Answer / dondapati ramesh

For High speed rotating equipment only,we are using this
negative voltage for vibration sensor.This negative voltage
is always set as gap voltage between the sensor and surface
of the rotating equipment.
This gap voltage is always set as -10Vdc.
This because of Ratio frequency, which is created by the
controller of vibration sensor. These frequencies are
generated under negative voltage only. which is also varies
with respect to eddy current flow. The amplitude of this
ratio frequency is measured as vibration.
For your information, Noice level is very less in negative
voltage compare than positive voltage which will not affect
the sensitive range of measurement of vibration.
For your information:-This concept is used in Bently neveda
vibration monitoring system.



The principle is that analyzer supplies a negative voltage
to transducer, who converts it into a frequency & sends it
across the probe. Now as the probe comes into the area of
rotor it creates a magnetic field & with it eddy current is
produced on the rotor side. This changes the amplitude of
the frequency. It induces the voltage & remaining frequency
comes to the transducer through the return way. Transducer
converts it into the voltage.
Now coming back to the question The gap voltage has a
negative relation with the voltage induced i.e. as the gap
increases the induced voltage decreases. So to make it to
into a linear relationship, negative voltage is supplied.
i.e

Gap Gap voltage
0 mm 0 vtg
1 mm -9 voltage
2 mm 18 voltage
& when the probe is in air it will have no induced voltage
so all -24 voltage without induced voltage will return back.

Answer / r.k

the concept of 4-20 mA & -ve voltage in vibration are
totaly different.The proximitty probes by the Bentley
Neveda and the Proximitter works on -ve bias volatge as it
made conventionaly.the devices which transmit the signal as
AC&DC Voltage depends upon measurement with less
trasmission loss to the Input Module.Formation of eddy
current w.r.t vibration is proportional to the changes in
voltage which is measured by the system.

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