A THERMAL AND FATIGUE ANALYSIS OF A BRAZED VERSUS A
SPOT WELDED ELECTRICAL CONTACT
by
Gabriela Contreras Plummer
An Engineering Project Proposal Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Advisor: Professor Ernesto Gutierrez-Miravete
ABSTRACT
The purpose of this project is to understand the performance differences
between an electrical contact that followed a brazing process versus an electrical
contact that followed a spot welding process. To accomplish this purpose, a
thermal and fatigue analysis using FEM will be used to compare both contacts.
A fatigue analysis will predict the mechanical life of the contact after a specific
number of operations. A thermal analysis will tell us the performance of the
contact after certain number of operations when a high current/load is applied to
the contacts for a specific period of time after a specific number of operations.
Therefore, the development of this project will answer questions on the
mechanical and electrical performance. This analysis can also help to identify
and predict the potential failures that the contacts can suffer during the normal
contactor operation, which can include welding, burning or pitting.
INTRODUCTION/BACKGROUND
A relay is an electrical operated switch. When a relay is used to switch a large
amount of electrical power through its contacts, it is designated by a special
name: contactor. A contactor physical construction will include multiple contacts
(power contacts, auxiliary contacts). The contacts are the current carrying part of
the contactor. These contacts are usually (but not always) normally-open, so that
power to the load (motors, lighting systems, etc) is shut off when the contactor
coil is de-energized. Perhaps the most common industrial use for contactors is
the control of electric motors.
Typical Three-Phase Motor Control Diagram
Unlike general-purpose relays, contactors are designed to be directly connected
to high-current load devices. Devices switching more than 15 amperes or in
circuits rated more than a few kilowatts are also usually called contactors.
The contact is a key element in the performance of the contactor, therefore, for
any change in the geometry, material or welding process of the contacts we can
sure expect an impact on the performance of the contactor.
Electrical contacts are typically welded to its arm or base by different welding
processes, being the most common: brazing and spot welding. If we were to
compare the performance of a brazed contact versus a spot welded contact we
would need to submit the contactor to different testing such as Overload,
Electrical Endurance, Mechanical Endurance, etc. These tests could require
either a large number of samples and/or a very long time to complete. An ANSYS
simulation is an alternative way to predict and determine the potential life and
failure a contact that followed a brazing process versus a spot- welded contact.
PROBLEM DESCRIPTION/METHODOLOGY
Brazing is a metal-joining process whereby a filler metal or alloy is heated to
melting temperature above 450 °C (840 °F) and distributed between two or more
close-fitting parts by capillary action.
Spot welding is a process in which contacting metal surfaces are joined by the
heat obtained from resistance to electric current flow.
By the nature of the welding process differences we can obviously expect a
difference in the performance. For instance, a contact that followed a better
welding process will probably have a stronger bond between the contact and the
base and therefore we can probably predict a higher life expectancy (in terms of
numbers of operations) while a contact with a welding defect, will for sure present
issues to complete the same number of operations.
A typical electrical contact will be a silver cadmium oxide welded to a copper
arm/base. For the development of the project I will consider a contact made out
of this alloy with a circular shape.
To perform the analysis of the contacts, Microsoft Excel and ANSYS will be used
for the computations. Data/Results will be analyzed in the post-processing
module of ANSYS.
The plan for the completion of the project is presented as follows:
Task
Plan
Start
Plan
Finish
Project Proposal
Research Literature
1/18/2010
1/18/2010
2/5/2010
2/12/2010
3D Modeling of Contact for Simulation
Ansys Fatigue Simulation (First Report)
Ansys Thermal Evaluation (Second Report)
2/12/2010
2/17/2010
2/26/2010
2/17/2010
2/26/2010
3/19/2010
Analysis Performance Comparison
3/19/2010
4/9/2010
Rework simulation differences
Conclusions (Final Report)
4/9/2010
4/15/2010
4/15/2010
4/21/2010
REFERENCES
[1]
Ragnar Holm. Electrical Contacts
[2]
Milenko Brauvonic. Electrical Contacts – Fundamentals, Applications and
Technology
[3]
Herman A Nied. The thermostructural Analysis of Electric Contacts using
FEM, ARTICLE,
[4]
UL508, Standard for Industrial Control Equipment