Effects of Recirculating BEMF to Coil

[MileHigh]

Hey Poynt,

Thanks a lot for doing that plot.  I am more confused now!  lol

For starters, I am under the impression that the relay connects the charged cap to the coil, you get the magnet jump and the ringdown.  The relay is on for 1/2 second or thereabouts.

Are you modeling the actual magnet-coil interaction in PSpice?  I am assuming that you can't do that, but I know next to nothing about the actual program.

I can't make heads or tails of the coil current taking that big triangular waveform with the turn-around at 120 us.  Nor do I get the parasitic capacitance retaining some energy.  Isn't the capacitance in parallel with the coil and the coil has it's own innate series resistance?  Are you adding resistors for the wire resistance?

Since I am so confused, just a few thoughts, only discussing the "no diode" circuit.

The end of the Step 1 discharge gives you current in the coil and delta-h and delta-v energy in the moving mass of the magnet.  This means that there will be less current flowing in the coil as compared to a no-magnet case.

At the beginning of Step2 you have a "launched" magnet and a ringdown event.  Yes, the NET work done on the launched magnet is more or less zero (the ringdown is not a constant amplitude sine wave), because the push and pull cancel each other out.

If you ignore the gravitational acceleration, the magnet velocity is wobbling 90 degrees out of phase with the AC magnetic field.

The modelling is a little bit tricky here.  The mass of the magnet is experiencing an sinusoidal AC force.  This is equivalent to an sinusoidal AC current source connected to a capacitor.  The higher the frequency of the AC current source the lower the resultant AC voltage across the capacitor will be.  That translates into a lower AC velocity of the wobbling magnet.  So indeed there is no net energy expended here, the mass of the capacitor is a purely reactive load.  By the same token, there is a tangible force that the mass experiences.

If you are still with me, it means that the rigdown event is being split up.  When the coil starts its discharge cycle, the energy goes into the electrical capacitor and the mechanical capacitor.  When the coil has depleted all of it's energy in terms of a cycle in the ringdown event, let's assume that most of the energy is stored in the electrical capacitor, and some of the energy is stored in the moving magnet (a.k.a. mechanical capacitor).   Not forgetting all the time that we are factoring out the acceleration of the moving magnet due to gravity.

I am not going to the other half of the cycle, the electrical capacitor part is obvious, and the moving magnet will induce current flow in the coil.

Ultimately, at the start of Step 2, the magnet continues with it's "launch phase" with this energy-neutral magnetic-field-induced wobble as it continues skyward.  When it reaches the peak (you assume the ringdown event is long gone), a certain proportion of the energy that originally came from the charged cap is now happily sitting in the mass of the magnet in the form of delta-h.

The magnet then drops back down and induces a final little rush of current through the coil and has a hard landing.  That's your heat creation event.  Of course when the magnet was going up during the launch phase that in itself was inducing current in the coil in the opposite direction as compared to the drop down.  That was superimposed on top of the cap discharge current also.

Anyway, either you think this makes sense or I am nuts!  lol

MileHigh

[poynt99]

Here are your two coil current shots. I eliminated some superfluous circuitry that Luc had before, so the two peak currents come out the same now. The circuit is back to "basic" now. There is no "load" on the coil, but I will try to add one.

If I can manage, then we should be able to see if the "load" is geting any energy during the ringdown.

I don't think it is necessary to fully simulate all that Luc is doing in this test with the discharging cap etc, but if you are so inclined, I can do that. Right now I am just pulsing the coil with 170V supply and using 120us pulse width. Just one single pulse.

.99

[MileHigh]

Hey Poynt,

Now I get it.  It's funny because now going back to your previous posts I get it, and it seems so obvious that you were just pulsing the coil to get some juice going through it.

There is a lesson for all here:  Document! Document! Document!  lol

Can you see how I made a disconnect when going from Luc's clip to your plots?  I was expecting something different and it threw me off.  I think Glen also went back and annotated some plots and stated what his probes were connected across for his scope shots.  It really helps.  When you do an experiment you have to step outside of your fishbowl and try to see what it looks like from an observer's perspective.

As far as the plots go, yes, very cool.  Notice everybody that the current does not follow a standard exponential decay curve for the case with the fly-back diode?   That's because the diode is a non-linear device where it's voltage drop is not proportional to the current flowing through it.  The net result is that the current flows for a longer time and takes a more or less linear drop with respect to time.

I am not a fan of your red average current plot for these two cases though.  It looks like they are the "average current over all time" and I don't think they are conveying too much information.

Anyway I hope somebody appreciated the detailed energy analysis I did in the previous posting.  I think that explains a phenomenon that I think we have all seen in a YouTube clip or two.  When someone has a self-resonating oscillator based on a transistor and coil + capacitor and are listening to the sound it makes on a speaker, sometimes you hear the frequency change when they bring a magnet close to it.  By bringing a magnet close to the coil, you are effectively adding to the capacitance of the LC oscillator via magnetic coupling with the external magnet, which should lower the resonant frequency.  Is that far out or what?  lol

Going back to the plots, you can easily see how you have much more upwards pushing power on the launched magnet when the diode is in the circuit.

100% percent efficiency would be defined by having all of the energy in the capacitor being used to lift the magnet up to a certain height.   In other words,  1/2 C v-squared = Mgh

Therefore the maximum height the magnet could be raised is as follows:

h =  (1/2 C v-squared)/Mg

If you measure how high the magnet jumps and compare it to "h" then you can calculate the efficiency for each of the two variations of the circuit.

MileHigh

[poynt99]

Sorry MH.

You're right I should have stated the difference in the setup.

The end result is pretty much the same though for both.

.99