MachineryWatch
Mechanical
I positioned a photocell speed sensor, that generates a TTL signal, to read the passing vanes of a cooling fan on an engine driven generator. The speed was 3600 RPM (60 Hz). There were 14 vanes on the fan. I ran the signal to an FFT analyzer. The FFT is 0-1000 Hz with 3200 FFT lines.
The FFT shows 60 Hz harmonics with a dominant peak at 840 Hz (60 X 14). The FFT also shows sidebands spaced 90 Hz from 840 Hz (750 Hz and 930 Hz).
The "Vane Pass Frequency" at 840 Hz is certainly expected. The 60 Hz harmonics I guess would be a result of slight differences in blade spacing that repeat each time the rotor makes 1 complete revolution.
What could cause the 90 Hz sidebands? Torsional rotor vibration would cause Frequency Modulation, correct? Would this appear as sidebands in the FFT? Is it proper to assume that a good TTL signal would have no Amplitude Modulation?
What further processing could I do on the captured waveform to try to extract torsional vibration information from this signal?
Thanks for your help.
Skip Hartman
The FFT shows 60 Hz harmonics with a dominant peak at 840 Hz (60 X 14). The FFT also shows sidebands spaced 90 Hz from 840 Hz (750 Hz and 930 Hz).
The "Vane Pass Frequency" at 840 Hz is certainly expected. The 60 Hz harmonics I guess would be a result of slight differences in blade spacing that repeat each time the rotor makes 1 complete revolution.
What could cause the 90 Hz sidebands? Torsional rotor vibration would cause Frequency Modulation, correct? Would this appear as sidebands in the FFT? Is it proper to assume that a good TTL signal would have no Amplitude Modulation?
What further processing could I do on the captured waveform to try to extract torsional vibration information from this signal?
Thanks for your help.
Skip Hartman