Electromagnetic and thermal model for Brushless PM motors

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Julius Shelton
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1 22 December 2017 Motor-CAD Software Tutorial: Electromagnetic and thermal model for Brushless PM motors Contents 1. Description Model Definition Machine Geometry Winding Definition Materials Calculation Results Coupled Electromagnetic / Thermal model Solving the coupled electromagnetic thermal model Conclusions Description The Motor-CAD allows the machine performance, losses and temperatures to be calculated for electrical machines. In this tutorial will describe how to model the electromagnetic performance of a BPM machine and then combine this electromagnetic model with a thermal model to calculate the full machine performance.

2 2. Model Definition Motor-CAD has both electromagnetic and thermal models. These models can be used separately or as a combined electromagnetic + thermal model. In this case we will start by creating an electromagnetic model by selecting the option shown below. Page 2 of 34

3 3. Machine Geometry The BPM machine type is selected from the option shown below: The default Motor-CAD BPM machine geometry will be used for this tutorial. The number of poles is change to 6 to model a 18 slot 6 pole machine. The radial geometry view is shown below: Page 3 of 34

4 4. Winding Definition Motor-CAD will automatically set up the winding pattern for the slot / pole combination of this machine. In this example our motor will have a double layer winding with 40 turns per coil. Page 4 of 34

5 The phasor diagram shows the sum of the MMF voltages from each coil. It can be used to check that the winding pattern is balanced: Page 5 of 34

6 For this machine design will have 4 strands in hand for each turn in the slot. In this machine the conductor separation distance between the conductors should be reduced to 0.05mm to distribute the conductors in the slot. A coil divider width of 1mm will also be used. With these changes made the slot is as shown below: Page 6 of 34

7 5. Materials Motor-CAD has a materials database populated with commonly used materials. Other materials can be added as required. Page 7 of 34

8 In this model will use the default materials provided from the materials database: Page 8 of 34

9 6. Calculation The calculation page shown below allows the machine control (speed, current, voltage and winding connections) to be defined. For this machine will set the DC bus voltage to 500V as shown below: Page 9 of 34

10 The drive mode can be modelled as: ideal sinusoidal current, voltage driven square wave and custom current waveforms. In this example we will use a sinusoidal current: Page 10 of 34

11 The BPM machine is always supplied and controlled via an inverter. Different PWM control strategies for the inverter are available in the Input Data->Settings->Drive page as shown below: The ratio between the output line-line RMS voltage of the inverter and the DC bus voltage is given in the table below: PWM Modulation strategy Ratio Vll(rms)/DC bus voltage SixStep Hexagon tracking :piecewise linear Hexagon tracking : secant Circle tracking SixStep Maximum linear range of sine/triangle Page 11 of 34

12 This machine will have parallel magnetization: Page 12 of 34

13 As this model is a electromagnetic model without the coupled thermal model then the temperatures for the electromagnetic performance will be specified by the user. For this first case we will use the default values of 20C. Page 13 of 34

14 Skew can be used to reduce cogging torque and torque ripple. No stator or rotor skew will be used in this design. Page 14 of 34

15 The performance test options section allows the user to select which calculations to run. The options allow the user to speed up the calculation time by removing performance tests not required. Page 15 of 34

16 The model is then solved by clicking on the Solve E-Magnetic Model button: Page 16 of 34

17 The Motor-CAD electromagnetic module uses finite element analysis to calculate the electromagnetic performance. The minimum solution based on symmetry is automatically selected. The finite element model and results can be viewed while solving by selecting the magnetics tab: Page 17 of 34

18 7. Results When the calculation is completed then the result waveforms can be viewed as shown below: Can see the torque for this machine: Page 18 of 34

19 Can see the cogging torque for this machine: Can also see the torque / speed profile for this machine with different phase advance angles: Page 19 of 34

20 Can look at harmonic analysis of current, voltages and torque values. The torque harmonics are shown below. Note the characteristic 6th and 12th harmonics. Page 20 of 34

21 The finite element results can be played back using the option shown below. Clicking on any region with the mouse will display the field and flux density values. Page 21 of 34

22 The total loss densities and loss components can be displayed as shown below: Page 22 of 34

23 The output sheets provide information on the machine performance and losses: Page 23 of 34

24 The losses have been calculated at 20C as can be seen below. Need now to couple in the thermal model to use the calculated losses and use the temperatures calculated in the thermal model in the electromagnetic model. Page 24 of 34

25 8. Coupled Electromagnetic / Thermal model In the first section we have produced an electromagnetic model for the machine but this has assumed that the winding, lamination and magnet temperatures are at 20C. This model has calculated the winding, magnet and iron losses so these can now be used in a thermal model to calculate the machine temperatures. To change to the thermal user interface view of the model select the Thermal option shown below: Page 25 of 34

The radial cross section now shows the machine housing and machine model can now be set up with housing type, ducts, materials and cooling options etc. that are important for the thermal model.

26 The radial cross section now shows the machine housing and machine model can now be set up with housing type, ducts, materials and cooling options etc. that are important for the thermal model. For this example will use the default machine with a finned housing and natural convection cooling and so will make no changes to the thermal model. After the thermal model has been set up you can then return to the magnetic interface view of the model as shown below: Page 26 of 34

27 9. Solving the coupled electromagnetic thermal model To set the model to have a combined Electromagnetic and Thermal model now select the Iterate to Converged Solution as shown below. To speed up the iteration we will also remove the Performance Tests and use only the On Load single operating point loss calculation. For more detailed analysis the Torque calculation can be run for calculating the machine losses. Page 27 of 34

28 Click on the Solve E-Magnetic Model button as shown below: The dialog box below should then be displayed: Page 28 of 34

29 Clicking on the OK button should then run an iterative solution between the electromagnetic and thermal models. This will pass the losses to the thermal model that will calculate the machine temperatures. The machine temperatures will then be returned to the electromagnetic model that will then recalculate the performance and losses. This process will repeat until the temperature and loss values converge. In this case the convergence took 3 iterations as shown below. In this model the final winding and magnet temperatures are quite low so the performance of the machine has not varied significantly from the initial predictions. Page 29 of 34

30 The machine temperature calculated in the thermal model that are used in the electromagnetic model are now shown in the control sheet: The losses are calculated based on the machine temperatures: Page 30 of 34

31 The loss of magnet flux with the increase in temperature of the magnets is shown below: Page 31 of 34

the losses that have been calculated in the

32 The thermal model can be viewed by selecting to view the thermal interface using the option shown below: Can see the losses that have been calculated in the electromagnetic model being used for the thermal model: Page 32 of 34

33 The Schematic overview shows the main points in the thermal model including the winding, stator, rotor and magnet temperatures. Now that have combined electromagnetic and thermal models can now study the electromagnetic and thermal model separately and couple them to transfer the results when necessary. Page 33 of 34

34 10. Conclusions This tutorial shows how to create a combined electromagnetic and thermal model in Motor- CAD. This model considers the machine temperatures and losses when calculating the machine performance and allows different electromagnetic and thermal design concepts to be fully evaluated. Page 34 of 34

Motor-CAD Brushless PM motor Combined electromagnetic and thermal model (February 2015)

Motor-CAD Brushless PM motor Combined electromagnetic and thermal model (February 2015) Description The Motor-CAD allows the machine performance, losses and temperatures to be calculated for a BPM machine.