What is a magnetic Load Case (LC)?

Definition of magnetic Load Case

In Manatee software, the electromagnetic excitations due to Maxwell forces are linearly decomposed into Load Cases. To do so, magnetic excitations are generally expanded in Fourier series along the airgap (for Maxwell stress) or along tooth distribution, resulting in a certain number of wavenumbers.

A magnetic Load Case is defined by four characteristics:

  • its type (force or moment)
  • its direction of application (radial, circumferential, axial for forces)
  • its structure of application (rotor, stator)
  • its wavenumber (r=0, r=1, r=2, etc) along the airgap (this notino can be used for both radial & axial flux machines)
 
A Load Case is therefore a special way to excite the structural dynamics of the electrical system. The excitation frequency is not included in the definition of the Load Case, although there is a correlation between wavenumber and frequency in electrical machines. A Unit Load Case (ULC) is a Load Case with a unit magnitude (e.g. 1 N, 1 N/m2 or 1 Nm). ULC are generally applied at variable frequency to perform Electromagnetic Vibration Synthesis. An Operational Load Case (OLC) is a Load Case with an excitation magnitude associated to a particular Operating Point (typically a torque speed point for traction motors). A magnetic Load Case is not a lumped force. As an example, a stator tooth radial lumped force is a radial force waveform which contains the effect of several wavenumbers and harmonics, so several Load Cases.

Application to e-NVH physics

Most important Load Case for air-borne magnetic noise in radial flux machines are generally radial forces applied to the external structure (stator most of the time) with “low” wavenumbers (r=0, r=1, r=2 for instance). In particular, in electric vehicles applications, electric traction motors vibroacoustics are often dominated by radial r=0 forces at high speed (air-borne noise) and by tangential r=0 forces at low speed (structure-borne noise).

Most important Load Cases for structure-borne magnetic noise are generally rotor torque ripple (r=0, tangential) and Unbalanced Mangetic Pull (r=1, radial).

For more information, see EOMYS technical note on the main transfer paths of magnetic noise and vibrations.

Application to Manatee e-NVH software

In Manatee software, magnetic noise and vibraitions can be calculated by Load Case decomposition using Electromagnetic Vibration Synthesis. Load Case can be visualized using Operation Force Shape environement.

Electromagnetic Vibration Synthesis is also an efficient e-NVH root cause analysis tool, as a first analysis of the origin of noise can be done at the end of the calculation. In most cases, Load Case separation also also to significantly speed up e-NVH computations.

Jaguar Load Case contribution from EVS
Example of Magnetic Load Case contribution on Jaguar I-Pace IPMSM using Manatee e-NVH software