Motor-CAD: How to Calibrate the Thermal Model (2D Calibration Method)

Motor-CAD provides a 2D Calibration Model feature to align the analytical Cuboidal Winding thermal model with detailed FEA thermal simulations.

A. Background: Cuboidal Winding Thermal Model

Motor-CAD uses a Cuboidal Winding Model to represent the complex 3D heat transfer within the stator windings. 
Each winding section (front end winding, active region, and rear end winding) is divided into cuboidal elements—volumes with different thermal conductivities in the radial, tangential, and axial directions. 

Key Features: 

• Supports both Stranded and Hairpin winding configurations. 
• Automatically calculates effective thermal conductivities (kradial,ktangential,kaxialkradial,ktangential,kaxial) based on: 

• Conductor material and geometry 
• Slot fill factor 
• Impregnation and insulation properties 
• Each cuboid includes 7 nodes: six on the faces and one at the center (average temperature node). 
• Heat conduction is modeled between cuboid faces and adjacent machine components. 

Reference: R. Wrobel and P. H. Mellor, "A General Cuboidal Element for Three-Dimensional Thermal Modeling," IEEE Trans. Magn., 2010.

B. Purpose of 2D Calibration

While the Cuboidal Model in Motor-CAD is three-dimensional, most FEA-based thermal analyses are performed in two dimensions (2D)—considering only radial and tangential heat transfer. 

The 2D Calibration Model enables direct comparison between Motor-CAD’s analytical cuboidal model and a 2D FEA simulation of the same slot. 
This helps validate and fine-tune the cuboid size and thermal conductivity values before running full 3D machine-level simulations. 

   

   C. How the 2D Calibration Model Works

When the 2D Model for FEA Calibration option is enabled in Motor-CAD: 

1. Axial heat transfer paths to the end windings and endspaces are removed. 
2. The Motor-CAD circuit automatically simplifies to a 2D representation, isolating radial and tangential conduction paths. 
3. The same copper loss distribution used in Motor-CAD is applied to the FEA model to ensure consistent boundary conditions. 
4. The FEA boundary temperatures on the stator outer diameter and airgap are set equal to those in the Motor-CAD model. 

This configuration allows engineers to check the temperature distribution across the slot and ensure the analytical model matches the FEA results. 

D. Calibration Procedure

Follow the steps below to perform 2D calibration: 

1. Open the Thermal Calculation Page 
2. Navigate to: Calculation tab 
3. Enable the 2D Model Option 
4. Select “2D Model for FEA Calibration”. 
5. The title bar will indicate that the model is in calibration mode. 

1. Run the Thermal Model 
2. Click “Solve Thermal Model” to compute the reduced 2D circuit. 
3. The graphical circuit diagram will show the removed axial paths as grayed-out or missing blocks. 
   1. 1. Run the 2D FEA Slot Model 
         Click “Solve Slot FEA” to compute the 2D finite-element model of the winding slot. 
         The solver automatically applies the same losses and temperature boundary conditions as in the Motor-CAD model. 

E. Compare Results 
Examine the temperature distribution along the slot in both models: 

• • • Good agreement indicates accurate thermal conductivity and cuboid sizing. 
    • If discrepancies exist: 
    • Check cuboid sizes in the Winding Editor to ensure they match the actual conductor layout. 

• 1. • If needed, adjust the radial and tangential thermal conductivities in the Winding Settings. 
       (This is rarely required.) 
  2. Optional Validation Tip 
     Increasing the copper losses during this comparison can improve temperature contrast and make deviations easier to detect. 

F. Important Notes

• • • The 2D Calibration Model is only for slot-level validation and should not be used for complete motor thermal predictions. 
    • Once the 2D model is calibrated: 
    • Revert to the normal 3D model for all production simulations. 
    • Keep any tuned cuboid dimensions or thermal conductivities from the calibration stage. 
    • Ensure that the 2D FEA mesh aligns accurately with the Motor-CAD cuboid segmentation to avoid artificial mismatch. 

 

1. Common Calibration Scenarios

Scenario	Recommended Action
Slot temperature gradient differs between FEA and Motor-CAD	Adjust cuboid height distribution (especially near slot opening).
FEA shows higher radial conduction	Verify radial  kk -value and impregnation assumptions.
End winding temperature deviation	Confirm end winding convection boundaries (excluded in 2D calibration).
Non-uniform flux density in FEA	Increase cuboid count or use skewed distribution option in Cuboidal Model settings.

 

1. Post-Calibration: Returning to 3D Thermal Model
2. After achieving good 2D agreement: 
3. Disable “2D Model for FEA Calibration.” 
4. Re-run the full 3D thermal model using your calibrated winding setup. 
5. Use the results for motor-level analyses including end winding, shaft, housing, and cooling jacket heat transfer. 

 

References 

1. Motor-CAD Documentation: Cuboidal Winding Model and 2D Calibration Model sections 
1. R. Wrobel & P. H. Mellor, “A General Cuboidal Element for Three-Dimensional Thermal Modeling,” IEEE Trans. Magn., 2010.