What is spectrogram synthesis technique (synthesized or ideal spectrogram)?

Spectrogram synthesis is a technique to accelerate the calculation of variable noise / vibration (called spectrogram) based on a limited number of single speed calculations (from 1 to 6 single speed calculation of harmonic forces).

Illustration of spectrogram synthesis technique in MANATEE software
Illustration of spectrogram synthesis technique in MANATEE software

This calculation is activated in MANATEE with

Input.Simu.is_spectro_synthesis=1

It is an alternative to the variable speed mode, where all electromagnetic and vibroacoustic calculations are repeated at each different speed.

A variable speed vibroacoustic spectrum can be obtained based on a single speed vibroacoustic simulation when the evolution of the magnetic excitation with speed can be predicted ni terms of magnitude and frequencies (including rotation direction).

As an example, the magnitude of the flux density and resulting Maxwell stress in an open circuit permanent magnet synchronous machine is independent of speed: the frequencies of the magnetic force harmonics change with speed but their magnitude is unchanged. This means that the vibroacoustic behaviour of the electrical machines can be quickly calculated at variable speed without significant additional calculations.

At partial load for PMSM, some specific single speed calculations are run with different current angles to capture the evolution of forces with current, and the evolution of magnetic force harmonics with speed can be deduced from torque and current angle maps as a function of speed and current magnitude. This technique is also used to quickly generate acoustic noise maps on all the operational points of the machine in torque / speed plane.

The synthesized spectrogram technique can also be used at no-load on induction machine.

Such algorithms are used in MANATEE to speed up the calculation process and include the vibroacoustic criterion in early design phases. The validation test ALG_001 favorably compares the synthesized sonagram technique with single speed calculations.

Some specific warnings are linked to this technique. In particular if the number of time steps defined at nominal speed is not enough then a part of the spectrum is missed at low speed. Generally vibrations are proportional to speed so at low speed the magnetic excitation is at low frequency, and the frequency limitation does not affect the vibroacoustic results. The warning message might look like "WARNING, synthesized spectrograms may not be valid below 9.3796 Hz / 187.5916 rpm (try increasing Nt_tot above 7684)".

This technique is independent from both the electromagnetic and structural models. It can therefore be used when importing the flux density calculated with another FEA software, or when using MANATEE coupling with external structural FEA software such as Optistruct and Ansys (e.g. during Electromagnetic Vibration Synthesis process).

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