Star-delta motor starter panel

Star-delta starter panel

http://electrical-engineering-portal.com/star-delta-motor-starter January 3, 2013

Star-Delta Motor Starter

Introduction to Star-Delta motor starter

Most induct ion motors are started direct ly on line, but when verylarge motors are started that way, they cause a disturbance ofvoltage on the supply lines due to large start ing current surges. Tolimit the start ing current surge, large induct ion motors are startedat reduced voltage and then have full supply voltage reconnectedwhen they run up to near rotated speed.

Two methods used for reduct ion of start ing voltage are: Stardelta start ing and Auto transformer start ing.

Working Principle of Star-Delta StarterThis is the reduced voltage start ing method. Voltage reduct ion during star-delta start ing isachieved by physically reconf iguring the motor windings as illustrated in the f igure below. Duringstart ing the motor windings are connected in star conf igurat ion and this reduces the voltage

Scheme - Working Principle of Star-Delta Starter

across each winding 3. This also reduces the torque by a factor of three.

After a period of t ime the winding arereconf igured as delta and the motor runsnormally. Star/Delta starters are probably themost common reduced voltage starters.They are used in an at tempt to reduce thestart current applied to the motor duringstart as a means of reducing thedisturbances and interference on theelectrical supply.

Tradit ionally in many supply regions, therehas been a requirement to f it a reducedvoltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta) starter isone of the lowest cost electromechanical reduced voltage starters that can be applied.

The Star/Delta starter is manufactured from three contactors, a t imer and a thermal overload.The contactors are smaller than the single contactor used in a Direct on Line starter as they arecontrolling winding currents only. The currents through the winding are 1/root 3 (58%) of thecurrent in the line.

There are two contactors that are close during run, of ten referred to as the main contractorand the delta contactor. These are AC3 rated at 58% of the current rat ing of the motor. Thethird contactor is the star contactor and that only carries star current while the motor isconnected in star.

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

Star-delta Starter Consists following units

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

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

3. Three-pole thermal overcurrent release 1 No.

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

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

Power Circuit of Star Delta StarterThe main circuit breaker serves as the main power supply switch that supplies electricity to thepower circuit .

The main contactor connects the reference source voltage R, Y, B to the primary terminal ofthe motor U1, V1, W1.

In operat ion, the Main Contactor (KM3) and the Star Contactor (KM1) are closed init ially, and

Power circuit of Star-Delta starter

then af ter a period of t ime, the star contactor is opened, and then the delta contactor (KM2) isclosed. The control of the contactors is by the t imer (K1T) built into the starter. The Star andDelta are electrically interlocked and preferably mechanically interlocked as well.

In effect, there arefour states:

The star contactorserves to init iallyshort the secondaryterminal of the motorU2, V2, W2 for thestart sequenceduring the init ial runof the motor f romstandst ill. Thisprovides one third ofDOL current to themotor, thus reducingthe high inrushcurrent inherent withlarge capacitymotors at startup.

Controlling theinterchanging starconnect ion and deltaconnect ion of an ACinduction motor isachieved by meansof a star delta or wyedelta control circuit .The control circuitconsists of pushbutton switches,auxiliary contactsand a t imer.

Control Circuitof Star-DeltaStarter (OpenTransition)

The ON push button

Scheme - Control Circuit of Star-Delta Starter (Open Transit ion)

The ON push buttonstarts the circuit byinit ially energizingStar Contactor Coil(KM1) of star circuitand Timer Coil (KT)circuit . When StarContactor Coil (KM1)energized, Star Mainand Auxiliarycontactor change itsposit ion f rom NO toNC.

When Star AuxiliaryContactor (1) (whichis placed on MainContactor coil circuit) become NO to NCit ’s complete TheCircuit of Maincontactor Coil (KM3)so Main ContactorCoil energized andMain Contactor’s Main and AuxiliaryContactor Changeits Posit ion f rom NOto NC. Thissequence happens ina f rict ion of t ime.

After pushing the ONpush button switch,the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ONpush button will become NO to NC, thereby providing a latch to hold the main contactor coilact ivated which eventually maintains the control circuit act ive even af ter releasing the ON pushbutton switch.

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

Once the t ime delay is reached its specif ied Time, the t imer’s auxiliary contacts (KT)(3) in StarCoil circuit will change 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 connect ion change from star todelta connect ion.

A normally close auxiliary contact f rom both star and delta contactors (5&6)are also placedopposite of both star and delta contactor coils, these interlock contacts serves as safetyswitches to prevent simultaneous act ivat ion of both star and delta contactor coils, so that onecannot be act ivated without the other deact ivated f irst . Thus, the delta contactor coil cannotbe act ive when the star contactor coil is act ive, and similarly, the star contactor coil cannot alsobe act ive while the delta contactor coil is act ive.

The control circuit above also provides two interrupt ing contacts to shutdown the motor. TheOFF push button switch break the control circuit and the motor when necessary. The thermal

overload contact is a protect ive device which automat ically opens the STOP Control circuit incase when motor overload current is detected by the thermal overload relay, this is to preventburning of the motor in case of excessive load beyond the rated capacity of the motor isdetected by the thermal overload relay.

At some point during start ing it is necessary to change from a star connected winding to adelta connected winding. Power and control circuits can be arranged to this in one of two ways– open transit ion or closed transit ion.

What is Open or Closed Transition Starting

1. Open Transition Starters

Discuss ment ion above is called open transit ion switching because there is an open statebetween the star state and the delta state.

In open transit ion the power is disconnected from the motor while the winding are reconf iguredvia external switching.

When a motor is driven by the supply, either at full speed or at part speed, there is a rotat ingmagnet ic f ield in the stator. This f ield is rotat ing at line f requency. The f lux f rom the stator f ieldinduces a current in the rotor and this in turn results in a rotor magnet ic f ield.

When the motor is disconnected from the supply (open transit ion) there is a spinning rotorwithin the stator and the rotor has a magnet ic f ield. Due to the low impedance of the rotorcircuit , the t ime constant is quite long and the act ion of the spinning rotor f ield within the statoris that of a generator which generates voltage at a f requency determined by the speed of therotor.

When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generatorand this result in a very high current and torque transient. The magnitude of the transientis dependent on the phase relationship between the generated voltage and the linevoltage at the point of closure can be much higher than DOL current and torque and can resultin electrical and mechanical damage.

Open transit ion start ing is the easiest to implement in terms or cost and circuit ry and if thet iming of the changeover is good, this method can work well. In pract ice though it is dif f icult toset the necessary t iming to operate correct ly and disconnect ion/reconnect ion of the supplycan cause signif icant voltage/current t ransients.

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 is open. The motor is connected in star and will produce one third of DOLtorque at one third of DOL current.

3. Open State: This type of operat ion is called open transit ion switching because there isan open state between the star state and the delta state. The Main contractor is closedand the Delta and Star contactors are open. There is voltage on one end of the motorwindings, but the other end is open so no current can f low. The motor has a spinningrotor and behaves like a generator.

4. Delta State: The Main and the Delta contactors are closed. The Star contactor is open.

The motor is connected to full line voltage and full power and torque are available

2. Closed Transition Star/Delta Starter

There is a technique to reduce the magnitude of the switching transients. This requires the useof a fourth contactor and a set of three resistors. The resistors must be sized such thatconsiderable current is able to f low in the motor windings while they are in circuit .

The auxiliary contactor and resistors are connected across the delta contactor. In operat ion,just before the star contactor opens, the auxiliary contactor closes result ing in current f low viathe resistors into the star connect ion. Once the star contactor opens, current is able to f lowround through the motor windings to the supply via the resistors. These resistors are thenshorted by the delta contactor.

If the resistance of the resistors is too high, they will not swamp the voltage generated by themotor and will serve 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 windingchangeover. A fourth contractor is required to place the resistor in circuit before opening thestar contactor and then removing the resistors once the delta contactor is closed.

These resistors need to be sized to carry the motor current. In addit ion to requiring moreswitching devices, the control circuit is more complicated due to the need to carry out resistorswitching

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 is open. The motor is connected in star and will produce one third of DOLtorque at one third of DOL current.

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

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

5. Delta State. The Main and the Delta contactors are closed. The transit ion resistors areshorted out. The Star contactor is open. The motor is connected to full line voltage andfull 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 contactor switch is oncorrect . This is because Star contactor must be reliably disconnected before Delta contactor isact ivated. It is also important that the switch over pause is not too long.

For 415v Star Connect ion voltage is ef fect ively reduced to 58% or 240v. The equivalent of 33%that is obtained with Direct Online (DOL) start ing.

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

When connected to Delta conf igurat ion the phase voltage increases by a rat io of V3 or 173%.The phase currents increase by the same rat io. The line current increases three t imes its valuein star connect ion.

During transit ion period of switchover the motor must be free running with lit t le decelerat ion.While this is happening “Coast ing” it may generate a voltage of its own, and on connect ion tothe supply this voltage can randomly add to or subtract f rom the applied line voltage. This isknown as transient current. Only last ing a few milliseconds it causes voltage surges and spikes.Known as a changeover t ransient.

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 protect ion in two posit ions, inthe line or in the windings.

Overload Relay in Line:

In the line is the same as just putt ing the overload before the motor as with a DOL starter.

The rat ing of Overload (In Line) = FLC of Motor.

Disadvantage: If the overload is set to FLC, then it is not protect ing the motor while it is indelta (set t ing 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 to thecontactors are split into main and delta. The overload then always measures the current insidethe windings.

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

Disadvantage: We must use separate short circuit and overload protect ions.

2. Size of Main and Delta Contractor

There are two contactors that are close during run, of ten referred to as the main contractorand the delta contactor. These are AC3 rated at 58% of the current rat ing 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 contactorcan be AC3 rated at one third (33%) of the motor rat ing.

Size of Star Contactor= IFL x 0.33

Motor Starting Characteristics of Star-Delta StarterAvailable start ing current: 33% Full Load Current.

Peak start ing current: 1.3 to 2.6 Full Load Current.

Peak start ing torque: 33% Full Load Torque.

Advantages of Star-Delta starterThe operat ion of the star-delta method is simple and rugged

It is relat ively cheap compared to other reduced voltage methods.

Good Torque/Current Performance.

It draws 2 t imes start ing current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter1. Low Start ing 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% start ing torque and if the load connected to the subject motorrequires higher start ing torque at the t ime of start ing than very heavy transients andstresses are produced while changing from star to delta connect ions, and because ofthese transients and stresses many electrical and mechanical break-down occurs..

6. In this method of start ing init ially motor is connected in star and then af ter change overthe motor is connected in delta. The delta of motor is formed in starter and not on motorterminals..

7. High transmission and current peaks: When start ing up pumps and fans for example,the load torque is low at the beginning of the start and increases with the square of thespeed. When reaching approx. 80-85 % of the motor rated speed the load torque is equalto the motor torque and the accelerat ion ceases. To reach the rated speed, a switch overto delta posit ion is necessary, and this will very of ten result in high transmission andcurrent peaks. In some cases the current peak can reach a value that is even bigger thanfor a D.O.L start ..

8. Applicat ions with a load torque higher than 50 % of the motor rated torque will not beable to start using the start-delta starter..

9. Low Start ing Torque: The star-delta (wye-delta) start ing method controls whether thelead connect ions from the motor are conf igured in a star or delta electrical connect ion.The init ial connect ion should be in the star pattern that results in a reduct ion of the linevoltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of thecurrent at full voltage, but the start ing torque is also reduced 1/3 to 1/5 of the DOLstart ing torque..

10. The transit ion f rom star to delta t ransit ion usually occurs once nominal speed is reached,but is somet imes performed as low as 50% of nominal speed which make transientSparks.

Features of star-delta starting1. For low- to high-power three-phase motors.

2. Reduced start ing current

3. Six connect ion cables

4. Reduced start ing 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 StarterThe star-delta method is usually only applied to low to medium voltage and light start ingTorque motors.

The received start ing current is about 30 % of the start ing current during direct on line start andthe start ing torque is reduced to about 25 % of the torque available at a D.O.L start . Thisstart ing method only works when the applicat ion is light loaded during the start .

If the motor is too heavily loaded, there will not be enough torque to accelerate the motor upto speed before switching over to the delta posit ion.