Proximity probe vibration in relation to magnetism?

[kmg1983]

Please excuse me if I present this information in an unfamiliar manner, as I am very unfamiliar with Electric Motors and Magnetism.

We have been experiencing times of high vibrations being read by proximity probes installed within a 3600 RPM, 3500 HP, Electic Motor. However, taking readings at the bearings housings yields little vibration. Can issues related to magnetism within a motor shaft lead to misleading readings at the proximity probes? I know magnetism is the theory driving their operation, however, what effect can be seen if a motor shaft is not de-magnetized after being reworked?

 

[Skogsgurra]

Usually not much.

The proximity probe usually works on eddy current principles and the frequencies used are in the 100 - 1000 kHz range. External magnetic fields does not influence readings a lot as long as you keep a fair distance between iron and transducer so that the magnetic field doesn't bring the ferrite in the transducer into saturation or non-linear operation.

There is also the possibility that a badly designed transducer picks up inductively induced voltage in the "search coil". But that should not find its way to the transducer output if the transducer is well designed.

It does depend a lot and the best thing to do is to check with magnetic fields. Static and dynamic ones.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

If you search for "glitch" on the Bently Nevada website, I think they have an article that discusses several possible sources of prox probe error, including magnetism if I remember.

But of course, the movement of the shaft might not be well transmitted to the housing. That's one reason some people prefer prox probes for sleeve bearing machines.

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[kmg1983]

Thanks for shedding some light on this for me...I'll check out those articles you mentioned, electricpete. Thanks again!