WAZIPOINT Engineering Science & Technology: Star Delta Starter Line Diagram and Its Working Principle

Thursday, March 31, 2022

Star Delta Starter Line Diagram and Its Working Principle

Fig: Three Phase Motor connection diagram with a star-delta starter.

What is Star Delta Starter for Motor Starting?

Star-Delta starter is an electrical motor starting device, generally uses in big size motors to overcome some technical limitations. Star and Delta mean here 2 separate states of motor running, first Star connection and then Delta connection.

The starting current of any heavy electric motor can be more than 4 times the normal load current it draws when it has gained speed and has reached its normal running condition. 

To overcome this initial high current enchanting problem, such arrangement needs Star connection at starting time and if Star connection has sufficient torque to run up to 75% to %80 of full load speed, then the motor can be connected in Delta mode.

When the motor is connected to the Delta configuration the phase voltage increases by 173% and phase currents increase by the same ratio. The line current increases three times its value in star connection.

What Problem Starting Motor without Star-Delta?

We already know, if the motor starts simply when connected in Delta, the starting current would be huge just to be able to start the motor, not to running condition. To do this would require -

v Large capacity circuit breakers to allow the start-up surge current to pass without immediately shutting the motor off.
v Oversized 3-phase power service cables require just for starting time, but the normal running time is not necessary.
v Large size of coils and contacts on the relays or contractors need to control the motor, but normal running time needs smaller sizes of them.

Sometimes, utility company also do not permits the big motor to start without a Star-Delta starter because of system instability due to switching transients. 

How Wiring a Star-Delta Starter for Motor?

Following the figure below, the power connection between magnetic contact for Main, Star, Delta and motor is shown in the schematic line diagram and wiring diagram for easy wiring.
Star-Delta Starter Connection
Star-Delta Starter Connection with Control wiring

What is Important During Star-Delta Transition Period?

It is important that the break between the Star contractor switch OFF and the Delta contactor switch is ON because the Star contactor must be reliably quenched before the Delta contactor is activated. During the transition period of switch-over, the motor must be free running with little deceleration. It is also important that the switch over pause is not too long, it may generate a voltage of its own and this may add to or subtract from the applied line voltage.

That’s all for the Star-Delta starter for today; here in this article, we discussed only motor starting by 3 magnetic contracts Star-Delta, solid-state motor starters from different companies also are available in the market now.


  1. Nice,
    This is very important to every electrical engineer,

    1. Thanks for valuable comments

    2. every electrician must have to meet with this important guy.

    3. @Fazlur Rahman
      yes, you are absolutely right.

  2. very good,
    we want more

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  4. Very nice description regarding star delta starter. I got clear idea regarding this topic

  5. Why star delta used, what problem to use direct switch?

    1. off course ur question is right we can start without using any switches . But in large motors at the time of starting it draws very high current so in the intention of reducing starting crrent (which leads in damaging in cable , winding)we use star delta starter

  6. Replies
    1. DOL starting is simply switching the motor on in one operation, with a direct connection from the power source. It’s generally used for smaller loads that starting current can be up to ten times the normal running current of the motor without overloaded any switching devices or conducotor. A contactor is generally used to switch power, and often a thermal or electronic overload relay is provided for motor protection. Direct online starting is the simplest and cheapest method of motor starting.

      But star-delta starter used when need some current limiting in a larger motor. Some motors can be started using the star-delta method: the motor is first connected in ‘star’ configuration, which allows the motor to gather speed without drawing excessive current. Once the motor is up to speed (or a preset time is elapsed) it is then connected in the normal ‘delta’ configuration. This method can reduce the starting current demand by 30%, but is only suited to applications where the motor is starting without load (e.g. where a clutched gearbox is used).

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