Magnetically Soft Materials – Transformer Cores

Silicon Steel €“ laminations or tape wound

Iron has a very high saturation level. It saturates above 20 kilogauss, but requires a lot of magnetizing force above 17 kilogauss. Cobalt has a higher saturation level, but is very expensive. Silicon is added to iron to improve the irons electrical resistivity. Processes have been developed with which promote grain orientation in the metal. The grain orientation lowers the losses and extends the boundaries of useful operation. The high saturation level permits the building of smaller transformers. Silicon steel must be used in thin strips to minimize its eddy currents; hence it is used for laminated core stacks or for tape wound cores. Eddy current become excessive as the operating frequency climbs. Eleven to fourteen mil thick strips are used for 50 & 60 hertz and at 100 hertz with some gauss de-rating. Six to seven mils is used for 400 hertz applications. Two to four mils is used near 1000 hertz. Use above 1000 hertz is possible but requires strip thickness below 1 mil and requires operating at lower gauss levels. Silicon steel is very economical within its useful frequency range. Silicon steels can be process to optimize square loop type properties.

Core Types Used by Butler Winding