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star delta overview

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OPERATION PURPOSE OF STAR DELTA IN INDUSTRY

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Posted By Samuel Kehinde Adegun { follow user } 1 year ago 6/10/2019 1:43:52 AM in Electrical { follow category }

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STAR DELTA OVERVIEW

 

It can be used only for those motors with stator winding designed for delta connection during normal operation. In starting, the stator windings are connected in star, so that the voltage across each phase is 1/sqrt(3) times the normal value. As the motor picks up speed, the changeover switch disconnects the windings terminals and then reconnects them in delta across the supply terminals. This method reduces the current drawn by the motor to one-third. However, the starting torque too is reduced to one-third. This method is cheap but limited to applications where high starting torque is not necessary.

                                    WHY STAR DELTA IS NEEDED

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.

                                         APPLICATION

  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

 

More to be known about Contactor selection and overload selection and probably power circuit and control circuit. To be continued

CONTROL OF STAR DELTA

 

  1. The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (K1T) circuit.
  2. When Star Contactor Coil (KM1) energized, Star, Main and Auxiliary contactor change its position from NO to NC.
  3. When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.
  4. After pushing the ON push button switch, the auxiliary contact of the main contactor coil (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 activated which eventually maintains the control circuit active even after releasing the ON push button switch.
  5. When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact K1T (3) become NC to NO.
  6. Once the time delay is reached its specified Time, the timer’s auxiliary contacts (K1T)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.
  7. A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. 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.
  8. The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective 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 load beyond the rated capacity of the motor is detected by the thermal overload relay.
  9. At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.

 

 

CONTROL OF STAR DELTA

 

 

  1. The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (K1T) circuit.
  2. When Star Contactor Coil (KM1) energized, Star, Main and Auxiliary contactor change its position from NO to NC.
  3. When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.
  4. After pushing the ON push button switch, the auxiliary contact of the main contactor coil (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 activated which eventually maintains the control circuit active even after releasing the ON push button switch.
  5. When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact K1T (3) become NC to NO.
  6. Once the time delay is reached its specified Time, the timer’s auxiliary contacts (K1T)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.
  7. A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. 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.
  8. The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective 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 load beyond the rated capacity of the motor is detected by the thermal overload relay.
  9. At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.

CONTACTOR AND OVERLOAD RELAY SELECTION

1)   Size of Over Load Relay:

For a star-delta starter there is a possibility to place the overload protection in two positions, 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 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 is x1.732 too high).
  • Overload Relay in Winding:
  • In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then always measures the current 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:

  • 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.
  • 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.

 

 

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:

  • Low Starting Torque (Torque = (Square of Voltage) is also reduce).
  • Break In Supply – Possible Transients
  • Six Terminal Motor Required (Delta Connected).
  • It requires 2 set of cables from starter to motor.
  • 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.
  • 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.
  • 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.
  • Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
  • 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 .
  • 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.

 

Comments

Samuel Kehinde Adegun 1 year ago 6/10/2019 1:46:12 AM

Selection of Contractor and Overload to be continued.

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