The line frequency in my region is 50Hz. We have a number of cases where half of the line frequency are present at high amplitudes and some cases harmonics of this 1/2line frequency on electric motor/generators. This is not clear to us as we don't understand the meaning of 1/2line frequency in a vibration spectrum. Please I need your support in this regards. Thanks

Original Post

What is your actual diesel speed and is the diesel a 4-stroke?  4-stroke diesels usually have a large component of vibration at 1/2 of their rotating speed and at harmonics of 1/2X.  Sometimes that energy moves through the foundation/mounting and can be present at places well away from the diesel.

So a 2-pole generator for 50 Hz would run at 3000 RPM (50 Hz) and I would anticipate finding 25 Hz in the system.  A 4-pole generator for 50 Hz would run at 1500 RPM (25 Hz) and I would anticipate finding 12.5 Hz in the system.

Last edited by John from PA

The 1/2 order is not a sign of mechanical looseness. It is a characteristic of the DG itself.    https://www.mobiusinstitute.co...alysis%20Definitions

Your spectrum shows exactly what I would expect (1X orders 16.6 Hz and 1/2 orders spaced at 8.33 Hz as seen in the spectrum.

So, if you mean 25 hz as half-line frequency it is simply 1.5 orders of "rotation" speed - NOT 1/2 order of "line frequency"

Hope this helps

Jim P

Its normal on diesel engines to have multiples of 0.5x, sometimes one of this harmonics excite a frequency, trending is the key, in a place I worked one of the peaks was clearly increasing in generator side, after changing a rubber element of the coupling it decrease, we review several units and could see it on too, the manual had a recomendation to change this in a time base.

Last edited by fburgos