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. In this tutorial will describe how to model the electromagnetic performance of a machine and then combine this electromagnetic model with a thermal model to calculate the full machine performance. 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 using the electromagnetic model by selecting the option shown below. 1
Machine Geometry The standard default machine geometry for a BPM machine is used in this tutorial as shown below. 2
Control definition The control definition page shown below allows the machine control (speed, current, voltage and connection) to be defined. It also allows the magnetization and any skew to be defined. The drive mode can be modelled as: ideal sinusoidal current, voltage driven square wave and custom current waveforms. The BPM machine is always supplied and controlled via an inverter. Different PWM control strategies for the inverter are available in the Input/Settings/E-magnetic page. 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 SixStep 180 0.780 Hexagon tracking :piecewise linear 0.7446 Hexagon tracking : secant 0.7418 Circle tracking 0.707 SixStep 120 0.675 Ratio Vll(rms)/DC bus voltage Maximum linear range of sine/triangle 0.612 For this machine will set the DC bus voltage to 500V as shown below. 3
The performance test options section allows the user to select which calculations to run. The default settings are usually good, the options allow the user to speed up the calculation time by removing calculations not required. As this model is a electromagnetic model without the coupled thermal model then the temperatures for the electromagnetic performance will be given by the user. For this first case we will use the default values of 20C. 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. For this machine design will have 4 strands in hand for each turn in the slot. This is set using the edit box shown below. Can also view the winding factors, phasor diagrams and winding harmonics. Can then view the conductors in the slot. In this case change the separation distance between the conductors to 0.05mm to distribute the conductors in the slot as shown below. 5
Materials Motor-CAD has a materials database populated with commonly used materials. Other materials can be added as required. 6
Note: if you have an old materials database then this can be updated to include the new magnetic materials please by selecting "Create new database" and then selecting the existing database name (usually 'solids.mdb'). You will then get the prompt below: After selecting Yes the database will then be updated with the magnetic materials. 7
In this model will use the default materials provided from the materials database: 8
Solving The model can now be solved by clicking on the 'Solve E-Magnetic model' button: 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: 9
Results Once the calculation is completed then the result waveforms can be viewed as shown below: Can see the torque, reluctance, alignment torques and also the cogging torque for this machine: 10
Can also see the torque / speed profile for this machine with different angle of phase advance: Can look at harmonic analysis of current, voltages and torque values. The torque harmonics are shown below. Note the characteristic 6th and 12th harmonics. 11
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. The total loss densities and loss components can be displayed as shown below: 12
The output sheets provide information on the machine performance and losses: 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. 13
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 couple the electromagnetic and thermal models select the option shown below. This option shows the electromagnetic interface. The option "Magnetic + Thermal (interface)" will use the same model but will give the thermal interface view of the model. The thermal model can now be set up with housing type, ducts, materials and cooling options etc.. For this example will use the default machine values so will make no changes to the thermal model. Once the thermal model has been set up then can return to the magnetic interface view of the model as shown below. 14
Solving the coupled electromagnetic thermal model To solve the model now select the "Solve magnetic and thermal model" button shown below. Note: to speed up the iteration it can be useful to remove the "Back EMF", "Cogging Torque", "Torque Speed Curve" and Self and Mutual inductance calculation options as these are not required for calculating the machine losses. 15
The dialog box below should then be displayed. 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. 16
After pressing the OK button the iterative calculation will then run. 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. 17
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: 18
The loss of magnet flux with the increase in temperature of the magnets is shown below: 19
The thermal model can be viewed by selecting to view the thermal interface using the option shown below: 20
Can now see the losses that have been calculated in the electromagnetic model being used for the thermal model: The schematic view 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. 21
Conclusion This example shows how to create a combined electromagnetic and thermal model in Motor-CAD. This model takes into account the machine temperatures and losses when calculating the machine performance and allows different electromagnetic and thermal design concepts to be fully evaluated. 22