# Why the mode frequency is approximate in modal analysis?

I have used the Ansys workbench to carry the modal analysis of one motor stator and under the free modal analysis, the first 6 mode frequency are all zero, it is ok because of the first 6 mode does not consider the deformation.

However why the adjacent two mode frequency are almost the same, it is queer.(Under the free modal analysis.)

The following the is free modal analysis result:

1.  0.
2.  0.
3.  1.556e-003
4.  4.0131e-003
5.  5.1746e-003
6.  6.3977e-003
7.  875.86
8.  876.46
9.  1662.2
10. 1663.2
11. 2309.2
12. 2311.
13. 3541.8
14. 3542.9
15. 4058.6
16. 4062.2
17. 5459.2
18. 5461.6
19. 5844.8
20. 5852.9


The first non-zero modal is as this: • Did you use a symmetry boundary condition in your analysis? If you use symmetry and do not force the harmonic index to 0, you will see results very similar to this. Mar 29 at 9:29
• @TristanC. I did not use this condition, I use the whole motor stator model and without any constrain, and set the whole stator as one part.
– fhrl
Mar 29 at 10:15
• @AJN, I have submitted the topology of the stator, it is more than symmetric, so why the result is 2 plets rather than 18 plets?
– fhrl
Mar 29 at 12:27
• Is your question "why are the mode pairs not exactly the same frequency and only approximately the same"?
– AJN
Mar 29 at 14:19
• @AJN is on the right track. Heavily symmetrical systems have groups of very similar modes that differ by one 'feature'. They can't be at the same frequency as they would act as harmonic dampers, that is , the mode would split into two frequencies, differing by one feature. I suggest you do some hand calculations with 3 dof systems to get an idea. Mar 29 at 20:50