Claudio Pedrazzi

If some natural frequency of the structure (correctly constrained and with the correct masses distribution everywhere) is near the rotating frequency of the motor (or any other excitation source), of course you will get a resonance, this could cause high stresses and high vibrations.  Normally this means to try make the structure stiffer or more flexible, looking for it to have no natural frequency within +- 20% of the excitation frequency.  When this is practically impossible, one could try to increase damping. Hope this helps.