star delta starter

hello guys ....
lets talk about star-delta starter. The star-delta is very common in any industry. it is very basic and important.  most of interviewer asks about star-delta starter control wiring diagrams in interview. 
so lets see how it works and understand it.

         

WHAT IS STAR-DELTA STARTER?

→This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically re configuring the motor wingdings.To do so we need star delta starter.



Control wiring of star-delta starter:-

          
                       (This is star-delta starter contol wiring diagram)

Let me tell you easy steps to make this diagram:-

Parts we need in star-delta starter
→3 Contactors  
→1 Timer
→NO,NC push-buttons
→Auxillary contacts NO&NC

Procedure to make wiring:-
(1) Take neutral wire from 1phase connection to directly in all the three contactors terminal A2 through looping and also to the timer's A2 .(The role of neutral wire is now no more.)
(2) Take the phase wire to the NC push-button or we can also go through thermal over-load relay's NC contact. and back to NC push-button.
(3) output of NC push-button link it to NO push-buttons one terminal.
(4) the output from second terminal  link it to A1 contact of mains contactor.
(5) Now for the holding we need to connect it through Auxillary NO contact. so take link from A1 of main contactor to the auxillary NO as shown in figure. and output of that NO the wire goes to NO push-button as shown.
(6) Take one link to A1 of timer to A1 of main contactor or to the auxillary NO. get the phase from anywhere.
(7) Now we have to start mains contactor and star contactor first. so from the NC terminal of timer take the link to the delta contactors auxiliary NC contact and pass it to the A1 of star contactor, as shown in figure. 

NOTE- (We are passing the NC of timer throgh delta contactirs NC contactor and connecting it to stars A1 just for interlocking so that both star and delta can not operate same time. it may hazardous or blast so always do interlocking)

(8) same as take a link from timers NO to the NC of star contactor and connect it to the A1 of delta contactor. as shown in figure.

This is how star-delta control wiring has to be done.

procedure of power wiring:-

                       (This is star-delta starter power wiring diagram)
procedure for power wiring:-
(1) supply three phases to the main contactors input terminal.
(2) link these three phases to the delta contactors as shown in figure.
(3) from the motor side take u1,v1,w1 to the main contactors out-going and connect it.
(4) from the motor side take w2,v2,u2 to the delta contactos outgoing as shown in figure.
(5) short all three terminal of star contactors incoming terminal as shown in figure.
(6) the out-going of star contactor connect it the delta contactors out-going as shown in figure. make sure winding get two phases. or just connect the R-B Y-Y B-R

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 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 from starter to motor.
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5. It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.
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6. In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. The delta of motor is formed in starter and not on motor terminals.
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7. High transmission and current peaks: When starting up pumps and fans for example, 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 rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks. In some cases the current peak can reach a value that is even bigger than for a D.O.L start.
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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.
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9. Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor 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.
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10. The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed 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 from star to delta
6. Mechanical load on changeover from star to delta

Application of Star-Delta Starter

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

The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start. This starting method only works when the 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 before switching over to the delta position.

                                                                                                              (ELECTRICAL ENGINEER)