Software models motors in 3D

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Maxwell Version 10 may end the search. lt uses finite elements to model 3D fields in both translating and rotating electromagnetic devices. Claw-pole alternator/motors, for example, have highly 3D geometry and cannot be modeled in 2D. Previous analyses assumed a stationary rotor and currents that were all known and dc. Those assumptions allowed computing only the dc (static) magnet field. V10 of the software removes those restrictions. It simulates rotation of claw-pole rotors, passage of time-varying voltages through attached electric circuits, and the resulting magnetic fields, losses, and torques.

As usual with finite-element modeling, model creation is the biggest task, though Maxwell 3D makes it fairly straightforward. Geometry menus include simple 3D shapes such as boxes and cylinders. For common, complex shapes, 2D curves can be swept or extruded into 3D parts. The software draws each shape as it is entered (with no noticeable time lag) in four simultaneous views — top, front, side, isometric. Adaptive meshing eliminates manual entry of finite elements, while mesh "seeding" accommodates specific finite-element sizes. This is useful in regions that need high accuracy, such as in air gaps, the region between a stator and rotor.

To model motion, you must define a rotating or translating band in the air gap. Maxwell then prompts users to enter material properties of each part. A large materials catalog includes nonlinear B-H curves and conductivities, as well as structural properties such as Young's modulus, Poisson's ratio, and thermal-expansion coefficient. Core loss and efficiency are computed using loss curves for steel, because many types of motors and generators use steel laminations.

Voltage sources and boundary conditions are entered next. Sources can depend on time, position, or speed. For transient analysis, also specify the time period of interest. Time steps can vary during transient analysis, and the same finite-element mesh can be used while a rotor is turning, thus avoiding the compute time of remeshing. Material properties may also vary with time. This is useful because conductivity of steel and copper drops as temperature rises. Selecting "solve" starts the meshing and solutions steps. An alternator provides an example of the process.

To save compute time, users need model only one-pole pitch. Be careful to apply proper periodic boundary conditions on model surfaces one pole-pitch apart. In particular, when the number of poles modeled is odd, as it is in an alternator, then the periodic boundary conditions enforce flux on one boundary to be the negative of flux on the other. When modeling an even number of poles, the periodic boundary conditions require flux entering both boundaries be the same unknown value.

The simulation calculates motion-induced eddy currents in the solid steelclaw rotor poles and their effect on alternator efficiency. Computed efficiency also includes losses in the alternator rectifier circuit. Compute time in this case is 15 hr, 10 min, on a Pentium Xeon 2.8-GHz computer with 2.8-Gbytes RAM.

A few recent features in V10 that make the above analysis possible include allowing both rotation and translation. Magnetic actuators and switches involve a lot of translations. The software calculates magnetic torques and forces and their effects on mechanical stresses. It also couples electric circuits with solid or stranded conductors. Circuit sources can depend on time, position, or speed. Core losses are computed using loss curves for steel, thus providing accurate efficiency predictions for many types of motors and generators that use steel laminations. Time steps can vary during transient analysis, and the same finite-element mesh can be used while a rotor is turning thus avoiding the compute time of remeshing. Material properties can also vary with time. This is useful because conductivity of steel and copper drops as its temperature rises.

Maxwell 3D comes from Ansoft Corp., Four Station Square, Suite 660, Pittsburgh, PA 15219, (412) 261-3200, www.ansoft.com —

John R. Brauer

John Brauer ([email protected]) is Adjunct Associate Professor at the Milwaukee School of Engineering.

For further reading on electromechanical analysis

J. R. Brauer, G. A. Zimmerlee, T. A. Bush, R. J. Sandel, and R. D. Schultz, 3D Finite Element Analysis of Automotive Alternators Under Any Load, IEEE Trans. on Magnetics, Jan. 1988.

J. R. Brauer, E. A. Aronson, K. G. McCaughey, and W. N. Sullivan, Three Dimensional Finite Element Calculation of Saturable Magnetic Fluxes and Torques of an Actuator, IEEE Trans. on Magnetics, Jan. 1988.

J. R. Brauer and F. L. Zeisler, Automotive Alternator Electromagnetic Calculations Using Three Dimensional Finite Elements, IEEE Trans. on Magnetics, September 1986.