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Star-d e lta mo to r starte r p ane l

Star-d e lta starte r p ane l

by jiguparmar

Star-Delta Motor Starter

Introduction to Star-Delta motor starter

Most induction motors are started directly on line, but when very large motorsare started that way, they cause a disturbance of voltage on the supply linesdue to large starting current surges. To limit the starting current surge, largeinduction motors are started at reduced voltage and then have f ull supplyvoltage reconnected when they run up to near rotated speed.

Two methods used f or reduction of starting voltage are: Star delta start ingand Auto transformer start ing.

Working Principle of Star-Delta Starter

This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved byphysically reconf iguring the motor windings as illustrated in the f igure below. During starting the motor windings

Sche me - Wo rking Princ ip le o f Star-De lta Starte r

are connected in star conf iguration and this reduces the voltage across each winding 3. This also reduces thetorque by a f actor of three.

Af ter a period of t ime the winding are reconf iguredas delta and the motor runs normally. Star/Deltastarters are probably the most common reducedvoltage starters. They are used in an attempt toreduce the start current applied to the motor duringstart as a means of reducing the disturbances andinterf erence on the electrical supply.

Traditionally in many supply regions, there has beena requirement to f it a reduced voltage starter on allmotors greater than 5HP (4KW). The Star/Delta (orWye/Delta) starter is one of the lowest costelectromechanical reduced voltage starters that can be applied.

The Star/Delta starter is manuf actured f rom three contactors, a t imer and a thermal overload. The contactorsare smaller than the single contactor used in a Direct on Line starter as they are controlling winding currentsonly. The currents through the winding are 1/root 3 (58%) of the current in the line.

There are two contactors that are close during run, of ten ref erred to as the main contractor and the deltacontactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the starcontactor and that only carries star current while the motor is connected in star.

The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) ofthe motor rating.

Star-delta Starter Consists following units

1. Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (CloseTransient Starter).

2. Time relay (pull- in delayed) 1 No.

3. Three-pole thermal overcurrent release 1 No.

4. Fuse elements or automatic cut-outs f or the main circuit 3 Nos.

5. Fuse element or automatic cut-out f or the control circuit 1No.

Power Circuit of Star Delta Starter

The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.

The main contactor connects the ref erence source voltage R, Y, B to the primary terminal of the motor U1, V1,W1.

In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed init ially, and then af ter aperiod of t ime, the star contactor is opened, and then the delta contactor (KM2) is closed. The control of thecontactors is by the timer (K1T ) built into the starter. The Star and Delta are electrically interlocked and

Po we r c ircuit o f Star-De lta starte r

pref erably mechanically interlocked as well.

In effect, there are fourstates:

The star contactor servesto init ially short thesecondary terminal of themotor U2, V2, W2 f or thestart sequence during theinit ial run of the motorf rom standstill. Thisprovides one third of DOLcurrent to the motor, thusreducing the high inrushcurrent inherent with largecapacity motors at startup.

Controlling theinterchanging star connection and delta connection of an AC induction motor is achieved by means of a stardelta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and atimer.

Control Circuit of Star-Delta Starter (Open Transition)

Sche me - Co ntro l Circuit o f Star-De lta Starte r (Op e n Transitio n)

The ON push buttonstarts the circuit by init iallyenergizing Star ContactorCoil (KM1) of star circuitand Timer Coil (KT) circuit.When Star Contactor Coil(KM1) energized, Star Mainand Auxiliary contactorchange its posit ion f romNO to NC.

When Star AuxiliaryContactor (1) (which isplaced on Main Contactorcoil circuit ) become NO toNC it ’s complete TheCircuit of Main contactorCoil (KM3) so MainContactor Coil energizedand Main Contactor ’s Main and AuxiliaryContactor Change itsPosition f rom NO to NC.This sequence happens ina f riction of t ime.

Af ter pushing the ON pushbutton switch, the auxiliarycontact of the maincontactor coil (2) which isconnected in parallelacross the ON pushbutton will become NO to NC, thereby providing a latch to hold the main contactor coil activated whicheventually maintains the control circuit active even af ter releasing the ON push button switch.

When Star Main Contactor (KM1) close its connect Motor connects on STAR and it ’s connected in STAR untilTime Delay Auxiliary contact KT (3) become NC to NO.

Once the time delay is reached its specif ied Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit willchange its posit ion f rom NC to NO and at the Same Time Auxiliary contactor (KT) in Delta Coil Circuit(4)change its Posit ion f rom NO To NC so Delta coil energized and Delta Main Contactor becomes NO To NC.Now Motor terminal connection change f rom star to delta connection.

A normally close auxiliary contact f rom both star and delta contactors (5&6)are also placed opposite of bothstar and delta contactor coils, these interlock contacts serves as saf ety switches to prevent simultaneousactivation of both star and delta contactor coils, so that one cannot be activated without the other deactivatedf irst. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, thestar contactor coil cannot also be active while the delta contactor coil is active.

The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF pushbutton switch break the control circuit and the motor when necessary. The thermal overload contact is aprotective device which automatically opens the STOP Control circuit in case when motor overload current is

detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive loadbeyond the rated capacity of the motor is detected by the thermal overload relay.

At some point during starting it is necessary to change f rom a star connected winding to a delta connectedwinding. Power and control circuits can be arranged to this in one of two ways – open transit ion or closedtransit ion.

What is Open or Closed Transit ion Start ing

1. Open Transit ion Starters

Discuss mention above is called open transit ion switching because there is an open state between the starstate and the delta state.

In open transit ion the power is disconnected f rom the motor while the winding are reconf igured via externalswitching.

When a motor is driven by the supply, either at f ull speed or at part speed, there is a rotating magnetic f ield inthe stator. This f ield is rotating at line f requency. The f lux f rom the stator f ield induces a current in the rotorand this in turn results in a rotor magnetic f ield.

When the motor is disconnected f rom the supply (open transit ion) there is a spinning rotor within the statorand the rotor has a magnetic f ield. Due to the low impedance of the rotor circuit, the time constant is quite longand the action of the spinning rotor f ield within the stator is that of a generator which generates voltage at af requency determined by the speed of the rotor.

When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generator and this resultin a very high current and torque transient. The magnitude of the transient is dependent on the phaserelationship between the generated voltage and the line voltage at the point of closure can be much higherthan DOL current and torque and can result in electrical and mechanical damage.

Open transit ion starting is the easiest to implement in terms or cost and circuitry and if the timing of thechangeover is good, this method can work well. In practice though it is dif f icult to set the necessary timing tooperate correctly and disconnection/reconnection of the supply can cause signif icant voltage/currenttransients.

In open transition there are four states:

1. OFF State : All Contactors are open.

2. Star State: The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor isopen. The motor is connected in star and will produce one third of DOL torque at one third of DOLcurrent.

3. Open State: This type of operation is called open transit ion switching because there is an open statebetween the star state and the delta state. The Main contractor is closed and the Delta and Starcontactors are open. There is voltage on one end of the motor windings, but the other end is open sono current can f low. The motor has a spinning rotor and behaves like a generator.

4. Delta State: The Main and the Delta contactors are closed. The Star contactor is open. The motor isconnected to f ull line voltage and f ull power and torque are available

2. Closed Transit ion Star/Delta Starter

There is a technique to reduce the magnitude of the switching transients. This requires the use of a f ourthcontactor and a set of three resistors. The resistors must be sized such that considerable current is able tof low in the motor windings while they are in circuit.

The auxiliary contactor and resistors are connected across the delta contactor. In operation, just bef ore thestar contactor opens, the auxiliary contactor closes resulting in current f low via the resistors into the starconnection. Once the star contactor opens, current is able to f low round through the motor windings to thesupply via the resistors. These resistors are then shorted by the delta contactor.

If the resistance of the resistors is too high, they will not swamp the voltage generated by the motor and willserve no purpose.

In closed transit ion the power is maintained to the motor at all t ime.

This is achieved by introducing resistors to take up the current f low during the winding changeover. A f ourthcontractor is required to place the resistor in circuit bef ore opening the star contactor and then removing theresistors once the delta contactor is closed.

These resistors need to be sized to carry the motor current. In addition to requiring more switching devices,the control circuit is more complicated due to the need to carry out resistor switching

In close transition there are four states:

1. OFF State. All Contactors are open

2. Star State. The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor isopen. The motor is connected in star and will produce one third of DOL torque at one third of DOLcurrent.

3. Star Transit ion State. The motor is connected in star and the resistors are connected across the deltacontactor via the aux [KM4] contactor.

4. Closed Transit ion State. The Main [KM3] contactor is closed and the Delta [KM2] and Star [KM1]contactors are open. Current f lows through the motor windings and the transit ion resistors via KM4.

5. Delta State. The Main and the Delta contactors are closed. The transit ion resistors are shorted out.The Star contactor is open. The motor is connected to f ull line voltage and f ull power and torque areavailable.

Effect of Transient in Starter (Open Transient starter)

It is Important the pause between star contactor switch of f and Delta contactor switch is on correct. This isbecause Star contactor must be reliably disconnected bef ore Delta contactor is activated. It is also importantthat the switch over pause is not too long.

For 415v Star Connection voltage is ef f ectively reduced to 58% or 240v. The equivalent of 33% that isobtained with Direct Online (DOL) starting.

If Star connection has suf f icient torque to run up to 75% or %80 of f ull load speed, then the motor can be

connected in Delta mode.

When connected to Delta conf iguration the phase voltage increases by a ratio of V3 or 173%. The phasecurrents increase by the same ratio. The line current increases three times its value in star connection.

During transit ion period of switchover the motor must be f ree running with litt le deceleration. While this ishappening “Coasting” it may generate a voltage of its own, and on connection to the supply this voltage canrandomly add to or subtract f rom the applied line voltage. This is known as transient current. Only lasting a f ewmilliseconds it causes voltage surges and spikes. Known as a changeover transient.

Size of each part of Star-Delta starter

1. Size of Over Load Relay

For a star-delta starter there is a possibility to place the overload protection in two posit ions, in the line or inthe windings.

Overload Relay in Line:

In the line is the same as just putting the overload bef ore the motor as with a DOL starter.

The rating of Overload (In Line) = FLC of Motor.

Disadvantage: If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting isx1.732 too high).

Overload Relay in Winding:

In the windings means that the overload is placed af ter the point where the wiring to the contactors are splitinto main and delta. The overload then always measures the current inside the windings.

The sett ing of Overload Relay (In Winding) =0.58 X FLC (line current).

Disadvantage: We must use separate short circuit and overload protections.

2. Size of Main and Delta Contractor

There are two contactors that are close during run, of ten ref erred to as the main contractor and the deltacontactor. These are AC3 rated at 58% of the current rating of the motor.

Size of Main Contactor= IFL x 0.58

3. Size of Star Contractor

The third contactor is the star contactor and that only carries star current while the motor is connected in star.The current in star is 1/ √3= (58%) of the current in delta, so this contactor can be AC3 rated at one third(33%) of the motor rating.

Size of Star Contactor= IFL x 0.33

Motor Starting Characteristics of Star-Delta Starter

Available starting current: 33% Full Load Current.

Peak starting current: 1.3 to 2.6 Full Load Current.

Peak starting torque: 33% Full Load Torque.

Advantages of Star-Delta starter

The operation of the star-delta method is simple and rugged

It is relatively cheap compared to other reduced voltage methods.

Good Torque/Current Perf ormance.

It draws 2 times starting current of the f ull load ampere of the motor connected

Disadvantages of Star-Delta starter

1. Low Starting Torque (Torque = (Square of Voltage) is also reduce).

2. Break In Supply – Possible Transients

3. Six Terminal Motor Required (Delta Connected).

4. It requires 2 set of cables f rom starter to motor..

5. It provides only 33% starting torque and if the load connected to the subject motor requires higherstarting torque at the time of starting than very heavy transients and stresses are produced whilechanging f rom star to delta connections, and because of these transients and stresses many electricaland mechanical break-down occurs..

6. In this method of starting init ially motor is connected in star and then af ter change over the motor isconnected in delta. The delta of motor is f ormed in starter and not on motor terminals..

7. High transmission and current peaks: When starting up pumps and f ans f or example, the load torqueis low at the beginning of the start and increases with the square of the speed. When reaching approx.80-85 % of the motor rated speed the load torque is equal to the motor torque and the accelerationceases. To reach the rated speed, a switch over to delta posit ion is necessary, and this will very of tenresult in high transmission and current peaks. In some cases the current peak can reach a value that iseven bigger than f or a D.O.L start..

8. Applications with a load torque higher than 50 % of the motor rated torque will not be able to start usingthe start-delta starter..

9. Low Starting Torque: The star-delta (wye-delta) starting method controls whether the leadconnections f rom the motor are conf igured in a star or delta electrical connection. The init ial connectionshould be in the star pattern that results in a reduction of the line voltage by a f actor of 1/√3 (57.7%) tothe motor and the current is reduced to 1/3 of the current at f ull voltage, but the starting torque is alsoreduced 1/3 to 1/5 of the DOL starting torque..

10. The transit ion f rom star to delta transit ion usually occurs once nominal speed is reached, but issometimes perf ormed as low as 50% of nominal speed which make transient Sparks.

Features of star-delta starting

1. For low- to high-power three-phase motors.

2. Reduced starting current

3. Six connection cables

4. Reduced starting torque

5. Current peak on changeover f rom star to delta

6. Mechanical load on changeover f rom star to delta

Application of Star-Delta Starter

The star-delta method is usually only applied to low to medium voltage and light start ing Torque motors.

The received starting current is about 30 % of the starting current during direct on line start and the startingtorque is reduced to about 25 % of the torque available at a D.O.L start. This starting method only works whenthe application is light loaded during the start.

If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed bef oreswitching over to the delta posit ion.