Saturday 30 January 2016

STAR DELTA STARTER


Working Principle of Star-Delta Starter

This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three.
Scheme - Working Principle of Star-Delta Starter
Scheme – Working Principle of Star-Delta Starter

After a period of time the winding are reconfigured as delta and the motor runs normally. Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.
There are two contactors that are close during run, often referred to as the main contractor 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 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%) of the motor rating.

Power Circuit of Star Delta Starter


Control Circuit of Star-Delta Starter


WHEN PUSH BUTTON FOR START IS PRESS POWER FLOW START FROM TIMER COIL KT ,STAR COIL KM1,SO STAR CONTECTOR &AUX CONTACTOR IN CHANGE FROM NO TO NC .

NOW  TIMER RUNNING AND CURRENT FLOW IN MAIN COIL KM3 FROM STAR  AUX CONTACT.SO MAIN AUX CONTACT COME INTO NC FORM .SO IF WE RELEASE ON PUSH BUTTON POWER FLOW FROM KM3 LATCH CIRCUIT.

AFTER SOME TIME STAR CONTACTOR DEENERGIZE DUE OPEN OFF TIMER CONTACTOR AND DELTA CONTECTOR ENRGIZE SO POWER FLOW THROUGH KM2 & KM3 ONLY.

 IF WE PUSH STOP THAN POWER FLOW TO MOTOR STOP .

The rating of Overload (In Line) = FLC of Motor.
The setting of Overload Relay (In Winding) =0.58 X FLC (line current).
Size of Main Contactor= IFL x 0.58

Size of Star Contactor= IFL x 0.33
  • 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 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.

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