Performance of the NASA Digitizing Core-Loss Instrumentation

The ‘standard method’ of magnetic core loss measurement was implemented on a high frequency digitizi ng oscilloscope in order to explore the limits to accuracy when characterizing high Q cores at frequencies up to 1 MHz. This method computes core loss from the cycle mean of the product of the exciting current in a primary winding and induced voltage in a separate flux sensing winding. It is pointed out that just 20% accuracy for a Q of 100 core material requires a phase angle accuracy of 0.1 ° between the voltage and current measurements. Experiment shows that at 1 MHz, even high quality, high frequency cur rent sensing transformers can introduce phase errors of a degree or more. Due to the fact that the Q of some quasilinear core materials can exceed 300 at frequencies below 100 kHz, phase angle errors can be a problem even at 50 kHz. Hence great care is nec essary with current sensing and ground loops when measuring high Q cores. Best high frequency current sensing accuracy was obtained from a fabricated 0.1 -ohm coaxial resistor, differentially sensed. Sample high frequency core loss data taken with the setup for a permeability -14 MPP core is presented.