75hp motor (please send my email addresses)

  28. ram kumar mishra says

    sir please help me mai star Delta starters se ek starters se do motor ko on off karna hai please give me diagram

  29. prem singh says

    why relay burns again and again i have 30hp motor on tubewell .can it burn because of nutrul from transformer is weak.i use jyoti star delta starter

  30. Justus Turyatunga says

    I have a motor connected in Delta but two times now it has been making contacts get jammed in whenever there is a phase failure ie when one phase goes off.This happens before the over load relay trips yet the over load is set at the minimum.Motor full load is 58A/40hp.The supply is direct on line.What do you think is the cause of this.Can the star connection solve the problem.

  31. eliseo hoover says

    Good post . I Appreciate the facts , Does anyone know where my assistant can get a blank IRS W-2 form to fill out ?

  32. murali.c says

    hello sir,
    my name is murali I need to to know how to identify star delta timer failure.

  33. Melvin says

    Hi i have a granulator connected in star delta. The second contactor keeps on blowing. What could the cause of this be?

  34. 75 HP/55k/92 amp- star delta starter for 55 HP motor

  35. please say
    Me how do select 3 magnetic contractor and over load on star delta operation 75 hp motor . please gave me calculation
    3 phase
    75 hp


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