StarDelta Motor Starter

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    electrical-engineering-portal.com http://electrical-engineering-po rtal.com /star-delta-mo tor-s tarter

    Star-de lta motor starter panel

    Star-delta starter panel

    by jiguparmar

    Star-Delta Motor Starter

    Introduction to Star-Delta motor starter

    Most induction motors are started directly on line, butwhen very large motors are started that way, they cause adisturbance of voltage on the supply lines due to largestarting current surges. To limit the starting current surge,large induct ion motors are started at reduced voltage andthen have full supply voltage reconnected when they runup to near rotated speed.

    Two methods used for reduction of starting voltage are:Star delta starting and Auto transformer starting.

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    Scheme - Working Principle of Star-Delta Starter

    Working Principle of Star-Delta Starter

    This is the reduced voltage starting method. Voltage reduct ion during star-deltastarting is achieved by physically reconfiguring the motor windings as illustrated inthe f igure below. During starting the motor windings are connected in starconf iguration and this reduces the voltage across each winding 3. This also reduces

    the torque by a factor of three.

    Af ter a period of time the winding arereconf igured as delta and the motorruns normally. Star/Delta starters areprobably the most common reducedvoltage starters. They are used in anattempt to reduce the start current

    applied to the motor during start as ameans of reducing the disturbancesand interference on the electricalsupply.

    Traditionally in many supply regions, there has been a requirement to fit a reducedvoltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta)starter is one of the lowest cost electromechanical reduced voltage starters that canbe applied.

    The Star/Delta starter is manufactured f rom three contactors, a timer and a thermaloverload. The contactors are smaller than the single contactor used in a Direct onLine starter as they are controlling winding currents only. The currents through thewinding are 1/root 3 (58%) of the current in the line.

    There are two contactors that are close during run, of ten referred to as the maincontractor and the delta contactor. These are AC3 rated at 58% of the current rating

    of the motor. The third contactor is the star contactor and that only carries starcurrent while the motor is connected in star.

    The current in star is one third of the current in delta, so this contactor can be AC3rated at one third (33%) of the motor rating.

    Star-delta Starter Consists following units

    1. Contactors (Main, star and delta contactors) 3 Nos (For Open State Starter)or 4 Nos (Close Transient Starter).

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

    3. Three-pole thermal overcurrent release 1 No.

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

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

    Power Circuit of Star Delta Starter

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

    The main contactor connects the reference source voltage R,Y, B to the primaryterminal of the motorU1, V1, W1.

    In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closedinitially, and then after a period of time, the star contactor is opened, and then thedelta contactor (KM2) is closed. The control of the contactors is by the timer (K1T)built into the starter. The Star and Delta are electrically interlocked and preferablymechanically interlocked as well.

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    Power circuit of Star-Delta starter

    In effect, there

    are four states:

    The starcontactor servesto initially shortthe secondaryterminal of themotor U2, V2, W2for the startsequence duringthe initial run ofthe motor fromstandstill. Thisprovides onethird of DOLcurrent to themotor, thusreducing the highinrush currentinherent withlarge capacitymotors atstartup.

    Controlling theinterchangingstar connectionand deltaconnection of anAC induction

    motorisachieved bymeans of a star delta or wye delta control circuit. The control circuit consists ofpush button switches, auxiliary contacts and a timer.

    Control Circuit of Star-Delta Starter (Open Transition)

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    Scheme - Control Circuit of Star-Delta Starter (Open Transition)

    The ON pushbutton starts thecircuit by initiallyenergizing StarContactor Coil(KM1) of star

    circuit and TimerCoil (KT) circuit.When StarContactor Coil(KM1) energized,Star Main andAuxiliarycontactor

    change itsposition from NOto NC.

    When StarAuxiliaryContactor (1)(which is placedon MainContactor coilcircuit ) becomeNO to NC its complete The Circuit of Main contactor Coil (KM3) so Main ContactorCoil energized and Main Contactor s Main and Auxiliary Contactor Change itsPosition f rom NO to NC. This sequence happens in a friction of time.

    Af ter pushing the ON push button switch, the auxiliary contact of the main contactorcoil (2) which is connected in parallel across the ON push button will become NO to

    NC, thereby providing a latch to hold the main contactor coil act ivated whicheventually maintains the control circuit active even af ter releasing the ON push buttonswitch.

    When Star Main Contactor (KM1) close its connect Motor connects on STAR and itsconnected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO.

    Once the t ime delay is reached its specif ied Time, the timers auxiliary contacts

    (KT)(3) in Star Coil circuit will change its position f rom NC to NO and at the SameTime Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position f rom NO ToNC so Delta coil energized and Delta Main Contactor becomes NO To NC. NowMotor terminal connection change from star to delta connection.

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    A normally close auxiliary contact f rom both star and delta contactors (5&6)are alsoplaced opposite of both star and delta contactor coils, these interlock contactsserves as safety switches to prevent s imultaneous activation of both star and deltacontactor coils, so that one cannot be activated without the other deactivated f irst.Thus, the delta contactor coil cannot be active when the star contactor coil is active,and similarly, the star contactor coil cannot also be active while the delta contactor

    coil is active.

    The control circuit above also provides two interrupting contacts to shutdown themotor. The OFF push button switch break the control circuit and the motor whennecessary. The thermal overload contact is a protective device which automaticallyopens the STOP Control circuit in case when motor overload current is detected bythe thermal overload relay, this is to prevent burning of the motor in case ofexcessive load beyond 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 windingto a delta connected winding. Power and control circuits can be arranged to this inone of two ways open transition or closed transition.

    What is Open or Closed Transition Starting

    1. Open Transition Starters

    Discuss mention above is called open transition switching because there is an openstate between the star state and the delta state.

    In open transit ion the power is disconnected from the motor while the winding arereconf igured via external switching.

    When a motor is driven by the supply, either at full speed or at part speed, there is arotating magnetic f ield in the stator. This f ield is rotating at line f requency. The fluxfrom the stator field induces a current in the rotor and this in turn results in a rotormagnetic f ield.

    When the motor is disconnected from the supply (open transition) there is a spinningrotor within the stator and the rotor has a magnetic f ield. Due to the low impedance

    of the rotor c ircuit, the time constant is quite long and the action of the spinning rotorf ield within the stator is that of a generator which generates voltage at a frequencydetermined by the speed of the rotor.

    When the motor is reconnected to the su l , it is reclosin onto an uns nchronized

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    generator andthis result in a very high current and torque transient.Themagnitude of the transient is dependent on the phase relationship between the

    generated voltage and the line voltage at the point of closurecan be much higherthan DOL current and torque and can result in electrical and mechanical damage.

    Open transition starting is the easiest to implement in terms or cost and circuitry and

    if the timing of the changeover is good, this method can work well. In practice thoughit is dif f icult to set the necessary timing to operate correctly anddisconnection/reconnection of the supply can cause significant 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 thedelta [KM2] contactor is open. The motor is connected in star and will produceone third of DOL torque at one third of DOL current.

    3. Open State: This type of operation is called open transition switchingbecause there is an open state between the star state and the delta state. TheMain contractor is closed and the Delta and Star contactors are open. There isvoltage on one end of the motor windings, but the other end is open so nocurrent 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 Starcontactor is open. The motor is connected to f ull line voltage and full powerand torque are available

    2. Closed Transition Star/Delta Starter

    There is a technique to reduce the magnitude of the switching transients. Thisrequires the use of a fourth contactor and a set of three resistors. The resistorsmust be sized such that considerable current is able to f low in the motor windingswhile they are in circuit.

    The auxiliary contactor and resistors are connected across the delta contactor. Inoperation, just before the star contactor opens, the auxiliary contactor closesresulting in current f low via the resistors into the star connection. Once the star

    contactor opens, current is able to flow 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

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    generated by the motor and will serve no purpose.

    In closed transition the power is maintained to the motor at all time.

    This is achieved by introducing resistors to take up the current f low during thewinding changeover. A fourth contractor is required to place the resistor in circuitbefore opening the star contactor and then removing the resistors once the deltacontactor is closed.

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

    In close transition there are four states:

    1. OFF State. All Contactors are open2. Star State. The Main [KM3] and the Star [KM1] contactors are closed and the

    delta [KM2] contactor is open. The motor is connected in star and will produceone third of DOL torque at one third of DOL current.

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

    4. Closed Transition State. The Main [KM3] contactor is closed and the Delta

    [KM2] and Star [KM1] contactors are open. Current f lows through the motorwindings and the transition resistors via KM4.

    5. Delta State. The Main and the Delta contactors are closed. The transitionresistors are shorted out. The Star contactor is open. The motor is connectedto full line voltage and full power and torque are available.

    Effect of Transient in Starter (Open Transient starter)

    It is Important the pause between star contactor switch of f and Delta contactorswitch is on correct. This is because Star contactor must be reliably disconnectedbefore Delta contactor is activated. It is also important that the switch over pauseis not too long.

    For 415v Star Connection voltage is ef fectively reduced to 58% or 240v. Theequivalent of 33% that is obtained with Direct Online (DOL) starting.

    If Star connection has suff icient torque to run up to 75% or %80 of full load speed,then the motor can be connected in Delta mode.

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    When connected to Delta conf iguration the phase voltage increases by a ratio of V3or 173%. The phase currents increase by the same ratio. The line current increasesthree times its value in star connection.

    During transit ion period of switchover the motor must be free running with litt ledeceleration. While this is happening Coasting it may generate a voltage of its own,

    and on connection to the supply this voltage can randomly add to or subtract fromthe applied line voltage. This is known as transient current. Only last ing a fewmilliseconds 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 twopositions, in the line or in the windings.

    Overload Relay in Line:

    In the line is the same as just putting the overload before the motor as with a DOLstarter.

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

    Disadvantage: If the overload is set to FLC, then it is not protecting the motor whileit is in delta (setting is x1.732 too high).

    Overload Relay in Winding:

    In the windings means that the overload is placed af ter the point where the wiring tothe contactors are split into main and delta. The overload then always measures thecurrent inside the windings.

    The setting 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

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    There are two contactors that are close during run, of ten referred to as the maincontractor and the delta contactor. These are AC3 rated at 58% of the current ratingof 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 themotor 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 Performance.

    It draws 2 times starting current of the full load ampere of the motorconnected

    Disadvantages of Star-Delta starter

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

    2. Break In Supply Possible Transients3. Six Terminal Motor Required (Delta Connected).

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

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    .

    5. It provides only 33% starting torque and if the load connected to the subjectmotor requires higher starting torque at the time of starting than very heavytransients and stresses are produced while changing from star to deltaconnections, and because of these transients and stresses many electricaland mechanical break-down occurs.

    .

    6. In this method of starting initially motor is connected in star and then afterchange over the motor is connected in delta. The delta of motor is f ormed instarter and not on motor terminals..

    7. High transmission and current peaks: When starting up pumps and fans forexample, the load torque is low at the beginning of the start and increases with

    the square of the speed. When reaching approx. 80-85 % of the motor ratedspeed the load torque is equal to the motor torque and the accelerationceases. To reach the rated speed, a switch over to delta position is necessary,and this will very of ten result in high transmission and current peaks. In somecases the current peak can reach a value that is even bigger than for a D.O.Lstart..

    8. Applications with a load torque higher than 50 % of the motor rated torque will

    not be able to start using the start-delta starter..

    9. Low Starting Torque: The star-delta (wye-delta) starting method controlswhether the lead connections f rom the motor are conf igured in a star or deltaelectrical connection. The initial connection should be in the star pattern thatresults in a reduction of the line voltage by a factor of 1/3 (57.7%) to themotor and the current is reduced to 1/3 of the current at full voltage, but the

    starting torque is also reduced 1/3 to 1/5 of the DOL starting torque..

    10. The transit ion from star to delta transition usually occurs once nominal speedis reached, but is sometimes performed as low as 50% of nominal speed whichmake transient Sparks.

    Features of star-delta starting

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

    2. Reduced starting current

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    3. Six connect ion 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 lightstarting Torque motors.

    The received starting current is about 30 % of the starting current during direct online start and the starting torque is reduced to about 25 % of the torque available at aD.O.L start. This starting method only works when the application is light loadedduring the start.

    If the motor is too heavily loaded, there will not be enough torque to accelerate themotor up to speed before switching over to the delta position.