AC losses in rare earth-iron transducer materials

Abstract

The magnetisation and magnetostrictive behaviours of Terfenol-D depend on applied uniaxial pressure, DC field bias and AC drive field. In the initial design of a transducer these factors are critical in reducing the principal source of loss, namely eddy current heating. This is particularly important in high power transducers where losses and power amplifier requirements must be minimised. The objective of this paper is to look at the relative merit of frequency doubling (due to the quadratic nature of the strain-field curve) in terms of the losses caused. An experimental investigation has been conducted in which a 6mm diameter rod of grain oriented Terfenol-D has been subjected to a uniaxial prestress of 5.4MPa and driven with a moderately strong AC field of 5.5kA/m. Three conditions of frequency and magnetic bias were used; 770 Hz at 0 and 20kA/m bias field and 1.54kHz with a bias of 20kA/m. The heat generated has been measured under each of these conditions and it is found to be three times greater in the first compared to the last case. In either case the drive frequency is well below the ‘critical’ frequency (i.e. the rod diameter is sufficiently small to allow full field penetration) but the material's permeability is significantly different under the two bias field conditions. Calculations show that the heating without a bias field can be accounted for by macroscopic eddy currents and although the permeability is higher without a bias field, there appears to be little or no contribution to the heating due to domain wall movement (anomalous loss). The results of this work indicate that under high field conditions (typically 75kA/m) for high power applications, regions of the magnetisation curve which have high permeability and low magnetostrictive activity must be avoided by the use of correct bias field: frequency doubling is a highly inefficient use of this transducer material.