View Full Version : Eddy Currents
03-08-08, 04:36 PM
"When a conductor moves relative to the field generated by a source, then EMFs can be generated around loops within the conductor. These EMFs acting on the resistivity of the material generate a current around the loop, in accordance with Faraday's law of induction. These currents dissipate energy, and create a magnetic field that tends to oppose the changes in the field.
Eddy currents are created when a moving conductor experiences changes in the magnetic field generated by a stationary object, as well as when a stationary conductor encounters a varying magnetic field. Both effects are present when a conductor moves through a varying magnetic field, as is the case at the top and bottom edges of the magnetized region shown in the diagram. Eddy currents will be generated wherever a conducting object experiences a change in the intensity or direction of the magnetic field at any point within it, and not just at the boundaries."
Was curious. Opinions on these eddies. Which at one point I was harassed about their existence. I'm needing to know the last terms of the last statement.
Would, say a detector, for emissions of such fields be possible. We all know that an AM radio giving off certain frequencies of distortion in the low-end range is an indicator that your computer is not properly shielded. Could you, say, take a gauge for detecting variances, a tweaked Ohm meter for instance, and determine if someone was "hot"?
Something like, I don't know: Continuity check, drop, apply mA, then continuity again. In addition, add a coupler of resistance coils. These should not increase, unless something is attempting to siphon the feed of the fields.
Within a case, these could impose a soft-shutdown, yes/no/maybe?
I'm led to believe that metal detectors work by inducing eddy currents and detecting the corresponding fields.
03-09-08, 09:20 AM
Technically. Your basic one emits radio waves. Since metals tend to reflect radio-waves more efficiently than surrounding what would be called inert materials, they give a reception.
Some newer ones which I can't wait to become affordable utilize ultra-sound. This will give an approximate shape, as well as depth. The ultra-sound technology will also, in my opinion, yield to the average consumer a method for detecting stress fractures. Say, in a wreck where the insurance company is trying to rip you off: You could show that your car frame is weakened.
03-10-08, 10:32 AM
Back to the original question of an 'eddy current' detcctor, there is (or at least was) a test instrument along those lines called a "tunnel dipper". It was a tunnel diode wide band calibrated oscillator with a sensitive meter measuring the oscillator supply current. When anything conductive was within the field of the inductor of the oscillator, it would cause inductive loading of the oscillator which would then pull more current. In practice the tunnel dipper wasn't used as an eddy detector, it was used to check the frequency response of a tuned circuit. When the resonant frequency matched the oscillator frequency, inductive loading occured and the meter would 'dip'.
03-10-08, 12:17 PM
...When anything conductive was within the field of the inductor of the oscillator, it would cause inductive loading of the oscillator which would then pull more current. ...And also caused a frequency shift, especially large if the inductance or capacitance of the antenna was part of the oscillator's frequency determining "tuned circuit." This effect saved the US navy from the Kama Kazi attacks in WWII.
APL, Applied Physics Laboratory made in very short time an extremely rugged vacuum tube that could survive being fired at the from Navy guns at the attacking Kama Kazi planes. There was a small propeller (no battery to go dead) driving a tiny generator. That propeller was part of the "tuned circuit" -only had to pass near the plane, not actually hit it, to explode the shell and as the plane was loaded with HE, a shell fragment hitting anywhere had high probably of blowing up the plane.
That began a now 66 year long relationship between APL and the Navy in which APL solves tough problems* -like the recent conversion of the APL designed standard missile into a system that can shoot down LEO satellite as recently demonstrated (on the first shot, just as I predicted it would) the destruction of a failed US spy satellite. BTW the critical parts of the Aegis ship's no-rotation full-coverage radars that fired that SM-3 were developed at APL as was the entire standard missile system over a 50 year period. The list of items APL has designed, and in the case of satellites often built (like the fastest ever now on way to Pluto) which have played critical roles in the nation's defense and/or broken new frontiers of technology is very long.
... In practice the tunnel dipper wasn't used as an eddy detector, it was used to check the frequency response of a tuned circuit. When the resonant frequency matched the oscillator frequency, inductive loading occurred and the meter would 'dip'. Yes, I had one back when I was a "ham" (radio armature). With its set of plug-in coils it covered very large frequency range. Back then they were called "grid dip meters" (GDM). Like the proximity fuse described above, they were a single tube oscillator and had (as all oscillators must) positive feedback (to the vacuum tube control gird in this case).
When some external circuit resonated with the frequency of the oscillator (you were constantly changing it to find this resonance) part of the oscillator's output was dissipated in the external circuit so less was feed back to drive the oscillation. A simple DC meter in the grid circuit of the GDM would indicate the frequency at which the resonance occurred by a pronounced decrease in the grid current (less energy feedback). Hence the name "grid (current) dip meter." Only us old relics and a few working with high power RF applications or old TVs know what vacuum tubes were and how they worked in this solid state era.
*I had a well paid 30 year carrier there in which I was allowed to mainly work with JHU hospital doctors and on several different energy related projects I was intrerested in, as I can think from an original POV and creatively, so was valuable when needed on some new problem. Most of APL's professional staff is well paid, well educated, and hard working as they enjoy their work and are good at it. NASA's current director is from APL.
03-10-08, 05:06 PM
I never used a GDM, but Heathkit had one they called the "Tunnel dipper" that I wish I had gotten before they went defunct. So called because it was based on a tunnel diode oscillator, as tunnel diodes work very well for a wide frequency low power oscillator.
Popular electronics had a simple metal detector project that worked on the eddy effect. It had two oscillators and two tanks. Anything conductive (or inductive) in proximity to the head coil (which was part of one tank) would pull the frequency of that tank. The two oscillators were heterodyned together and fed to an audio amp. As you swept around you would hear a varying whistle, whose pitch indicated a find. I think some of the cheaper metal detectors on the market still work that way.