# frequencies in frequency spectrum with no correlation together

I have a lack of understanding of the following questions: If I have a signal from a motor that is recorded with an accelerometer. And the rotating speed of the motor is 150Hz(rpm 9000 ), I can see in the frequency spectrum harmonics of the fundamental frequency(150hz). The harmonics are integer multiples of the fundamental frequency. But what are the other frequencies for example peaks at other frequencies that are not integer multiples of the fundamental? Shouldn't all frequencies that occur from a motor actually be something that is related to the rotating speed?(and i need to understand the building blocks of a motor to understand the frequencies to find a correlation to the non-integer harmonics?) Can there also frequencies with no correlation to the fundamentals? That would mean i have multiple fundamental frequencies? and there is something next to the roational speed of the motor, that also has his own frequency or what?

• It could be because of non linearities in the captured signal, it could be noise. Are the amplitudes of frequency that are not at integer multiples of 150 comparable to those at integer multiples of fundamental frequency? May 1, 2020 at 15:25
• they amplitudes are not comparable they are higher than the fundamental (~ 5 times higher amplitude) and they occur at ~ 650-750 hz.
– Khan
May 2, 2020 at 11:19
• Then this looks like some unintended signal being coupled during acquisition of the intended signal May 2, 2020 at 12:19