THERMAL ANALYSIS OF TWO BRAZE ALLOYS TO
IMPROVE THE PERFORMANCE OF A CONTACTOR
DURING THE TEMPERATURE RISE TEST
By
Gabriela Contreras Padron
A Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the Degree of
MASTER OF ENGINEERING
Major Subject: MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
April 2010
(For Graduation May-2010)
i
© Copyright 2010
by
Gabriela Contreras Padron
All Rights Reserved
ii
CONTENTS
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
NOMENCLATURE ......................................................................................................... vi
ACKNOWLEDGEMENT ............................................................................................... vii
ABSTRACT ................................................................................................................... viii
1. INTRODUCTION AND BACKGROUND ................................................................ 1
1.1
Brazing ............................................................................................................... 2
1.2
Material Considerations ..................................................................................... 3
1.3
Industry Standards .............................................................................................. 3
1.3.1
Temperature Rise Test ........................................................................... 3
1.3.2
Overload and Electrical Endurance Test ................................................ 4
1.3.3
Welding ………………………………………………………………...4
2. THEORY AND METHODOLOGY ........................................................................... 5
2.1
Joule Heating...................................................................................................... 5
2.2
Arc Heating ........................................................................................................ 6
2.3
Validation ........................................................................................................... 6
2.3.1
2D Axis-symmetric Model (Arcing-Transient) ..................................... 7
2.3.2
3D Model (Joule Heating-Steady State) .............................................. 10
3. RESULTS AND DISCUSSION ................................................................................ 12
3.1
Arcing............................................................................................................... 12
3.2
Temperature Rise Test (experimental) ............................................................. 13
3.3
Steady State Joule Heating (3D Model) ........................................................... 14
4. CONCLUSIONS ....................................................................................................... 18
REFERENCES ................................................................................................................ 19
GLOSSARY OF TERMS ................................................................................................ 20
iii
LIST OF TABLES
Table 1
Material Properties Used as Input…...……………………………………3
Table 2
Material Properties Used as Input in 2D Model……………………….…9
Table 3
2D Model Boundary Conditions Setup...……………………………..…11
Table 4
3D Model Boundary Conditions Setup………..………………………...12
Table 5
Summary of Temperature Rise Results………………………………....18
iv
LIST OF FIGURES
Figure 1
Typical Three-Phase Motor Contact Diagram……...…………………….1
Figure 2
Typical Electric Contacts…………………………………………………2
Figure 3
Schematic Representation of 2D Model…..……………………………...7
Figure 4
2D Axis-symmetric Finite Element Model………………..……………...8
Figure 5
2D Boundary Conditions Setup Model....…………...……………..……..9
Figure 6
Electric Contact 3D Model……….…………..…………………………10
Figure 7
Schematic Representation of 3D Model……...…………………………10
Figure 8
3D Model Boundary Conditions Setup…………..……………………...11
Figure 9
Computed Temperature Rise Distribution (arcing)……………………..12
Figure 10
Temperature Rise Plot………………….……………………………….13
Figure 11
Contact using Braze 750 and zero defect bonding.………..……………14
Figure 12
Contact using Braze 750 and imperfect bonding....………..……………15
Figure 13
Contact using Silfos and zero defect bonding……………..……………16
Figure 14
Contact using Silfos and imperfect bonding....……...……..……………16
v
NOMENCLATURE

Electrical Resistance, -m

Electrical Conductivity, S/m
Je
Electric Current Density, Amp/m2
h
Coefficient of Heat Transfer, Watts/m2*K
K
Degrees Kelvin
C
Degrees Celsius

Density, kg/m3
Cp
Heat Capacity, Joule/kg*K

Ohms
Q
Resistive Heat Source, W/m3
q
Heat flux due to arcing, W/m2

Thermal Expansion Coefficient, m/m*C
k
Thermal Conductivity, Watt/mK
vi
ACKNOWLEDGEMENT
I would like to give thanks to Teresa and Heriberto, my beloved parents, for
devoting their lives to my brother and I. It is because of their support that I have
accomplished one more important step in my life. Thanks to the creator for the great
experiences and the great people that have been put in my life. I would also like to give
thanks to my adviser, Prof. Ernesto Gutierrez-Miravete, professor of Mechanical
Engineering at Rensselaer Polytechnic Institute, for his support and guidance.
vii
ABSTRACT
The purpose of this study was to investigate the performance of electrical
contactors using mathematical models and experiments. The study employed Finite
Element Analysis using COMSOL to simulate coupled electro-thermal phenomena in
typical contacts used in industry. The FE models were calibrated and validated by
comparison with the results of previous studies and with experimental measurements
obtained in laboratory experiment. Predictions from the models were in good agreement
with both prior studies and laboratory measurements.
viii
1. INTRODUCTION AND BACKGROUND
A relay is an electrically 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 includes multiple contacts (power contacts,
auxiliary contacts, etc). The contacts are a critical component of the contactor since
they are current carrying components and they facilitate the make and break
operations of the electrical circuit.
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. The most common industrial use for contactors is the
control of electric motors. Figure one shows the control diagram of a three phase motor.
Figure 1: Typical Three-Phase Motor Control Diagram [Reference 5]
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 of the contactor, therefore any change in the contact
geometry, material or welding process, can have an impact on the performance of the
contactor.
1
Figure 2: Typical Electric contacts [Reference 6]
Electrical contacts usually consist of a rectangular or circular tip joined to a copper or
brass carrier. The tip is joined to the base by different processes being brazing and
resistance spot-welding some of the most common (see Figure 2).
1.1 Brazing
Brazing is a metal-joining process whereby a filler metal or alloy is heated to its
melting temperature, usually above 450 °C (840 °F) and distributed between two or
more close-fitting parts by capillary action. The filler metal is brought slightly above its
melting temperature while protected by a suitable atmosphere or flux. It then interacts
with a thin layer of the base metal (known as wetting) and is then cooled rapidly to form
a sealed joint. By definition, the meting temperature of the braze alloy is lower
(sometimes substantially) than the melting temperature of the materials being joined.
A variety of alloys are used as filler metals for brazing depending on the intended use or
application method. The filler metal for a particular application is chosen based on its
ability to wet the base metals, withstand the service conditions required, and melt at a
lower temperature than the base metals or at a very specific temperature.
2
1.2 Material Considerations
Typical electrical contacts used in motors consist of a Silver-Cadmium Oxide tip
brazed to a brass base. Silver-Cadmium Oxide offer high resistance to welding,
resistance to electrical erosion and good arc interruption characteristics. It also has low
contact resistance which permits the carrying of substantial currents without excessive
heating. The present analysis will be made using a Silver-Cadmium Oxide tip brazed to
a carrier made out of brass. The materials properties used for the analysis are shown on
Table 1.
PROPERTIES
AgCdO
Electrical Resistivity (ohm-meter)
3.3 x
10-8
Brass
5.39 x
10-8
Braze 750
17.4 x
10-8
Silfos
3.2 x 10-8
Temperature Coefficient of Resistance (1/K)
0.004
0.001
0.00369
0.00375
Thermal Conductivity (W/m*K)
386.17
140
40
30
(kg/m3)
10000
8670
8440
9945.67
Heat Capacity (J/kg*K)
238.48
380
343.25
260
Density
Table 1: Material Properties used as input
1.3 Industry Standards
Industrial Control Equipment is a term commonly used to represent discrete
components ranging from selector switches, relays, contactors, motor starters, pilot
lights, etc. The Underwriters Laboratories Standard for Industrial Control Equipment is
UL508, which covers the safety requirements.
1.3.1
Temperature Rise Test
The purpose of the Temperature Rise Test (TRT) is to determine the maximum
temperature reached by the contact terminal (carrier) after passing the rated current of
the contactor. Based on a specific application of the contactor, the rated current to be
used in this analysis will be 30 Amperes. At the conclusion of the test, the temperature
rise of the contact terminal shall not exceed the maximum temperature rise specified.
The present analysis will consider the maximum temperature rise allowed as 65 °C. In
this work, the temperature rise will be considered as the difference between its stabilized
3
test temperature and the test ambient. The TRT is typically run for 4 or 5 hours to ensure
the terminals reach steady state.
1.3.2 Overload and Electrical Endurance Test
During an overload test one evaluates if the equipment (contactor) can withstand
high values of voltage, current and power without failure. For the contactor as a whole,
there shall be no electrical or mechanical breakdown. Moreover the contacts should
undergo no undue burning, pitting or welding. Depending on the ultimate application of
the contactor, the industry specifications determine the number of operations and the
electric circuit parameters.
1.3.3
Welding
The question of the nature of the actual contact material involves four aspects: (i)
they must be able to pass high currents when in the closed positions without appreciable
heating; (ii) they must not suffer excessive loss in the arc which occurs in opening; (iii)
in large-scale work the contacts must be able to withstand considerable mechanical
wear; and (iv) when they are large they must not require large quantities of precious
metal. These aspects play a high role in the selection of the material. The material has to
withstand the heat generation without welding. The welding can occur because a large
current is forced to flow through the small area of contact where the contacts touch.
When the current density is high enough melting will occur. However, with melting the
area of contact increases so the current density decreases. Eventually a point is reached
where not enough heat is supplied to offset the heat removed by conduction and the
liquid solidifies. There is then some interest in determining the maximum temperatures
that can be obtained in a contact during the normal making and breaking operation.
4
2. THEORY AND METHODOLOGY
The finite element methodology was used to develop coupled Electro-Thermal
models of electrical contacts with the objective of investigating their thermal response
when subjected to conditions encountered during the making and breaking process and
also during the temperature rise test. The analysis is also compared data from actual tests
run at a laboratory. The electro-thermal interaction module from COMSOL 3.5 was used
to conduct the analysis.
2.1 Joule Heating
The first process considered is Joule Heating. In Joule heating, the temperature
increases due to the resistive heating from an electric current. The generated resistive
heat Q is proportional to the square of the magnitude of the electric current density J e.
Current density, in turn, is proportional to the electric field, which equals the negative
gradient of the electric potential V
(2.1)
The coefficient of proportionality is the electric resistivity ρ = 1/σ, which is also the
reciprocal of the temperature-dependent electric conductivity σ = σ(T). Combining the
above yields:
(2.2)
The resistive heating source term Q is the well-known Joule heat due to current flow. In
the electro-thermal interaction module in COMSOL, this term is predefined as the source
term when using the Joule-Heating predefined multiphysics coupling.
5
It is important to note that the quantities other than σ also vary with temperature.
Although the electric conductivity σ is a function of temperature T, this dependency has
been neglected in this study.
Under extreme conditions (as when large current surges flow through the contact-see
next section), melting can take place. Therefore the effect of the latent heat of fusion (L)
must be taken into account in those cases. The latent heat effect was incorporated into
the analysis by converting the latent heat into appropriate units of specific heat (Cp) over
the temperature range that heat is absorbed or liberated for melting or solidification,
respectively (effective specific heat method).
2.2 Arc Heating
The second type of heating encountered in contacts is the one due to arcing. The
arc heating can be determined from the power dissipation in the arc. The arc heating was
simulated in this study as a heat flux. The combination of joule and arc heating produces
large temperature gradients in the contacts, which generate thermal distortion and
thermal stresses.
2.3 Validation
Nied, et al. and Robertson [References 2 and 3] have presented results of coupled
electro-thermal analysis of contacts using the Finite Element methodology. Both works
use a 2D Axis-symmetric model to analyze the performance of an electrical contact
under typical tests that UL508 defines for Contactors, such as Overload and Locked
Rotor Endurance.
Nied presents the analysis of a Silver-Cadmium Oxide Contact that is brazed to a Copper
carrier using Silver as the braze alloy. A two dimensional axis-symmetric model was
developed using the COMSOL system with the objective of reproducing the results of
this earlier studies and therefore to validate the applicability of the Finite Element
Methodology in COMSOL for the analysis of electrical contacts.
6
2.3.1
2D Axis-symmetric Model (Arcing-Transient)
The 2D-axisymmetric model presented by Nied consists of a mesh using an
Isoparametric element with quadratic shape functions. The thermal-electric solid element
(STIF67) from ANSYS was used to compute the temperatures. Nied performed the
analysis of a contact assembly submitted to the Locked Rotor Endurance Test. The
Locked Rotor Endurance test has high values of current which depending on the rating
of the contactor can be either four, five or six times the normal operation current of the
contactor. The paper presented the analysis of this test for a contactor that is rated 40
amps as its normal operation current. The locked rotor value used was 240 amps (six
times the normal current). To model the making and breaking of the contacts, Joule
Heating was imposed as current coming into the model, arc heating was imposed on the
model as heat flux. The analysis shows the temperatures reached on the contact surface
when arc heating is applied for the duration of 3 milliseconds. A model of the same
contact assembly was developed in COMSOL using the electro-thermal module using
triangular quadratic elements. A schematic representation of the system is shown in
Figure 4.
Figure 3: Schematic representation of 2D model
7
The material properties used in the simulation of the 2D model are shown in Table 2.
PROPERTIES
AgCdO
Silver
Copper
3.3 x 10-8
1.6 x 10-8
1.72 x 10-8
Temperature Coefficient of Resistance (1/K)
0.004
0.0038
0.0039
Thermal Conductivity (W/m*K)
Electrical Resistivity (ohm-meter)
386.17
429
400
(kg/m3)
10000
10490
8700
Heat Capacity (J/kg*K)
238.48
235
385
Density
Table 2: Material Properties used as input in 2D model
The meshed geometry produced by COMSOL is shown in Figure 4.
Figure 4: 2D Axis-symmetric Finite Element Model
Figure 5 and Table 3 summarize the setup of the boundary conditions. Axial symmetry
was imposed in Boundaries 1, 2, 3. Electrical and thermal insulation was assumed in
boundaries 4 through 9. Inward current of 240 Amps and associated heat flux were
imposed on Boundary 10.
8
Figure 5: 2D Model Boundary Conditions Setup
Conductive
Boundaries Media
Heat Transfer
B1
Axial Symmetry
Axial Symmetry
B2
Axial Symmetry
Axial Symmetry
B3
Axial Symmetry
Axial Symmetry
B4
Electric Insulation Thermal Insulation
B5
Electric Insulation Thermal Insulation
B6
Electric Insulation Thermal Insulation
B7
Electric Insulation Thermal Insulation
B8
Electric Insulation Thermal Insulation
B9
Electric Potential Thermal Insulation
B10
Inward Current
Heat Flux (q=2x109)
Table 3: 2D Model Boundary Conditions Setup
9
2.3.2
3D Model (Joule Heating-Steady State)
A 3D model (Figure 6) of the contact assembly was also built to simulate steady
state conditions obtained during the temperature rise test. The effect of the current used
during the temperature rise test was analyzed using the Joule Heating effect.
A
schematic representation of the system is shown in Figure 7.
Figure 6: Electric Contact 3D Model
Figure 7: Schematic Representation of 3D model
Figure 8 and Table 4 summarize the setup of the boundary conditions. Heat is assumed
to be lost by convection into the environment through all external surfaces except
surfaces 1 and 2. The conduction loss through the grounded surface was simulated by
means of an enhanced value of heat transfer coefficient.
10
Figure 8: 3D Model Boundary Conditions Setup
Conductive
Boundaries Media
Heat Transfer
B1
Ground
Heat Flux (h=55)
B2
Inward Current
Themal Insulation
Others
Electric Insulation Heat Flux (h=1)
Table 4: 3D Model Boundary Conditions Setup
11
3. RESULTS AND DISCUSSION
As stated before, the purpose of the present work is to understand the performance
of a contact assembly during the Temperature Rise Test. The results for the experimental
test are presented along with the FEA analysis using the methodology previously
validated.
Instead of a 2D axis-symmetric model, a 3-D model is presented. The
analysis is made for a 6mm in diameter circular silver-cadmium oxide tip that is brazed
to a brass carrier. Two different braze alloys (Braze 750 and Silfos) were considered in
the analysis. The material properties have been presented before. The analysis is
presented for a zero defects, perfect brazing process (100% bonding area) versus a
realistic brazing process (imperfect bonding area).
3.1 Arcing
A transient analysis was performed to evaluate the arcing of the contact by
applying a heat flux in the magnitude of 105 W/cm2 for 3 milliseconds. The computed
temperature field is shown in Figure 9. The application of the heat flux for this time
produced a temperature of 975 C on the top surface of the contact. This result was
calculated with great accuracy in comparison to the results obtained by Neid where a
temperature of 961 C is obtained.
Figure 9: Computed Temperature Distribution (arcing)
12
3.2 Temperature Rise Test (experimental)
The temperature rise test was run on a contactor containing contact tips made of
silver-cadmium oxide and brazed to brass arms. The temperature rise test was run for
four hours to ensure the contacts would reach steady state. The contactor was operated
under normal conditions and passing the rated current of 30 Amperes through the
contacts. Thermocouples were placed on the surface of the carrier and temperature
measurements were taken every 15 minutes. For the test, a temperature was considered
to be constant when three successive readings that were taken at intervals of 10 percent
of the previously elapsed duration of the test but not less than 10 minute intervals
indicated no change in the temperature rise. As mentioned before, the temperature rise
was considered as the difference between its stabilized test temperature and the test
ambient.
Figure 10 presents typical results of a temperature rise test. It is important to note that
the test was run on a contactor which construction includes multiple contact assemblies.
The plot shows the results on the contact that had the highest temperature readings.
Temperature Rise Test
SILFOS
Temperature Rise Above Ambient ( °C )
100
BRAZE 750
TEMPERATURE RISE LIMIT
90
80
70
70
65
60
49.4
50
40
30
20
10
0
0
1.25
2.5
3.75
5
6.25
Time (hours)
Figure 10: Temperature Rise Plot
13
7.5
Experimental data indicates that the material used for the brazing process of the contact
has an impact on the contactor performance during the test. At the end of the test,
contacts using Braze750 showed a better performance.
3.3 Steady State Joule Heating (3D Model)
Four different scenarios were investigated using the 3D Model. These were as
follows:

Contact Using Braze 750 and zero defect bonding

Contact Using Braze 750 and imperfect bonding

Contact Using Braze Silfos and zero defect bonding

Contact Using Braze Silfos and imperfect bonding
In all cases an inward current of 30 amps was applied on the top of the contact surface
area. The current was not applied on the entire surface area but rather on a small section.
This was achieved by defining a small cylinder on top of the contact to define the area
where to apply the current. Figure 11 shows the calculated temperature of the contact
assembly using Braze 750 reached a temperature of 73 C.
Figure 11: Contact using Braze 750 and zero defect bonding
14
As mentioned on the temperature rise test, the thermocouple is placed on the surface of
the contact carrier (brass block); therefore the temperature taken as a result is the one on
the surface of the carrier. In reality no braze layer will have one hundred percent
coverage of the surface of the contact. To create the simulation of the contact with a
realistic bond the bonding area of the braze layer was reduced by 30 percent.
Figure 12 shows the results of the same contact with an imperfect bonding. For this case
the temperature reached is 74.2 C. The results for the simulation of the electric contact
using the Silfos as a braze alloy are presented as follow.
Figure 12: Contact using Braze 750 and imperfect bonding
Figure 13 shows the results of the simulation of the contact using Silfos as the braze
alloy and a perfect bonding model. The temperature reached on the brass carrier was
79C. Finally, Figure 14 shows the simulation of the contact using Silfos and an
imperfect bonding. The temperature reached on the brass carrier is 81.7 C.
15
Figure 13: Contact using SilFos and perfect bonding
Figure 14: Contact using Silfos and imperfect bonding
16
Table 5 summarizes the temperature rise results in degrees Celsius. As
mentioned before, the temperature rise is considered as the difference between the
maximum Temperature reached during the temperature rise test minus the ambient
temperature. A room temperature of 22 C has been considered for the computations of
the temperature rise values.
Perfect Bonding Realistic Bonding Experimental Test
Silfos 15
57 °C
60 °C
68 °C
Braze 750
51.2 °C
52 °C
53 °C
Table 5: Summary of Temperature Rise Results
The summary of results indicates that during the experimental test as well as during the
simulation the material selection for the braze alloy of the contact has an impact on the
performance of the contactor in the Temperature Rise Test. Braze 750 presents a better
performance than Silfos. Also, the simulation is more accurate when we consider the fact
that no brazing can achieve a perfect, zero defect bonding area.
17
4. CONCLUSIONS
After validating the Finite Element Model by reproducing the results presented in
the References 2 and 3, a 3D Model of the Silver-Cadmium Oxide contact brazed to a
brass carrier was developed to simulate the steady state conduction during a
Temperature Rise Test. Experimental data demonstrated that contact assemblies using
Braze 750 will perform better during temperature rise test. The simulation results
support this statement by predicting lower steady state temperatures. The quality control
of the brazing process to produce a good bonding area plays also a role in the
performance of the contactor. The simulation with an imperfect bonding produced
temperatures slightly closer to the experimental values. Industry Requirements for
Bonding Area in electric contacts varies from 70 to 90 percent depending on the
application of the contact.
This paper presents an evaluation on two braze alloys used to join the contact to the
carrier, but other alloys can be also evaluated as well as other materials on the contact
tip. The present job makes this analysis with Silver-Cadmium Oxide tips. Though SilverCadmium Oxide is known as a material with good performance in electric contact
applications, the industry is moving away in the use of Cadmium due to RoHS
(Restriction of Hazardous Substances) requirements. Extensive testing has to be
performed to ensure new materials can meet the specific contactor and ratings
requirements. FEA demonstrated to be a useful tool to assist in the analysis of the
performance of electrical contacts.
18
REFERENCES
[1] F. Llewellyn Jones; The Physics of Electrical Contacts, 1st Edition; Oxford
Publications.
[2] Herman Nied, Cheryl Schlansker, The Thermostructural Analysis of Electric
Contacts Using a Finite Element Mode, IEEE Journal, Vol. CHMT-7, NO. 1, March
1984
[3] Struan R. Robertson, A Finite Element Analysis of the Thermal Behavior of
Contacts, IEE Journal, Vol. CHMT-5, NO. 1, March 1982
[4] UL508 Standard, “Industrial Control Equipment”, section 43
[5] Motor Control Protection: http://www.kilowattclassroom.com/Archive/MotorOL.pdf
[6] Google Electric Contact Pictures
19
GLOSSARY OF TERMS
Contactor An electrical relay used to control the flow of power in a circuit.
FEM Finite Element Methodology
FEA
Finite Element Analysis
RoHS Restriction of Hazardous Substances
20