×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Unbalanced magnetic pull

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.

RE: Unbalanced magnetic pull

Yes, the unbalanced magnetic would act to reduce the total effective stiffness of the rotor support and decrease the natural frequency.

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

(OP)
Thank you for the informationbut, sorry, i don't understand what do you mean with dc and 2*LF components. In the only formula i have for calculating the radial force produced by the unbalance it depends on some geometrical parameters and coefficients, the square of the magnetic flux (constant for me) and the eccentricity. This way i obtain a linear stiffness law (linear spring).

RE: Unbalanced magnetic pull

If you move the rotor off-center, there are two components acting on it:

#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

(OP)
Thanks.

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

Hmm, interesting. One problem you'll have is that the magnet is a non linear negative spring, the more it deflects the higher the rate. Therefore simple modal models will struggle with this, ie you'll need to do a non linear dynamic analysis.





Cheers

Greg Locock

RE: Unbalanced magnetic pull

Damn, hit reply too quick.

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources