What are the stacking methods for transformer cores?

The magnitude of the main magnetic flux in a transformer core is determined by the excitation current. Since the no-load current of a transformer actually varies with its operating voltage, a higher voltage results in a larger no-load current, which in turn increases the excitation current. Conversely, a lower voltage leads to a smaller no-load current.

There are three different types of joint methods used when stacking transformer cores.

The first is the butt joint. Its characteristics include convenient processing and stacking, as well as a large overlapping area. Therefore, the stacked core has good structural strength and integrity, and is not easily deformed. However, this method can only be used with hot-rolled silicon steel sheets. If applied to cold-rolled grain-oriented (CRGO) silicon steel sheets—which have excellent magnetic properties along the rolling direction—the magnetic flux at the joints does not pass along the rolling direction. This degrades the magnetic performance and causes an increase in no-load loss.

The second type is the half-miter (or half-direct, half-oblique) joint, which is used for CRGO silicon steel sheets. The characteristic of this joint is that butt joints and miter joints alternate within the core stacking. When the width of the core limb laminations is consistent with that of the yoke laminations, the miter joint is at a 45° angle, accounting for 50% of the corner overlap area. This offers a significant improvement in magnetic performance compared to the butt joint, along with reliable structure and convenient shearing and stacking, resulting in a high utilization rate of the silicon steel sheets. However, since a portion of the joints are still butt joints, the directionality of the CRGO silicon steel is not fully utilized, so the no-load loss of the core cannot be reduced to a minimal level.

The third type is the full-miter (or full-oblique) joint. To further reduce the no-load loss of the core and fully utilize the properties of CRGO silicon steel sheets, the full-miter joint should be adopted. This ensures that the direction of the magnetic flux is consistent with the rolling direction of the silicon steel sheets, minimizing core loss. However, this joint process is complex, and the overall integrity of the stacked core is relatively poor.