[SEL] Recharging of Maytag coils

Elden DuRand edurand at mchsi.com
Mon Jun 8 16:48:00 PDT 2009


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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