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. lekaokaop says:

    dear sir,
    please help me
    i have a compressor with 7.5kw motor, it starts star/delta.when motor is starting in star it can not rotation. after timer set it change to delta and running ok
    i have checked the current of three phases when it was starting in star are 27a ( for all 3 phases. at this time motor did not rotate)and then it changed to delta mottor rotates nomaly the current of three phases are 12a/ 14.5a nominal.

  14. I’m going to right away learn your feed because i can not to discover your e-mail monthly subscription link and also e-newsletter program. Do you have every? Nicely make it possible for my family recognize in order that I really could sign up to. Thanks.

  15. It is, however, becoming more plus much more common for
    any shipper to supply cars directly on the house in the buyer.

    Is the entire cost as it can be, or will there be additional charges.
    Make sure that the business is approximately date on their own license and insurance.
    Rachel recently posted..RachelMy Profile

  16. BrianS says:

    There is a good discussion at the below site about sizing & operation of Start Delta starters.

    Interesting comments:
    1. The use of smaller contactor for the Star (Y, Wye) Contactor.
    The Main & Delta Contactor will be the same size with the Star Contactor either the same size or one size smaller.
    2. The Star Contactor may be operated before the Main Contactor.
    If the Star Contactor is one size smaller, it lasts longer if the switching on of the start current is done by the bigger, Main Contactor. However, since contact erosion on Make operation is not high, so you can switch on Main and then Star if absolutely essential.
    The Main Contactor may be operated by an Auxiliary Contact on the Star Contactor.
    3. Timing discussions.
    A “transition delay” when changing from Star to delta prevents any ionised gases in the star contactor causing a short-circuit, especially with higher current ratings, 50 hp and above. For <50hp, 30 ms is enough (although figures of 50-100ms are also given), and above that 100 ms, although special circumstanced may require larger motors & higher voltage motors to have a longer delay.
    The delay is a compromise because it allows the motor to slow down during the transition delay & if it slows down too much in the transition time, then when it is switched ON in delta, there will be a large current surge again… so the very reason for using a Star/Delta (Wye/Delta, ΥΔ) starter will be lost.
    4. The safety aspects for using Mechanical Interlocks as well as the Electrical Interlocks.
    Interesting comment was that the short activation distance of some Auxiliary Contacts allow them to operate when the Main Contactor contacts are welded together so that an electrical interlock can allow the Delta to Close while the Start is still closed. A welded electrical Auxiliary Contact can also allow the closing of both contactors. That type of fault is a bolted short and could destroy all the contactors, the conductors & motor as well.
    Mechanical Interlock makes that type of faulty operation impossible… even by manually mechanically pushing it!
    5. Related to item 4, only some contactors such as the Sprecher CA7 will allow you to Mechanical Interlock two contactors of different sizes. Many only allow two identical contactors to be Mechanical Interlocked.

    Discussion was at:

  17. I am really impressed with your writing skills and also with
    the structure in your weblog. Is that this a paid topic
    or did you modify it your self? Anyway keep up the nice high quality writing, it’s uncommon to see a nice weblog like this one these days..
    louis vuitton handbags recently posted..louis vuitton handbagsMy Profile


  1. [...] **Show Us Your Installs!!!** Hi..I want share my Star Delta Starter Control Panel For Chiller Water Pump.. 310720122846.jpg [...]

  2. [...] most active post in Electrical Engineering Centre blog is Star delta starter and Star Delta motor connection.I received many comments and request for Star Delta circuit [...]

  3. [...] you want connect the induction motor with star delta starter,what you must to do is take out all cooper bar from motor terminal.It mean we not use a cooper [...]

  4. [...] each phase ( red,yellow,blue).If you not read yet,please click on Star Delta motor connection and Star Delta Starter for further [...]

  5. [...] How many second we need to setting for changeover timer from STAR to DELTA sequence for Star Delta Starter?This question is very popular among electrician around the world when handling the Star [...]

Speak Your Mind


CommentLuv badge