Core Types of Control Transformers: Shell-Type and Core-Type

The core of a transformer is made by stacking silicon steel sheets with a thickness of 0.28–0.35 mm. Both sides of each sheet are coated with insulating varnish to insulate the sheets from each other, which helps reduce eddy current loss. In addition, to minimize magnetic reluctance and exciting current, there should be no obvious gaps in the core. The joints of each layer of the stacked sheets need to be staggered. Since silicon steel sheets have higher magnetic permeability and lower loss along the rolling direction, full skew joints are now widely used to avoid magnetic lines of force being perpendicular to the rolling direction of the silicon steel sheets. This can reduce the no-load current by 20%–30% and the core loss by 7%–10%.

According to how the coils are sleeved onto the core, transformer cores are further divided into two types: shell-type and core-type. The shell-type structure features good strength, easy heat dissipation, high material consumption, and relatively complex manufacturing. Generally, this structure is mostly used in small dry-type transformers.

Core-Type Core Control Transformers

The primary and secondary windings of a core-type transformer are respectively sleeved on the two core columns of the core. This structure has the advantages of simple structure, easy assembly, and good insulation performance. It is suitable for transformers with large capacity and high voltage, and core-type structures are mostly selected for general power transformers.

Different core structures are used for different transformers: BK control transformers use EI cores or skewed EI cores, while BKC control transformers use C-type cores.