Star-Delta Starter


Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. Two methods are used for reduction of starting voltage are star delta starting and auto transformer stating.

Working Principal of Star-Delta Starter:

  • This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs normally.
  • Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.
  • Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltage starters that can be applied.
  • The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.
  • There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.

Star-delta Starter Consists following units:

1)     Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).

2)     Time relay (pull-in delayed) 1 No.

3)     Three-pole thermal over current release 1No.

4)      Fuse elements or automatic cut-outs for the main circuit 3 Nos.

5)     Fuse element or automatic cut-out for the control circuit 1No.

Power Circuit of Star Delta Starter:

  • The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.
  • The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1, V1, W1.
  • In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period of time, the star contactor is opened, and then the delta contactor (KM2) is closed. The control of the contactors is by the timer (K1T) built into the starter. The Star and Delta are electrically interlocked and preferably mechanically interlocked as well. In effect, there are four states:


  • The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup.
  • Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of a star delta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and a timer.

Control Circuit of Star-Delta Starter (Open Transition):


  • The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit.
  • When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.
  • When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.
  • After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch.
  • When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO.
  • Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.
  • A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active.
  • The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective device which automatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay.
  • At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.

What is Open or Closed Transition Starting

(1)   Open Transition Starters.

  • Discuss mention above is called open transition switching because there is an open state between the star state and the delta state.
  • In open transition the power is disconnected from the motor while the winding are reconfigured via external switching.
  • When a motor is driven by the supply, either at full speed or at part speed, there is a rotating magnetic field in the stator. This field is rotating at line frequency. The flux from the stator field induces a current in the rotor and this in turn results in a rotor magnetic field.
  • When the motor is disconnected from the supply (open transition) there is a spinning rotor within the stator and the rotor has a magnetic field. Due to the low impedance of the rotor circuit, the time constant is quite long and the action of the spinning rotor field within the stator is that of a generator which generates voltage at a frequency determined by the speed of the rotor. When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generator and this result in a very high current and torque transient. The magnitude of the transient is dependent on the phase relationship between the generated voltage and the line voltage at the point of closure can be much higher than DOL current and torque and can result in electrical and mechanical damage.
  • Open transition starting is the easiest to implement in terms or cost and circuitry and if the timing of the changeover is good, this method can work well. In practice though it is difficult to set the necessary timing to operate correctly and disconnection/reconnection of the supply can cause significant voltage/current transients.
  • In Open transition there are Four states:
  1. OFF State: All Contactors are open.
  2. Star State: The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
  3. Open State: This type of operation is called open transition switching because there is an open state between the star state and the delta state. The Main contractor is closed and the Delta and Star contactors are open. There is voltage on one end of the motor windings, but the other end is open so no current can flow. The motor has a spinning rotor and behaves like a generator.
  4. Delta State: The Main and the Delta contactors are closed. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available

(2)   Closed Transition Star/Delta Starter.

  • There is a technique to reduce the magnitude of the switching transients. This requires the use of a fourth contactor and a set of three resistors. The resistors must be sized such that considerable current is able to flow in the motor windings while they are in circuit.
  • The auxiliary contactor and resistors are connected across the delta contactor. In operation, just before the star contactor opens, the auxiliary contactor closes resulting in current flow via the resistors into the star connection. Once the star contactor opens, current is able to flow round through the motor windings to the supply via the resistors. These resistors are then shorted by the delta contactor. If the resistance of the resistors is too high, they will not swamp the voltage generated by the motor and will serve no purpose.
  • In closed transition the power is maintained to the motor at all time. This is achieved by introducing resistors to take up the current flow during the winding changeover. A fourth contractor is required to place the resistor in circuit before opening the star contactor and then removing the resistors once the delta contactor is closed. These resistors need to be sized to carry the motor current. In addition to requiring more switching devices, the control circuit is more complicated due to the need to carry out resistor switching
  • In Close transition there are Four states:
  1. OFF State. All Contactors are open
  2. Star State. The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
  3. Star Transition State. The motor is connected in star and the resistors are connected across the delta contactor via the aux [KM4] contactor.
  4. Closed Transition State. The Main [KM3] contactor is closed and the Delta [KM2] and Star [KM1] contactors are open. Current flows through the motor windings and the transition resistors via KM4.
  5. Delta State. The Main and the Delta contactors are closed. The transition resistors are shorted out. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available.

Effect of Transient in Starter (Open Transient starter)

  • It is Important the pause between star contactor switch off and Delta contactor switch is on correct. This is because Star contactor must be reliably disconnected before Delta contactor is activated. It is also important that the switch over pause is not too long.
  • For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained with Direct Online (DOL) starting.
  • If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.
  • When connected to Delta configuration the phase voltage increases by a ratio of V3 or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection.
  • During transition period of switchover the motor must be free running with little deceleration. While this is happening “Coasting” it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causes voltage surges and spikes. Known as a changeover transient.

 Size of each part of Star-Delta starter

(1)   Size of Over Load Relay:

  • For a star-delta starter there is a possibility to place the overload protection in two positions, in the line or in the windings.
  • Overload Relay in Line:
  • In the line is the same as just putting the overload before the motor as with a DOL starter.
  • The rating of Overload (In Line) = FLC of Motor.
  • Disadvantage: If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting is x1.732 too high).
  • Overload Relay in Winding:
  • In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then always measures the current inside the windings.
  • The setting of Overload Relay (In Winding) =0.58 X FLC (line current).
  • Disadvantage: We must use separate short circuit and overload protections.

(2)   Size of Main and Delta Contractor:

  • There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor.
  • Size of Main Contactor= IFL x 0.58

(3)   Size of Star Contractor:

  • The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is 1/ √3= (58%) of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.
  • Size of Star Contactor= IFL x 0.33

Motor Starting Characteristics of Star-Delta Starter:

  • Available starting current: 33% Full Load Current.
  • Peak starting current: 1.3 to 2.6 Full Load Current.
  • Peak starting torque: 33% Full Load Torque.

Advantages of Star-Delta starter:

  • The operation of the star-delta method is simple and rugged
  • It is relatively cheap compared to other reduced voltage methods.
  • Good Torque/Current Performance.
  • It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter:

  • Low Starting Torque (Torque = (Square of Voltage) is also reduce).
  • Break In Supply – Possible Transients
  • Six Terminal Motor Required (Delta Connected).
  • It requires 2 set of cables from starter to motor.
  • It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.
  • In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. The delta of motor is formed in starter and not on motor terminals.
  • High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks. In some cases the current peak can reach a value that is even bigger than for a D.O.L start.
  • Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
  • Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque .
  • The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which make transient Sparks.

Features of star-delta starting

  • For low- to high-power three-phase motors.
  • Reduced starting current
  • Six connection cables
  • Reduced starting torque
  • Current peak on changeover from star to delta
  • Mechanical load on changeover from star to delta

 Application of Star-Delta Starter:

  •  The star-delta method is usually only applied to low to medium voltage and light starting Torque motors.
  • The received 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. This starting method 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.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

246 Responses to Star-Delta Starter

  1. Manish says:

    Very good Article

  2. Gopal says:

    Thank You Sir ji

  3. Gopal says:

    Oh Sir ji again thanks
    I want be electrical design engineer in the field of power plant/substations/etc.
    please guide me and provide more stuff for kind of calculations.


  4. ABI says:

    hi veryvery nice article
    and ji i need some datas regarding transformers request

  5. siva says:


  6. Correction is to be required in ‘Disadvantages of Star-Delta Starter: Six Terminal Motor Required (Delta connected). Six Terminal Motor is required, but not Delta Connected Motor. If Delta connected Motor is placed, when you press On Push-button, Three Phases Short- circuit and Flash-over takes place in Terminal box as soon as Main Contactor and Star Contactors got energized.

  7. Somenath says:

    Exceptional artical…..helped me a lot….thanx….

  8. juber says:

    I have 60HP motor in my chiling plant, while in running condition motor suddenly trip, sometime it run continues, sometime trip frequently, I already fluctuate relay and timer setting in star-delta starter, and take some action, but gain same result, PLEASE TELL ME SOLUTION…

  9. teja says:

    Hello sir, i am teja working in control panel designing and i am just passed out be tech EEE so please kindly help me how to connect on/off/trip indication lamps for star-delta contactors and one thing i did not understand from your article why we are connecting km1 and km2 contactors i,e(5,6) from control drawing so please help me reg this. thank you.

  10. anil says:

    thanks sir the artical is unique and described in simple way .thanks again

  11. sandip Thummar says:

    thank u sir.
    sir I want totely thierical deatils of y d starter so please email me on address.

  12. Rajureddy.M says:

    Thank u sir, plaese update your contact detail,,,,

  13. Alexander says:

    You know what you are doing. Your article is short and sweet

  14. Maston Mwenda says:

    Well sir,thankyou for your work. Well am from Kenya en working in electrical field. Please tell me how i can use contactors to protect a motor connected in star delta against single phasing THANKYOU.

  15. musa jibrin says:

    this write up really helps me ‘cos am writing a project that involved star/delta. It really help alot thank. We expecting more it kind.

  16. muni yandi says:


  17. absee says:

    very clear n got all the info, thanks

  18. swash says:

    awesome article…i am ding my industrial training in muscat i am a btech 3rd year year electrical engineering student i would like to know few college project ideas related to starters which could be possible for me to understand an ddo and economic………….thank you

  19. Venki says:

    Thank u sir…

  20. Augustine says:

    It ‘s helped me a lot thank you.

  21. Saravanan says:

    sir can u explain me about reverse direct online transformer

  22. rajesh says:

    good artical

  23. vaibhav garg says:

    thanks sir, your schematic diagram and method of explanation is so practical. i am currently designing a starter for 10hp to 20 hp.can you plz suggest that how can we connect a single phase preventer and an Autoswitch (which can restart the motor automatically even after a power supply failure.

  24. samuel says:

    this is really gr8 it is simple and precise. thank u so much.

  25. chirag says:

    Thank u sir ji….

  26. D Arun kumar says:

    Thank you very much sir.This article is very useful for me

  27. Shoukath says:

    I got new in my life . Thank you

  28. PAZHANI. P says:

    Respected Sir..,,
    it is great Job .. your intention towards knowledge sharing take you to ‘GURU’ status.
    It is so much useful to all young electrical engineers..

  29. Sezu says:

    great info

  30. Garry says:

    HI. Please check the motor connections diagram should the terminals V2 AND U2 be swooped.
    Many thank for a good explanation

  31. ujjwal agarwal says:

    is the phase sequence shown in the circuit diagram correct?? m not sure about it!

  32. MUDASIR says:

    Very good artical.

  33. Engr dhnishan says:

    sir this is very good process.

  34. Satya Narayan Padhy says:

    Actually its a good article…….I need ur help ,so can u please give me ur contact no.

  35. Uncle E says:

    may U remain blessed n Properous. I luv dis Publication. I’ve gaind alot. Tanx.

  36. Nitesh mishra says:

    Thanks sir

  37. wasim says:

    please Guide me for a Good books on Distribution n Transmition of Electrical power.

  38. ratindra Kumar says:

    Very helpful artical…thankx

  39. pradeep says:

    thanks mr. parmar

  40. david kevazi says:

    excellent information

  41. ram k says:

    this is best info. to learn new engg.student

  42. sandeep says:

    itss great sir ji

  43. Akmal says:

    Akmal Sardar Awan[Elec sup] i think you helping lot of peopels its great Mr Parmar God bless you

  44. Manford says:

    thanks for your helpful article .
    i have a question: the Star-Delta starter is an electro-michanichal starter what is the altirnative power electronic starter same as soft starter we can use for the same purpose ? and what are the main advanteges of the latter of the former?

  45. bilalahmed khan says:

    very thanks

  46. Cobus Jacobs says:


    What will happen if the electrical interlock doesnt work and the delta contactor energises before the star contactor de-energises.

  47. farida says:

    what happens when both star and delta are operated at a time

  48. yogender says:

    star delta starter used only in induction motor or any other type of motor

  49. Veronika Petronella Mazorodze says:

    great stuff enjoyed reading.

  50. S. P.Majumder says:

    S.Majumder saus :
    March 11, 2013 at 15: 05 pm.
    My question is that Overload Relay has been put in mainline ( before main contactor), so how could i set the relay it in (0.58x F.L), because when motor will run in Delta full load current of motor will flow through o/L relay.

  51. shiva says:

    I reely like it

  52. SIR,What are the possible causes of a delta star control circuit fail to start at times when powered on

  53. Nikhil Yadav says:

    I want to start twin drives (90 kw each for lime stone belt conveyor) through Delta-Star starter, what happens (in the panel and in the motor) if i short both motor windings at star contactor.

  54. Ian Turner says:

    I have not studied star delta for 23years so thank you for your clear insight

  55. Muhammad Jamshid Khuram(MJK) says:

    Thanks for sharing star delta information with your trade mates.

  56. Dennis Ronoh says:

    Thankyou very much.You always equip me in the field of electrical engineering and make my studies enjoyable without difficulties.God bless you.I have gained alot.

  57. Rakib says:

    Very good.

  58. up to what HP the star delta starters can be used

  59. k.pratap says:

    good article

  60. Musa Nkomo says:

    A very good & interesting subject.Sir i hv read the differences between a star-delta starter & a DOL starter.can u please provide me with the difference of a soft starter,VSD’s & PLC’s.thanks

  61. Subin idicula says:

    Does the control diagram changes according to conditions or requirements.?
    Why control diagram is not universal as power diagr in start delta?

    • Subin idicula says:

      Please tell me how to detect the mpcb and mccb of particular motors
      Iam working as an electrical engineer in a oil plant.sometimes in mcc panel the name of the motor is not given in the feedewhat is r,so our seniors detect.the specified motors either by relay or current given in. mpcb or mccb
      Tell me what is the logic n how should I find it?

  62. Kelvin says:

    Good artical

  63. lawrence says:

    u have done a wonderful analytical approach to a star-delta starter . i have gain a lot & pls keep it up. sir what must be the cause of frequent timer’s failure in star dellta starter.

  64. yadvendra sharma says:

    why we give supply to star winding first…?

  65. Poundal says:

    motor connection sequence is in U1,V1,W1 and W2,V2,U2

  66. Poundal says:

    how to change the direction of rotation of motor for star delta starter?

  67. Ankit says:

    sir why in india 230v and fix 50 hz frequency use?

  68. ramesh r says:

    very lucid article…thanks for that

  69. Balaji says:

    it is really very good article .Many Thanks for providing such a very good technical info.

  70. sanjeet kumar says:

    I ask for you
    1Kv=??mm cable (copper=aluminium)

    plz tell me in my mail id

    • Not Getting Your Point !!!!
      Amp depends upon Voltage so I can not Answer 1amp=Watt
      1KV=?? mmCable…
      Actually Amp depends on Size of Cable (copper=aluminium)!!!
      You can not predict Kv from Size of Cable!!

  71. love kumar says:

    why we short the star contactor in star delta starter

  72. Devaraju M says:

    nice one for all

  73. sunny rai says:


  74. Angelraj says:

    Good and detailed explanation

  75. jayanand mantri says:

    Sir, we want to go for automation for waterpump please give dealers in mumbai India

  76. riyal says:

    please give me your mobile no,

  77. praveen says:

    its really very very useful

  78. dilesh says:

    sir,good notes

  79. delca says:

    Thank sir,wanna become sa engineer one day, do you have any guidance for me

  80. Patel brijesh says:

    I am happy sir thanks

  81. Anowar says:

    I want to select HT Cable of 3x50rm,Type- 2xSEYFGY,Uo/U(Um)-18/30(36)Kv) for 2.5MVA,33/.440kV,Z=7% oil type Transformer, Please tell me is it ok or nok.

  82. Gokhan says:

    Thank you for sharing your knowledge, I came across a star delta starter for the first time yesterday, your notes are useful as always. Keep up the good work


  83. Audrey Douglas says:

    I went for an interview the other day and was asked what was The Star. I went on to tell how Star Delta worked but the interviewer said that was not the correct answer, he said I obviously knew about the Star Delta but was not telling him what the Star actually was. I am to go back to a second interview and still can’t work out the answer. Can you help please.

    • Don’t be so confused or depressed .. Actually You can not judge any body by interview or You can not judge your self from success ratio of interview.
      Actually success ratio of interview is depend upon how much your knowledge match with the knowledge of interviewer..
      As per my personal view 90% of interviewer doesn’t know how to take interview of candidate . He just ask the question what he know but actually he has to ask the question regarding the topic in which candidate feel himself comfortable.
      As per my personal experience ,I got rejection in one interview before six month . The reason which interviewer told me that I have not sufficient knowledge in Electrical…The Question asked to me that “What is IR Value of Substation”..I told him to give me name of equipment of S/S and for which voltage.but he did not convince. I also told interviewer that Substation do not have IR value but it has Earth Resistance value. After interview I quested him to gave me answer for my knowledge but he could not give the answer and told me to search that !!!!
      There is only star connection (in winding ) which i know in electrical engineering other STAR I have no idea..Please let me know if you get idea for that

    • maybe if you answered THE SUN IS A STAR then you might get it correct.

  84. prasad says:

    Don’t be so confused or depressed .. Actually You can not judge any body by interview or You can not judge your self from success ratio of interview.
    Actually success ratio of interview is depend upon how much your knowledge match with the knowledge of interviewer..

    nice line sir…….

  85. PVSN Murty says:

    Thanks alot Sir. Article helped me a lot. Very simple explaination on Starters.



    this is really a good post

  87. Ravi Parmar says:

    Dear Sir,
    I want to ask about topic related to Star Delta Connection. What is rating difference between Star delta starter and DOL starter of 75 HP motor.

  88. Ravi Parmar says:

    Dear Sir,
    I want to ask about topic related to Star Delta Connection. What is rating difference of Over Load Relay of Star delta starter and DOL starter of 75 HP motor.

  89. sibikumat says:

    In our company 75HP motor working above 20years in DOL starter .then y star delta ..

  90. Ranjan says:

    thank you sir it’s very help full notes for me and for my future in electrical side……


    This is a very nice article

  92. shahi shakir says:

    very good sir and than you sir

  93. shahi shakir says:

    sory thankyou sir for providing information

  94. Metodi says:

    I want to ask you something about the main scheme
    What will happen if U1, V1, W1 and U2, V2, W2 you replace the places or the startup U1, V1, W1 join on the KM1 contactor?

  95. souvik roy says:

    Thank u sir shearing your knowledge….

  96. bakerms keint says:

    Its nice

  97. sai shankar says:

    good knowdged base

  98. Naveen says:

    If your distribution system permits starting Voltage Dip than You can use DOL without any problems..

    Pls explain this sir.

  99. rahul says:

    its nice

  100. M A Kallon says:

    wow wonderfully done

  101. Ajay says:

    Very in depth article about star delta starter. Nice😎

  102. Ajay says:

    Very in depth article on star delta starter. Nice😎

  103. ALIKHAN100 says:

    you are great sir, good information for electrical engineer

  104. Hemant Kumar Tripathi says:

    please guide me that how we will decide a actual capacity of over lode relay and timer capacity for over 5 k.w. Motors. Please give me the method.

  105. Maheshgautam says:

    Dear sir

    I’m mahesh gautam for the. thinks

    Very good thanks sir ji

  106. Wondimu kibret says:

    Very good explanetion with detail!

  107. Blessing says:

    this is good staff

  108. Janith says:

    I am confused with the explanation of control circuit of the star delta starter and the diagram.But this is very useful article.thank you sir…

  109. rakesh varma says:

    hi sir
    i want to know about power cable
    i have 5hp and above and 10hp and above 50 hp and above motors
    how much of cable is used for these motors………

  110. Ritesh says:


  111. Abhijeet Unde says:

    ur article was to be good

  112. sathish says:

    Dear Sir,

    Please give clearances required for transformers,DGs,panels as per Andhra pradesh EB rules.We are having transformers in row wise in b/w walls.And also plz say how much clearance required top of panel to ceiling.pls give me an idea

  113. Atul Thakur says:

    This is very good artical. I have below mention question regarding power cable selection for star-delta starter motor, please answer the same.
    Is it correct, power cable size required for same distance & same rating star-delta starter motor is less than require for DOL starter? if yes, why?

  114. Hardik says:

    thnks fr this good knowledge…

  115. Mohamed iqbal says:

    I have a three phase induction motor in HVAC system (AHU) it connected in DOL rating= 37kw,415V ,50Hz, 1481 rpm ,68A, 0.81pf and S1 Duty,make ABB. When We are commissioning this motor (solo run) starting current only getting 40 Amperes,each phase. I know the starting current will take four five times of full load current I using clamp meter -make fluke for measure, why getting the low value ? is it a super motor? where is the mistake sir

  116. Mohamed iqbal says:

    sir, please upload closed transition diagram

  117. Naftal sika says:

    Plz publish how interlock works

  118. Nagendra says:

    Sir, i jus wanted to know if suppose the change over fails for some reasons then, what will happen to the motor which continues to to run in star connection??

  119. sir can u answer me these questions
    1. what are the maintenance done on starters
    2. safeties on starter
    3. in star delta starter why star first then delta, why not delta first then star

  120. trushal says:


  121. amit says:

    one question i have regarding the star delta concept is that qhy star delta starter is not used below 5HP motor

  122. Hardik says:

    Basic definition of automatic star- delta starter

  123. Hardik says:

    overview of automatic star-delta starter

  124. hasan says:

    Dear sir… v.nice work and good efforts… sir i am from Pakistan i wana purchase electrical notes (Hard copy)… how can i purchase?

  125. Philip D. says:

    Hi Mr. Parmar,

    Thank you for your time, in providing this excellent resource online. I’m an electrician in mid-west USA, and occasionally run across “Star / Delta” motors & starters.

    On first read, your article looks excellent – I will certainly refer to it again, in the future!

    However, the first thing that struck me was in your second paragraph, and I quote: “During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three.”

    If I am not mistaken, perhaps the voltage is reduced by sqrt(3), or 1.732 – which would reduce the torque by a factor of 3.

    In other words: your example shows the stator/field voltage as 230V in the star configuration, and 400V in the delta configuration. This is a factor of 1.732, or sqrt(3).

    I’m sure this was a “typo”…

    Again thank you Sir for your time & efforts. Please address as you wish


  126. Jonhny Magtulis says:

    I have saame rated of motor capacity 2kw Wye connection voltage is 440v in delta same 440v.
    my question is what this advantage and disadvantage of thist wo connection in application?
    Using in accros the line connection only.

  127. Dr. Chander Mohan says:




  128. ram says:

    very nice frd thanks

  129. Stewart nangete says:

    Nyc notes

  130. thamer says:

    sir i want to know whats the use of safety relay in this circuit please ?

  131. Junsuu says:

    What is IFL? how to find ?

  132. Bharat says:

    why induction motor starting current high consume?

  133. vimal monasara says:

    why star current is 1/1.73 of delta current?in star delta load?

  134. spraggah says:

    Nice article sir I really appreciate your help

  135. Abraham mamo says:

    It’s nice article.

  136. h.s.choudhary says:

    thanku sir jeevery smple

  137. aloyce says:

    thank a lot !

  138. Ankonga says:

    Good electrical explanation and guidance!

  139. zeeshiii says:

    thanks a lot

  140. Danny says:

    I m electrician and I want lean motor ,contactors conetion.

  141. Alec says:

    very helpful do you offer on line courses

  142. Nathan says:

    thanks Sir,
    Any reference please?

  143. Anshul thakur says:

    thanks sir ji u r so helpful to med

  144. somashekar says:

    nice information. sir please tell me about LIFTS working procedure and service.

  145. vikram ahuja says:

    Dear Jignesh sir,
    You provide me excel sheet of soft starter and auto transformer starter
    thanks and regards
    Vikram ahuja

  146. naveen joshi says:

    Sir Very very Thanks For easy explanation.

  147. NAUSHAD ALI says:

    3 phase mother star delta . sir give to 440 this moter running for star running for delta

  148. Asefa Marye says:

    Very nice for your explanation and wiring diagram, I need the limitation their voltage,current and motor size to pump up fixed water! thanks from electrical ana computer engineering dep.t

  149. abc says:

    why we use fuse and over current relay? , and what is type of overcurrent relay we use in star delta circuit ?
    2. what we check after over speed of generator
    3.what is procedure of flashing up a dead generator
    4how could a old motor overheat and how we rate a fuse for circuit breaker

  150. Alfred. says:

    good stuff.

  151. star delta starter

  152. bhanu says:

    Beautiful description.. Covered all the basic points.The best thing is,explained in a very simple manner. Thanks a lot..very very much helpful.:-)

  153. ppz give details of why realay setting full load of motor current *1.732 n watt to amp n amp to watt single phase n three phase

  154. avinash says:

    which connection use in star delta connection either star or delta or both?please explain

  155. Dear Sir,
    Please prove that the current in star is 1/3 times the rated current.
    and also current delta is 1/root 3 of rated current

  156. আশরাফুল says:

    Thanks a lot, this is very helpful for me. There is no other site can explain Star Delta controlling circuit easily like this site. Thanks again.

  157. Sir why we are using current convertor in satar delta starter panel..?

  158. pravin parekh says:

    sir DGpwr. ane GEB pwr O/G busbar ma “sort circuit “na thai e mate kai rely instell karvi jaruri chhe athwa pl..temate drowing /diagram batavso. Aapno viswasu pravin. Thanks

  159. rasheed says:

    How does power failure occure in electric motor I just don’t understand

  160. Arun albert says:

    Its very useful….

  161. sandeep verma says:

    Thanks for giving good knowledge

  162. McKhaoya Titus says:

    Good article for those specializing in Electrical machines

  163. McKhaoya Titus says:

    Thanks alot for the information

  164. Paul Pollock says:

    Hi there
    I’ve connected a 15 kw motor to a star delta. On star I got a reading of 46 amps and delta 20 amps. But on change over for a second it reads 124 amps which interferes with phone lines and data. Dose my time ratio on change over need to be changed or what.

  165. Nandish says:

    It helped me a lot to remind basics. Explained very nicely. Thank you!

  166. Pako Ngake says:

    Thank you for the good explanation but I have the following questions;

    1. How long should the pause time be when switching from star to delta connection?
    2. How do I make sure that I maintain the same direction of rotation of the motor when switching from star to delta connection?

  167. Balvant says:

    Dear sir I have direct online starter and starting current is 113 Amps. So when it draws high voltage during starting though the coil does not hold and it starts hold and release very fast. Please give solution.

  168. pratha says:

    how many contactors in star delta starter?

  169. Erric says:

    Has any one implemented Stall, Ground fault and under Voltage protection on Star Delta starter. Is the principal still the same the Stall current , Ground Current and Voltage divided by √3?

  170. MAHESHLAL says:

    Dear sir ….i have one doubt….in which condition we use vfd instead of star delta starter..In my work i saw for chiller primary pump motor they used star-delta starter and for secondary pump motor they used VFD…please explain the differences??????????

  171. sunil says:

    Very nice to understand.

  172. Ahmed says:

    thank you

  173. Rohan chepe says:

    Very great articlesir
    Thank you sir for help me

  174. Subhash Chitnawis. says:

    Thank you for explaining the functioning,/ principle of operation of star delta starter in very simple words. In fact I was searching for my below query which remained unanswered.

    My query is how long the transition period (Period between Break from star connection & Make in Delta connection can be harmless.)

    I am looking for 20 MS.?? Suppose there is no much Voltage / current spike generating if transition period reduced to less than 20 Milliseconds, I will be happy to think some thing on improving Star Delta starters. Your alternative given as resistor with fourth cont actor for reducing bad effects of open transition are good but but I am thinking in reducing the time to Harmless period and minimum is 11 – 20 Milliseconds.

    If find time, pl answer at least in yes / NO permissible time. etc.

    Warm regards.

  175. Subhash Chitnawis. says:

    Thank you for explaining the functioning,/ principle of operation of star delta starter in very simple words. In fact I was searching for my below query which remained unanswered.

    My query is how long the transition period (Period between Break from star connection & Make in Delta connection can be harmless.)

    I am looking for 20 MS.?? Suppose there is no much Voltage / current spike generating if transition period reduced to less than 20 Milliseconds, I will be happy to think some thing on improving Star Delta starters. Your alternative given as resistor with fourth cont actor for reducing bad effects of open transition are good but but I am thinking in reducing the time to Harmless period and minimum is 11 – 20 Milliseconds.

    If find time, pl answer at least in yes / NO what is permissible safe time. etc.

    Warm regards.

  176. D rajender says:

    Thbnk you sir


    your stardelta starter article was very useful. again one doubt how possible TOR blast in stardelta starter when the motor starting time.

  178. PETER JOHN says:

    pls sir ,i need 10 refrences on star and delta electric motor

  179. dereje says:

    tank u very much i need how to configur pabx(automatic branch exchange)

  180. Tin Duong says:

    Thanks alot for the information

  181. kanake kenedy says:

    nice article,please i require an article on wiring of soft starts

  182. Mohamed Edrees says:

    I have a chilled water pump at this conditions 41.5 lps @ 65 mtr , we selected the motor 50 HP , starting method is Y/D , and also for another pump we used VFD , Both motors are getting very hot after only 2 hours running reached to 82 Deg.C ….. kindly advise what should we do here …. shall we trim the impeller or what ?

  183. Aseem says:

    I have a simple doubt. Is the star-delta starter used because it causes voltage fluctuations in the supply line or for the protection of the motor windings?

  184. mac says:

    good day people what is the effect of reducing or increasing time delay on star
    delta starter

  185. japhet chirchir says:

    Well understood

  186. Edgar Mwambwa says:

    Thanks, the message was so impotant to me.

  187. rajanish bhandari says:

    what Is the size of MCB.

  188. dharmendra Chauhan says:

    Very good

  189. Sonuga says:

    Thanks sir

  190. Tamesh says:


  191. tubai raha says:

    thank you sir.

  192. jadhav amit says:

    what is DOL and RDOL starter.

  193. Dimple tundon says:

    Pls clearify …
    Ist supply us giving to delta contactor ..why…

    It require start firstly thn running…
    It means supply shoult be given to star contactor thn delta means running …..

    Pls explain any 1

  194. neel says:

    hi sir…as per your way i calculated current for delta contactor and the value is how i select model of delta contactor???
    reply fast

  195. Tarun mewada says:

    it is so good thank for this information

  196. Maxwell says:

    Thanks sir this article has been very useful to me.
    Please l want to know more about motors. Can you help me?

  197. sonuswami says:

    Thanks for your help

  198. Ankur Barot says:

    Very good Article sir! Fully knowledgeable and understating point of view! Sir we want to invite you for Expert Session in our Vocational Training Institute! Sir, may I’ve your email id? Or send me your contact detail on my mail id

  199. BUKENYA JAMES says:

    Very good notice, easly understandable thks.

  200. boby says:

    Thak you Google team

  201. I am electrical maintenance engineer.

  202. arch says:

    how much current flow at starting?

  203. Gothard Mwango says:

    Very nice lesson

  204. Pratheep kumar says:

    Thank you for your detailed about star delta.

  205. tamer says:

    DEAR SIr
    your remarks regarding disadvantages of each method is wrong and confusing
    In first method overload sees I line and thus setting will be motor In not as you mentioned while in second method setting will be In*0.58

    Source Schneider electric

  206. MISS says:

    hi may i ask what will be the overcurrent protection rating of star delta motor if the power is 55kw. Thank You!

  207. zeka kaphamtengo says:

    the article is very good .

  208. Abhinash chauhan says:


  209. ddemambro says:

    Excellent article, thank you!

  210. where is close transition system ?

  211. sagar sarak says:


  212. Lakshman says:

    Very Good Article you will get more knoweledge

  213. Dave Bolt says:

    Wonderful article, thanks for the information. Is it possible to put delta connections on the star contactor to further reduce the current rating required of the star contactor? This would be done by connecting terminals 2 to 3, 4 to 5, 6 to 1 with wires. Can anyone think of any detrimental effects? Cheers.

    • Dave Bolt says:

      I’ve realised that this will not work. Does anyone know what it means to “delta” the star contactor?

  214. Curtis says:


  215. Xavier says:

    Your diagram is wrong. This motor would run fine in star but would loose a phase in delta.

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