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Star Delta Connection of 3 phase induction motor

 The star-delta starter is a common method used to reduce the starting current and torque of three-phase induction motors during startup. This method initially connects the motor windings in a star (Y) configuration and later switches to a delta (Δ) configuration after the motor reaches a certain speed. Here's an in-depth look at the star-delta connection, including its working principle, wiring, operation, advantages, and applications.


Working Principle

The star-delta starter reduces the starting current and torque by initially connecting the motor windings in a star configuration, where each winding receives a reduced voltage. Once the motor accelerates and reaches a certain speed, the connection is switched to a delta configuration, allowing the motor to run at full voltage and develop full torque.

Key Components and Circuit Diagram

  1. Power Supply:

    • Three-phase power supply lines: L1, L2, L3

  2. Motor Windings:

    • Start terminals: U1, V1, W1
    • End terminals: U2, V2, W2

  3. Contactors:

    • Main Contactor (KM1): Connects the motor to the three-phase supply.
    • Star Contactor (KM2): Connects the motor windings in star configuration.
    • Delta Contactor (KM3): Connects the motor windings in delta configuration.

  4. Timer (T):

    • Controls the switching time from star to delta configuration.


Star-Delta Connection Diagram


Star (Y) Configuration:

In the star configuration, the ends of each winding (X, Y, Z) are connected together to form a neutral point, and the line voltage (V_L) is applied across each phase winding. This reduces the voltage across each winding by a factor of √3 (V_L/√3).

Delta (Δ) Configuration:

In the delta configuration, each winding is connected end-to-end, forming a closed loop. The line voltage (V_L) is applied directly across each winding.

Wiring Diagram

Here is a simplified wiring diagram for a star-delta starter:

  1. Three-Phase Supply: L1, L2, L3
  2. Motor Windings: U1, V1, W1 (start terminals); U2, V2, W2 (end terminals)
  3. Contactor:
    • Main Contactor (KM1): Connects the motor to the three-phase supply.
    • Star Contactor (KM2): Connects the motor windings in star configuration.
    • Delta Contactor (KM3): Connects the motor windings in delta configuration.
  4. Timer (T): Controls the switching time from star to delta.

Operation Sequence

  1. Starting in Star Configuration:

    • When the motor starts, the main contactor (KM1) and the star contactor (KM2) are energized.
    • The motor windings are connected in star configuration, and each winding receives a reduced voltage (V_L/√3). (line voltage divided by √3).
    • The starting current is reduced to one-third of the direct-on-line starting current.

  2. Transition to Delta Configuration:

    • After a preset time delay (typically 5-10 seconds) controlled by the timer (T), the star contactor (KM2) is de-energized, and the delta contactor (KM3) is energized.
    • The motor windings are now connected in delta configuration, and each winding receives the full line voltage (V_L).
    • The motor runs at full voltage, developing full torque.

Advantages of Star-Delta Starter

  1. Reduced Starting Current: Limits the starting current to about one-third of the direct-on-line starting current, reducing electrical stress on the supply network.
  2. Reduced Starting Torque: Provides lower starting torque, which is suitable for applications where high starting torque is not required.
  3. Cost-Effective: Simple and economical method for reducing starting current and torque.

Disadvantages of Star-Delta Starter

  1. Reduced Starting Torque: May not be suitable for applications requiring high starting torque.
  2. Mechanical and Electrical Stresses: Transition from star to delta can cause mechanical and electrical stresses due to the sudden change in torque.
  3. Complexity in Wiring: Requires additional contactors, wiring, and a timer, increasing the complexity of the starter circuit.

Applications

  1. Pumps: Suitable for centrifugal pumps where reduced starting torque is sufficient.
  2. Fans and Blowers: Commonly used in fans and blowers to reduce the starting current and torque.
  3. Compressors: Used in compressors to minimize electrical stress during startup.
  4. Conveyors: Ideal for conveyors where smooth acceleration is required.

  5. Textile Machinery:

    • Used in textile machinery to avoid sudden jerks and ensure a smooth start.

  6. HVAC Systems:

    • Employed in HVAC systems for fans and compressors to reduce starting current and mechanical stress.

Detailed Operational Steps and Timing Sequence

  1. Initial Start (Star Connection):

    • When the start button is pressed, the main contactor (KM1) and the star contactor (KM2) are energized.
    • The motor windings are connected in star configuration, reducing the voltage across each winding to V_L/√3.
    • The motor starts with reduced current and torque.

  2. Timer Activation:

    • The timer (T) is activated simultaneously with the main and star contactors.
    • The timer is set to a specific delay time (typically 5-15 seconds) based on the motor's characteristics and the load requirements.

  3. Transition to Delta (After Timer Delay):

    • After the preset delay, the timer deactivates the star contactor (KM2) and energizes the delta contactor (KM3).
    • The motor windings are now connected in delta configuration, receiving the full line voltage (V_L).
    • The motor runs at full voltage and torque for normal operation.

Safety and Protection Measures

  1. Overload Protection:

    • Thermal overload relays are used to protect the motor from overheating due to excessive current.

  2. Short Circuit Protection:

    • Circuit breakers or fuses are installed to protect the motor and starter components from short circuits.

  3. Phase Failure Protection:

    • Relays to detect phase loss or imbalance to prevent motor damage due to unbalanced voltages.

  4. Under-Voltage Protection:

    • Prevents the motor from operating at dangerously low voltages, which can cause overheating and reduced efficiency.

Design Considerations

  1. Timer Settings:

    • The timer must be set accurately based on the motor’s startup characteristics to ensure a smooth transition from star to delta.

  2. Contactor Ratings:

    • The contactors must be rated to handle the motor’s starting and running currents to ensure reliable operation.

  3. Wiring and Connections:

    • Proper wiring practices must be followed to ensure reliable and safe operation, avoiding loose connections and ensuring correct phase sequence.

Comparison with Other Starting Methods

  1. Direct-On-Line (DOL) Starter:

    • Pros: Simple and inexpensive.
    • Cons: High starting current and torque, causing electrical and mechanical stress.

  2. Autotransformer Starter:

    • Pros: Provides higher starting torque compared to star-delta starter.
    • Cons: More expensive and complex.

  3. Soft Starter:

    • Pros: Smooth and gradual increase in voltage, reducing electrical and mechanical stress.
    • Cons: Higher cost and complexity compared to star-delta starter.

  4. Variable Frequency Drive (VFD):

    • Pros: Precise speed control, reduced starting current, and torque.
    • Cons: High initial cost and complexity.

Summary

The star-delta starter is an effective and economical method for starting three-phase induction motors with reduced starting current and torque. It is widely used in various industrial applications where high starting torque is not essential. Understanding the working principle, wiring, and operation of the star-delta starter helps in selecting the right starting method for specific applications, ensuring optimal performance and longevity of the motor.



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