Magnetic field on an electrical motor 1

[LeoEb]

Hey everybody!

I have a question concerning the magnetic field produced by an electrical motor.

Is it possible to measure that magnetic field outside of the motor?

Would it be possible to place a sensor outside on the motor to determine its activity?


Thanks for the help in advance !

 

[3DDave]

It would be so useful to do that it would be done regularly if it worked.

This article implies that it is only detectable when the motor has a failed winding.

https://www.scielo.br/scielo.php?script=sci_arttext&pid=S2179-10742012000100006

 

[LeoEb]

Thank you for your reply 3DDave.

Would there then maybe be another way of measuring a motors activity from outside i.e. without having to place a sensor inside the motor (like a tachometer)?

 

[itsmoked]

The typical way is to place a current monitor coil around any one lead to the motor to detect current flowing.

Something like this

You give no information as to what you need this for.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

In a perfect motor there is no magnetic field around the motor.
Any magnetic flux outside the motor will be leakage flux and is a loss and a source of inefficiency.
However, there will always be some leakage flux.
The windings are inducing a magnetic flux in surrounding material, both the iron core and the surrounding space.
Given that the flux path in the space surrounding the core is generally longer than the flux path through the core and given that the permeability of the core is about 50,000 times the permeability of the space, the leakage flux may be considered to be less than 1/50,000 of the working flux.
Detecting and measuring leakage flux may be a valuable check on a design.
A poor design may result in restricted areas of the core being magnetically saturated.
In the event of saturation, the 50,000:1 advantage is lost and the leakage flux may approach 1/2 of the flux in excess of saturation.
This condition may be inferred by accurately measuring the motor current as the applied voltage is raised.
At the onset of saturation of any part of the magnetic circuit, the current increase will be disproportionate to the increase in the applied voltage.
A suggestion:
A small transformer with one side of the core removed so as to form a "C" or a horseshoe, with thin non-magnetic spacers on the resulting pole faces may detect leakage flux.
This may be tested by increasing the voltage applied to the motor to the point to the onset of local magnetic saturation.
This test would also identify the areas most prone to leakage flux.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

In regards to the information linked by Dave, it appears that they are measuring changes in the normal leakage flux caused by defective rotors.
It should be noted that while the changes in the leakage flux relative to changes in the core flux may not be exactly the permeability ratio (1/50,000) of the core to free space, the leakage will be related to this ratio.
I don't know but I wonder if carefully raising the applied voltage may result in a better signal to noise ration.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

What is the purpose of your wanting to measure this? It would be nice not to have to guess and ASS-u-me*...

*Assume makes an ass out of u (you) and me...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[IRstuff]

Would it be possible to place a sensor outside on the motor to determine its activity

It seems to me that a Hall sensor on the shaft would easily tell you whether the motor is active or not.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[jraef]

If it's an AC motor, and it needs an external OL relay anyway, you can use a Solid State OL relay that determines the actual motor kW. That becomes useful because kW is an indicator of the actual LOAD on the motor, whereas magnetic field detection only tells you that the motor is energized. Using current alone is not as good, because current varies with voltage swings, but kW remains being indicative of actual load. I use kW all the time to not only determine if a belt or shaft is broken, but also to detect bearing wear, because if I know what the kW / load SHOULD be under normal conditions, and I see the kW increase without a corresponding change in the work being done, that means the motor is having to work harder to do the same task, ergo bearings are going bad.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden