Direct On Line Starter


  • Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.
  • The simplest form of motor starter for the induction motor is the Direct On Line starter. The DOL starter consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.
  • Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.

Principle of DOL:

  •  To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.
  • As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.
  • The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.
  • Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.
  • The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia.
  • DOL starting have a maximum start current and maximum start torque. This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always experience a voltage drop when starting a motor. But if this motor is not a high power one it does not affect much.

Parts of DOL Starters:

(1)   Contactors & Coil.

  • Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits.
  • Magnetic motor controllers use electromagnetic energy for closing switches. The electromagnet consists of a coil of wire placed on an iron core. When a current flow through the coil, the iron of the magnet becomes magnetized, attracting an iron bar called the armature. An interruption of the current flow through the coil of wire causes the armature to drop out due to the presence of an air gap in the magnetic circuit.

  • Line-voltage magnetic motor starters are electromechanical devices that provide a safe, convenient, and economical means of starting and stopping motors, and have the advantage of being controlled remotely. The great bulk of motor controllers sold are of this type.
  • Contactors are mainly used to control machinery which uses electric motors. It consists of a coil which connects to a voltage source. Very often for Single phase Motors, 230V coils are used and for three phase motors, 415V coils are used. The contactor has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the control circuit. A contact is conducting metal parts which completes or interrupt an electrical circuit.
  • NO-normally open
  • NC-normally closed

(2)   Over Load Relay (Overload protection).

  • Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.
  • Under any condition of overload, a motor draws excessive current that causes overheating. Since motor winding insulation deteriorates due to overheating, there are established limits on motor operating temperatures to protect a motor from overheating. Overload relays are employed on a motor control to limit the amount of current drawn.
  • The overload relay does not provide short circuit protection. This is the function of over current protective equipment like fuses and circuit breakers, generally located in the disconnecting switch enclosure.
  • The ideal and easiest way for overload protection for a motor is an element with current-sensing properties very similar to the heating curve of the motor which would act to open the motor circuit when full-load current is exceeded. The operation of the protective device should be such that the motor is allowed to carry harmless over-loads but is quickly removed from the line when an overload has persisted too long.
  • Normally fuses are not designed to provide overload protection. Fuse is protecting against short circuits (over current protection). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless inrush current and a damaging overload. Selection of Fuse is depend on motor full-load current, would “blow” every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.
  • The overload relay is the heart of motor protection. It has inverse-trip-time characteristics, permitting it to hold in during the accelerating period (when inrush current is drawn), yet providing protection on small overloads above the full-load current when the motor is running. Overload relays are renewable and can withstand repeated trip and reset cycles without need of replacement. Overload relays cannot, however, take the place of over current protection equipment.

  • The overload relay consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves, directly or indirectly, to break the circuit.
  • Overload relays can be classified as being thermal, magnetic, or electronic.
  1. Thermal Relay: As the name implies, thermal overload relays rely on the rising temperatures caused by the overload current to trip the overload mechanism. Thermal overload relays can be further subdivided into two types: melting alloy and bimetallic.
  2. Magnetic Relay: Magnetic overload relays react only to current excesses and are not affected by temperature.
  3. Electronic Relay: Electronic or solid-state overload relays, provide the combination of high-speed trip, adjustability, and ease of installation. They can be ideal in many precise applications.

Wiring of DOL Starter:

(1)   Main Contact:

  • Contactor is connecting among Supply Voltage, Relay Coil and Thermal Overload Relay.
  • L1 of Contactor Connect (NO) to R Phase through MCCB
  • L2 of Contactor Connect (NO) to Y Phase through MCCB
  • L3 of Contactor Connect (NO) to B Phase through MCCB.
  • NO Contact (-||-):
  • (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)
  • Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is connected to Common wire of Start/Stop Button.
  • NC Contact (-|/|-):
  • (95-96) is a normally closed NC contact (opens when the thermal overloads trip if associated with the overload block)

(2)   Relay Coil Connection:

  • A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermal over Load Relay’s NC Connection (95).

(3)   Thermal Overload Relay Connection:

  • T1,T2,T3 are connect to Thermal Overload Relay
  • Overload Relay is Connecting between Main Contactor and Motor
  • NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and Common Connection of Start/Stop Button.

Wiring Diagram of DOL Starter:

Working of DOL Starter:

  • The main heart of DOL starter is Relay Coil. Normally it gets one phase constant from incoming supply Voltage (A1).when Coil gets second Phase relay coil energizes and Magnet of Contactor produce electromagnetic field and due to this Plunger of Contactor will move and Main Contactor of starter will closed and Auxiliary will change its position NO become NC and NC become (shown Red Line in Diagram)  .
  • Pushing Start Button:
  • When We Push the start Button Relay Coil will get second phase from Supply Phase-Main contactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2).Now Coil energizes and Magnetic field produce by Magnet and Plunger of Contactor move. Main Contactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) from NO to NC .
  • Release Start Button:
  • Relay coil gets supply even though we release Start button. When We release Start Push Button Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) – Auxiliary contactor (54)-Stop Button-96-95-Relay coil (shown Red / Blue Lines in Diagram).
  • In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95.
  • Pushing Stop Button:
  • When we push Stop Button Control circuit of Starter will be break at stop button and Supply of Relay coil is broken, Plunger moves and close contact of Main Contactor becomes Open, Supply of Motor is disconnected.

 Motor Starting Characteristics on DOL Starter:

  • Available starting current:    100%.
  • Peak starting current:           6 to 8 Full Load Current.
  • Peak starting torque:            100%

Advantages of DOL Starter:

  1. Most Economical and Cheapest Starter
  2. Simple to establish, operate and maintain
  3. Simple Control Circuitry
  4. Easy to understand and trouble‐shoot.
  5. It provides 100% torque at the time of starting.
  6. Only one set of cable is required from starter to motor.
  7. Motor is connected in delta at motor terminals.

Disadvantages of DOL Starter:

  1.  It does not reduce the starting current of the motor.
  2. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).
  3. Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.
  4. Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same lines and therefore not suitable for higher size squirrel cage motors
  5. High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes.

Features of DOL starting

  • For low- and medium-power three-phase motors
  • Three connection lines (circuit layout: star or delta)
  • High starting torque
  • Very high mechanical load
  • High current peaks
  • Voltage dips
  • Simple switching devices

DOL is Suitable for:

  • A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW.
  • DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts.

DOL is not suitable for:

  • The peak starting current would result in a serious voltage drop on the supply system
  • The equipment being driven cannot tolerate the effects of very high peak torque loadings
  • The safety or comfort of those using the equipment may be compromised by sudden starting as, for example, with escalators and lifts.

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.

109 Responses to Direct On Line Starter

  1. Lazarous says:

    This page has very useful information. You are doing a tremendous job

  2. mozes says:

    Thanks for the info, man..Its so brief and to the point. How do you choose the right cable, MCB, Contactor, overload sizes for induction motors. Any method?

    • Shafiq Ur Rehman says:

      How do you choose the right cable, MCB, Contactor, overload sizes for induction motors. Any method?

  3. hira says:

    in many industry dol starter used for upto 15 or 20 hp motor. but there also used upto 30 hp motor. why ?

    • As Torque is directly proportional to Current, for loads of higher torque requirements( like hammering metal industries)even for 50HP squirrel cage motors, where the electric installation network is much stronger – still use DOL starting, mostly in Asian countries.

    • Sajal Mukherjee says:

      when the source is strong, then high capacity motors can also be started with DOL mode as in this case the dip in voltage will not be affected

  4. Deepak Dhiman says:

    Very helpfull to know working even i am a fresher diploma engineer.


  5. A Pedersen says:

    How would an e-stop be incorporated into this circuit, through the stop connection and contactor?

    • Badri Narayanan says:

      By e-stop, if you mean emergency stop, then use this instead of the stop push button which is generally a part of the control wiring of a DOL Starter. A simple emergency stop (either push-pull / twist to release / key release) can be used with an NC contact. But generally speaking, Em stop is not really required for starters. If you have safety issues, then pls go ahead with em stop buttons with key or padlock.

  6. Balaprakash says:

    Cant we use dol starters for Axilary Switching of Working motor to spare motor Automatically.

  7. Janardhan says:

    The above topic was very interesting & use full for me. Now I understood what is DOL starter.
    Thank you very much.

  8. shreedevi says:

    thanks for your information amazing knowledgeable gift

  9. Oryech Sedrick says:

    Splendid yo description is so understandable n it made it so easy 4 me to understand DOL starters

  10. naveen bodaballa says:

    thanks a lot, this page was very use ful

  11. Vignesh says:

    Superb.its very useful to me

  12. Vignesh says:


  13. too informative & very useful Sir …keep rolling such a nice stuff…thanks a lot.

  14. Trevor Hobart says:

    I have a question I would like to ask I am an electrician but I do not have any experience in control wiring I have installed a DOL starter on a pump but they now want a float switch and a flow switch to control the circuit can you please draw a circuit for me.

    • Shane Gobey says:

      could you please send me also the diagram for the DOL starter with float switch that switches on and off.

    • sadiq says:

      if you must include a float swich into the circuit..let the control life wire that supply to the thermal overload relay pass through the float swich N.C contact before reaching contact 95 of the that the float swich wil hv the ability to open the contact that power of the control starts from

  15. Pankaj patil says:

    I am fresher electrician plese help in dol+leval sencer and relay wring

  16. Badri Narayanan says:

    I have a situation wherein the motor is initially started using a start push button. But in case of a power cut, the motor has to start automatically (in an instant with minimum delay), i.e. without me having to use the start push button again. I tried using On delay timers but the result is not very satisfactory. Let me know your views.

  17. shambho more says:

    It is very useful information on the page u have do very tremendous job

  18. Muhammad says:

    Excellent stuff…. U r very generous in spreading knowledge. Mashallah

  19. Pradeep Vyas says:

    excellent sir…salute to u…

  20. nilesh kapuriya says:

    its a really very important and useful for me…and everyone…

  21. Aura table says:

    You smashed it once more my buddy carry on the excellent work I generally get excitement from your articles!

  22. Albert Yanzu says:

    how can i connect direct online starter to a flourescent lamp.PLZ what is the difference in drawing of 3phase and single phase direct online starter

    • DOL Starter for Fluorescent Lamp !!!!!!!!!!!!!!!!!!!!!!!!!!!
      Not getting your point..

      • Albert says:

        what i meant was that can u hint me on single phase direct online starter controlling a fluorescent lamp?was thinking DOL was meant for controlling motors, pumps,compressors and conveyors but was asked to draw the control diagram in one of my interviews as regard controlling fluorescent lamp

      • A.Nijamudeen says:

        Why not we can control lightings too,through Direct Online Starters, like in stadium,auditorium etc. This is a coommon practice nowadays to switching on group of lights together.Insteed of going for each and every switch.

  23. malebogo moupi says:

    am realy interested on the criteria used on the selection of motor overload relay

  24. deodat siriram says:

    I’ve learn a lot from the information provided

  25. salim says:


    One of my consultant proposed DOL starter for 150 hp motor. Is it possible.
    Awaiting for your kind reply.

  26. Daniel meresha says:

    thanks for sharing your knowledge

  27. bijay says:

    Very nice work…itz clear cut…thanks a lot..

  28. Warren Lang says:

    does it make a difference how far the DOL starter is from the motor?

  29. Orlando Candido says:

    Is it possible to use DOL starter for 600 KW motor.

  30. Eng Douglas says:

    You are so wonderfuf. Please send email about autotransfomer: diagram and its operation. Thanks and be blessed.

  31. vijay says:

    massive explanations you done a good job. thanks for this article

  32. Alexander isoe says:

    a powerful explanation

  33. Mogeni Bob says:

    thanks for the notes.

  34. Sandeep Anbazhagan says:

    Thanks for the information. Can you please explain how a auto, local DOL starter works?

  35. Tayo Taiwo says:

    Thanks 4 ur help!

  36. Parth says:

    D.o.l starter is which motor will in used???

  37. biju says:

    now actualy it is confused that DOL only used upto 5 kw otherwise it may be connected with VFD.and more than 5kw we are using star delta. pls comment for this

  38. savanty says:

    hanks for the notes

  39. Ibrah washo says:

    Hi,it’s good

  40. Mohamed Abdelsameea says:

    Water pump 60 kw we can use the DOL methode to start this pump and please u can calculate the circuit breaker ,contractor and the cable size if the total length is 120 meter?

  41. darwin says:

    thanks for sharing your knowledge. Now i understand about dol.

  42. ruchit joshi says:

    thanks i have verify this concept practically.

  43. suthar jaimin says:

    thanks sir my big halp to this site
    very very thanks

  44. SAHAD.A.F says:


    Please inform. upto how much KW we can use DOL starter. Also please give the range of Star-Delta starter and starting KW for preferring VFD.

  45. visu says:

    hello sir,
    i have 4 motor 3.7 kw,0.85 kw,0.1 kw & 0.2kw, all this motor are work in 3Ph 200V AC
    so i have used 3Ph 15KVA step down transformer(415/200V), i am using DOL stater to on this motor ,when we switch on the 3.7KW motor it staring current is 6to7 time of the rated current
    my worries is this motor will start and stop frequently will affect the transformer and how to reduce the started current of the motor ?

  46. Donald Ruhangarinda. says:

    i have enjoyed this page

  47. Prakash says:

    How can i check the DOL starter? Whether it is in good condition or not..

  48. D J says:

    Dear sir
    I can’t understood this language so translate to marathi other wise hindi and send me my mail address all star delta & dol starter circuit diagram & power diagram…
    Deepak Jadhav

  49. kenong says:

    Do DOL starter gives energy saving?

  50. The post was satisfactory.

  51. Omutinyu D. Shitandi says:

    Big up, You are wonderful.

  52. Good job you are doing

  53. aloyce says:

    It is good then ,how about revising and forward contactor wiring drawing ?

  54. niten lalwani says:

    very nicely explained. thanks……..

  55. niten lalwani says:

    dear sir my starter on off switch is not working. i have to press contactor manually by tester.

    i think may be coil is not energising. . is there any solution ? please guide me.

    i am using this starter at presure switch of compressor.

  56. Bill Hewitson says:

    Dear Jignesh,

    Thank you very much for your useful information regarding direct on line starters. I was not familiar with IEC contactors and scoured the Internet trying to locate a wiring diagram for one. Your information provided the answer I needed and your explanation of the operation of DOL starters was very helpful as well. Keep up the good work!

  57. Niger Mnimbo says:

    thanks sir

  58. Ruhi says:

    Have you already prepare any concerning electrical installation? if yes how can i get them?

  59. Ruhi says:

    Have you already prepare any notes concerning electrical installation? if yes how can i get them?

  60. Joseph Jackson says:

    Really bin helpful. Thnx alot

  61. pradeesh says:

    Is that possible to run two 5HP motor with one DOL starter ?

  62. Ahmed says:

    Do any one have references as per IEC or NEC for what range Min to Max kW ratings which type of Starter to use 1). DOL 2).Start Delta 3).Softstarter , kindy please if any one have any references please provide.


  63. Gabriel Bimpong says:

    This site is very good

  64. Gabriel Bimpong says:

    This site is very good and every information here is accurate

  65. Denis says:

    hi can I connect a three phase motor of 10 HP using direct on line starter? and how do I connect it because the contacter has T1 T2 T3 and the motor has 6 connectors of windings u1 v1 w1 u2 v2 w2

  66. raju joshi says:

    hello sir!

    i have a great honours for you for providing us with such a good knowledge about the motors and the contactors–they are all good to learn for the people like me!!

    I’m working as an electrical helper in jagdamba steel simara nepal. it would be great to me if you provide me with a power and control diagram of dol and inching.

  67. sameer says:

    very usrfull page

  68. Manawar Hussain Anjum says:

    Please let me know about dol starter use upto which hp motor capacity

  69. suresh says:

    Hi sir,

    I have an question,if i use indicating light at outgoing to motor side using star connection,is there any problem when the motor running time? my outgoing lights can works?

  70. Garethbrown says:

    Hi I have argued phase pump that needs to be wired via a contactor with afloat switch that activates the contactor. I have a contactor and overload but I’m a bit puzzled with how I wore the float switch in to stop and start the contactor? I have a 400v coil

  71. George K. says:

    Your guide was very informative and helped me change a new lathe that was originally wired for three phase to a single phase supply with a VFD. With your help, I was able to properly wire the contactor and overload relay to control the main power supply to the lathe. Thank you very much for your instruction!

  72. Gulshan Hanspal says:


    Can u please guide me. I’m preparing a control circuit for two individual DOL starters with an interlock where one dol works and if the pressure drops the other dol switch on automatically and and first one should be off manually. and this should be possible in both of them.

  73. majid khan says:

    i need to ask that can i connect DOL starter with 125kw motor in any condition. help me plz if any one knows…

  74. Majid Khan says:

    i need to ask that can i connect DOL starter with 125kw motor at any condition. plz help me out.

  75. Godfrey says:

    very helpful notes,wel done sir for grdat job.

  76. ratu says:

    thanks for the infor…..very helpful in the compilation of my project

  77. actually the details u posted helped me to pass my course exams…thnx alot

  78. nambi joan says:

    thank u very much is it possible to hv the above info on ma mail

  79. Bhuumz says:

    Thanks you for the useful info

  80. john says:

    What’s the max size in kw should I start a scim

  81. pranay says:

    my DOL starter connected with compressor, starter makes frequent noise after some interval, can you help me find out the problem.

  82. maruti says:

    hi, i need help on how to connect 3 phase motor with 4 heater, individually heater is working.
    when temp in the tank reaches 80 centigrade, heater coil should stop, but motor pump is running.
    when pump motor is off, heater should not start

  83. I need
    Schematic diagram
    Wiring diagram
    Oneline diagram of DOL Starter of 3-phase induction Motor.

  84. Alpha says:

    your doing a wonderful job, your providing useful information in the very easy way. Can you plz help me with the explanation on changeover switch?

  85. Myo Saint says:

    Thks, useful for me.

  86. Shri Anjan Kumar Maity says:

    Very useful indeed. Projection through video will also provide Filip to the mission.

  87. Ayo E. Peters says:

    Jignesh.Parmar you are good and have really help me a big time in my finals project all the way from NIGERIA. i am now able to structure the theory/introduction page very well

  88. mohanlal says:

    when load connected on a single phase DOL starter and pushes the start button, it does not starts and removing the output load from terminals it works properly. starter contains 2 capacitors connected

  89. Oyoo Ochola says:

    I found it helpful to me as an electrician.

  90. balamurugan says:

    very useful information about starters

  91. anil says:

    Sir,can we use MCB in place of contactor?As contactor is Also applying the rated voltage

  92. Can it be used for frequent starting and operating in forward and reverse directions.

  93. Issac.a says:

    Hi, can i know what will cause a DOL starter to fail?

  94. Jerome says:

    Useful information, Thanks

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