Platinum Metals Rev., 1971, 15, (3), 100
A High Temperature Electric Motor
Use of Nickel-clad Silver-palladium Wire
General Electric research workers at Schenectady, New York, have constructed an electric motor which has operated successfully at temperatures as high as 725°C and have thereby demonstrated the possibilities of a new high temperature electrical conductor, namely nickel-clad 50 per cent silver-palladium wire.
The conductor was developed by C. S. Tedmon and co-workers during work to produce cathode leads capable of resisting the corrosion and temperatures of up to 1000°C met with in the zirconia electrolyte fuel cell. The clad wire was made by drawing down a composite formed by inserting a silver-palladium rod into a nickel tube. The nickel cladding was thermally oxidised to NiO at 1100°C. Some interdiffusion of nickel and silver-palladium occurred at their boundary so that the cladding became well bonded to the substrate.
The conductor is in practice NiO-clad silver-palladium and it has the advantages that the NiO is inert with respect to both an oxidising atmosphere and to the conducting core, that the NiO prevents volatilisation of silver or oxidation of palladium, that the NiO provides some insulation at low voltage loads, and finally that core and cladding have similar coefficients of thermal expansion. Thus the wire can be repeatedly subject to thermal cycling without degradation of mechanical and electrical properties. The conductor can be used up to the melting point of 50 per cent silver-palladium, which is above 1200°C.
The high temperature electric motor is an hysteritic shaded-pole synchronous a.c. motor with four principal components: the field windings, the field pole-pieces, the armature, and the shading elements. It is the field windings which consist of a 24-turn coil made from 0.030 in. NiO-clad 50 per cent silver-palladium wire.
Field pole-pieces consist of laminated 6 per cent silicon-iron. The armature incorporates 6 per cent tungsten steel laminations with shaft and bearings of Lucalox and housing of sintered alumina. The shading elements are made of silver.
Cycling of the motor to 400°C presented no difficulty and subsequent sustained operation at 575°C was also successful. The motor proved to be synchronous and self-starting up to the Curie temperature of the silicon-iron field laminations at about 725°C. Above this temperature the motor would stop but it restarted on cooling through the Curie point. There were some lubrication difficulties with the motor but the efficacy of the NiO-clad silver-palladium conductor at high temperatures was proved without a doubt.
General Electric claim that this “red hot” motor has operated at the highest-ever temperature for an electric motor. They also claim that the alloy on which the device was based exhibits excellent consistency in its electrical and mechanical characteristics over relatively long periods. The electrical resistance, for example, remained unaltered during eight months’ testing at high temperature.
While there is no obvious demand for an electric motor operating in such conditions, it is clear that the excellent properties of the platinum metals at high temperatures make them prime candidates for consideration in other devices for severe environments.