autotransformer starter

Autotransformer Starters: A Smooth Start for Induction Motors

Starting large induction motors directly on full line voltage can lead to high starting currents, excessive voltage drops, and potential damage to the motor itself. Autotransformer starters provide a solution by offering a controlled, reduced-voltage start for these motors, reducing stress on the system and extending motor life.

How it Works:

An autotransformer starter utilizes a single three-phase autotransformer or three single-phase transformers to reduce the voltage applied to the motor during startup.

Single Three-Phase Autotransformer: * A single three-phase autotransformer with a tapped winding acts as a variable voltage source. * During starting, the transformer is connected to a tap with a lower voltage, reducing the voltage supplied to the motor. * As the motor accelerates, the tap is switched to a higher voltage position until full line voltage is reached, providing a smooth transition.

Three Single-Phase Transformers: * Three separate single-phase autotransformers are used, one for each phase of the motor. * Each transformer has a tapped winding, offering independent voltage control for each phase. * This approach allows for finer control over the starting voltage and offers a more balanced start for the motor.

Advantages of Autotransformer Starters:

• Reduced Starting Current: The reduced voltage results in lower starting current, minimizing voltage dips and reducing stress on the electrical system.
• Reduced Starting Torque: While the starting torque is also reduced, it is typically sufficient for most industrial applications.
• Improved Motor Life: The reduced stress on the motor during starting extends its lifespan.
• Simple Design and Operation: Compared to other starting methods like star-delta starters, autotransformer starters are simpler in design and operation.
• Cost-Effective: Autotransformer starters are generally more cost-effective than other methods for starting larger motors.

Disadvantages:

• Limited Voltage Reduction: Autotransformer starters can only reduce the voltage to a certain level, which might not be suitable for motors with extremely high starting currents.
• Harmonics: The use of transformers can introduce harmonics into the electrical system, requiring potential mitigation strategies.
• Higher Cost: Compared to direct-on-line starting, autotransformer starters are more expensive to install and maintain.

Applications:

Autotransformer starters are commonly used in various applications where high starting currents are undesirable, such as:

• Large industrial motors
• Centrifugal pumps
• Fans and blowers
• Compressors

Conclusion:

Autotransformer starters offer a reliable and efficient method for starting large induction motors at reduced voltage. They effectively mitigate high starting currents, minimize voltage drops, and extend motor life. While they have some limitations, their advantages make them a widely used solution in various industrial applications.