I have a number of small 1 KW electric motors running at 240V 50Hz. these are usually attached to pumps, but I am fairly sure this noise isn't emanating from the pump, but the motor itself.
The noise we're getting is a beating (adding/subtracting of multiple frequencies), not a single frequency. it's not all that loud, but because it's fluctuating, it's very annoying.
The noise of one particular motor can be changed from good to bad by loosening one of the screws that hold the stator shell to the pump wet end (axial screw, 1 of many). It's interesting too that this causes the noise to go from beating to a high frequency whine at Rotor Bar Pass Frequency. I'm unsure what frequencies are causimg the beating.
(see attachment, black is beating, red is the high frequency RBPF)
I understand that both these noise conditions can be caused by eccentricity of some sort, but can anyone explain why there'd be such a difference in tightening one mounting screw (it moves the mounting foot by only 0.2mm axially).
Thanks in advance for any assistance.