Question

Why star delta starter is preferred with induction motor?

Answer 1

The star-delta starter is commonly used with induction motors to reduce the initial inrush current and make the motor start smoothly. Here's why it's preferred:

1. Reduced Starting Current:
   

   When an induction motor starts directly (across-the-line), the inrush current can be up to 6-8 times the full-load current. This high current can cause voltage dips in the power supply and can damage the motor windings. In the star-delta starter, the motor initially starts in the star connection, which reduces the voltage across the windings. Since the voltage is reduced by about 1/√3 (approximately 58%) in the star connection, the starting current also reduces significantly to about 1/3 of the direct-on-line starting current.

1. Reduced Starting Torque:
   

   The torque is proportional to the square of the voltage. By starting the motor in a star configuration, the torque is reduced to about 1/3 of the full starting torque, which makes the motor start more gradually and prevents mechanical stress on the motor and connected equipment.

1. Energy Efficiency:
   

   By reducing the starting current, the star-delta starter helps prevent overheating of the motor during startup. This makes the motor more energy-efficient and reduces the likelihood of damaging components.

1. Cost-Effective:
   

   Star-delta starters are simpler and cheaper compared to other soft starting methods, like autotransformer starters or VFDs (Variable Frequency Drives), while still offering significant reduction in inrush current and torque.

How it Works:

1. Star Connection (Initial Start): In the first stage, the motor is connected in a star (Y) configuration, which reduces the voltage and current.
   

1. Delta Connection (After Motor Picks Up Speed): After the motor has reached a certain speed (typically 70-80% of its full speed), the starter switches the motor to the delta connection, which applies the full voltage to the motor, and it runs at full torque and speed.
   

This combination of reduced inrush current and smooth transition to full-speed operation makes the star-delta starter a widely used and reliable method for starting induction motors.