Direct-On-Line (DOL) starter, Star-Delta starter, and Autotransformer starter are three common methods used for starting three-phase induction motors. Each method has its own characteristics and advantages. Here are the main differences between them:

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

   • DOL starters are the simplest and most common method for starting induction motors.
   • In a DOL starter, the motor is directly connected to the full supply voltage at the start, which results in a high inrush current.
   • DOL starters are suitable for small- to medium-sized motors where the starting current does not cause excessive voltage drop or mechanical stress.
   • They are cost-effective and easy to install but may not be suitable for larger motors due to the high starting current.

2. Star-Delta Starter:

   • Star-Delta starters are used for medium- to large-sized induction motors to reduce starting current and torque.
   • In a Star-Delta starter, the motor is initially connected in a star configuration, which reduces the voltage applied to each winding and thus reduces the starting current and torque.
   • After a predetermined time (typically a few seconds), the motor is then switched to a delta configuration for normal operation at full voltage and rated current.
   • Star-Delta starters are more complex and require additional control circuitry, but they offer smoother starting and reduced mechanical stress on the motor and connected equipment.

3. Autotransformer Starter:

   • Autotransformer starters are used for medium- to large-sized induction motors to reduce starting current and voltage.
   • In an Autotransformer starter, the motor is initially connected to a tapped autotransformer, which reduces the applied voltage during starting.
   • The motor starts at a reduced voltage and current, reducing the starting torque and stress on the motor and connected equipment.
   • Autotransformer starters are more efficient than Star-Delta starters in terms of reduced voltage and current during starting, but they are more complex and expensive.

In summary, the main differences between DOL, Star-Delta, and Autotransformer starters lie in their starting methods, complexity, cost, and suitability for different motor sizes. DOL starters offer simplicity and low cost but result in high starting current. Star-Delta starters reduce starting current and torque but require additional control circuitry. Autotransformer starters offer reduced voltage and current during starting, but they are more complex and expensive. The choice of starter depends on factors such as motor size, starting requirements, cost considerations, and application-specific requirements.