Hi guys, i would like to give me your opinions about a problem i have. In our university laboratory we have designed and built a permanent magnet electrical machine and when we test it the Voltage/Frequency characteristic is not constant as we expected. You can find the graph on the link below. We thought that the rotor is vibrated due that some of the rotation frequencies coincides on rotor's eigenfrequencies, but i have strong doubts about that. What is your opinion?