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120 Hz vibration peak

120 Hz vibration peak

120 Hz vibration peak

(OP)
I am doing vibration analysis on some of the fans our company produces.  I am finding a large contributor occurs at 7200 cpm or 120Hz.  The motors are less than 1hp, single phase, 60Hz, 115V induction motors.  Could someone explain this peak at 120 Hz?  I know it's a 2nd harmonic of 60Hz, but that's about it.

RE: 120 Hz vibration peak

2* line frequency is a common result of magnetic forces.

Most common in 2-pole motors (3600 rpm or 3000 rpm).

Biggest thing to look at is the motor feet. If they cause distortion of the motor casing than 2*LF can occur.

Thinking about small motors, if the driven component is putting a bend on the shaft that can cause 2*LF.

Operating a motor at higher votlage or lower frequency than intended can increase 2*LF vibration.

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RE: 120 Hz vibration peak

(OP)
thanks for the reply.  I see this peak with the motor loaded or unloaded (it is larger when loaded).  They are 4 pole motors however.  

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