Designing a heating conductor for seat heaters has many challenges, the principal ones being mechanical flexing and temperature control. The two materials most widely used today for the conductors are:
The insulation on the wires varies from fluropolymer to enameling or just bare copper. Typically, the conductors are then encapsulated within a fabric/foam substrate creating a shell either side of the conductor, usually secured by adhesives.
Conductor dynamics
An occupant's entry onto and exit from the seat necessitates a sliding action over the side bolsters. This action produces a gradual rubbing and scuffing of the bolster's internal heating conductors, inducing considerable stress forces. Conductor stress is also produced by a person twisting and moving around in the seat.
For metallic conductors, logic may suggest that the greater the cross sectional area of the wire, the greater the tensile strength—resulting in improved flex performance. Nothing could be further from reality!
Tensile strength is important but equally so are the ductile properties of the wire. This can be seen in existing seat heaters that use stranded copper wires. Such heaters are constructed of many narrow gauge wires bunched together. These smaller wires are more ductile than one large wire of the same electrical resistance. Unfortunately, the stranding is longitudinal in direction, resulting in less than optimum flex performance.
A design refinement is to spirally (helically) wrap the conductors around a flexible core. This winding enhances the ductile properties of the conductor producing considerable improvements in its dynamic performance. Spirally wrapped conductors consistently outperform stranded conductors by upward of 10 times.
Thermocable has developed its own flex test (see below). This test's objective is to apply a more robust and severe test in an effort to design spirally wrapped conductors that are fit-for-purpose regarding the practical dynamics being experienced in the field. This test is in addition to International Electrotechnical Commission (IEC) tests.
