Unbalanced magnetic pull
Unbalanced magnetic pull
(OP)
Hello all.
I am studying the dynamic behaviour of a squirrel-cage rotor. I have some reports where its natural frequencies have been determined experimentally in a free suspended state (only the rotor, out of the motor). Trying to make a relation of this results with the ones that coud be expected with the rotor installed and running i know that bearing system would stiffen the shaft and increase the values of the natural frequencies. However, i have a doubt about other fact: unbalanced magnetic pull. From information i have gathered it is possible to assume it as NEGATIVE STIFFNESS SPRINGS (for FEM simulations, for example). ¿Would this fact lower the values of the natural frquencies in an unstiffen-like phenomenon?.
I would thank any kind of information about the calculation of the forces produced by the Unbalanced Magnetic Pull.
Thank you.
I am studying the dynamic behaviour of a squirrel-cage rotor. I have some reports where its natural frequencies have been determined experimentally in a free suspended state (only the rotor, out of the motor). Trying to make a relation of this results with the ones that coud be expected with the rotor installed and running i know that bearing system would stiffen the shaft and increase the values of the natural frequencies. However, i have a doubt about other fact: unbalanced magnetic pull. From information i have gathered it is possible to assume it as NEGATIVE STIFFNESS SPRINGS (for FEM simulations, for example). ¿Would this fact lower the values of the natural frquencies in an unstiffen-like phenomenon?.
I would thank any kind of information about the calculation of the forces produced by the Unbalanced Magnetic Pull.
Thank you.





RE: Unbalanced magnetic pull
Note that if we offset the rotor from the centerline there is a dc component of the force and a 2*LF component. I believe it is the dc component that would act to modify (reduce) the effective spring constant. The 2*LF component would need to be considered separately.
RE: Unbalanced magnetic pull
RE: Unbalanced magnetic pull
#1 "dc" = force with no time variation
#2 "2*LF" = sinusoidal time varying force with frequency = twice line frequency.
A basic idea of these two components would come from assuming a fixed geometry displaced from the center.
F ~ B^2 ~ [Bmax*sin(2*Pi*f*t)]^2
~ Bmax^2*sin^2(2*Pi*f*t)
~ Bmax^2 * [0.5-0.5*cos(4*Pi*f*t)]
The 0.5 part is the dc part. The 0.5*cos(4*Pi*f*t) is the 2*LF part. Although the simple equation above suggests that these components have equal maximum magnitudes, they are not equal when the full problem is modeled.
RE: Unbalanced magnetic pull
Any bibliographycal reference or standar for the calculation of this force??? Sorry but i don't use to deal with electrical problems...
RE: Unbalanced magnetic pull
Cheers
Greg Locock
RE: Unbalanced magnetic pull
You may find that an energy method (Rayleigh Ritz) may give you a more tractable insight into the change in modal frequency, if you can write a good equation for PE.
Rayleigh Ritz is adequately described in William Thomson's vibration book.
Cheers
Greg Locock