Working Principle of Star-Delta Starter

 Most induction motors start directly on the mains, but when very large motors are started this way, they can cause voltage disturbances on the mains due to large starting current surges. To limit inrush current, large induction motors are started at a reduced voltage and then reconnected to full supply voltage as they approach speed. There are two methods to reduce the starting voltage: star-delta starting and autotransformer starting.

The working principle of the star-delta starter:

  • This is the buck start method. The voltage reduction during star-delta startup is achieved by physically reconfiguring the motor windings as shown in the figure below. During startup, the motor windings are connected in a star configuration, which reduces the voltage on each winding 3. This also reduces torque by a factor of three. After some time, the windings will reconfigure to delta and the motor will operate normally. 

 The star-delta starter is probably the most common type of step-down starter. They are used to reduce the starting current applied to the motor during starting to reduce noise and disturbances in the power supply.

The star-delta starter consists of three contactors, a timer and thermal overload protector. Contactors are smaller than the single contactors used in direct in-line starters because they only control the winding current. The current through the winding is 1/root 3  of the current in the line.

Star-delta startup circuit:

  • The main circuit breaker acts as the main power switch to supply power to the circuit.
  • The main contactor connects the reference source voltages R, Y, B to the motor primary terminals U1, V1, W1.
  • During operation, the main contactor (KM3) and star contactor (KM1) are initially closed, after a while the star contactor opens and then the delta contactor (KM2) closes. The contactor is controlled by a timer (K1T) built into the starter. Star and Delta are electrically interlocked and preferably mechanically. There are actually four states
  •     The star contactor is used to initially short the motor secondary terminals U2, V2, W2 when the motor is started from standstill to achieve the start sequence. This provides one-third of the DOL current to the motor, reducing the high inrush current inherent in bulk motors at startup.
  • Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of a star delta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and a timer.

Control Circuit of Star- Delta Starter

  • The ON drive button starts the circuit by originally amping Star Contactor Coil( KM1) of star circuit and Timer Coil( KT) circuit.
  • When Star Contactor Coil( KM1) amped , Star Main and Auxiliary contactor change its position from NO to NC.
  • When Star Auxiliary Contactor( 1)( which is placed on Main Contactor coil circuit) came NO to NC it’s complete The Circuit of Main contactor Coil( KM3) so Main Contactor Coil amped and Main Contactor’s Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a disunion of time.
  • After pushing the ON drive button switch, the supplementary contact of the main contactor coil( 2) which is connected in parallel across the ON drive button will come NO to NC, thereby furnishing a latch to hold the main contactor coil actuated which ultimately maintains the control circuit active indeed after releasing the ON drive button switch.
  • When Star Main Contactor( KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT( 3) come NC to NO.
  • Once the time detention is reached its specified Time, the timekeeper’s supplementary connections( KT)( 3) in Star Coil circuit will change its position from NC to NO and at the Same Time supplementary contactor( KT) in Delta Coil Circuit( 4) change its Position from NO To NC so Delta coil amped and Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.
  • A typically close supplementary contact from both star and delta contactors( 5 & 6) are also placed opposite of both star and delta contactor coils, these interlock connections serves as safety switches to help contemporaneous activation of both star and delta contactor coils, so that one can not be actuated without the other killed first. therefore, the delta contactor coil can not be active when the star contactor coil is active, and also, the star contactor coil can not also be active while the delta contactor coil is active.
  • The control circuit above also provides two interposing connections to arrestment the motor. The OFF drive button switch break the control circuit and the motor when necessary. The thermal load contact is a defensive device which automatically opens the STOP Control circuit in case when motor load current is detected by the thermal load relay, this is to help burning of the motor in case of inordinate cargo beyond the rated capacity of the motor is detected by the thermal load relay.
  • At some point during starting it's 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. 

Post a Comment

Previous Post Next Post