How does an Axial Thrust or Axial Gap Transmitter work

How does an Axial Thrust or Axial Gap Transmitter work

Axial Thrust or Axial Gap Transmitters are used to find out the Displacement of a Shaft in the axial direction. This helps operators of the equipment understand whether or not their equipment - be it a pump, fan, turbine, etc. is safe in axial tolerances.

Let us try to understand how IRD7100 - our Machinery Protection Transmitter monitors Axial Gap and how does it provide the proportionate 4-20 mA output.

Let us assume that we have an Eddy Current Probe mounted axially to a shaft and connected to our IRD7100 Machinery Protection Transmitter. Let us assume the range of the transmitter to be +/- 1 mm. We want to monitor the gap in our Control Room and thus wish to use the 0-5 V DC output of the Transmitter or the 4-20 mA output of the transmitter. What shall we take as 4 mA and what shall we consider as 20 mA. The answer for this can be clearly seen through the simple sketch below:

Thus, we can see that when the sensor is set at an initial gap from the sensor. This gap should be -10 to -12 V DC. In our example we have considered -10 V DC to be the initial gap. This is denoted as position 0 mm for axial gap measurement. The target (shaft) can move closer or move away from the sensor while running. If it moves closer, then the movement is denoted as negative with -1 mm being the closest it can get for the transmitter. If it moves away, then the movement is denoted as positive with +1 mm being the farthest it can get from the transmitter.

Thus, when axial gap displayed on transmitter is -1 mm, it will mean that the sensor gap voltage is -2 V DC and the current output will be 4 mA. When the axial gap displayed on transmitter will change to 0 mm, it will mean that the sensor gap voltage is -10 V DC and the current output will be 12 mA DC. Last but not the least, when the axial gap displayed on the transmitter will change to +1 mm, it will mean that the sensor gap voltage is -18V DC and the current output will be 20 mA.


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