[SEL] Recharging of Maytag coils

[Bill Dickerson]

Yes, I was trained by Sun on the use of he scope, own a scope now, and got
my first job by showing the service manager what was wrong with the Ford he
ws working on.
You can see it all in the scope pattern...... And tell a LOT about an
engine, even mechanically, if you can read the scope.
The full operating of the system would take pages............

I've still got all my Sun training guides downstairs......... Pages and
pages. 

-----Original Message-----
From: sel-bounces at lists.stationary-engine.com
[mailto:sel-bounces at lists.stationary-engine.com] On Behalf Of Elden DuRand
Sent: Monday, June 08, 2009 6:48 PM
To: The SEL email discussion list
Subject: Re: [SEL] Recharging of Maytag coils

Jerry & Bill:

A couple of years ago, I did a study of point/coil/condenser (battery or
mag) and ignitor ignition electrics.

The condenser is, in fact, needed in order to produce a "hot" spark at the
plug.  As a side effect, it also supresses arcing at the points.

What you have with this type of ignition is a parallel resonant L/C circuit.
The "L" is the inductance of the primary of the coil plus the reflected
inductance of the secondary of the coil.  The "C" is the capacitance of the
condenser.

What happens is that when the points are closed, there is a magnetic field
produced by the primary in the core of the coil.  At this time, the
condenser does not exist in the circuit, being short-circuited by the
points.

The fun begins when the points open.  As soon as the points open, the field
of the coil begins to collapse, inducing a voltage across the condenser
which is now in the circuit.  The rate at which the voltage rises is
dependent upon the capacity of the condenser and the inductance of the coil.
Once the coil's field has collapsed completely (and made one spark at the
plug), the condenser is charged up and has nowhere else to discharge itself
but back into the primary of the coil, which it does, causing the magnetic
field to build back up in the coil (albeit at a lower voltage|).  When the
condenser has discharged itself, the field of the coil again collapses
(making another, weaker, spark) and charges the condenser again.  This goes
on for several cycles and is called a "damped oscillation".

The reason the points burn if you don't have a condenser across them is that
the voltage rises faster than the points can open and a series of small arcs
occur at the points as they relatively slowly open, adding up to a big
heating event.

There is actually a damped oscillation in the coil even without the
condenser.  This is caused by the distributed capacitance of the coil and
wiring.  The frequency of this oscillation is very high and the coil, as
designed, is very inefficient at producing a high secondary voltage at this
frequency.

If you want to see oscilloscope pictures of what happens in a
point/coil/condenser, go to my web page (below my signature) then go to
"Otherstuff" and pick your subject.

Oh, yes.  You can get very scientific about the value of the condenser used
but most of the coils aren't very picky about just what resonant frequency
they resonate at.  As long as the value is between 0.15 microfarad and 0.47
microfarad, you should be okay.  If you get a non-automotive condenser, get
a film type (NOT ELECTROLYTIC!) capacitor rated at 630 Volts DC and you will
be fine.

Please note that this is just an audit class.  No quizzes will be given.

Take care - Elden
http://www.oldengine.org/members/durand 

> -----Original Message-----
> From: sel-bounces at lists.stationary-engine.com
> [mailto:sel-bounces at lists.stationary-engine.com]On
>  Behalf Of
> bill at antique-engines.com
> Sent: Monday, June 08, 2009 02:14 PM
> To: The SEL email discussion list
> Subject: Re: [SEL] Recharging of Maytag coils
> 
> 
> Having a degree in automotive technology, earned when points were 
> still in use to some degree, here's why I state that it MAY, mind you 
> MAY not start and/or run, or run properly:
> 
> When the points open, the sudden loss of primary current causes a 
> quickly collapsing magnetic field in the coil which induces a larger 
> voltage in the secondary windings. The voltage traveling through the 
> secondary winding in turn induces a current flow, in reverse, in the 
> primary circuit
> - enough to cause the points to arc.
> This "back emf" is due to the design of the ignition coil and its 
> inherent inductance.  This feedback of electrical energy causes a 
> spark to occur at the points that pits and erodes the contact 
> surfaces.  To suppress this energy, a condenser is used.  It acts like 
> a shock absorber (absorbing the back emf), ** and in so doing it helps 
> to interrupt the primary current as quickly as possible when the 
> points open.
> Without the condenser, the back emf would form an arc across the 
> points as they separate, ** slowing the collapse of the coil's field, 
> thus delaying and thereby reducing the voltage rise in the secondary 
> coil winding - ~possibly~ reducing it enough to cause it to not span 
> the plugs gap.  The condenser suppresses most, but not all of the back 
> emf. So points still burn or erode.
> 
> Bill
> 
> > It should start and run without the condenser
> but only till the
> > points burn which can be fairly quickly. The
> condenser has little to
> > do with the actual generation of spark, more to
> do with the
> > protection of the points from arching and
> burning quickly. If you try
> > it without a condenser gap the points a bit
> wider then the spec .020
> > to get a clean break or so that the arc flame
> between the points does
> > quit due to width of gap.        Mark


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