Which classification is NOT used to categorize transformers?

Transformers are typically classified based on various criteria that define their design and functionality. One of the common ways to categorize them is by winding style, which refers to how the windings are arranged within the transformer, impacting its construction and electrical characteristics. Frequency range is another relevant classification, as transformers are designed to operate efficiently at specific frequency ranges, such as low frequency or high frequency. Power application refers to the specific use of the transformer, including power transformers, isolation transformers, and audio transformers, among others.

Phase angle, while an important concept in AC circuit theory that relates to the timing difference between voltage and current in a circuit, is not a standard classification for transformers themselves. Transformers operate in terms of voltage and current levels, winding turns, and magnetic coupling, rather than phase angle as a classification. This is why phase angle does not serve as a category for classifying transformers, making it the correct choice in this context.