Heater cable having a tapered profile
A self-regulating heater cable includes a PTC core formed of positive temperature coefficient material and disposed in electrical contact with at least two bus wires. The PTC core may encapsulate the bus wires and space the bus wires apart a predetermined distance via a connecting portion of the PTC core. The connecting portion has a tapered profile, and is thinner at the ends approximate the bus wires and thicker in a portion between the ends, which portion may be toward or at the center of the connecting portion. The thicknesses of the ends and the thicker portion are selected to produce a ratio that is within a range at which the heater cable produces heat at its outer surface with a substantially uniform profile, and primary heat generation of the heater cable has not shifted from the center of the connecting portion to the ends of the connecting portion.
1. A self-regulating heater cable comprising:
a heating element including a first bus wire and a second bus wire both encapsulated, in a cross-sectional profile of the heater cable, by a monolithic positive temperature coefficient (PTC) core having a connecting portion that extends between and spaces apart the first and second bus wires, the connecting portion having a first thickness at each of a first end of the connecting portion approximate the first bus wire and a second end of the connecting portion approximate the second bus wire, and the connecting portion further having a second thickness at a center of the connecting portion, the second thickness being greater than the first thickness at a ratio of at least 1.5.
2. The self-regulating heater cable of claim 1 , wherein the connecting portion tapers from the center to each of the first end and the second end in a substantially planar manner.
3. The self-regulating heater cable of claim 2 , wherein the connecting portion tapers to a point at the center.
4. The self-regulating heater cable of claim 2 , wherein a top surface and a bottom surface of the connecting portion both taper from the center to each of the first end and the second end.
5. The self-regulating heater cable of claim 1 , wherein a top surface and a bottom surface of the connecting portion both are curved and convex.
6. The self-regulating heater cable of claim 1 , wherein the first thickness and the second thickness are selected such that the ratio is below a threshold at which primary heat generation of the self-regulating heater cable shifts from the center of the connecting portion to one or both of the first end and the second end of the connecting portion.
7. A heater cable comprising:
at least two bus wires;
at least one positive temperature coefficient (PTC) core in contact with each of the at least two bus wires, the PTC core including at least one connecting portion extending between the at least two bus wires, the at least one connecting portion having a tapered cross-sectional profile wherein a first portion of the at least one connecting portion has a first thickness that is greater than a second thickness of the at least one connecting portion at one or more ends of the at least one connecting portion, each end approximate one of the at least two bus wires, wherein a ratio of the first thickness to the second thickness is greater that approximately 1.3.
8. The heater cable of claim 7 , wherein the ratio is further less than or equal to approximately 5.0.
9. The heater cable of claim 7 , wherein a ratio of the first thickness to the second thickness is within a range at which:
the heater cable produces heat at an outer surface of the heater cable with a substantially uniform profile; and
primary heat generation of the heater cable has not shifted from the center of the connecting portion to one or more of the ends of the connecting portion.
10. The heater cable of claim 7 , wherein the first portion is toward the center of the at least one connecting portion.
11. The heater cable of claim 7 , wherein one or both of a top surface and a bottom surface of the at least one connecting portion has a taper that forms the first thickness and the second thickness.
12. The heater cable of claim 11 , wherein the taper is planar.
13. The heater cable of claim 7 , further comprising a polymer jacket surrounding the at least one PTC core and the at least two bus wires and providing dielectric separation from the at least one PTC core while allowing conductance of heat away from the at least one PTC core.
14. The heater cable of claim 13 , further comprising an outer jacket surrounding the polymer jacket, the outer jacket formed from an environmentally resilient material.
15. A method of manufacturing a heater cable, the method comprising:
forming a positive temperature coefficient (PTC) material into electrical contact with at least two bus wires to produce a heating element having a connecting portion of PTC material extending between a first bus wire and a second bus wire of the at least two bus wires; and
forming a tapered cross-sectional profile in the connecting portion, wherein the tapered cross-sectional profile produces a first thickness in one or both of a first end and a second end of the connecting portion, and produces a second thickness in a thicker portion of the connecting portion between the first end and the second end, wherein the second thickness is greater than the first thickness, the second thickness and the first thickness being produced at a ratio greater than approximately 1.3.
16. The method of claim 15 , wherein forming the tapered cross-sectional profile comprises forming the thicker portion about the center of the connecting portion.
17. The method of claim 15 , wherein forming the PTC material into electrical contact with the at least two bus wires comprises passing the at least two bus wires and the PTC material through an extruder mold having a tapered extrusion mold profile designed to create the tapered cross-sectional profile.