Hello All,

I have a question on a waveform.

What would one expect to see in the spectrum based on this waveform. 1X, 2X, high frequency, subsynchronous, etc.?

Not a trick question or anything like that. I just wonder what you guys might think.

It's from a 3600 rpm motor.

You have a beat (modulation) occurring with a frequency near twice line frequency. Beyond the typical 1X and 2X I would anticipate that if you zoomed about 120 Hz you would find some sidebands.

Loose rotor bars often will cause 1X with fp sidebands, but may also demonstrate 2 times line frequency sidebands (2*fline) about rotor bar passage frequency (fbar pass) and its harmonics, with high vibration seen at 2* fbar pass also possible. Seismic vibration levels exceeding 0.05 in/sec peak in the fbar pass region warrant concern.

Uneven air gap or eccentric rotors produce an uneven variable air gap. Typical responses are seen at 2 times line frequency (2*fline),with sidebands of pole pass frequency (fp).2*fline seismic vibration levels exceeding 0.05 to 0.10 in/sec warrant concern. Sidebands at 2*fp indicate a more serious condition. Pole pass frequency sidebands may also be seen around rotational speed, 1X. An audible “beat” is also a typical characteristic of this malfunction.

John

My guess would be RBPF with multiple 2*LF sidebands.

(does the winning guess get a prize?)

quote:

(does the winning guess get a prize?)

I wish.

Ralph ,

The true data taken wich you show is the sum of two frequencies lightly different the one of the other of about the same amplitude.

In the other hand, this two frequencies has a beat two times for each revolution.

If this two frequencies were the only, each beat lowest level would be zero, but like it is not, it mean that there is or there are very low amplitudes at higher frequency than the two mentioned.

This sum of these two high frequencies with almost equal frequency is a potential amplitude helper for 2X misaligment problems and I believe that this is the source of your post.

My best regards.
Miguel Kovac

Ralph,

Most common cause of beating vibrations and sound for 3600 rpm induction motor is 2xSS from shaft misalignment beating with 7200 cpm from 2x line frequency. A high resolution spectru would show two frequencies. A coarse live spectrum with both frequencies in the same bin would show time changing amplitude.

Walt

From what could be seen from the TWF with a naked eye it appears that some high frequency (likely RBPF) is modulated by, likely, 2xLF.

I wonder why would one want to guess? A spectrum (is it not available?) will show much more then visually feasable in this TWF pattern.

This message has been edited. Last edited by: David_G,