What is Star Delta Starter? with Types & Working Principle

Star Delta Starter

In the ever-evolving world of electrical engineering, certain components stand the test of time due to their efficiency and reliability. One such component is the Star Delta Starter. This article, brought to you by Linquip, delves deep into its intricacies, explaining its components, working, applications, and more. Whether you’re a seasoned engineer or just curious about the subject, let Linquip guide you through this comprehensive breakdown.


  • What is it? An electrical device that reduces the starting current of large motors by initially connecting them in a star configuration, then shifting to delta as the motor speeds up.
  • Key Components: Main contactors, star contactors, delta contactors, timer, and overload protection relays.
  • Advantages: Reduced starting current, smooth motor start, cost-effective, and built-in overload protection.
  • Applications: Used in pumps, compressors, conveyor systems, large fans, and crushers, among others.
  • Working Principle: Starts the motor in a star configuration (reducing voltage and current) and shifts to delta configuration (full power) as the motor reaches about 75-80% of its full speed.
  • Types: Manual (requires manual switching) and Automatic (uses timers for switching).
  • Safety & Troubleshooting: Regular maintenance, proper sizing, and keeping in a clean environment are key. Common issues include failure to start in star mode, no transition to delta, and overheating.
  • Comparisons: Different from Direct Online (DOL) Starters and Soft Starters in terms of operation, complexity, starting torque, and cost.
  • Challenges: Limited starting torque in star mode and torque drop during star-to-delta transition.
  • Real-world Applications: Found in water treatment plants, manufacturing industries, agriculture, and mining operations.
  • Relevance: Continues to be a vital component in electrical engineering due to its blend of simplicity, cost-effectiveness, and efficiency.

For in-depth knowledge and further insights on electrical equipment, turn to Linquip.


What is a Star Delta Starter?

A Star Delta Starter is an electrical device that reduces the starting current delivered to the windings of induction motors. It primarily helps in the smooth starting of a large motor by initially connecting them in a star configuration and then changing over to a delta run once the motor reaches a specified speed.


Components of a Star Delta Starter

The major components include:

  • Main Contactors (3 Nos): These control the main motor winding.
  • Star Contactors (3 Nos): They manage the motor’s star connection.
  • Delta Contactors (3 Nos): These handle the delta connection of the motor.
  • Timer: To ensure that the star to delta changeover happens after a predefined time.
  • Overload Protection Relays: They protect motors from potential overload.


Why Use a Star Delta Starter?


  • Reduced Starting Current: By starting the motor in a star connection, the starting current is reduced to one-third of the direct starting current.
  • Smooth Start: Helps in starting large motors smoothly, preventing mechanical shocks to the system.
  • Cost-Effective: Star Delta Starters are relatively cheaper compared to other starters.
  • Overload Protection: Built-in overload protection mechanisms ensure the longevity of the motor.


  • Pumps
  • Compressors
  • Conveyor systems
  • Large fans & blowers
  • Crushers


The Working Principle of Star Delta Starter

Initially, when the motor starts, the starter keeps the motor in a star configuration. This means that the voltage across each phase is reduced, leading to a reduced starting current. Once the motor reaches about 75-80% of its full load speed, the starter transitions the motor to a delta configuration. In this state, the full line voltage is applied across each motor phase, allowing the motor to run with full power.


The Difference between Star and Delta Connection

Parameter Star Connection Delta Connection
Voltage Reduced (V/√3) Full Line Voltage
Current Reduced to 1/3 Full Current
Power Reduced during the start Full Power during operation


Significance of the Star Delta Starter in Industrial Applications

Star Delta Starters are a mainstay in numerous industrial settings. Their ability to effectively minimize the inrush current during the starting of large motors ensures that the power grid or the source isn’t excessively burdened. This is particularly crucial in industries where sudden surges in power can lead to disruptions or damages to sensitive equipment.


Types of Star Delta Starters

There are primarily two types of Star Delta Starters:

  • Manual Star Delta Starter: This requires manual intervention to switch the motor from the star connection to the delta connection.
  • Automatic Star Delta Starter: Integrated with timers, these starters automatically shift the motor from star to delta after a predetermined time.


Safety Precautions with Star Delta Starters

  • Regular Maintenance: Like any electrical component, regular maintenance ensures the longevity and safety of Star Delta Starters.
  • Proper Sizing: Ensure that the Star Delta Starter is appropriately sized for the motor it is controlling. An undersized or oversized starter can lead to inefficiencies or potential hazards.
  • Environment: Keeping the starter in a clean, dust-free, and moisture-free environment will reduce risks of short circuits or failures.


Troubleshooting Common Star Delta Starter Issues

  • Motor Fails to Start in Star Mode: This could be due to a malfunctioning timer or defective star contactor. Check both components for faults.
  • Motor Doesn’t Transition to Delta: Often, this is because of a faulty delta contactor or timer setting issues.
  • Overheating of the Starter: This might occur due to prolonged usage, short circuits, or using an incorrectly sized starter for the motor.


Star Delta Starter vs. Direct Online Starter

While both are popular starters, they serve different purposes:

Parameter Star Delta Starter Direct Online (DOL) Starter
Starting Current Reduced to about 1/3 of DOL Full inrush current
Complexity More complex due to star to delta transition Simple with direct connection
Cost Slightly more expensive Cheaper
Applications Large motors where smooth start is crucial Smaller motors, uncomplicated starts


Challenges with Star Delta Starters

While the Star Delta Starter is incredibly efficient, it’s not without its challenges:

  • Limited Starting Torque: In the star configuration, the starting torque is only one-third of the torque in delta. This could be insufficient for some heavy-load applications.
  • Transition Phase: The momentary period of disconnection during the transition from star to delta can cause a drop in torque, which might not be suitable for some applications.


Star Delta Starter vs. Soft Starter

Another common motor starter is the Soft Starter. Here’s a brief comparison:

Parameter Star Delta Starter Soft Starter
Operation Mechanical switching between star and delta Reduces voltage gradually
Complexity Relatively simple Requires electronic components
Starting Torque Reduced in star mode Can be adjusted as per requirements
Cost Moderately priced Generally more expensive
Wear and Tear More due to mechanical switching Less, due to electronic operation
Applications Heavy-duty motors where transition is acceptable Where precise control of starting torque is needed


Real-world Applications of Star Delta Starters

  1. Water Treatment Plants: The large pumps used in these facilities often employ Star Delta Starters to ensure a smooth start without causing a surge in the power system.
  2. Manufacturing Industries: Large machines, especially in the metal and cement industries, utilize these starters to prevent mechanical shocks to the machinery.
  3. Agriculture: For high horsepower irrigation pumps, Star Delta Starters can be an ideal solution.
  4. Mining Operations: Heavy machinery and conveyors in mining operations often benefit from the reduced starting current and smooth start provided by these starters.


Tips for Effective Use of Star Delta Starters

  • Regular Inspection: Always ensure that the contactors and relays are free from damage or wear.
  • Stay Updated: As technologies evolve, newer versions or models might offer enhanced features. Stay connected with trusted platforms like Linquip to be informed.
  • Training: Ensure that personnel handling the starters are adequately trained, understanding the transition from star to delta and its implications.



Star Delta Starters have found their niche in the world of electrical engineering and continue to remain relevant despite the advent of newer technologies. Their unique blend of simplicity, cost-effectiveness, and efficiency makes them a favorite in various industries. As technologies advance, perhaps we’ll witness even more refined versions of this starter, but its foundational principles will likely remain the same.

For professionals seeking deeper insights, expert consultations, or looking to explore more about motor starters and other electrical equipment, Linquip is always at your service. We’re committed to bridging knowledge gaps and presenting the latest in the world of engineering and beyond.


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