PURPOSE: To obtain the rotary transformer using a ferrite core satisfying both aspects of strength and characteristics by using the quality of material, initial permeability thereof is higher than a stator core by 30%, as a rotor core.

CONSTITUTION: The quality of material of large initial permeability is used as the rotor core not screwed, and the quality of material of excellent strength is used as the stator core screwed. The graphs of the values of combined inductance to the arithmetical means values of the initial permeability μR of the quality of material used for the rotor core and the initial permeability μS of the quality of material used for the stator core are shown on each case of μR/μS of 1, 2, 3 and 4. So far as μR≥500, μS≥300 and μR/μS≥3.0 are formed, it is recognized that practically the same effect as the same quality of material is used for the rotor core and the stator core is obtained when the transformer is designed by using the arithmetical mean values of μR and μS.