Which statement about auto-transformers is correct?

Auto-transformers are designed to be more efficient in certain applications compared to traditional transformers, particularly in how they use copper in their construction. Due to their design, auto-transformers can operate with a single winding that acts as both the primary and secondary side of the transformer. This setup means that they require less copper for the same power rating because there is only one winding coil, rather than two distinct coils that would be found in traditional transformers.

This efficiency results in a lighter, smaller, and more economical transformer. The reduced amount of copper not only leads to cost savings but also reduces energy losses associated with resistance in the winding. Therefore, stating that auto-transformers can use less copper accurately reflects their inherent design benefits and operational efficiency.

The other statements do not encapsulate the primary advantages of auto-transformers. For instance, they do not have an equal transformation ratio as compared to traditional transformers, nor are they typically used for large transformation ratios. Additionally, they actually use less copper rather than more, which distinguishes their design in the realm of electrical engineering.