Motor-CAD Induction Motor - Combined electromagnetic and thermal model (February 2015)

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1 Motor-CAD Induction Motor - Combined electromagnetic and thermal model (February 2015) Description The Motor-CAD allows the machine performance, losses and temperatures to be calculated for an Induction 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

2 Machine Geometry The standard default machine geometry for an induction machine is used in this tutorial as shown below. This is a 4-pole, 50Hz, 18 slots, 26 bars induction machine. 2

3 Control definition The control definition page shown below allows the calculation of the machine performance (speed, frequency, slip, voltage and connection) to be defined. It also allows any skew angle to be defined. Drive Mode (use in conjunction with Performance test/torque-speed characteristics) Motor: The induction machine operates between zero speed (slip = 1) and synchronous speed (slip = 0). The corresponding performance curves are plotted when selecting Torque/see Generator: The induction machine operates between synchronous speed (slip = 0) and double value of the synchronous speed (slip = -1) Motor/Generator: The induction machine operates between zero speed (slip = 1) and double value of the synchronous speed (slip = -1) Brake: The induction machine operates between negative synchronous speed (slip = 2) and zero speed (slip = 1) 3

4 Operating Point Definition Use to define one singular operation point that is of interest. By using sensitivity analysis it is possible to calculate the performance for a given range of speed, frequency or slip values Shaft Speed - the rotational speed of the machine Slip - The relative difference between the synchronous speed of the revolving magnetic field, and the shaft speed Frequency - the fundamental supply frequency NOTE: The relation between the shaft speed, slip and frequency is given below, where Poles is the machine s number of magnetic poles: 4

5 Supply Definition Two methods are available to control the supply voltage of the induction machine AC Mains the machine is connected directly on the mains line Inverter Fed the machine is 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 Ratio Vll(rms)/DC bus voltage SixStep Hexagon tracking :piecewise linear Hexagon tracking : secant Circle tracking SixStep Maximum linear range of sine/triangle Voltage Definition for AC Mains, assumes sinusoidal voltage for which the peak or the rms value of the line-line voltage is given for Inverter Fed, the DC bus voltage is available in conjunction with the PWM modulation strategy. 5

6 Set the supply voltage for this machine to 430V rms as shown below. The performance test options section allows the user to select which calculations to run. We will select to run all performance tests as shown below. The calculation time can be speeded up 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. In this case we will use the default temperature values. 6

7 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 100 turns per coil. For this machine design will have 1 strand in hand for each turn in the slot. This is set using the edit box shown below. You 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 0.1mm to distribute the conductors in the slot as shown below. The wire size is defined using the copper slot fill factor and this values is set to

8 Materials Motor-CAD has a materials database populated with commonly used materials. Other materials can be added as required. 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'). 8

9 You will then get the prompt below: After selecting Yes the database will then be updated with the magnetic materials. In this model will use the default materials provided from the materials database. 9

10 Solving The model can now be solved by clicking on the 'solve' button: The Motor-CAD electromagnetic module uses both analytical algorithms and 2D finite element analysis to calculate the electromagnetic performance. The minimum solution based on symmetry is automatically selected for the finite element calculations. The finite element model and results can be viewed while solving by selecting the E-magnetics tab: 10

11 Results Once the calculation is completed then various graphs can be viewed as shown below with analytical estimation of the airgap flux-density (Bgap), saturated and unsaturated values. 11

12 The graphics results can be grouped according to the modelled performance test: Torque-speed characteristics Can display the steady-state results for current, torque, power, losses, efficiency, power factor variation with slip for this machine: 12

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15 Self and mutual inductance 15

16 Acceleration 16

17 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 (*Torque.mes files). The total loss densities and loss components can be displayed as shown below (*Loss.mes files): 17

18 The output sheets provide information on the machine performance and losses: 18

19 The losses have been calculated at assumed temperatures as can be seen above. 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. 19

20 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 rotor temperatures. 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 "Thermal" 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. 20

21 Solving the coupled electromagnetic thermal model To solve the model now select the "Solve E-Magnetic model" button shown below. Note: to speed up the iteration it can be useful to keep just Single load point test, as the other tests are not required for calculating the machine losses. 21

22 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. 22

23 After pressing the OK button the iterative calculation will then run. This is normally quite a fast calculation. In this case the convergence took 3 iterations as shown below. In this model the final winding and rotor temperatures are quite low so the performance of the machine has improved slightly from the initial predictions. 23

24 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: 24

25 The thermal model can be viewed by selecting to view the thermal interface using the option shown below: Can now see the losses that have been calculated in the electromagnetic model being used for the thermal model: 25

26 The radial and axial views shows the main points in the thermal model including the winding, stator, rotor iron and rotor cage 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. 26

27 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. 27