Collapsing magnetic fields in high inductance coils
In what form is the energy contained in a high inductance coil released when a current source to that coil is suddenly removed?
This is a practical question that I have been considering. Using a traditional (old fashion) coil and points ignition system as a model and basis for my question.
This is how I understand the working of a points and coil ignition system. Relatively high inductance coils (many turns) have been used to produce short duration high voltage pulses of electricity. A low voltage source of current is used develop a magnetic field in and around the coil. The low voltage source, typically 12 volts, in connected across the coil when the points close. The inductance in the coil will result in impedance that will limit the rate of change of current through the coil. Over a predictable period of time the current flow through the coil will increase to a level determined by the resistance in the coil. As long as the points remain closed that current flow in the coil and the magnetic field produce by the coil will remain constant. The instant that the points open the 12-volt current source is remover from the circuit. As the current diminishes through the coil the magnetic field begins to collapse. The collapsing magnetic field will induce an increasing voltage level across the leads to the coil. At some point the increasing voltage level become great enough to ionize the air in the gap of a spark plug that is now connected across the coil leads. With the high voltage pulse connected to the spark plug there is a release of the magnet/electrical energy in the form of an arc (spark) at the gap of the spark plug. This all seams clear and fairly straight forward to me. My question is what happens to the magnet/electrical energy in the coil if there is no spark plug?
Suppose I construct a circuit with a very high inductance coil connected to a current source. I then disconnect the current source using a very fast and very complete method. The leads of the coil are now completely isolated and insulated from each other. If no connection or arcing is possible between the leads where does the energy dissipate to in this scenario? It seems to me that the theory might indicate that the voltage level between the coil leads would increase to infinity, I don’t imagine that would actually happen. What does happen? Perhaps the coil winding's short out internally producing heat? Perhaps it in physically impossible to break the connection fast enough to prevent arcing? That seams unlikely, since the switching system could be of any type and in any environment, perhaps in a vacuum. I don’t know, I am just curious what other believe happens in this scenario?
Note by
Darryl Dennis
2 years, 2 months ago
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Suppose the experiment is done in a vacuum, ruling out the possibility of arcing through the air. There is parasitic capacitance between the wire leads, and presumably some resistance in the metal conductor. So the current may be a sinusoid with a decaying envelope, as energy sloshes back and forth between the inductance and the parasitic capacitance, with some of it being dissipated as heat by the conductor resistance. The geometry of the coil and wire leads would determine the oscillation frequency, with the resistance determining the rate of damping of the envelope. And I suppose the system might also act as an antenna, radiating away some of the energy as EM waves. This sort of reminds me of a dipole antenna.
That explanation does include a consideration I have overlooked. The potential difference between the coil leads and throughout the coil itself may become very high. I had not considered that at these high voltage levels the capacitance ,although quite small, between the leads would become significant. I like the explanation. It does resolve the question to great extent. It seems to me, in a actual experiment depending on the construction of the coil and the insulating material between adjacent coil windings it may be the case that insulation breakdown could be a factor in some circumstances. i.e. short between windings within the coil at locations where because of the coil's design there may be high voltage differences between adjacent coil windings.
Continuing with my ignition system model as an example. It is very clear that older points and coil ignition system could be very EM wave noisy at times, especially if the ignition points capacitor was bad. I have experienced engines that would interfere with radio and television reception blocks away from the engines location. I suppose that the point capacitor is intended to eliminate or at least reduce the oscillations in any frequency that is likely to be a problem. This explains why a bad capacitor can result in a engine that may be radio frequency noisy.
Thanks