120 Hz vibration peak

[drmayer]

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.

 

[electricpete]

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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[drmayer]

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.