Effects of Recirculating BEMF to Coil

[poynt99]

Luc:

Luc.

In my tests I've confirmed all your findings, except the magnet elevation, but since the magnetic field in a coil is proportional to the current through that coil, I think it's safe to say that is covered as well.

What you've built is essentially a DC-DC converter. You've taken a high voltage low current source and converted it to low voltage / high current in a load. In this case the load and conversion element are one in the same, the coil. The diode completes the appropriate circuit path that allows the conversion to actually take place.

Some numbers I obtained from my tests:

                         Ave VS    Ave IS    Ave WS    Ave VC    Ave IC    Ave WC
Without Diode:     170V       15.5mA   2.635W    157mV     15.5mA   2.43mW
With Diode:         170V       10.6mA   1.802W    2.932V     285mA    0.836W

Where:
VS = Supply Voltage
IS = Supply Current
WS = Supply Power
VC = Coil Voltage
IC = Coil Current
WC = Coil Power
(these are averaged values)

Indeed the current from the supply decreases with the diode in-circuit, but the output power from the coil never exceeds the input from the source.

Notice the huge increase in coil current with the diode in-circuit compared with it out of circuit? This explains the force it has on your neo magnet. With coils, it's all about current. A substantial amount of power has been transferred to the coil with the diode present (about 46% of the input), as opposed to nearly none without the diode.

Are there any gains in power or energy? No, in fact there is quite a substantial loss due to the DC resistance of the coil and connecting wires.

I hope this explains all that is happening with your experiment, but I'd be happy to expand on or run tests on any aspect if you want. Scope shots are also available if you wish.

Regards,
.99

Hi Luc.

Monitoring a coil's voltage does not always indicate what is going on there in terms of its current. The fact that the bottom portion of the coil voltage disappears when the diode is in-circuit, actually does indicate that a conversion is taking place. The conversion is from high voltage/low current, to high current/low voltage. If you zoom in when the diode is in-circuit, you will probably see a negative swing of about -0.65 Volts (from the previous -300V or so), but the current has shot up significantly. This is the indicator that "something" different is happening in the coil that is causing the increased magnetic power.

With no load (i.e. without a flyback diode), the coil sees almost an open circuit during its inductive kickback cycle. The same amount of energy (minus losses) must be conserved, so the coil voltage extends quite high in the reverse direction, but the current is quite small.

When the coil IS loaded during its inductive kickback cycle by placing the diode across it in reverse, the diode creates nearly a short circuit across the coil during this cycle, so the current has no choice but to increase by a large amount. It is this increase of current that you can not see by looking at the coil voltage, but this increased current is what is responsible for the much larger magnetic force being applied to your neo magnet.

Remember with coils, it is current that energizes them and produces a corresponding magnetic field, not voltage. With a heavy enough wire you could produce quite a strong magnetic field with only a few volts, as long as the source can supply a large current.

Hope that helps,
.99

and with the document:
http://www.overunity.com/index.php?topic=7713.msg191135#msg191135

Peter L. is the only one that is somewhat close so far, but not quite. It's simple: Adding the flyback diode causes the inductor current to be "rectified", similar to how voltage is that's used to charge a capacitor. Because of this rectified current, the average, or mean current in the inductor increases substantially, and as such, so does the magnetic force from the coil which kicks the magnet. Remember also that the flyback diode helps convert voltage to current, which is what is needed for a stronger mag field. See the comparison of the coil current wave forms at the end of the document.

.99

[MileHigh]

Just for fun. let me put my two bits in here.

Without the fly-back diode in place, you have an LC resonator that also pushes the magnet up when it first engages.  Keep that in mind.

For both setups you can split the sequence of events into Step 1, from initial relay closure to the point in time where the cap has discharged down to zero volts, and Step 2, whatever happens after that.

Case A:  Without the diode in the circuit.

Step 1:  The cap discharges, some of the energy pushes the magnet up, some of the energy gets stored because the coil gets energized with current.

Step 2:  The current reverses and the cap and the coil and wire resistance form an RLC oscillator which starts a ring down at a high frequency.  The ring down alternately pushes and pulls on the magnet for a net zero upwards push.  Therefore none of the energy stored in the coil at the beginning of Step 2 helps push the coil upwards and simply becomes resistive heat in the wires.


Case B:   With the diode in the circuit

Step 1:  The cap discharges, some of the energy pushes the magnet up, some of the energy gets stored because the coil gets energized with current.  (The same as Step 1 above)

Step 2:  The "LC oscillator" is now charged with current flowing through the inductor.  The capacitor voltage starts to reverse but the moment it hits 0.6 volts, the diode starts to conduct and the coil starts to recirculate the current flow through itself and the diode.  This sustains the magnetic field longer and you assume that this keeps a steady upward push on the magnet for a certain amount of time.  This is in contrast to a net zero upward push on the magnet in Case A.

The net result:  In Case A all of the available energy at the start of Step 2 becomes heat in the wires.  In Case B some of the available energy at the start of Step 2 pushes up on the magnet, and some of the available energy becomes heat in the wires.

I think that's the way the cookie crumbles.

MileHigh

[wattsup]

@gotoluc

Always good video and very well explained.

Also, @MileHigh came up with a pretty good explanation, but maybe about the LC resonator part is not right if there is only one pulse (one pulse, plus lots of time charging, then another pulse, etc.).

I have a question about the relay. Is the relay switching both sides of the caps or just one side? What I mean by that is does the first cap, second cap and the coil share any common connection or are they all isolated by the relay switching two sides?

[poynt99]

Wattsup,

MH and I are mostly saying the same thing, and he is correct about the oscillation. However, I will correct him on one point. Without the diode, the ringdown is indeed there, but it has no ability to do work in this phase. It does not push and pull the magnet in oscillation because the current and voltage are not in the right alignment to make real power. The real push only comes right at the start as he said.

When the flyback diode is introduced, the initial current pulse is recirculated within the coil and not only is it present for a longer duration, but it is higher in amplitude as well. It's that rectified current I spoke of.

See the two plots here of average coil current with a single discharge pulse, one without, and one with the flyback diode present.

.99

[MileHigh]

There are some great follow-up questions to this test.  What is the true efficiency or COP for each of the two configurations?  That leads to the following question, how do you define the efficiency?

The best place to start answering these two questions is to define what the goal is for this experiment.  I'll answer that one to put my line of reasoning into a proper context.  The goal is to lift the magnet as high as possible.

MileHigh