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
Can you post a photo of the graph
Unusual, are you certain it is exactly 1/2 of line (25 Hz)?
Just wondering,,,,, Are these running an electrical setup through a convertor producing DC current to run a true DC motor?
Or, are they just running unaltered AC current with an AC motor, and also no variable frequency device present?
Just a thought, are these generators driven by a diesel? 4-stroke diesels are known to amplitudes related to the 1/2 Orders.
Thanks John. You are quite correct. This is a diesel engine driven generator. I have attached spectrum screenshot. Though the highest peaks are orders of the diesel engine/generator speed of 100RPMm, we want to understand the meaning of the 1/2 of the line frequency harmonics.SPECTRUM
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.
Thanks for all that have responded to my post. I am new here and struggling to learn how to reply individual responses. Please see my earlier response with more details. Thanks once more.
The diesel speed is 1000rpm and same with that of the generator.
Thanks so much. The speed of the generator is 1000rpm (16.6Hz appx), and there are harmonics of 1X, 2.5X, 3.5X, 4X and 5.5X which we think is a sign of mechanical looseness. However, the presence of 1/2line frequency harmonics cannot be explained. Does this indicate another problem in addition to the mechanical looseness?
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
Exactly 25, 50, 75, etc
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.