Building factor: why an assembled transformer core loses more than the datasheet says
Mill core loss is measured on single strips under ideal conditions. In a finished core, joints, cutting and clamping add loss. Knowing where it comes from helps designers choose grades and specify processing.
Every Baosteel CRGO grade carries a guaranteed maximum core loss, for example 0.90 W/kg at 1.7 T and 50 Hz for B27R090. That value is measured on prepared strips magnetised exactly along the rolling direction. A finished core never works under those ideal conditions, so its no-load loss per kilogram is higher. The ratio between the two is called the building factor.
Where the extra loss comes from
- Joints: at corners and T-joints the flux has to turn and cross small air gaps, so part of the steel is magnetised away from the rolling direction, where grain-oriented steel performs much worse.
- Cutting stress and burr: slitting and cutting strain the edges, and a high burr can bridge the insulation between laminations and create local eddy currents.
- Clamping and handling: excessive clamping pressure, bending or dropped laminations put stress into the steel and raise its loss.
- Flux distribution: in three-phase cores the flux is not uniform between limbs and yokes, and rotating flux appears near the T-joint.
Joint design makes the biggest difference
Mitred joints keep the flux closer to the rolling direction than butt joints. Step-lap joints go further: by staggering the joint positions over several layers, the flux can pass each gap through neighbouring sheets. This is why step-lap designs are widely used in distribution and power transformers; they also tend to lower no-load current and noise.
What it means for grade selection
A better grade only pays off if the core is built well. Moving from a conventional grade to a domain-refined grade lowers the strip loss, but a core with high burrs or poorly fitted joints can lose much of that gain. When comparing offers, look at the guaranteed loss of the grade and at the processing behind it.
- Specify the joint type, and the number of steps for step-lap cores.
- Agree a maximum burr height and a width tolerance for slit strip.
- If the core will be annealed after cutting, check the grade with the supplier first: heat treatment can remove the domain-refining effect of some types.
- Keep laminations flat and avoid bending them during stacking.
How we can help
We supply Baosteel CRGO and cut it into mitred or step-lap laminations at the Hunan branch of Guangzhou Huaxin Industrial Co., Ltd. Send us your core drawing and target loss, and we will propose grade and processing options together.
