Question:
Good day to all,

Did anybody came across on vibration analysis services on 2250KW(leroy somers) motor/74Hz, variable drive. 1480 rpm,sleeve bearings.How to diffrentiate mechnical or electrically induced vibration other than switching off the power. What are the common problem, i.e if any excessive clearance of the bearing , can it result in resonance condition,etc..

I did perform coast down for this equipment , but high vibration at 8 mm/s observed on 1480 to 1200 rpm, but then reduce to 1 mm/s at 1000rpm. Is there any other thought than resonance, Phase change right at 90 deg in 1480 rpm, can rotor thermal bow cause this problem.. when heat up due to high speed 1480 rather than at 1000 rpm.. . how many rotor bar for this type of motor , as found 0.7 mm/s at 103 X rpm.. can i conclude this as the # rotor bar

Thanks & best regards..

Joe...

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One way to differentiate mechanical from electrical vibs which is possible on a sync motor (but not an induction motor) would be to vary the excitation. As you progressively increase excitation going from from lagging p.f.(vars in) toward unitity p.f. and further toward leading power factor (vars out), the flux increases and electric related vibrations should increase.

A word of caution - I believe sometimes vibrations of mechanical original are exacerbated by the electrical field so it is not a pure mechanical or pure electrical but result of both. I was warned of that by a salty Siemens OEM... don't know if it's correct but I picture the combined effects of movement of rotor centerline at 1x within sleeve bearing clearances as well as flexing of rotor in the middle can reduce the airgap noticeably at 1x and magnetic force pulls more at the location of the minium airgap and tends to reinforce the 1x vib.

If you see 2*LF spacing among peaks in the high frequency it is usually safe to assume it is a RBPF +/- 2*LF pattern.

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