----- Original Message -----
From: "Rob Pearson" <aussierob@odyssey.net>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Monday, November 20, 2000 5:03 PM
Subject: Re: [MV] 24v COIL OHMS TEST - Lisa jeepcj8Qhome.com
> The COIL
> From my "coil" experience I have measured the current in numerous "12v"
> coils over the years and found that when running, the current / amps flow
> was around 1.5amps approx!
> When not running the current with the points closed was around 2.5amps or
> higher.
>
When running you are measuring the _average_ current flow ie, something
between zero points open and max with them closed, the value being about
half the standing current and dependant on the dwell angle which is the
period the points are closed.
> I have also discovered that many manufacturers installed coils that were
> designed to run on 8-10volts and if you replaced the coil, it must be with
a
> same operating voltage type ...possibly, most probably designed with a
> ballast resistor in series with coil?
>
Very common if not universal on later conventionally fired vehicles,
occasionally the ballast resistor is a specially made and defined length of
resistance wire from the switch to the coil itself, this is common on Ford
of England.
>
> COIL does NOT get too hot when engine runs! SO,
> Why or how, would the coils resistance in ohms seemingly change from when
> the points are "closed" (Ign. ON - engine sitting stationary)... as
compared
> to when points start "opening and closing" (engine turns over and runs)
> Well, the coil resistance ...does not change. The coil itself does not
> physically / magically change its resistance, measured in Ohms, ...or
does
> it?
>
The coil resistance doesn't change, simply that the current is being pulsed,
in fact as the revs rise the dissipation of the coil goes up.
> What happens here, occurs at the "speed of light" nearly "too" fast to
> imagine!
> During the absolute instantaneous moment when the points close, a current
> starts to flow through the copper wire which is wound around the steel
core
> inside the coil unit. . . . . . . . . . . . . . . .
>
> The INDUCTANCE "only" occurs when the points close and, at any instant of
> time, the magnetic flux builds within the coil.
>
No, the inductance is always there, its effect is only apparent when
changing a steady state.
> However, it also "Only" occurs at a second instant in time, that is, the
> moment current-amps flow from the battery, stops. Which is, when the
points
> OPEN ! ..and then again, the flux collapses. It's the changing flux that
> does it.
>
As above, however at this point in time the collapsing flux no only induces
the very high spark voltage in the secondary HT winding but also induces a
high voltage in the primary circuit as it attempts to maintain the steady
state which was current flowing in the primary. The capacitor across the
points absorbs most of this reverse pulse of energy or the point sparking
would quickly weld or burn them away, further, the ionised gas (spark) at
the points effectively slows the flux collapse and thus reduces the actual
HT energy. If you don't believe it grab hold of the _LT_ contact breaker
terminal on the distributor or coil (NOT THE HT) with the engine running and
see if you don't get a good belt. . . . . . . . .
> So, it is the rising and falling of the magnetic flux in each turn or
> winding, that causes the Induction... and this in turn causes a higher
> Impedance (ohms) ...and this higher impedance (ohms) allows for the
> lower amps in the coil when the engine is running !
> Therefore, "normal" coil temerature... No excessive heat No breakdown !
>
What you say for self induction is broadly true and applicable to an
ignition coil however it does not come into play as suggested, if it did
misfiring would occur.
The effect of self-induction in any coil of wire is to resist the _change of
state_ of the current in the coil, however in an ignition coil you need the
primary current to build to its maximum as defined by the DC resistance of
the primary winding, when this can't happen the spark voltage drops and
misfires occur.
This is typical of the problem in an 8 cyl, four stroke engine needing
266.667 sparks a second at 4000 rpm for instance, some means of dwell angle
extension between sparks is needed to allow more time for the current, and
therefore magnetic flux, to build to its maximum value. This can be
achieved with twin points or indeed splitting the system into two four
cylinder arrangements with twin coils.
The design of the ignition coil (which is actually a transformer) has to
allow it to get hot at lower revs where you have to accept the dwell angle
is much more than is required and primary current is simply turned into
heat.
Look at something that really needs a heap of sparks, now you Americans
aren't allowed howling multi-cylinder two-stroke (two-cycle) motorcycles any
more but you do have triple, four and six hole two-stroke outboard motors,
these need twice the spark rate of a four-stroke and you'll find a coil for
each cylinder, very high revving Jap fours have had twin coils since
whenever. Big, multi cylinder aero engines are the same both for
reliability and spark rate.
Modern capacitive discharge systems, however, are not so inhibited by
self-inductance problems, these are very similar to a camera flash-gun in
operation where the ignition "coil" takes the place of the flash tube.
> The interesting fact here is proven that when you leave the ignition On
> ...and the coil get hot. Remember, that impedance Only occurs when the
> magnetic flux rises and falls, all very quickly at the speed of light.
>
Simply because with the motor stationary you are over-running the coil
dissipation by 100% typically, the coil has an intended _duty_ cycle.
Magnetic flux and AC impedance do not happen at the speed of light, if that
were true you could fire a 12 cyl engine at hundreds of thousands of revs on
a single coil, which you can't.
The time taken for current to stabilise in any coil and its impedance
(reactance + DC resistance) is a standard electronic sum which is
inappropriate here, however, a typical ignition coil will show a time
constant of several milliseconds which looks like several centuries when
compared to the speed of light.
Richard
Southampton - England
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