comparision of dol and star-delta motor starting _ eep

Comparision of Direct-on-line (DOL) and Star-deltaComparision of Direct-on-line (DOL) and Star-deltaMotor StartingMotor Starting

Motor starting methods //Motor starting methods //

Direct-on-line (DOL) startingDirect-on-line (DOL) starting

AdvantagesAdvantages

DrawbacksDrawbacks

Star-delta startingStar-delta starting

AdvantagesAdvantages

DrawbacksDrawbacks

Comparision of DOL and Star-delta startingComparision of DOL and Star-delta starting

Direct-on-line starting (DOL)Direct-on-line starting (DOL)

As the name suggests, direct-on-line starting means that As the name suggests, direct-on-line starting means that the motor is started bythe motor is started by

connecting it directly to the supplyconnecting it directly to the supply at rated voltage. Direct-on-line starting, (DOL), is at rated voltage. Direct-on-line starting, (DOL), is

suitable for suitable for stable suppliesstable supplies and mechanically stiff and well-dimensioned shaft systems and mechanically stiff and well-dimensioned shaft systems

– and pumps qualify as examples of such systems.– and pumps qualify as examples of such systems.

Comparision of DOL and Star-delta Motor Starting | EEP http://electrical-engineering-portal.com/comparision-of-direct-on-line-do...

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Where:Where:

K1K1 – Main contactor – Main contactor

MV1MV1 – Overload relay – Overload relay

Go back to Methods ↑Go back to Methods ↑

Advantages of DOLAdvantages of DOL

DOL starting is the simplest, DOL starting is the simplest, cheapestcheapest and and most common starting methodmost common starting method..

Furthermore it actually gives the lowest temperature rise within the motor during start upFurthermore it actually gives the lowest temperature rise within the motor during start up

of all the starting methods.of all the starting methods.

It is the obvious choice wherever the supply authority’s current limitingIt is the obvious choice wherever the supply authority’s current limiting

restrictions allow for its use.restrictions allow for its use.

Power plants may have varying rules and regulations in different countries. Power plants may have varying rules and regulations in different countries. For example:For example:

Three-phase motors with locked-rotor currents above 60 A Three-phase motors with locked-rotor currents above 60 A must not use direct-on-linemust not use direct-on-line

starting in Denmarkstarting in Denmark. In such cases, it will obviously be necessary to select another. In such cases, it will obviously be necessary to select another

starting method.starting method.


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Motors that start and stop frequently often have Motors that start and stop frequently often have some kind of control systemsome kind of control system, which, which

consist of a consist of a contactorcontactor and and overload protectionoverload protection such as a such as a thermal relaythermal relay..


Drawbacks of DOLDrawbacks of DOL

Small motors which do not start and stop frequently need Small motors which do not start and stop frequently need only very simple startingonly very simple starting

equipmentequipment, often in the form of a hand-operated motor protection circuit breaker., 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 startingFull 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 torque will be 150% to 300% of the full-load value, while the starting current will be 300%300%

to 800% of the full-load currentto 800% of the full-load current or even higher. or even higher.


Star-delta startingStar-delta starting

The objective of this starting method, which is used with three-phase induction motors, The objective of this starting method, which is used with three-phase induction motors, isis

to reduce the starting currentto reduce the starting current..

In starting position, current supply to the stator windings is connected in In starting position, current supply to the stator windings is connected in star (Y) forstar (Y) for

startingstarting. In the running position, current supply is reconnected to the windings in . In the running position, current supply is reconnected to the windings in deltadelta


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(∆) once the motor has gained speed(∆) once the motor has gained speed ..


Advantages of Y-ΔAdvantages of Y-Δ

Normally, Normally, low-voltage motors over 3 kWlow-voltage motors over 3 kW will will

be dimensioned to run at either be dimensioned to run at either 400 V in400 V in

delta (∆) connectiondelta (∆) connection or at or at 690 V in star (Y)690 V in star (Y)

connectionconnection . The flexibility provided by this. The flexibility provided by this

design can also be used to start the motordesign can also be used to start the motor

with a lower voltage. Star-delta connectionswith a lower voltage. Star-delta connections

give a low starting current of only about onegive a low starting current of only about one

third ofthird of

thatthat

foundfound

withwith

direct-direct-

on-lineon-line

starting.starting.

Star-deltaStar-delta

startersstarters

areare

particularly suited for particularly suited for high inertiashigh inertias, where the load are initiated after full load speed., where the load are initiated after full load speed.


Drawbacks of of Y-ΔDrawbacks of of Y-Δ

But they also reduce the starting torque to about 33%.But they also reduce the starting torque to about 33%. The motor is started in The motor is started in


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Y-connection and accelerated and switched toY-connection and accelerated and switched to

the star-delta connection. This method canthe star-delta connection. This method can

only be used with induction motors that areonly be used with induction motors that are

delta connecteddelta connected to the supply voltage. to the supply voltage.

If the changeover from star to delta takes place at too low a speed, this canIf the changeover from star to delta takes place at too low a speed, this can

cause a cause a current surgecurrent surge which rises almost as high as the corresponding DOL which rises almost as high as the corresponding DOL

value. During the even small period of switch over from start to deltavalue. During the even small period of switch over from start to delta

connection connection the motor looses speed very rapidlythe motor looses speed very rapidly, which also calls for , which also calls for higherhigher

current pulsecurrent pulse after connection to delta. after connection to delta.

The two illustrations to the right show two features which should be taken intoThe 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 consideration when using star-delta starting. The starter first connects the motor in starstar

(contactor K1 and K3). After a time period – which depends on individual needs – it(contactor K1 and K3). After a time period – which depends on individual needs – it

connects the motor in connects the motor in delta contactor K3delta contactor K3 open and open and contactor K2contactor K2 close. close.


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Starting torque and current are considerably lower at star-delta starting than at direct-Starting torque and current are considerably lower at star-delta starting than at direct-

on-line starting: one third of the equivalent DOL value.on-line starting: one third of the equivalent DOL value.

Mismatching of motor torque speed curve and load torque speed curve. In the exampleMismatching of motor torque speed curve and load torque speed curve. In the example

shown here, the motor would slowly accelerate shown here, the motor would slowly accelerate up to approximately 50 per cent ratedup to approximately 50 per cent rated

speedspeed..


Comparision of DOL and Star-delta startingComparision of DOL and Star-delta starting

The following graphs illustrate currents for a The following graphs illustrate currents for a Grundfos CR pumpGrundfos CR pump started with a started with a

Grundfos MG 7.5 kW motorGrundfos MG 7.5 kW motor by means of DOL and star-delta starting, respectively. As by means of DOL and star-delta starting, respectively. As

you will see, the DOL starting method features a you will see, the DOL starting method features a very high locked-rotor currentvery high locked-rotor current which which

eventually flattens and becomes constant.eventually flattens and becomes constant.

The star-delta starting methodThe star-delta starting method features a lower locked-rotor current, but peaks during features a lower locked-rotor current, but peaks during

the starting process as the changeover from star to delta is made.the starting process as the changeover from star to delta is made.

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


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However, when However, when switching over from star to deltaswitching over from star to delta (at t = 1 .7 s), the current pulse (at t = 1 .7 s), the current pulse

reaches the same level as the locked-rotor current seen with direct-on-line starting. reaches the same level as the locked-rotor current seen with direct-on-line starting. TheThe

current pulse can even get highercurrent pulse can even get higher, because the motor during the switching period is, because the motor during the switching period is

un-powered which means it reduce speed before the full voltage (delta voltage) areun-powered which means it reduce speed before the full voltage (delta voltage) are

supplied.supplied.


Reference //Reference // The Motor Book – Grundfos ( The Motor Book – Grundfos (DownloadDownload))


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Share with engineers //Share with engineers //

Recommended EE articles //Recommended EE articles //

About Author //About Author //

Edvard CsanyiEdvard Csanyi

EdvardEdvard - - Electrical engineer, programmer and founder of Electrical engineer, programmer and founder of EEPEEP..

Highly specialized for design of LV high power busbar trunkingHighly specialized for design of LV high power busbar trunking

(<6300A) in power substations, buildings and industry fascilities.(<6300A) in power substations, buildings and industry fascilities.

Designing of LV/MV switchgears.Professional in AutoCADDesigning of LV/MV switchgears.Professional in AutoCAD

programming and web-design.Present on programming and web-design.Present on Google+Google+

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comparision of dol and star-delta motor starting _ eep

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