Direct on line starter

Around the world it famous with name DOL starter or direct on line starter.This is the most common starting method available on the market and at basic stater for all motor application.The starting equipment consists of only a main contactor and thermal or electronic overload relay.The disadvantage with DOL starter is that it gives the highest possible starting current.

A normal value is between 6 to 7 times the rated motor current but values of up to 9 or 10 times the rated current exist.Besides then DOL starter draw the starting current there also exists a current peak that can rise up to 14 times the rated current since the motor is not energized from the the first moment when starting.

What is DOL starter or Direct on line starter?

The values are dependent on the design and size of the motor, but in general, a smaller motor gives higher values than a larger one.

During a direct on line start, the starting torque is also very high, and is higher than necessary for most applications.The torque is the same as the force, and an unnecessary high force gives unnecessary high stresses on couplings and the driven application.

Naturally, there are cases where this starting method works perfectly and in some cases also the only starting method that works.

For more detail,please refer the MOTOR CONTROL HANDBOOK :-


  1. A 15kW compressor motor use DOL starter and MCB rating is 40Amp. Is it OK?

    • Dear Yushar,use this formula to calculated the ampere,
      kW = I x V x 1.732
      I = 15kW / 415 x 1.732
      I = 20.86 @ 21
      MCB rating = 21 x 0.25 + 21 = 26.5 amp ( safety factor )

      u can used 40 problem..u also can use 32 ampere of MCB.

      • sir if we use 40A and 32A mcb in the dol. so what is the meaning of breaking. and in the over load motor does not trip…that is wrong sir
        if i m wrong pls clearify
        i ask u apersonal question u r indian @admin

        • Virender says:

          Miss priya in d o l starter there is a contactor, over load relay, start stop push button no NC and mccb. In over load relay we can set the current limit according to motor parameter. If motor overloaded due to any cause then over load relay tripped and circuit break. There is no concern of Mcb whether it is 40A or more. Mcb limit should be more than the motor, s current limit. I don’t know it is satisfactory answer to you or not inform me.

      • anderson says:

        please can u explain how we got 1.7312

    • vishnu says:

      Dear sir in 15 kw motor how can we use a DOL starter .it can be use only a star delta starter so is it a correct question?

      • DOL starter will generate high in rush current for big size motor.For better application, I suggest you to use VFD @ inverter.Easy to setup but quite expensive.Alternative you can use star delta starter.

  2. Amazing info. I’ve bookmarked this web site.

  3. What is the type of MCB (B,C,D etc) recommended in the above calculation?. What is the reference (book)used to select In+0.25xIn as the rating of MCB?. A MCB manufacturer says you should refer the tripping characteristics of MCB(type B,C,D) and make sure that for the starting current (say 6In)and duration of starting(say 4secs) the MCB will not trip. Does your formula satisfies this principle.

  4. I’m young and fresh graduate of electrical engineering who wish to know more and promote techology inthe nworld.
    Please reply on how I can get a copy of this text-books

  5. why in calculation kw=V*I*1.73 the power factor not used

    • Hi mahdi,
      power factor is optional value for kW calculation,if have the power factor value,you can included in formula.also you can put efficiency value.So the answer more accurate :D that my 10 cent opinion…

      P = I x V x 1.73 x Pf x efficiency

    • if pf given then we use and where not given then assumed unity pf or approximately of respective

  6. Think’s for the information it helps a lot



  8. why dol is use below 5 hp motor

  9. Bhavik says:

    Dear Sir,
    we can will be use 100hp motor in Dol Starter, This motor rating in safe of Dol Starter.

    Pls. reply

  10. Bhavik says:

    Dear Sir,
    we can will be use 100hp motor in Dol Starter, This motor rating in safe of Dol Starter of Hudraulic Powerpack ?

    Pls. reply

  11. gilbert says:

    what is the application phasor algebra to a.c circuit

  12. what is the appplication of phasor alegebra to a.c circuit

  13. Krishna Ch. Pradhan says:

    what is the application phasor algebra to a.c circ

  14. vill-nasirpur math Post-kotwa narayanpur dist-ballia U.P 277501

  15. why does a motor draw 5 to 8 times the rated current when starting with DOL

  16. Prajwith says:

    please let me know. Why generally In DOL starter control diagram OLR terminal stated in the order of 96, 95.. Some one telling that it is usef siemans OLR ‘s. Why it is like that.?

    • Hi,
      Terminal 95 & 96 for OLR is normally closed contact (NC).This contact used to open the control circuit when overload trip.So the starter can stop due to overload relay trip.We also call this as interlock system.

  17. ali Livcollins says:

    Please sir have an issue on lathel machine the machine is run on star delta connection it has two transformation,relay breaker,and fuses power enter the circuit but the starting contactor can energize when start button is press what will be the cause

  18. sir what is relation in between air flow and any electrical machine? can we operate machine with opposite flow of air direction???


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