Design strategies (Electrical Machine)


The development and design of electromagnetic devices reflects a complex process. Originating from an initial idea, the construction runs through different phases. This procedure is terminated when a final concept is selected and considered to be designed, subject to various targets and constraints. As a whole, the task of the design engineer is to find solutions for technical problems. On the way to the latter physical and technical product, certain aspects have to be considered. Technological and material-dependent questions as well as cost effectiveness and ecological constraints have to be taken into consideration. A cut-set of the mentioned boundary conditions controls the feasibility of the final design. With emphasis on electromagnetic devices, Fig. 2.9 shows a simplified scheme of interdependences of targets and constraints. This simple pattern clarifies that the design process is strongly dependent on the experience of the engineer and reflects an optimization procedure with often contradictory aims. Therefore, the necessity of a systematic and strategic design with engineering tools is obvious. Here, solution strategies using modern numerical methods to accelerate and ensure a high-standard technical product in an overall design process are discussed.

Interdependencies in the design of electromagnetic devices.

Fig. 2.9. Interdependencies in the design of electromagnetic devices.

Designing electromagnetic devices includes the calculation and analysis of the electromagnetic field distribution. From the local field quantities forces, torques and losses can be derived to make predictions concerning global quantities such as converted power and efficiency. For complicated geometries analytical field solutions are non-existent or very hard to obtain. Using numerical field computation techniques of a general application range, the microscopic field solution leads via a lumped parameter approach to the desired time-dependent behaviour of the device (Fig. 2.10).

The microscopic field solution itself delivers important knowledge regarding the material utilisation. Such results offer the opportunity to reduce material, weight and the costs of the latter product. To accelerate development, extensive field computations with various types of material can be performed avoiding expensive prototyping. It is even possible to predict system behaviour before new materials are actually available on the market. With this knowledge, the design engineer can order special material to be developed at the material manufacturer or, vice versa, if the material supplier uses such numerical tools he can suggest and offer the right choice of material for a particular device.

Analysis scheme using the finite element method.

Fig. 2.10. Analysis scheme using the finite element method.

Lumped parameter models are essential for the development of control strategies for electromechanical devices such as electrical drive systems. To be able to perform real time control schemes, lumped parameter models are used to form an observer control. Here, very accurate field computations are recommended to determine the concentrated elements of such models.

FEM model of the end-winding area to compute the leakage reactance of a servomotor with d-q model in vector diagram representation.

Fig. 2.11. FEM model of the end-winding area to compute the leakage reactance tmp1015_thumbof a servomotor with d-q model in vector diagram representation.

For example, the computation of the leakage reactance of electrical motors can be performed using a three-dimensional FEM model (Fig. 2.11). The knowledge of this reactance is essential for the optimum control of a permanent magnet-excited servomotor.

Knowledge-based design

The main aspect of the structured development of novel technical products is analysis followed by a detailed synthesis. Analyzing means obtaining information on partial functions of the desired overall function, by investigating single elements and their mutual interactions. In this way overall links between various principles of the partial functions are found.

In Fig. 2.12, a structured and knowledge-based development process is illustrated in a simplified scheme. In this example, the final technical product has to be designed, able to fulfil three partial functions. Those individual functions to be connected to the overall task of the product are a linear motion, a continuous rotation and some reverse operation. After the analysis phase, in the synthesis step different physical working principles are selected and evaluated. The selection process is governed by simple qualitative rules. In this way the partial functions are evaluated with regard to their feasibility with respect to the given constraints and limitations (Fig. 2.12). In this step, the feasible principles are ranked qualitatively by weighted constraints and limitations.

Knowledge-based and structured design.

Fig. 2.12. Knowledge-based and structured design.

The process of synthesis (Fig. 2.13) leads from qualitative decisions to quantitative statements in a following design step. The whole process is accompanied by the consultation of experts and expert knowledge (Fig. 2.12). A detailed investigation and ranking, i.e. the precise calculation of the operating conditions, leads by a comparison to the final technical product. In this loop of iterations, between validation and the performance of detailed predictions of qualified concepts, a numerical optimisation combined with field computation methods is found as an important and powerful engineering tool for the design of electromagnetic devices.

The quantification and ranking of the working principles is governed by the choice of materials or other components such as electronic circuits. Their interdependency on the studied principle can be distinguished into an object and a rule world. The various, for instance ferromagnetic, permanent magnet, conductive or dielectric materials, and respectively components such as the electronic hardware have, considered inside an object world, particular properties and characteristics. To employ such object properties in order to obtain a physical working principle fulfilling a desired function, appropriate rules determining the function of the object have to be considered. In both object and rule world, constraints are found to govern a decision to consider the principle further in the ranking or to reject it. Numerical techniques can help to employ the rules accurately to the studied object.

Process of synthesis.

Fig. 2.13. Process of synthesis.