Electrical calculations

Electrical design

The electrical design of standard machines based on magnetically equivalent circuits, know-how and measured values is fast and easy to perform. After a rough first dimensioning, the optimization can be carried out with the calculation of different geometry variants and operation points. Developers of large generators prefer to use their in-house program mostly written in Fortran or programmed with Simulink; these have been tuned with measurements results.

The magnetic field of new electrical machines should be calculated with the Finite-Elements Analysis. For the concept of a long motor, these calculations can be made in 2D. For a more complex and detailed analysis, the calculations should be made in 3D using a commercial software.


     Magnetic field of an asynchronous motor
     (© Institute for energy and automation, technical university in Berlin)

Localization of the losses

Losses in electric machines are ohmic losses (winding copper losses), re-magnetization (iron losses) and mechanical losses (ventilation losses and bearing losses); the copper losses are for most designs the largest. The allocation of the electrical losses to the places of production is not a subject of focus during the concept phase; the project members are mostly interested in the values of the total losses and the corresponding efficiency. A good knowledge of the locations of production is however required for temperature calculation.

The copper losses of the stator winding and pole winding can be directly calculated based on the current and the voltage waveforms including skin effect. The magnetic field and iron losses can be analytically calculated based on the Steinmetz equation.

In order to localize the distribution of iron loss densities precisely. It is recommended that 2D calculations of the magnetic field be performed prior to a thermal calculation. If this is not possible then the supplementary losses can be located using formulas from freely-available literature.

  Distribution of the flux density for a machine with pole salients


Coupling of the Electromagnetic and Flow Calculations

Commercial tools usually advertise for Multiphysics, do we really need it? The iron losses hardly depend on the temperature, only the copper losses increase with the temperature as the resistance increases. The losses should be calculated with measured values of the temperature, if available; otherwise, an initial temperature must be guessed. If the calculated temperature differs then one can simply recalculate the resistance and the copper losses with the calculator. The thermal calculation should be then restarted.
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