Electro-Mechanical star delta starter

Star Delta starter is the most widely used application especially for Air-compressor and HVAC system.This starter used for 6 or 12 wire for motor terminal depend on electric motor that chosen for the application.

Basically this starter used 3 unit contactor,1 unit timer delay,1 unit overload relay and start/stop button to control the star delta motor.Around the world,it have many types of control circuit design for star delta.

It have different design and technique  for the control circuit but the main objective is same,to reduce starting inrush current.

For safety condition for the star delta control circuit,it include with interlock for star and delta sequence to avoid serious damage for electric motor.That mean during star sequence,contactor for delta cannot energize or vise versa.

This post i want share about two types typically used for star delta starter control circuit.It have a pro and contra for each design and it depend on our application and objective for installation.

Types of Star Delta control circuit


1) Star Delta Open Transition

Firstly i explain why it call OPEN transition because the electrical motor is disconnected from main line when star sequence change to delta for momentarily condition.It include with interlock system for this control circuit.

For this open transition,the advantage is it depend on timing for transition when loading the electric motor.It cause the current surge and the high torque can effected electrical and mechanical due to shock on the application system.

It also can cause the instantaneous current peaks exceed the limit even the locked rotor current for short cycle of rotation.For the electrical aspect it can happen for the instantaneous peaks and could resulted the power fluctuations or losses.

Mechanical aspect it could cause the damage for system component such as drive shaft,bearing or coupling due to torque increased from current spike.



2) Star Delta Close Transition

This CLOSE transition is an alternative type for star delta starter.The method is same for basic operation but it have additional a few devices to reduce and remove the surge in current matter that faced by OPEN transition type.

For this type,the design have the additional device and component such as a magnetic contactor and power resistors. The advantage of this method is it can eliminated the surge in current problem with high efficiency.

It can reduce a damage for motor winding and mechanical part.Theoretically it can make electric motor have a long life span and save cost for replacement part for mechanical items.For the clear comparision between OPEN and CLOSE transition,please refer to the graph below :



Comparison for OPEN and CLOSE transition Star Delta Starter


Lemau is Owner of Electricneutron.com .Involved in Electrical & Automation Engineering since 2001. Passionate to shared all experience & knowledge about Electrical.

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  1. haitham hashim says:

    This topic is very important Thanks for the detailed explanation

    • Ur welcome…thanks for read my post my friend :D

      • Can you recommend how to select value of power resistor (Res.)in the closed Star-Delta starter?


        • The resistors must be sized such that considerable current is able to flow in the motor windings while they are in circuit.
          The resistors should be sized such that the current through the resistors when in series with the delta configured windings, should be similar to the current through the resistors when connected in star.

          Now in market available for Soft starter and VSD to replaced star delta starter..my suggestion is to change to modern starter :D

  2. michael says:

    i would like to know why a star delta motor can trip after running like 5 min?

  3. bilal says:

    What are the reasons for a star delta to chatter(switch on & off) rapidly after 10 hours of normal operation

    • NerdEng says:

      the reason is simple, voltage drop in your control circuit, especially when your control transformer has the same feeder as your load.

      “there is no prima-donna in engineering and possibly cant learn electrical engineering online (source of electrocution)”

  4. keep up, Mr.Lemau, you are doing a lot of help.

  5. sreeraj says:

    dear sir i have a 13 kw dual speed motor which type control panel can use?soft start or normal

  6. Vijay says:

    but how can we decide the time when we switch over from star to delta.

    • Normal practice is around 8 to 10 seconds

      • PerKriM says:

        The timer should be set to 80-90% of the time the motor needs to reach nominal rpm (normal operating speed under normal load)
        The acceleration period must thus be timed, because it will vary depending on the work load.
        Set the timer to a minute, and start the motor, with normal load. Time the acceleration very carefully, from start to nominal rpm. set the timer to 90% of acc.time.
        If this takes more than 15 sec. to reach nominal rpm, I’d say the motor is too small – power-wise. Normal should be 8 to 12 sec.

        Bare in mind that if the motor is connected through an ordinary Star-Delta wiring, there needs to be a short power-pause between switching Star power off and Delta power in. This is due to the fact that the motor needs to get rid of the standing magnetic field in its windings. It will do so in approx 5 periods. If the AC frequency is 50Hz, the pause should be 1/50Hz x 5 = 0,08s = 80ms.
        If you leave out this pause, you will get a major current transient. This will bug the electrical neighborhood, and cause wearing on the motor.

  7. Thats good update

  8. Thanks for details.

  9. PerKriM says:

    I have a rather tricky question – regarding the calculation of voltage loss in the Star-Delta main motor wiring.
    Motor current FLC : 10A
    Efficiency factor: 0,87
    Voltage 230V
    Cabling distance: 40m
    Max allowed V drop: 3% (6,9V)
    Cable 2 cu cables à 3×2,5mm2
    Resistivity for Cu: 0,0175

    deltaU = (0,0175 x rot3 x (10A x 0,58) x 0,87 x 40) / 2,5mm2 = 4,22V
    This is well inside the demand.

    Or should both calbes count? 40m x 2
    deltaU = (0,0175 x rot3 x (10A x 0,58) x 0,87 x (40×2)) / 2,5mm2 = 8,44V
    Now the voltage drop is too high. We need to raise the the copper square mm2 of the cables.

    Any tip will be appreciated.
    A detailed explanation of the why and how will be deeply appreciated.

    • PerKriM says:

      The calculations got the best of me – they are some what wrong.
      My calc’s is done with I=10A not (10A x 0,58)
      If we raise the distance to 60m the problem arises correctly.
      Then the V.drop for 60m is under the limit and the V.drop for 120m is above.

      Sorry for that :-D

  10. ChilesheZaccheriour says:

    thanks for the information, you are realy doing a great job.

  11. Macky says:

    Please I need a detail explanation about star connection , delta connection and stardelta connection and how they work in an electric panel in controlling an industrial Motor for pumping

    • Per Kr. Meyer says:

      All you ask is answered earlier in this thread. My first suggestion would be that you read it all from the top. And then perhaps post Your q again, a little more spesific.
      These two videos give a Nice graphic explanation of how these motors are constructed.
      This is basic knowledge you neeed for understanding any of the postings in this topic.

      • Per Kr. Meyer says:

        What kind of job the motor does is irrelevant. How ever only really large pumps would need a Automatic star/delta solution. In such a case the star/delta solution would be used to aviod the Heavy starting current that you get from a Direct star or delta wiring.
        In a Direct coupling, the motor can draw as much as 10 times (worst case) the nominal current (data plaquet on morot).
        As you might know, all inductive load will, when fired, pose only a small ohmsk resistance. When the coil/winding builds up the magnetic Field the impedance increases due to the inductive resistance from the magnetic Field.
        A induction motor is an inductive load.

        All standard induction motors are constructed to be complient to two voltage Levels.
        Here in Norway motors are usually made for 230V and 400V.
        Delta : 230V (Current D)
        Star : 400V (Current Y= Current D/squareroot of 3)
        Motor gives same output in both realms, Power wise. (P is Power)
        P(D) = 230 x Current D
        P(Y) = 400 x Current D / Sqr3
        Result: P(D) = P(Y)

        • Per Kr. Meyer says:

          For all the professors out there:
          The mathematics here is some what simplified :-)

  12. Yagnik says:

    i have a 30 HP 3 phase squirrel cage Induction motor and it is connected in Star-Delta Starter,when i start the motor its main and star contactor on but when it comes in delta it suddenly trip,the MCCB,Thermal Overload Relay does not trip in the process (Time delay b/w star/delta is 15 sec).I checked all the possible causes and wiring but its found OK.so pls give me the solution and reson of the problem as soon as possible.

    • Per Kr. Meyer says:

      The most common problem with the star/delta starter is fase-fault.
      A faulty order of fases will cause the motor to run fine in star-mode but stop due to tripping overload relay when in delta-mode.
      You have to pay close attention to how the fases are connected.
      If you have got the fases wrong, the motor will start just fine in star mode.
      This is because in star-mode, the motor will run ok regardless the order of the fases.
      The only impact of the fase order in star mode is of wich way the rotor will turn.
      In delta mode, how ever, the fase order is of cruisial importance. If wrong – one of the windings (one of the three stator windings) will get the same fase Connected on both sides and this effectivly blocks for the current flow. No current, no magnetic Field from this winding. Then you will have only two working windings. This usually makes the motor hum and shiver.
      All overload relays will catch this and trip after a few seconds.

      How to check?
      Unconnect all 6 wires from the motor Connection panel.
      Correct order:
      L1 o o L1
      L2 o o L2
      L3 o o L3
      Set the starter in delta mode and measure the voltage.
      When measuring L1 and L1 the reading should be Zero voltage.
      Measure L2 and L3 in the same manner.

      Be careful of personnal danger when working with live wires.
      Let me know if this is the case.


    • HI,please check your outgoing cable connection..please refer my last post about star delta motor connection –> http://www.electricneutron.com/electric-motor/star-delta-motor-connection/

  13. Per Kr. Meyer says:

    I have scematics that distinct between main current and Control current.
    This makes it a little easier to grasp the concept.
    But I don’t know how to post them here.
    It doesn’t seem to be any room for graphics or attatchments in this forum.

    Anyhow…. Yagnik
    There’s no short cut around the basic knowledge of how these motors are constructed and how they work.
    You should be familiar to all items on the motors nameplate.
    Voltage, Current, Power, rpm, Frequency, IP value, cos phi and more….

  14. Per Kr. Meyer says:

    If this an old installation that has been working, we can rule out the “faulty order of fases” issue. If this motor suddenly has started to fail in delta mode, its more likely to assume a bad contact in the delta contactor.
    This will cause the motor to behave in the manner you describe.

  15. hi am

  16. Sir,

    How to calculate the winding resistance of a motor.

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