Design and test of a thermomagnetic motor using a Gadolinium rotor.

Abstract

The purpose of this paper is to show that a Thermomagnetic (Curie) Motor [1-3], which can rotate continuously and has useful mechanical characteristics, is feasible. A thermomagnetic motor can directly converts thermal energy into kinetic energy. In this type of motor force is generated by a thermally induced permeability difference in two areas of the rotor, which can generate a force if the rotor is placed in a magnetic field. This force can be enhanced if the hot side temperature of the rotor is above the Curie’s temperature of the magnetic material and the cold side under this temperature Unfortunately, the traditional ferromagnetic materials have very high Curie’s temperature and therefore their ferromagnetic phase transition cannot be used. As a result Curie motors built by using traditional materials have very poor performances. Only one ferromagnetic material, Gadolinium, has a Curie temperature which allows to obtain an easily usable Curie temperature. As a result, in this paper we present the result of the use of Gadolinium as the ferromagnetic material of a thermomagnetic motor. Gadolinium powder was used in the rotor of the machine. The Curie’s temperature of Gadolinium powder is 293 K. In this paper we present a novel approach to the description and design of the Curie motor and we use the design obtained to build a prototype. The approach is based on a thermal-magnetic coupled dynamic model of the motor. The motor is modeled in terms of both its magnetic as well thermal properties (magnetic permeability and thermal conductivity) and the thermal processes are supposed to be influenced by the thermal conductivity, the convection and the advection. An analytical expression of the generated torque, which links this quantity to the magnetic, thermal and geometrical parameters of the generated torque is given. The expressions of speed and torque are derived and related to the thermal properties of the machine and used as optimization indexes in an optimization procedure. The analytical results are verified by a 3D FEM analysis. This process leads to the design of the stator and the rotor of the machine. The rotor has been built and an experimental verification of the performances is reported.