Tesla's "COIL FOR ELECTRO-MAGNETS".

[TinselKoala]

Ah... I remembered another point from the original thread that I wanted to comment on.

AC electromagnets.

Yes.... you can get remarkable effects from AC electromagnets. If you place your poles right, you can levitate, that is, repel, non-ferrous metals like aluminum and copper. Some very fancy cooktops use this effect to levitate a frying pan while at the same time heating it by inductive power transfer.

But you can even levitate quite well using just plain 50 or 60 Hz line AC into a proper electromagnet, no other components necessary (except a Variac to control the power).

It is of course eddy current levitation; the pulsing field from the AC EM induces eddys in the metal, and the field from the eddys repels the field from the EM, and so the material gets pushed away from the EM.

If the pulsing field is at the right frequency and strength it will indeed induce an electric field across non-conductive bits of plastic, etc, and can cause them to be attracted to the EM (RF in this case).

[synchro1]

I wonder if I broadcast into an SB pancake antenna with a coventional C.B or Marine band radio, if I could drive a magnet with that frequency signal?

I believe the SBC spinner starts out as a pulse motor, then transitions to a synchronous a.c. motor powered by the SBC sine wave broadcast signal. The coil pulses one pole then the a.c phases in and drives both.

[MileHigh]

TK:

I liked your Arduino "ringing the bell" inductance meter clip.  Really cool.

On the other hand I did not like your other clip where you used your frequency generator.  You have to factor in the fact that I have no bench to work on and I doubt I ever did that test.  Nonetheless, I see big issues with your clip.  You are "brute force" driving the LC resonator with a direct connection from the function generator right across the resonator.  So you are heavily loading it down with the 50-ohm impedance of your function generator and the L-C of the cable itself.   On your scope display when you are at resonance you clearly see the straight lines in the waveform.  So that's probably the capacitance discharging and charging through the the 50-ohm alternating square wave voltage from the function generator.  It doesn't give me a good feeling.

I always assumed that you would put something like a 1K resistance between the signal generator output and he LC resonator.  That pretty much isolates the LC resonator from the signal source and the signal source just "tickles" the resonator.   Then you set your function generator to output a sine wave and you look at the voltage across the LC resonator to look for the peak response.  As you sweep the frequency all that you see is a pure sine wave across the LC resonator.  That's the way I envisioned you were supposed to do it.

MileHigh

[TinselKoala]

@synchro:
Well.... CB is 30 MHz, approx 10 Meter wavelength band. Your oscillations will be too fast to "catch" anything physical like a magnet, I think. But try it, by all means.

@MH: I suppose you are technically correct. I usually do use a series resistance when I put the FG's output into a coil, but maybe I didn't in that video.
But I think that the primary effect of having the FG and the probes attached is to reduce the Q of the tank; the associated capacitances are so small that the resonant frequency isn't being altered that much. I think. Otherwise.... how could I have arrived at a final value for the inductance of the coil, that so closely agrees with its measured value using a commercial inductance meter?

"As you sweep the frequency all that you see is a pure sine wave across the LC resonator. " That's not true (in my experience), even if you use a pure sine wave as stimulation. You don't see the pure sinusoidal output until you are close to the resonant frequency _or a harmonic_.

[Farmhand]

Quote from: MileHigh on April 21, 2013, 10:47:19 PM

(snip)
I strongly disagree with you though about using a square wave to find he coil self-resonant frequency.  We are modelling the coil as a parallel LC circuit and looking for the frequency corresponding to the the highest impedance, correct?  So why would you want to pump multiple simultaneous frequencies into the parallel LC filter if you are looking for a single resonant frequency?  When you characterize any filter it is easier to do it with a sine wave representing one frequency only, and then sweep that frequency.

I am not saying that it's absolutely wrong to use a square wave, but you could get fooled and see what you think is a peak response which is happening due to a harmonic and not due to the fundamental.  It's just "cleaner" to sweep the filter with a sine wave.

(snip)

You might be interested to know how my Arduino-based inductance meter works. I illustrate its operation in a video. I also show in another video how to determine the resonant frequency of a coil.

While the resonant frequency of a tank does correspond to the highest input impedance of the circuit, that is not the parameter I am observing, because that is not the parameter of interest. I want to know the resonant frequency. Hence I look for the purity and amplitude of the _output_ signal produced in a "receiving" coil placed around or near the inductor of the tank circuit I am testing. Take a look at Farmhand's scopeshots to see how the spectral purity of the output cleans up and becomes sinusoidal, and then peaks in amplitude, at the exact resonant frequency. There is no doubt when you get to the right frequency.

But the Arduino does it a little differently, but still corresponding to the same thing, and _still_ using a sharp-edged fast risetime square wave pulse to do it. The Arduino places the inductor under test into a tank circuit with a 2 microFarad cap, then rings the tank by applying a single sharp pulse, then it isolates the tank and observes the ringdown voltage peaks, times them and determines the frequency, then finds the inductance by calculation. A tank rings down at its resonant frequency, and the higher the Q the more countable peaks you will get in the ringdown before you have to "hit the gong" with a sharp pulse again.
(But of course a properly resonant TC hits the gong on every cycle in exactly the right timing.)
http://www.youtube.com/watch?v=S6N8ys8FiA4
http://www.youtube.com/watch?v=alkfoX62Na0
Any Tesla coiler knows that the key to getting the proper HV VRSWR effect happening in the secondary, is to make the "driving pulse" in the primary to have the most rapid rise and fall times possible: a rectangular pulse, not a sine. This is why Tesla spent so much time on spark gaps, and why the spark gap is so critical to a proper performing TC. And it's why my MOT DC SGTC works so well: I use compressed air to blow out the spark gap, to decrease the rise and fall times of the primary current.

ETA: Looking at the tank directly, as in the videos above, rather than using a second pickup coil, produces the same results but for slightly different reasons.

Wow one sleep and so many comments.

The way I see it the square wave does not actually contain all harmonics, it "possibly" contains all harmonics, a square wave cannot actually be all frequencies at the same time.
If we take some french curves I'm sure we could draw all the harmonics within the square wave but that is just for the theory. To explain things.

I usually always use a square wave to find resonance for my Tesla coils or whatever other coils/transformers. Showing the harmonic was merely to show that the harmonics can be there and can be used. Although proper resonance is more useful. I think when we tune a Tesla coil we don't actually want it tuned spot on, we want it tuned so that when it loads up it becomes in tune, which is not necessarily max impedance or whatever. It's maximum voltage or output we want mostly. I tune for max output power with an output arrangement and max secondary voltage with Tesla coils ( when loaded). The primary of my spark gap Tesla coil is only resonant when the gap is conducting because it has no resonance tuning caps across the primary all the time, only when the cap is discharging. All the Q is in the secondary and extra coils. But if the primary capacitance on discharge is not correct to give the primary "LC resonance" at the correct frequency it won't work so well I use tuning coils to find the right tune for a given experiment. I use a high speed rotary gap of sorting bar design.

I will repeat the tests with a sniffer coil, but I foresee that any difference will be small and the same for both coils, as well as they will ring longer. I like to see the harmonics come together and the sine wave form and peak then the waveform after resonant point is usually pointy, anyway we get a feel for it.

My concern was that if a bifilar coil was used as a DC resonant charging inductor the self capacitance might hold energy in the coil, now that I think about it not such an issue. I can try it and compare since I have comparative coils I can test them in a few different situations.

I didn't aim for any particular frequency, I just found a piece of wire almost 10 meters long that I stripped off a transformer to even up windings and strung it out folded it in half and went from there. Any similarity with the frequency of my Tesla coils is just coincidence.

Yes I might test some magnet lift tests and such, but I don't expect to be using smooth DC much.

The thing I see with resonance is that in some situations or uses we may reduce Q and damp outputs with load or other ways but a coil tuned with a capacitance for a specific frequency is more responsive than one without.

I call that tuning the coils to resonance, even though when in use the Q might be gone and the waves damped.

In my accelerating under load video we can see the setup is loaded up by the coils harmonics which loads the driving motor, then when the load is applied the harmonics and the main wave are flattened, I used a rectifier as well, if I loaded the output coil without the rectifier the effect would have been more pronounced.

The use of a lower resonant frequency of a coil wound to have it is many.

Cheers