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Home / Technical Articles / Comparision of Direct-on-line (DOL) and Star-delta Motor Starting

Motor starting methods


Direct-on-line starting (DOL)

As the name suggests, direct-on-line starting means that the motor is started by connecting it directly to the supply at rated voltage. Direct-on-line starting, (DOL), is suitable for stable supplies and mechanically stiff and well-dimensioned shaft systems – and pumps qualify as examples of such systems.

Comparision of Direct-on-line (DOL) and Star-delta Motor Starting
Comparision of Direct-on-line (DOL) and Star-delta Motor Starting (photo credit: Nigel David Brown)
Line diagram for Direct-on-line motor starting
Line diagram for Direct-on-line motor starting

Where:

  • K1 – Main contactor
  • MV1 – Overload relay

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Advantages of DOL

DOL starting is the simplest, cheapest and most common starting method. Furthermore it actually gives the lowest temperature rise within the motor during start up of all the starting methods.

It is the obvious choice wherever the supply authority’s current limiting restrictions allow for its use.

Power plants may have varying rules and regulations in different countries. For example: Three-phase motors with locked-rotor currents above 60 A must not use direct-on-line starting in Denmark. In such cases, it will obviously be necessary to select another starting method.

Motors that start and stop frequently often have some kind of control system, which consist of a contactor and overload protection such as a thermal relay.

DOL curve - Synchronous speed / Full-load torque
DOL curve – Synchronous speed / Full-load torque

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Drawbacks of DOL

Small motors which do not start and stop frequently need only very simple starting equipment, often in the form of a hand-operated motor protection circuit breaker.

Full voltage is switched directly onto the motor terminals. For small motors, the starting torque will be 150% to 300% of the full-load value, while the starting current will be 300% to 800% of the full-load current or even higher.

DOL curve - Synchronous speed / Full-load current
DOL curve – Synchronous speed / Full-load current

Go back to Methods ↑


Star-delta starting

The objective of this starting method, which is used with three-phase induction motors, is to reduce the starting current.

In starting position, current supply to the stator windings is connected in star (Y) for starting. In the running position, current supply is reconnected to the windings in delta (∆) once the motor has gained speed.

Line diagram for star-delta motor starter
Line diagram for star-delta motor starter

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Advantages of Y-Δ

Normally, low-voltage motors over 3 kW will be dimensioned to run at either 400 V in delta (∆) connection or at 690 V in star (Y) connection. The flexibility provided by this design can also be used to start the motor with a lower voltage. Star-delta connections give a low starting current of only about one third of that found with direct-on-line starting.

Star-delta starters are particularly suited for high inertias, where the load are initiated after full load speed.

Start-delta starter curve - Synchronous speed / Full-load torque
Start-delta starter curve – Synchronous speed / Full-load torque

Go back to Methods ↑


Drawbacks of of Y-Δ

But they also reduce the starting torque to about 33%. The motor is started in Y-connection and accelerated and switched to the star-delta connection. This method can only be used with induction motors that are delta connected to the supply voltage.

If the changeover from star to delta takes place at too low a speed, this can cause a current surge which rises almost as high as the corresponding DOL value. During the even small period of switch over from start to delta connection the motor looses speed very rapidly, which also calls for higher current pulse after connection to delta.

The two illustrations to the right show two features which should be taken into consideration when using star-delta starting. The starter first connects the motor in star (contactor K1 and K3). After a time period – which depends on individual needs – it connects the motor in delta contactor K3 open and contactor K2 close.

Star-delta starter curve – Synchronous speed / Full-load current
Star-delta starter curve – Synchronous speed / Full-load current

Starting torque and current are considerably lower at star-delta starting than at direct-on-line starting: one third of the equivalent DOL value.

Mismatching of motor torque speed curve and load torque speed curve. In the example shown here, the motor would slowly accelerate up to approximately 50 per cent rated speed.

Mismatching of motor torque speed curve and load torque speed curve
Mismatching of motor torque speed curve and load torque speed curve

Go back to Methods ↑


Comparision of DOL and Star-delta starting

The following graphs illustrate currents for a Grundfos CR pump started with a Grundfos MG 7.5 kW motor by means of DOL and star-delta starting, respectively. As you will see, the DOL starting method features a very high locked-rotor current which eventually flattens and becomes constant.

Direct-on-line starting of a Grundfos 7.5 kW motor installed on a Grundfos CR pump
Direct-on-line starting of a Grundfos 7.5 kW motor installed on a Grundfos CR pump

The star-delta starting method features a lower locked-rotor current, but peaks during the starting process as the changeover from star to delta is made.

When starting in star (t = 0.3 s), the current is reduced.

Star-delta starting of a 7.5 kW Grundfos motor installed on a Grundfos CR pump
Star-delta starting of a 7.5 kW Grundfos motor installed on a Grundfos CR pump

However, when switching over from star to delta (at t = 1 .7 s), the current pulse reaches the same level as the locked-rotor current seen with direct-on-line starting. The current pulse can even get higher, because the motor during the switching period is un-powered which means it reduce speed before the full voltage (delta voltage) are supplied.

Go back to Methods ↑

Reference // The Motor Book – Grundfos (Download)

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

22 Comments


  1. Nizar Obeid
    Jan 14, 2024

    Hello ,
    Sorry , just only forgot the

    FLC ( AMPS ) = 308 A
    Starting Method = Star – Delta


  2. Nizar Obeid
    Jan 14, 2024

    Good Morning Gents ,
    I have Fire Fighting Electrical Pump , the Technical Data : PH = 3 , Insul Class = F , HP = 250 , AMPS = 387 , SF AMPS = 387 , RPM = 3565 , HZ = 60 ,NEMA NOM EFF = 95 , DES = B , Guaranteed Efficiency = 94.1 , DUTY : CONT , WT = 1200 LBS , PF = 89

    And i have Diesel Generator : KVA = 513 , KW = 410 , COS = 0.8 , HZ =60
    V = 380 , AMPS = 779 , RPM 1800 REV / MIN
    Kindly requested to confirm us if could we start the Motor / Pump as per the above DATA .

    Thanks .


  3. Himanshu pahade
    Apr 19, 2022

    Your explanation is great and having clear and easy concepts
    But which i want is that:Dol,star/delta and rotor resistance starter


  4. Nazeer
    Jul 24, 2019

    Thank you very such a very useful information for me with details & Good Examples


  5. CHANDAN RAWAL
    Jul 12, 2019

    DOL is used in small rating/low cost motors only and only because of economic reasons.


  6. Winston Garcia
    May 27, 2019

    Generally in Y delta Starting, it was proven that Y reduces starting current. But when I experimented on centrifugal pumps, I found the reverse reality. Meaning , I started the centrifugal pump in Y and recorded the Starting current. Then I used the same pump and used DOL, meaning I started it in Delta. My record shows that the Delta starting was even lesser. Can you please explain this. I’ve been trying to search explanations for this but till now I haven’t seen any. No one seems to have noticed this either. I wonder, probably its due to the nature of the impeller mechanism. Hoping to find get answer for this question at last.


  7. Mumbere Jockim
    Mar 30, 2019

    Good article Edvard! Good work!


  8. Elhamy Naguib
    Mar 02, 2019

    I need to convert Star/ Delta to DOL to operate with auto transformer , shall I connect R1 with T2 , S1 with S2 and T1 with R2


  9. RAJASEKARAN
    Oct 24, 2018

    VERY HELPFUL


  10. Very good
    Oct 20, 2018

    Hello Edvard
    Can you send me your contact email adress or phone
    Thanks


  11. Sapan prakash Mukharjee
    May 18, 2018

    My requested question is, why should we have used to three contactor in star-delta


  12. M Tariq Zia
    Dec 06, 2017

    I have a confusion that How can we calculate motor current in delta and star.
    Suppose we have a motor rating 11kw, 380 volt PF 0.90,
    How can i calculate line current if motor is connected in delta? Also current in star?


  13. Jesus Santos
    Mar 04, 2017

    Muy buena información me gustaría subcribirme a su blog


  14. Ahmed
    Jun 25, 2016

    what is the advantage of using star delta starting as it causes the same high current of direct online during switching from star to delta?


    • Ahmed
      Jun 20, 2018

      Same question.


  15. jeramil
    Dec 09, 2015

    thank you very much… very useful information.


  16. surajit mishra
    Oct 12, 2015

    The decision to adopt DOL starting also depends on the capacity of the source, usually a transformer, and the distance from the source to the motor control centre. For example, if a 100 kW pump motor controller is located close to a power transformer of 1000 kVA, it can be started DOL. But if the same motor control centre is located 500 meter from the source transformer, it is better to adopt a star-delta starter because the starting current for this motor will cause sufficient voltage drop to affect starting of this motor.
    But then, if the motor is driving a centrifugal blower, it will be wise to use a star-delta starter even if the MCC is close to the transformer. As has been pointed out, a high inertia load like a centrifugal blower will draw enormous starting current and cause instability.


  17. kuppuraju E
    Aug 10, 2015

    your article is so much useful for me and also this article has been explained in a simple manner,so every electrical engineer can understand this type article…keep up


  18. Reyadh Mohammad
    Jul 25, 2015

    Very good and helpful article. Keep up.


  19. ULHAS VAJRE
    Jul 18, 2015

    Dear Edvard, in order to disseminate the knowledge to Electrical Engineering fraternity, can we publish these articles or present on other forums??. Pl let me know on my mail…thanks


    • Edvard
      Jul 25, 2015

      Sure, no problem. Just put the reference URL at the bottom.


  20. pankaj kumar
    Jul 18, 2015

    Hi, good artical again, your article is very helpful for practical analysis.
    Thank you

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