Extension Cable - GE Bently Nevada 86517 Operation Manual

Accelerometer Interface Module Operation Manual
The differential charge output of the accelerometer enters the interface module
through pins C and A of the three-pin locking connector while pin B connects to
the extension cable inner shield and to the interface module case. The charge
amplifier converts the differential charge input to a low impedance voltage signal
and the subsequent scaling stage adjusts the sensitivity to 10 mV/g pk (1.02
mV/m/s pk). The frequency response of the charge amplifier/scaling stage
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combination has an overall bandpass transfer characteristic. See Section 6 for
specifications. The interface module can be ordered to operate with
accelerometers having charge sensitivities of 50 pC/g or 100 pC/g. This option is
not field programmable.
The interface module filters are cascaded 6th order low pass and 6th order high
pass filters with a Butterworth response. Filter corner frequencies are determined
at ordering and are not field programmable. The corner frequencies are normally
chosen to pass the vibration component at the rotative speed of each rotor while
excluding higher and lower frequency signals which may be due to blade
passings or structural resonances. The standard corner frequency options
conform to requirements for General Electric LM 2500, LM 5000, LM 1600, and LM
6000 series aeroderivative gas turbines. Contact your Bently Nevada
representative for nonstandard corner frequencies. If you elect to use corner
frequencies not recommended by the engine manufacturer consideration should
be given to the fact that the acceleration signal will contain structural and/or
structural-borne noise that is not related to rotor vibration. Most common
machine malfunctions originate on the rotor and can be detected as a change in
rotor vibration. Choose corner frequencies carefully.
Both the acceleration and velocity signal outputs can be wired as either single-
ended outputs or as balanced two-wire outputs. In the single-ended
configuration, only the + ACCEL or + VELOCITY terminal is wired to the monitoring
or measuring instrumentation. In the balanced two-wire mode the + VELOCITY
and - VELOCITY outputs create a differential output and the + ACCEL and -
ACCEL outputs create a differential output. Inside the interface module the (-)
terminals are connected to signal common through an impedance which
matches the output impedance of its mate. The +VELOCITY and +ACCEL outputs
are biased at -10 Vdc nominal.

2.2 Extension Cable

The extension cable connects the accelerometer and the interface module and
may consist of a single softline cable or two cables where the cable on the
accelerometer side is a hardline, mineral-insulated cable rated for higher
temperatures. The extension cables must be constructed with special low noise
characteristics and resist high temperatures and corrosive environments. The low
noise construction is a conductive treatment on the surface between the center
conductors and the dielectric and between the dielectric and the inner shield. This
treatment minimizes the charge induced between the surfaces by motion or
vibration of the cable. This phenomenon is called triboelectric noise and can be
particularly troublesome in charge amplifier systems. The total pole-to-pole
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