Thermal Aspect in Operation of Inductive Current Transformers and Transducers

An increase in the temperature of the magnetic core causes narrowing of its hysteresis loop and reduction in the saturation magnetic flux density. Therefore, at the same operating point on the magnetization characteristic, the nonlinear effect may become stronger. In the case of the inductive current transformers, this may result in change in their transformation accuracy and increased self-generation of the low-order higher harmonics to the secondary current. Consequently, the equivalent methods used to determine their values of current error and phase displacement without operating conditions resulting from the presence of the secondary current provide less reliable results, which is particularly important for inductive current transformers with high transformation accuracy requirements and may also be significant in certain borderline cases when determining its accuracy class and the value of error is close to the limit. However, ambient temperature does not affect the transformation accuracy of conventional inductive current transformers, as their internal operating temperature is solely driven by the relatively high RMS values of the rated secondary current (1 A or 5 A) and the large number of secondary winding turns evenly distributed over the magnetic core. During thermal testing of a current transducer operating in a closed-loop feedback configuration with a Hall sensor, a deterioration of its conversion accuracy was observed at high ambient temperatures. This was caused primarily by the thermal expansion of the magnetic core, which leads to a change in the dimensions of the air gap where the Hall sensor is placed, and thus also to a change in the electrical parameters of the feedback loop circuit.