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Comparision of Direct-on-line (DOL) and Star-delta Motor Starting
Comparision of Direct-on-line (DOL) and Star-delta Motor Starting (photo credit: ukworkshop.co.uk)

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.

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

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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

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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

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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.

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Reference // The Motor Book – Grundfos (Download)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

8 Comments


  1. 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?


  2. jeramil
    Dec 09, 2015

    thank you very much… very useful information.


  3. 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.


  4. 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


  5. Reyadh Mohammad
    Jul 25, 2015

    Very good and helpful article. Keep up.


  6. 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
      Edvard
      Jul 25, 2015

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


  7. pankaj kumar
    Jul 18, 2015

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

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