Design and optimization of electrical contacts and thermal cutoffs
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This Dissertation is focused on the design and optimization of the two types of metal
based switching devices: rivet electrical contacts and S–type thermal cutoffs.
Theoretical basis of the electrical contacts design is employed for the adequate contact
material selection, determination of the proper set of contact geometry and dimensions,
and estimation of the minimum contact force value. Contacts of the solid and clad
type, with rounded head are considered within two supporting structures. Obtained
results of 3–D simulation of mechanical, electrical and thermal characteristics of the
selected contacts in a steady state regime are presented. Simulations are carried out in
the coupled physical domains by the direct solving method. Special emphasis is put
on the dependencies of the operating temperature and maximum equivalent stress in
the contacts under various working conditions. On the basis of the obtained results,
optimization of the rivet electrical contacts is realized... by the appropriate selection of
the contact material, determination of the geometry and dimensions of contacts and
definition of the rated current value. The optimization was conducted by functional,
economic, ecological and reliability issues. Feasibility of contacts implementation into
the specific switching devices with predefined rated currents and number of switching
cycles before the failure are outlined. Analyzed dependencies and optimization results
are described in details.
Theoretical basis of design and optimization procedures of thermal cutoffs are summarized.
Parameters of functionality, quality and reliability are specially considered,
since they are crucial for the appropriate choice of geometry, dimensions and materials
of cutoffs constitutive elements.
Thermodynamic and microstructure characterization of the low melting alloys for
thermal cutoffs purposes are elaborated. A new method for evaluation of the liquidus
surfaces of the ternary systems is proposed. It is based on the standard 3–D surface
modeling principles, and is applicable for any ternary system whose phase diagrams of
binary subsystems and a few appropriate experimental points are known. The liquidus
temperature can be determined from this 3–D surface by geometrical rules for any arbitrary
composition of the ternary system. Moreover, the range of compositions of ternary
alloys that have desired liquidus temperature can be easily extracted. Microstructure
characteristics of the two eutectic low–melting alloys are also presented. Investigation
was performed by SEMand EDS techniques, and the obtained results are discussed from
the aspects of thermal cutoff technology development.
The results of 3–D electrical and thermal simulation of thermal cutoffs with two
cutoff temperatures for the rated current of 12 A and rated voltage of 250 V (S–95 and
S–138 type) are analyzed. The functionality and quality parameters of thermal cutoffs are
determinedby the simulation. Optimization of thermal cutoffs geometry anddimensions
is proposed considering functional, economic and technological aspects.
The results of electro–thermal characteristics investigation of S–138 thermal cutoff
industrial prototypes by the thermovision imaging are presented. Thermal boundary
conditions for the simulation, which are crucial for the successful optimization of the
cutoffs, are set according to the obtained results. On the basis of the temperature distributions
in the cutoffs under different working conditions, the temperature rise due
to the self–heating is registered, the cutoff temperature is determined, and the response
time of the cutoffs is evaluated. Thermal characteristics of the housing and conductive
elements of the cutoffs are also analyzed.
As a result of the performed design and optimization procedures of thermal cutoffs,
a technology line for their industrial production is presented. Complete process flow
diagram for the S–type cutoffs is proposed. It includes preparation and assembling of
the constitutive elements and quality and reliability testing procedures.