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As far my knowledge, it just matter of choice between P-N-P & N-P-N transistors. In early days (that time instrumenation standard +24Vdc was not implemented), P-N-P transisters were more cheaper than N-P-N transisters. So, it was decided to use P-N-P transistor and as we all know that for P-N-P transistors, a -ve bias voltage was required instead of +ve voltage. So, that's why in vibration Monitoring system -ve voltage use and now a days it's compatible for all systems.
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 -Vdc. 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 voltage0 mm0 vtg1 mm -9 voltage2 mm voltage & when the probe is in air it will have no induced voltage so all - voltage without induced voltage will return back.
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