Noise minimization using notches

Notches (also called slits, grooves, or dummy slots) can artificially increase the number of slots from the airgap permeance point of view, so it is an efficient noise reduction technique that can be studied and optimized in MANATEE using model hybridation.

To introduce notches a new machine is first defined by copy/paste/rename machine_SCIM_A in machine_SCIM_D activating:

Input.Geometry.Nnotcher = 1;            % number of notches per rotor tooth
Input.Geometry.Wnotcher=6e-3;                  % rotor tooth tip notch width [m]
Input.Geometry.Hnotcher=3e-3;                % rotor tooth tip notch depth [m]
Input.Geometry.type_shape_notcher=1;    % rotor notch shape (1: rectangle)

The no-load vibroacoustic simulation project tuto_SCIM_01 can be copied/pasted/renamed in tuto_SCIM_28. The effect of notches cannot be calculated using analytical permeance function so the permeance calculation is going to be done with FEMM using

Input.Simu.type_perm_modelR = 2;          % rotor slotting permeance model type:
                                   % 0 to use analytical models
                                   % 1 to call FEMM and use finite element electromagnetic calculations for permeance calculation, and save the corresponding calculation
                                   % 2 to enforce call to FEMM

When running the simulation an elementary model of a rotor slot pitch is created in FEMM to calculate the permeance in non linear case:

FEMM submodel to calculate a rotor notch effect
FEMM submodel to calculate a rotor notch effect

At the end of the simulation the permeance function can be plot using plot_Per_slot post processing:

Permeance in front of each slot
Permeance in front of each slot

One can see that rotor notches have introduced an additional permeance variation that is very similar to the one of the rotor slot itself. In this case the first slotting permeance harmonic is then minimized while the second harmonic of the Fourier series is reinforced, as if the number of rotor slots was doubled.

The effect on the noise is very striking (plot_VS_ASWL_modal_cont):

Structural mode contribution (without and with the notches)
Structural mode contribution (without and with the notches)

On the left is the initial sound power level at variable speed without rotor notches, on the right is the new sound power level with rotor notches. The use of rotor notches gives a 15 dB reduction in this particular example where a strong resonance occurs due to the first rank of permeance harmonics.

The calculation of the variable speed acoustic noise due to magnetic forces including rotor notches effect takes less than 5 seconds of simulation with MANATEE. A sensitivity study can be easily run to optimize the notch number, shape, and dimensions.