Design and Optimization of S-Type Thermal Cutoffs

A design and optimization procedure for S-type thermal cutoffs is presented in this paper. This type of cutoff is widely used for thermal protection of windings in motors and transformers and in boiling systems. Cutoff temperature is determined by the melting point of the alloy which is used for breaking the electrical circuit. A method for obtaining phase diagrams for ternary alloys by using 3-D surface modeling was developed. Composition of the ternary alloys for the desired melting point could be obtained by using this method. Construction of the thermal cutoffs for 95 $^{\circ }{\hbox {C}}$ and 138 $^{\circ }{\hbox {C}}$ temperatures in terms of thermal, electrical, and mechanical properties is described. Design and optimization were carried out by using finite-element analysis (FEA) method for 3-D simulation of the devices' steady-state and transient behavior in coupled thermal and electrical domains. Technology for the production of experimental samples is also described. Characterization of the devices was done by using the thermovision technique and reasonable agreement with the simulation results was found. Further optimization steps based on the experimental data are suggested.