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AC Synchronous Reluctance Machine for Traction Application
Gallery spotlightMotors & Generators
The rise of rare earth permanent magnet prices due to their limited supply has revived a great deal of interest in AC synchronous reluctance machines, particularly, for traction applications. In this example, a 55 KW traction motor is designed using a stator that was originally designed for a squirrel cage induction motor for a similar output rating and application. The design of the new machine uses the stator of the induction machine and only the rotor geometrical parameters and configurations are used as free design parameters to achieve the target performance criterion.
METHODS and RESULTS
Model Performance Specifications
The target performance criterion for the synchronous reluctance machine are; operating speed range from 0 to 10Krpm, constant torque range from 0 to 5Krpm, DC Voltage, 650 V, max diameter, 250 mm. Shown in this figure is the default MotorSolve interface and model for reluctance motors which will be modified to design a motor that satisfies the desired specifications.
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Model Setup and Sizing
Initial sizing of motor models are generally done by selecting pole and slot combinations, material choice, setting rotor and stator outer diameters etc. A 36 slot stator is chosen that is based on an existing AC induction machine and a 4 pole, 4 layer angled barrier rotor template is selected for this example. Note that other design options such as the number of barriers as well as their shapes could also have been selected as a case study. The selection made here was based on design lessons reported in the literature for such machines and considering the target performance specifications. Also, 29 Gauge M19 lamination material is chosen for the rotor. The initial model configuration is shown in this figure which will be iterated to create an optimal design which satisfies the target performance specifications.
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Winding Design
The stator winding details of this model are as follows; lap winding, coil span, 9, phase offset 6, 16 turns with 6 parallel paths, double layered and rectangular cross section wires were used to obtain high fill factor. It is shown in this figure.
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Design Iterations
In this example, the stator geometry as well as the winding configuration was kept fixed, due to the desire to reuse an already existing induction machine. Hence, only the rotor configuration and its associated geometrical parameters were considered free design parameters. The main objective during design iteration was to achieve a reasonable flux and current density distribution at various load points. In addition, the design principle for synchronous reluctance machine is to aim for the lowest d-axis to q-axis inductance ratio for achieving high torque density. A reasonably low value of this was obtained through the iterative process applied here. It is not claimed that the optimal ratio has been obtained for this case. A simple comparison of the flux density distribution of the initial and final rotor geometry is presented in this image.
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Final Design Cross Section
This image presents the final design cross section and the torque-speed characteristics of the machine for a range of advance angles. The torque-speed characteristics clearly show that for advance angle values between 70 and 75 degrees, a reasonable constant torque and constant power region is obtained that satisfies the target design criterion.
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Losses
To optimize the design during fine tuning all sources of losses must be taken into account. These include friction, windage, stray, core and ohmic losses for typical electric machines. MotorSolve, as an FEA based simulation software, takes into account each of these losses in calculating machine efficiency. Core loss, an important loss factor for variable speed traction applications is calculated both locally and globally, and includes hysteresis and eddy current contributions. The losses are calculated based on the Steinmetz equation. This figure shows the loss distributions of the final model.
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Design Summary
The model performance summary is presented here. The values reported here were computed using the transient with motion simulation capability of the MotorSolve software. The design presented satisfies the target criterion set forth at the beginning.
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Concluding Remarks
A typical design sequence of an AC synchronous reluctance motor for traction application has been implemented using MotorSolve v2.6 and the results have been presented here.
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