AC losses in rare earth-iron transducer materials

The magnetisation and magnetostrictive behaviours of Terfenol-D depend on a pplied 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 importan t in high power transducers where losses and power amplifier requirements m ust be minimised. The objective of this paper is to look at the relative me rit 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 ro d of grain oriented Terfenol-D has been subjected to a uniaxial prestress o f 5.4MPa and driven with a moderately strong AC field of 5.5kA/m. Three con ditions of frequency and magnetic bias were used; 770 Hz at 0 and 20kA/m bi as field and 1.54kHz with a bias of 20kA/m. The heat generated has been mea sured under each of these conditions and it is found to be three times grea ter in the first compared to the last case. In either case the drive freque ncy is well below the 'critical' frequency (i.e. the rod diameter is suffic iently small to allow full field penetration) but the material's permeabili ty is significantly different under the two bias field conditions. Calculat ions show that the heating without a bias field can be accounted for by mac roscopic eddy currents and although the permeability is higher without a bi as 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 (typical ly 75kA/m) for high power applications, regions of the magnetisation curve which have high permeability and low magnetostrictive activity must be avoi ded by the use of correct bias field: frequency doubling is a highly ineffi cient use of this transducer material.