# [Beowulf] on powering off and its character

KevinO kevin at kevino.org
Thu Aug 7 12:35:43 PDT 2014

```On 08/07/2014 11:19 AM, H. Vidal, Jr. wrote:
> All,
>  Just curious if anyone here among those well-informed about the nature
> of electrical power might know about documented or canonical analysis
> of AC power as it's being turned off......
>
>  That is, is there existing and/or standard analysis or characterization of
> typical AC power as power is removed? Does it ring, or spike, or otherwise
> modulate in understood or at least studied ways?
>
This is very well understood, studied, and documented for well over a century, by
Engineers and Physicists.

Any shift in the amplitude (voltage) due to a circuit being opened is due to the
non-zero and complex impedance of the circuit supplying the power and the load.
(The voltage will not jump 'just because')

Maxwell's equations provide an accurate way to analyze to relationship of the
current through an inductor to both the value of the inductor (the inductance)
and the rate of change of the voltage across the inductor. (and the voltage
across an inductor as it relates to the amount of inductance and the
rate-of-change of the current).

In a nutshell: When the rate-of-change of the current through an inductor
approaches infinity (as when a switch in series is opened) the voltage across the
inductor will approach infinity. (This is limited by circuit capacitance and
resistance that can absorb some of the energy of the 'spike' caused by the rapid
collapse of the magnetic field of the inductor, or by arcing and high voltage
breakdown of the insulators in the circuit)

Simple wires have a small amount of inductance. Coils and transformers have a lot.

RC circuits (composed of series combinations of resistors and capacitors) are
often placed across switch contacts or coil windings to absorb this energy in a
controlled way.
--
KevinO
```