Frequency in a blender signal

[sybrenlaga]

We have a blender connected to a loadcell. This blender mixes chemical powders togheter. We discovered an oscillation in this weight signal, which makes sense considering the blender impellers are rotating. When analyzing this data further using Fourier Analysis, I found that for low rpm's, the frequency I got from Fourier analysis matches perfectly with the rpm of the motor. However when running at 600 rpm, I get a peak at 40 Hz instead of 60 Hz. Could there be any mechanical reason for this?

 

[IRstuff]

600 rpm is 10 Hz, so it might be something internal to the motor, like having 4 poles and the commutation might be generated a bunch of electrical noise that interferes with the load cell signal.

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[petb]

Blade pass frequency (BPF) or 2 x BPF of the mixer impeller maybe? How many impeller blades?

/PB