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Home / Technical Articles / Direct On Line (DOL) Motor Starter
Direct Online Motor Starter - Square D
Direct Online Motor Starter - Square D

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 Direct On Line Motor Starter (DOL) 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 Direct On Line Starter (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

Contactors & Coil

DOL part -Contactor
DOL part -Contactor

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

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.

DOL part - Termal Overload Relay
DOL part - Termal Overload Relay

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

Direct On Line Starter - Wiring Diagram
Direct On Line Starter - Wiring Diagram

Working principle 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.

DOL - Wiring scheme
DOL - Wiring scheme

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

Direct On Line Motor Starter (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.

Direct On Line Motor Starter (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.

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Surendra Sai
    Oct 17, 2020

    Hello EEP.

    Thanks for the article, it’s awesome.

    Here my question, Why do we call the DOL as a starting method of a motor?

    Basically, the starting method’s aim is to limit/reduce the starting current and its effect on the supply system.
    However, the DOL is not contributing anything in limiting the starting current or supporting the supply system.
    What’s your opinion on my view of understanding particularly the DOL starting of a motor?

  2. Appasaheb Patil
    Oct 06, 2020

    Respected Sir,

    I want one electrical device that is when we want start the T.V. and watching and if power fail T.V. will be off and again power has come I don’t want start the T.V. until we start the TV again manually. i.e. just like very small motor stator up to 5 AMP if any device or circuit board with fitting is available please inform me Sir.
    Sir. this is my humble request only.

    KLE College of ENgg & Tech,
    At & Post: Chikodi-591201
    Dist: Belgaum (Karnataka State)
    Phone :9449016366

  3. Susaan Ranasingha
    Jan 28, 2020

    I dosn’t recomand DOL system in this process.

  4. buddha bahadur gharti
    Aug 10, 2019


  5. yousuf khan
    Jun 01, 2019

    Combination DOL starter, NEMA Size 3, 480V, 3P, 60Hz, Draw-out type complete with Bucket, stubs, indicating lights (LED), handle and all the required accessories.
    For 30 kW motors
    please suggest me any model for this capicity in schnider , square D, siemens or ABB /GE as i am unable to find it out i laso need 2 in 11kw and one for 7.5 KW motor

  6. manoj kumar
    Mar 23, 2019

    thanks sir

  7. Ibrahim sobah
    Feb 22, 2019

    I need information about dol starter

  8. Vivekraj
    Aug 15, 2018

    I bought one DOL STARTER yesterday for my 5HP machine. But, I have one doubt. Can i use this starter continuously for more than 18 hours or non- stopped. I’m expecting your reply ASAP.


    Aug 11, 2018

    Please tell me the Power rating of different type starter??

  10. Tapan Samant
    Jul 19, 2018

    For 30Kw induction motor connected with Dol starter what will be staring current and till how many sec or min it will remain?

  11. Ravikumar
    Jul 14, 2018

    Hi Jignesh.. Its nice article…. i need to discuss more on the technical…. can you please share your contact number

    Ravikumar Talawar

  12. Jayanta Kar
    Apr 08, 2018

    I am an electrical contractor I am sufferlng a lots of trouble from a 220V D.O.L L&T starter. When I want to stop the motor by pushing red stop button, then the motor should have to stop but It is not happening. How can it solved? Ple

    • Mohd Shah
      May 27, 2018

      Hi, the best is using frequency inverter. It have decelerate time setting to stop the motor.

  13. Lokesh
    Jan 19, 2018

    Knowledgeable portal…….. ???????

  14. Ajay Mulay
    Jan 16, 2018


    I am facing problem of Operation of DOL starter, 3 Phase, 3 HP Motor through PLC.
    I have 24VDC Coil, 5 Amp resistive load on 8 QTY connected to my PLC card, This is smoke extraction system and there are five zone with five different PLC installed, each zone has Eight 3HP fans working in Primary and secondary configuration. When the PLC program starts based on Fire Signal 4qty of FANS starts, but I am getting a 110VAC P-P Surge on my 24 VDC power which is killing my PLC output card. Our Observation is that there is some problem in the starter panel, because of which the surge is getting transferred from Our relay Contact to Coil. The Contact are connected directly in the control circuit of DOL to switch on KM1 contactor. can you please explain this phenomenon or the reasoning of this problem.

    • Steve
      Mar 28, 2019

      I believe your PLC card is getting damaged by the reverse voltage surge when the contactor coil is turned off. The contactor (or relay) coil is a rather large inductor and when the drive current from your 24VDC PLC is opened, the coil (inductor) tries to keep the current flowing by creating a large (100’s of volts) voltage spike.
      The solution is to use a diode across the DC coils in a reverse direction (diode cathode goes to PLC output card source, Anode to -). Many contactors have a snubber option which plugs in and does this function. Research this.
      AC coils have a similar need but cant use a diode so they use another device as a snubber which is either a dual direction diode (TVS), an MOV, or a simple RC ( 47 Ω + 0.1 uF typ) — as with most things there is a reason for which device to use and the RC values so do your research.
      Good luck

  15. Shyna
    Nov 27, 2017

    really helpful ??

  16. Pragati Meshram
    Nov 06, 2017

    it’s very nice explanation

  17. Abdul Mannan Mollah
    May 06, 2017

    Very smartly and knowledgeably explained. We expect better engineering in life

  18. prakash kumar shukla
    Mar 04, 2017

    Sir,D.O.L stater kitne HP tak ke induction motors me use kiye jate hai

    • Abhishek Bhardwaj
      Nov 09, 2017

      Upto 10HP

    • Akshay Pawar
      Jan 11, 2019

      upto 15 kw

  19. paul
    Dec 28, 2016

    I am an electrical contractor and have a problem with an elevator soft start always dimming out the voltage at an apartment bldg. We have upgraded the xfmr and the wire from the utility but still have the brown out. Can you help me with the calcs and explaining the delta in the wye starter? thanks!

  20. hari
    Dec 27, 2016

    i having 2 hp motorpump by using dol starter again and again relay was damaged the realy setting value is 2.7 its range is 2.5-4.0the motor we are using for chemical water draining. i checked continuity to L-L,L-body its ok but when i started the motor in fraction of seconds the relay fused out what the reason i cant find it please tell me and suggestion me.

  21. Semazzi Zaake
    Dec 05, 2016

    Is an overload relay designed to take longer to trip for high inrush currents in a motor and to take a shorter time to trip for lower overload currents? Thank you!

  22. Juhi
    Jul 29, 2016

    Well I just wanna know that what is the approximate starting time of motor if it is using DOL starter/ soft starter.

  23. anup kumar
    Jul 13, 2016

    Very good.

  24. ASHOK
    Apr 19, 2016


  25. hussein omary
    Jan 10, 2016

    i want to know disadvantages of DOL method

  26. ELVIN
    Dec 09, 2015

    starter motor,direct on line pump,90kw,150A,550v with circuit breaker 250MA earth leakage relay start/stop push button.
    Kindly give a quote on above

  27. Hafiz kashif
    Nov 26, 2015

    very well
    D.A.E. Electrical & simple B.A. & Three years electric filed experience

    Nov 25, 2015

    Sir You are good, explanation was clear

  29. Soumendu Jana
    Nov 16, 2015

    It is very useful for me . This artical is very much suitable for industrial application as well as educational application .

  30. julian
    Oct 29, 2015

    good it is a good site we can ever have in the world

  31. vikas
    Oct 28, 2015

    Basic information about dol starter

  32. yvn tagoor
    Sep 20, 2015

    Dear sir
    I was suffering from my 3 point starter. I have 5 HP motor it is connected by 3 phase startor . it is used to ground water lifting. Start the motor, trip the starter ofter half on hour, then countnusly trip the starter. Please tell me how to phase my problem.

    • virendra kumar
      Apr 12, 2019

      Dear YVN

      pl change the contact of starter as it sparking and becoming hot. pl try and report.

  33. Daniel AP
    Sep 18, 2015

    I wish to know what is the KW diff between a motor connected with DOL starter & again connected with VFD drive assuming 1005 speed setting.

  34. chris
    Aug 24, 2015

    I want to connect a centurion g switch to my dol starter le1 d 253 is it possible and a diagram would be appreciated thanks

  35. cretto tembo
    Aug 14, 2015

    information is helpful to the leaner

  36. Agaba sulait
    Jun 01, 2015

    could you kindly help us with calculations about Dol starter and even symbols of main componets

  37. Robert Odhiambo
    May 29, 2015

    Would you also show how to calculate the DOL Starter rating calculations? That would be helpful…

  38. amit
    Apr 17, 2015

    its work in delta only

  39. Deogratius Mkina Bunoge
    Apr 14, 2015

    The electrical circuits are very helpful to the learners at all levels.

  40. Deogratius Mkina Bunoge
    Apr 14, 2015

    The circuit diagrams are very educative and very understable

  41. wasi hasan
    Mar 03, 2015

    very help full
    inshallah u get more success

    Feb 07, 2015

    The drawings are okay,well simplified

  43. sivapragasam.p
    Jan 27, 2015

    still explanatory answer with starting and running characetstics for steady load is expected.

  44. Ramesh
    Jan 04, 2015

    why we use DOL starter for below 5HP motor?

  45. prafull patel
    Nov 04, 2014

    Respected sir,

    how select fuse for 3 phase motor

  46. rahul kumar
    Sep 29, 2014

    sir, please tell me about the sizing of overload relay(i.e. how to select overload relay for a motor of specific capacity).

  47. hardik solanki
    Sep 24, 2014

    Dear sir,
    we are drilling machine manufacturer, I need help, we have fitted 3-Phase 1.5HP motor in machine controlled with Reverse/Forward Switch, now we wish to add Emergency Push Button Stop, where should I give connection for it? what other components we require for it? (someone suggested me pushbuttton with NC connector- but the NC has only 2 Nodes as 3-phase must have 3 nodes in it) kindly help me,
    Thanks in advance.

    • E A Fernandez
      Sep 25, 2014

      Just connect a mushroom-type industrial grade emergency stop button in series with the stop push in the control circuit of the motor. Please consult a certified technician / engineer for its proper installation and adherance to all safety norms.
      I guess the esteemed author of this page will also support my suggestion :)

      • Yusuff Kehinde Monsuru
        Sep 08, 2015

        Yes the drawing is understandable………………….Well done

  48. TOKELO
    Sep 14, 2014

    can a push button control three phase starter , work properly even if there is no main fuse and please send me functions all the components of DOL starter

  49. Bhavesh Machhi
    Aug 16, 2014

    Hi, my question is which starter recommended to be used with compressor? DOL or Star-Delta?
    i think it s better to go for star-delta for rating higher than 10hp?

    Jun 04, 2014

    How can I use three phase to run single phase motor. thanks

  51. navaneeth
    Apr 30, 2014

    Dol starters used for star connected motors or delta connected motors???

  52. Shabnam
    Jan 28, 2014

    my Q is , when overload relay is open condition then i put in yellow LED to indicate its overload causes this open condition but how to yellow LED glow when its open condition????

    • jackson
      Mar 01, 2014

      can i use asingle phase motor acontorctor?

  53. vittal
    Jan 20, 2014

    i would like to connect a 7.5hp motor to dol starter.what will happen

    • Somnath Mukherjee
      Jun 08, 2014

      Dear sir
      you can use DOL Starter for operate 7.5 Hp Motor.

      • Elvis luis
        Sep 05, 2014

        I am no expert but usually for motor rating above 5 HP we use star delta starter.
        One reason being higher the rating more is the starting inrush current.

  54. Bethuel
    Dec 28, 2013

    Nice articles, Thanks

  55. Anup
    Dec 12, 2013

    In case of 120V Single-Phase DOL Motor control 1)How neutral should be wired -Should it be wired through overload relay or not.
    2) Live phase -can it be wired through one contact of contactor and overload relay or it should be routed through all three contacts or any two contacts.

  56. Manish Gandhi
    Oct 02, 2013

    How cn I download it??

    • Edvard
      Oct 02, 2013

      Download what? Technical article?

  57. ndrw1977
    Apr 26, 2012

    Usually this is set to full load current for a DOL motor starter, and root 3 of the full load current for a star-delta starter. Dependent on the equipment being protected, electronic overload relays are available which can offer a better accuracy/reliability than the bimetallic strip with an thermal overload relay

    • mackulit3
      Sep 04, 2012

      Hello . Good day I am a bit confused about the DOL and Delta starter. Can anyone please help me to differentiate the difference? What is the basic formula for DOL, 3P? there was a question ,What size genset is needed to start a 3 phase electric motor Direct on Line (DOL) start? using 85HP ? can anyone give me the step by step answer? thank you

      • Mamun
        Sep 05, 2012

        85hp=85*746=63410watt=63.48kw You have need minimum 80 KVA Genset..85 hp motor is may be star delta starter use.

  58. ravendra.chand
    Apr 24, 2012

    What should be the setting on thermal overload device for a given running current of motor ie what percentage of running current.


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