8/13/2019 Star Delta Motor Starter

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8/13/2019 Star Delta Motor Starter

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ABB Australia Pty LimitedIssue date: 15 May 08

1VGA671022

There are two contactors that are closeduring run, often referred to as the maincontactor and the delta contactor. Theseare AC3 rated at 58% of the current ratingof the motor. The third contactor is the starcontactor and that only carries star currentwhile the motor is connected in star. Thecurrent in star is one third of the current indelta, so this contactor can be AC3 ratedat one third of the motor rating.

OperationIn operation, the Main Contactor (KM3)and the Star Contactor (KM1) are closedinitially, and then after a period of time, thestar contactor is opened, and then thedelta contactor (KM2) is closed. Thecontrol of the contactors is by the timer(K1T) built into the starter. The Star andDelta are electrically interlocked andpreferably mechanically interlocked aswell.

In effect, there are four states:1. OFF State. All Contactors are open2. Star State. The Main and the Star

contactors are closed and the deltacontactor is open. The motor isconnected in star and will produce onethird of DOL torque at one third of DOLcurrent.

3. Open State. The Main contactor isclosed and the Delta and Starcontactors are open. There is voltageon one end of the motor windings, butthe other end is open so no currentcan flow. The motor has a spinningrotor and behaves like a generator.

4. Delta State. The Main and the Deltacontactors are closed. The Star

contactor is open. The motor isconnected to full line voltage and fullpower and torque are available.

This type of operation is called opentransition switching because there is anopen state between the star state and thedelta state.

Open Transition StartersWhen a motor is driven by the supply,either at full speed or at part speed, thereis a rotating magnetic field in the stator.This field is rotating at line frequency. Theflux from the stator field induces a current

in the rotor and this in turn results in arotor magnetic field.

When the motor is disconnected from thesupply (open transition) there is a spinningrotor within the stator and the rotor has amagnetic field. Due to the low impedanceof the rotor circuit, the time constant isquite long and the action of the spinningrotor field within the stator is that of agenerator which generates voltage at afrequency determined by the speed of therotor. When the motor is reconnected tothe supply, it is reclosing onto an

Enquiries email: abb.zvc@au.abb.comSh.2

8/13/2019 Star Delta Motor Starter

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ABB Australia Pty LimitedIssue date: 15 May 08

1VGA671022

Enquiries email: abb.zvc@au.abb.com

In effect, there are five states:unsynchronised generator and this resultin a very high current and torque transient.The magnitude of the transient isdependent on the phase relationshipbetween the generated voltage and theline voltage at the point of closure, buttypically can be much higher then DOLcurrent and torque and can result inelectrical and mechanical damage.

1. OFF State. All Contactors are open2. Star State. The Main [KM3] and the

Star [KM1] contactors are closed andthe delta [KM2] contactor is open. Themotor is connected in star and willproduce one third of DOL torque atone third of DOL current.

3. Star Transition State. The motor isconnected in star and the resistors areconnected across the delta contactorvia the aux [KM4] contactor.

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4. Closed Transition State. The Main[KM3] contactor is closed and theDelta [KM2] and Star [KM1] contactorsare open. Current flows through themotor windings and the transition

resistors via KM4.5. Delta State. The Main and the Deltacontactors are closed. The transitionresistors are shorted out. The Starcontactor is open. The motor isconnected to full line voltage and fullpower and torque are available.

Closed Transition Star/Delta StarterThere is a technique to reduce themagnitude of the switching transients.This requires the use of a fourth contactorand a set of three resistors. The resistorsmust be sized such that considerablecurrent is able to flow in the motorwindings while they are in circuit.

The auxiliary contactor and resistors areconnected across the delta contactor. Inoperation, just before the star contactoropens, the auxiliary contactor closesresulting in current flow via the resistorsinto the star connection. Once the starcontactor opens, current is able to flowround through the motor windings to thesupply via the resistors. These resistorsare then shorted by the delta contactor. Ifthe resistance of the resistors is too high,they will not swamp the voltage generatedby the motor and will serve no purpose.

Source: L M Photonics Ltd