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Contact Switch
Introduction
A contact switch is used to regulate whether or not an electrical current is passing from a power source into an electrical device. These switches are found in many types of equipment and they are used to control, for example, the power output from a wall socket into a device when it is plugged in; the currents passing across the circuit board of a computer; or the electricity powering a light bulb when the switch is flipped on. Because of their prevalence, simulating contact switches is a fundamental step in designing electronic applications.
The working principle behind a contact switch is simple: two conductive pieces of metal with an electrical voltage difference across them are brought into contact, allowing a current to flow between them. The metallic surfaces of the two components that touch one another are called contacts, and when the connection between the two contacts is broken, the current stops flowing.
The current flow between the two contacts contributes to an increase in temperature in the switch due to the Joule heating effect.
Figure 1: A contact switch.
The heating of the contact switch can change the material properties of the metal as well as the surface area of contact, and therefore is an important effect to consider when modeling the switch. Letting the temperature become too high can even cause the switch to burn out, meaning the switch is no longer functional. Therefore, it is important to analyze its current-carrying capability in order to prevent this from happening. It is also important to consider that when the two metallic pieces come into contact, the surfaces touching each other experience a mechanical pressure or contact pressure. This mechanical pressure on the contacts can alter the electrical and thermal properties of the material locally around the region surrounding the contacts. Therefore, in order to accurately simulate the current-carrying capability and temperature rise in the switch, it is important to take a more comprehensive approach in the simulation and incorporate the effect of contact pressure to compute the electrical and thermal conductance of the contact surfaces.This tutorial illustrates how to implement a multiphysics contact. It models the thermal and electrical behavior of two contacting parts of a switch. The electric current and the heat cross from one part to the other only at the contact surface.
The contact switch device has a cylindrical body and plate hook shapes at the contact area (see Figure 1). There, the thermal and electrical apparent resistances are coupled to the mechanical contact pressure at the interface, which the application solves for.
The initial temperature is equal to the external room temperature. A potential difference between the left and right parts leads to heating through the Joule effect.
Model Definition
The geometry of the switch is shown in Figure 2. Only half of the device is represented due to symmetry considerations.
Figure 2: Switch geometry.
The switch is made of copper, with two fixed cylindrical elements and a central region where the contacts are located. On the end of each contact are plate hooks that enable contact between the two pieces. In the simulation, an electric potential of 1 mV is applied to the left side of the switch, while the right side is grounded.
The thermal and electrical contact conductances are assumed to be related only to the contact pressure.
The exposed surfaces of the switch lose heat due to their interaction with air via natural convection. In the simulation, this is modeled by specifying a heat transfer coefficient and the ambient temperature of the surrounding air (a more ambitious simulation might also include the fluid flow of the air). The application first solves for structural contact to obtain the contact pressure on the contact surfaces. These results are then used to compute the electrical and thermal conductance of the contact’s surfaces in a Joule heating simulation.
Results and Discussion
Figure 3 shows the electric potential distribution, ranging from the grounded right side to the applied 1 mV on the left.
Figure 3: Electric potential profile.
A potential difference across the two components in the switch creates a current flow, which in turn leads to Joule heating. This causes a rise in temperature in the switch. If you leave the switch on for a while, temperature distribution in the switch reaches an equilibrium. Figure 4 shows the temperature distribution in the contact switch. In this example, Joule heating causes the temperature in the switch to rise about 5 K above room temperature, although only a small temperature variation is seen within the switch itself.
Figure 4: Temperature distribution.
The internal temperature distribution is almost constant. Introducing the effect of electrical and thermal conductance allows us to predict the temperature rise more accurately. The simulation also shows that the switch gets slightly hotter at the contact region.
Finally, Figure 5 plots the temperature distribution at the contact region. Streamlines show the current density.
Figure 5: Temperature distribution (volume plot) and current density (streamlines) at the contact region.
Application Library path: Heat_Transfer_Module/Thermal_Contact_and_Friction/contact_switch
Modeling Instructions
From the File menu, choose New.
New
In the New window, click  Model Wizard.
Model Wizard
1
In the Model Wizard window, click  3D.
2
In the Select Physics tree, select Structural Mechanics>Solid Mechanics (solid).
3
Click Add.
4
In the Select Physics tree, select Heat Transfer>Electromagnetic Heating>Joule Heating.
5
Click Add.
6
Click  Study.
7
In the Select Study tree, select General Studies>Stationary.
8
Click  Done.
Geometry 1
Import 1 (imp1)
1
In the Home toolbar, click  Import.
2
In the Settings window for Import, locate the Import section.
3
Click Browse.
4
Browse to the model’s Application Libraries folder and double-click the file contact_switch.mphbin.
5
Click Import.
Form Union (fin)
1
In the Model Builder window, under Component 1 (comp1)>Geometry 1 click Form Union (fin).
2
In the Settings window for Form Union/Assembly, locate the Form Union/Assembly section.
3
From the Action list, choose Form an assembly.
4
From the Pair type list, choose Contact pair.
5
Clear the Create pairs check box.
6
In the Home toolbar, click  Build All.
Definitions
Contact Pair 1 (p1)
1
In the Definitions toolbar, click  Pairs and choose Contact Pair.
2
Select Boundaries 12 and 15 only.
3
In the Settings window for Pair, locate the Destination Boundaries section.
4
Select the  Activate Selection toggle button.
5
Select Boundaries 25 and 28 only.
Add Material
1
In the Home toolbar, click  Add Material to open the Add Material window.
2
Go to the Add Material window.
3
In the tree, select Built-in>Copper.
4
Click Add to Component in the window toolbar.
5
In the Home toolbar, click  Add Material to close the Add Material window.
Solid Mechanics (solid)
Fixed Constraint 1
1
In the Model Builder window, under Component 1 (comp1) right-click Solid Mechanics (solid) and choose Fixed Constraint.
2
Select Boundaries 4, 5, 34, and 35 only.
Symmetry 1
1
In the Physics toolbar, click  Boundaries and choose Symmetry.
2
Select Boundaries 2 and 22 only.
Contact 1
1
In the Physics toolbar, click  Pairs and choose Contact.
2
In the Settings window for Contact, locate the Pair Selection section.
3
Under Pairs, click  Add.
4
In the Add dialog box, select Contact Pair 1 (p1) in the Pairs list.
5
Click OK.
6
In the Settings window for Contact, locate the Contact Method section.
7
From the Formulation list, choose Augmented Lagrangian.
8
Locate the Initial Value section. In the Tn text field, type 1e7.
Heat Transfer in Solids (ht)
In the Model Builder window, under Component 1 (comp1) click Heat Transfer in Solids (ht).
Heat Flux 1
1
In the Physics toolbar, click  Boundaries and choose Heat Flux.
2
Select Boundaries 3 and 6–33 only.
3
In the Settings window for Heat Flux, locate the Heat Flux section.
4
Click the Convective heat flux button.
5
In the h text field, type 2.
Symmetry 1
1
In the Physics toolbar, click  Boundaries and choose Symmetry.
2
Select Boundaries 2 and 22 only.
Pair Thermal Contact 1
1
In the Physics toolbar, click  Pairs and choose Pair Thermal Contact.
2
In the Settings window for Pair Thermal Contact, locate the Pair Selection section.
3
Under Pairs, click  Add.
4
In the Add dialog box, select Contact Pair 1 (p1) in the Pairs list.
5
Click OK.
6
In the Settings window for Pair Thermal Contact, locate the Contact Surface Properties section.
7
From the p list, choose Contact pressure (solid/cnt1).
Electric Currents (ec)
In the Model Builder window, under Component 1 (comp1) click Electric Currents (ec).
Ground 1
1
In the Physics toolbar, click  Boundaries and choose Ground.
2
Select Boundary 34 only.
Electric Potential 1
1
In the Physics toolbar, click  Boundaries and choose Electric Potential.
2
Select Boundary 5 only.
3
In the Settings window for Electric Potential, locate the Electric Potential section.
4
In the V0 text field, type 1[mV].
Pair Electrical Contact 1
1
In the Physics toolbar, click  Pairs and choose Pair Electrical Contact.
2
In the Settings window for Pair Electrical Contact, locate the Pair Selection section.
3
Under Pairs, click  Add.
4
In the Add dialog box, select Contact Pair 1 (p1) in the Pairs list.
5
Click OK.
6
In the Settings window for Pair Electrical Contact, locate the Contact Surface Properties section.
7
From the p list, choose Contact pressure (solid/cnt1).
Mesh 1
Free Tetrahedral 1
In the Mesh toolbar, click  Free Tetrahedral.
Size 1
1
Right-click Free Tetrahedral 1 and choose Size.
2
In the Settings window for Size, locate the Geometric Entity Selection section.
3
From the Geometric entity level list, choose Boundary.
4
Select Boundaries 12, 15, 25, and 28 only.
5
Locate the Element Size section. Click the Custom button.
6
Locate the Element Size Parameters section. Select the Maximum element size check box.
7
In the associated text field, type 5e-4.
Size
1
In the Model Builder window, click Size.
2
In the Settings window for Size, locate the Element Size section.
3
From the Predefined list, choose Fine.
4
Click  Build All.
Study 1
Solve the model in two steps. The first step only computes for Solid Mechanics while the second solves for Joule Heating (Electric Currents and Heat Transfer in Solids).
Step 1: Stationary
1
In the Model Builder window, under Study 1 click Step 1: Stationary.
2
In the Settings window for Stationary, locate the Physics and Variables Selection section.
3
In the table, clear the Solve for check boxes for Electric Currents (ec) and Heat Transfer in Solids (ht).
Stationary 2
1
In the Study toolbar, click  Study Steps and choose Stationary>Stationary.
2
In the Settings window for Stationary, locate the Physics and Variables Selection section.
3
In the table, clear the Solve for check box for Solid Mechanics (solid).
4
In the Study toolbar, click  Compute.
Results
Stress (solid)
In this first default plot, the switch is slightly deformed due to the contact pressure. The von Mises stress is located at the switch base and at the contact area.
Follow the next steps to visualize the third and fourth default plots as in Figure 3 and Figure 4.
Study 1/Solution 1 (sol1)
1
In the Model Builder window, expand the Results>Datasets node, then click Study 1/Solution 1 (sol1).
2
In the Settings window for Solution, locate the Solution section.
3
From the Frame list, choose Spatial  (x, y, z).
Study 1/Solution Store 1 (sol2)
1
In the Model Builder window, click Study 1/Solution Store 1 (sol2).
2
In the Settings window for Solution, locate the Solution section.
3
From the Frame list, choose Spatial  (x, y, z).
Mirror 3D 1
1
In the Results toolbar, click  More Datasets and choose Mirror 3D.
2
In the Settings window for Mirror 3D, locate the Plane Data section.
3
From the Plane list, choose xy-planes.
Electric Potential (ec)
1
In the Model Builder window, click Electric Potential (ec).
2
In the Settings window for 3D Plot Group, locate the Data section.
3
From the Dataset list, choose Mirror 3D 1.
4
In the Electric Potential (ec) toolbar, click  Plot.
Temperature (ht)
1
In the Model Builder window, click Temperature (ht).
2
In the Settings window for 3D Plot Group, locate the Data section.
3
From the Dataset list, choose Mirror 3D 1.
4
In the Temperature (ht) toolbar, click  Plot.
To observe the temperature and current density only at the contact region (Figure 5), proceed as follows.
Surface 1
1
In the Results toolbar, click  More Datasets and choose Surface.
2
In the Settings window for Surface, locate the Parameterization section.
3
From the x- and y-axes list, choose xy-plane.
4
Select Boundaries 10 and 21 only.
Temperature (Contact Region)
1
In the Results toolbar, click  2D Plot Group.
2
In the Settings window for 2D Plot Group, type Temperature (Contact Region) in the Label text field.
Surface 1
1
In the Temperature (Contact Region) toolbar, click  Surface.
2
In the Settings window for Surface, click Replace Expression in the upper-right corner of the Expression section. From the menu, choose Component 1 (comp1)>Heat Transfer in Solids>Temperature>T - Temperature - K.
3
Locate the Coloring and Style section. From the Color table list, choose ThermalLight.
Temperature (Contact Region)
In the Model Builder window, click Temperature (Contact Region).
Streamline 1
1
In the Temperature (Contact Region) toolbar, click  Streamline.
2
In the Settings window for Streamline, click Replace Expression in the upper-right corner of the Expression section. From the menu, choose Component 1 (comp1)>Electric Currents>Currents and charge>ec.Jx,ec.Jy - Current density (spatial frame).
3
Locate the Streamline Positioning section. From the Positioning list, choose Uniform density.
4
In the Separating distance text field, type 0.02.
5
Locate the Coloring and Style section. Find the Point style subsection. From the Type list, choose Arrow.
6
In the Temperature (Contact Region) toolbar, click  Plot.