Star Delta Starter Controlling Diagram | Working of Star Delta Starter

Introduction of Induction Motor

• The most basic feature of an Induction motor is its self starting mechanism. Due to the rotating magnetic field, an emf is induced in the rotor, because of which current starts flowing in the rotor. As per the Lenz law, the rotor will start rotating in a direction so as to oppose the flow of electric current and this gives a torque to the motor. Thus the motor gets self started.

Connection of A 3 Phase Induction Motor Consists of

• A 3 Phase Induction motor consists of a stator.
• It contains 3 phase winding connected to the 3 phase AC supply.
• The arrangement of the winding is to produce a rotating magnetic field.
• The rotor of the Induction motor contains cylindrical core with parallel slots that contain conductors.

What Problems Can Encounter During Motor Starting

• During the motor self starting period, torque increases and a large amount of current flows in the rotor.
• To achieve this the stator draws a large amount of current and by the time the motor reaches its full speed
• A large amount of current is drawn and coils get heated up, damaging the motor.
• Hence there is a need to control the motor starting.
• One way is to reduce the applied voltage, which in turn reduces the torque.

The Objectives of Star-Delta Technique Motor Starter are

• Reduce high starting current and along these lines forestall motor from overheating.
• Provide over-burden and no-voltage assurance.

Star Delta Starter

• In star delta starting, the motor is connected in STAR mode throughout the starting period.
• When the motor reached the required speed, the motor is connected in DELTA mode.

Main Components of a Star-Delta Starter

• Contactors : The Star- Delta starter circuit comprises of three contactors: Main, star and delta contactors.
• Timer : The contactors are regulated by a timer incorporated with the started.
• Interlock Switches : Interlock switches are connected between star and delta contactors of the control circuit as a safety measure so one can't activate delta contactor without deactivating star contactor.
• Thermal Overload Relay : A thermal over-load relay is likewise consolidated into star-delta control circuit to ensure the motor from intemperate heat which might expedite motor finding fire or wearing out.

Working of Star-Delta Starter

• At first the primary contactor and the star contactors are shut.
• After a time interval the timer signs to the star contactor to head off to the open position.
• The primary, delta contactors to head off to the shut position, accordingly structuring delta circuit.
• At the time of starting, every stator stage gets voltage VL/v3, where VL is the line voltage.
• The line current drawn by the motor at starting is decreased to one-third as contrasted with starting current with the windings associated in delta.
• Since the torque advanced by an induction motor is corresponding to the square of the applied voltage.
• Star- delta starter decreases the starting torque to one- third of that possible by immediate delta starting.
• The timer controls conversion from star connection to delta connection.
• A timer in star delta starter for a 3-phase motor is intended to do the move from star mode.
• Utilizing which the motor runs on a decreased voltage and current and produces less torque.
• To the delta mode indispensable for running the motor at its full power.
• Utilizing high voltage and current to transform a high torque.

Terminal Connections in Star and Delta Configurations

      Star Configuration

• L1, L2 and L3 are the 3-phase line voltages, which are given to primary contactor.
• The main motor coils are U, V and W.
• In star mode of motor windings, the primary contactor associate the mains to essential winding terminals U1, V1 and W1.
• The star contactor shorts the auxiliary winding terminals U2, V2 and W2
• Notwithstanding when the primary contactor is shut, supply arrives at terminals A1, B1, C1.
• Consequently the motor windings are energized in star-mode.
• The timer is initiated in the meantime moment when star contactor is energized.
• After the timer achieves the specified time period.
• The star contactor is de-energized and delta contactor is energized.

The point when delta contactor closes.

• The motor winding terminals U2, V2 and W2 get associated with V1, W1 and U1 individually through the shut contacts of primary contactor.
• For delta association, fulfilling end of one winding is to be joined with beginning end of the other winding.
• The motor windings are reconfigured in delta by supplying line voltage L1 to winding terminals W2 and Ul1, line voltage L2 to winding terminals U2 and V1.
• The line voltage L3 to winding terminals V2 and W1.

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