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Magnetic Coupling Machine Application


Definition

A magnetic coupling is a coupling that transfers torque from one shaft, but using a magnetic field rather than a physical mechanical connection.

The couplings are synchronous and the output shaft speed is exactly equal to the input speed and these types of coupling can be designed with a maximum 100% efficiency. Magnetic couplings are widely used in pump systems to isolate the electrical motor from the liquid. Magnetic shaft couplings preclude the use of shaft seals, which eventually wear out and fail from the sliding of two surfaces against each another. This has the advantage of removing the dynamic seals which have a finite lifetime and are prone to leakage, which can cause failure of the machine and contamination of the working fluid. Magnetic couplings are also used for ease of maintenance on systems that typically require precision alignment, when physical shaft couplings are used, since they allow a greater off axis error between the motor and driven shaft.

A membrane/seal wall is located in the magnetic gap between the rotors providing complete isolation between the wet and dry systems. Magnetic couplings can also be used to introduce compliance into the drive train and can ultimately be used as a torque fuse to protect the system drive components. Some diver propulsion vehicles and remotely operated underwater vehicles use magnetic coupling to transfer torque from the electric motor to the prop. Magnetic gearing is also being explored for use in utility scale wind turbines as a means of enhancing reliability. The magnetic coupling has several advantages over a traditional stuffing box. 
 

Magnetic Coupling Machine

Figure 1 - Magnetic Coupling Machine

Description

The motor considered here consists of permanent magnet arrays in the form of a steel rotor containing 12 permanent magnets and a steel stator with another 12 permanent magnet. The magnets are polarized in an alternating fashion radially inwards and outwards from the axis of the cylinder. The array setup provides linear and rotational coupling from the outer array (stator) to the inner array (rotor). The rotor is driven to turn by magnetic forces resulting from the permanent magnets. In this analysis, the inner set of magnets and the outer set are separated by an offset of between 0 and 15 degrees in steps of 5 degrees (Figure 2). The torque applied on the inner rotor increases as the angular offset increases.  The following plots are the results of a magnetostatic analysis of magnetic coupling machine

Model of the Magnetic Coupling Machine

Figure 2 - 3D Model of the Magnetic Coupling Machine

Study

 
The Magnetostatic module of EMS coupled with the SolidWorks Motion is used to compute and visualize the flux density and the motion in the rotor. After creating a motion analyses in SW and a Magnetostatic study in EMS, four important steps shall always be followed: 1 - apply the proper material for all solid bodies, 2- apply the necessary boundary conditions, or the so called Loads/Restraints in EMS, 3 - mesh the entire model and 4- run the solver.

Materials

In the Magnetostatic analysis of EMS, the  required material property is the relative permeability (Table 1).

Table1 - Table of materials
 

Components / Bodies Material Relative permeability
Rotor Copper 0.999991
Outer Air Air 1
Rotor Mild Steel 2000
Band Air 1
Outer thimble Mild Steel 2000
 

The table shows all information related to the permanent magnets used in the model.

Table2 - Characteristics of magnets 
 
Components / Bodies Material Relative permeability Coercivity  Remanence
Permanent Magnets S2818 1.03884 819647   A/m 1.07 T

Loads and Restraints

Loads and restraints are necessary to define the electric and magnetic environment of the model. The results of analysis directly depend on the specified loads and restraints. Loads and restraints are applied to geometric entities as features that are fully associative to geometry and automatically adjusted to geometric changes. 

Table 3:  Force and Torque information
 
Name Torque Center Components / Bodies
Virtual Work At origin Rotor
 
 
The rotor
Figure 3 -The rotor

Meshing

Meshing is a very crucial step in the design analysis. EMS estimates a global element size for the model taking into consideration its volume, surface area, and other geometric details. The size of the generated mesh (number of nodes and elements) depends on the geometry and dimensions of the model, element size, mesh tolerance, and mesh control. In the early stages of design analysis where approximate results may suffice, you can specify a larger element size for a faster solution. For a more accurate solution, a smaller element size may be required
In the study with motion coupling we should use a component named Band around the moving parts. This technique allows the re-meshing of the moving parts and the Band in each step of simulation.

Mesh quality can be adjusted using Mesh Control (Table 4), which can be applied on solid bodies and faces. Below (Figure 4) is the meshed model after using Mesh Controls.

Table 4 -  Mesh control
 
Name Mesh size Components /Bodies
Mesh control 1 1.66666700 mm Permanent magnets of the rotor
Mesh control 2 2.00 mm Inner rotor
 
Meshed Model in the step 18
 
Figure 4 - Meshed Model in the step 18

Results

The regular flux, field, current, etc. plots are available in motion studies at each position, i.e. time step.  These results can be viewed at each step separately or animated to examine the effect of the motion.  Similarly, the tabular results such as force/torque, inductance, flux linkage, etc. can now be visualized at each time step.  They can also be plotted versus time, position, speed, and acceleration, e.g. torque vs. speed.  Furthermore, the kinematic results such as position versus time can also be visualized right in the tabular results.  A more complete motion and kinematics results are readily available in the SolidWorks Motion Manager.

After running the simulation of this example we can obtain many results. Magnetostatic Module generate the results of : Magnetic Flux Density (Figure 5,6), Magnetic Field Intensity ,Applied Current Density, Force density(Figure 7), and a results table which contains the computed parameters of the model, the force and the torque (Figure 8)… 2D plots and animation for motion also are allowed by EMS. 

Magnetic Flux Density in step 31
Figure 5 - Magnetic Flux Density in step 31
 
 Magnetic Flux density in step 13, vector plot
Figure 6 - Magnetic Flux density in step 13, vector plot
 
 Force Density with section plot
Figure 7 - Force Density with section plot
 
2D plot of the rotor torque
Figure 8 - 2D plot of the rotor torque
 

Conclusion

By creating multiple studies, the user can change the materials and the geometry of each part. EMS allows the user to keep the same assembly file and associate each study with a design table position. All these features are very helpful for designers and can be used to determine the motor parameters responsible for optimizing the motor performances. Hence, in addition of being fully integrated in SolidWorks and Inventor, EMS is also accurate and easy to use.