MAGNETO PARTS AND OPERATION
[Illustration: Fig. 56. Magnets--Pole Blocks]
The purpose of the magneto is to furnish electrical current at regular
intervals, to jump the spark plug gaps and to ignite the gas which has
been compressed in the combustion chambers. The discovery was made
years ago that, by placing a coil of wire between two magnetic poles,
current would be present at once. But it is only while the wire coil is
in motion that the current will flow or circulate, and while there are
many theories why this takes place only while the coil is in motion,
none seem to explain the fact satisfactorily. The strength of the
current depends on the size of the magnetic field, and the number of
wraps of wire in the coil. Consequently the larger the coil the more
intense the current. Fig. 56 represents the magnets, of which there are
from three to six. The U-shaped pieces are made of steel which has been
case hardened and charged with electricity which causes them to become
magnetized. Magnets have two poles or axes, one of which is positive
from which the current flows, and one of which is negative to which
the current flows or passes. Fig. 56A shows the pole pieces which are
located on the inside of the lower or open end of the magnets. The pole
pieces are channel ground, leaving a round space or tunnel in which the
armature revolves.
Fig. 57 shows the soft iron core which is shaped like the block letter
H, and wound with fine wire, making up the coil shown in Fig. 57A of
the wound armature.
[Illustration: Fig. 57. Armature Core--Wound Armature]
[Illustration: Fig. 58. Primary and Secondary Winding and Current
Direction]
Fig. 58 shows the primary and secondary winding. The primary or heavy
wire is wound on the core lengthwise, each strand being separated from
the other with rubber or tin foil insulation. The current passes from
the top of the left pole piece to the top of the core until it passed
out of range, crossing the upper gap between the two pole pieces. As
the top of the core leaves or breaks the contact flow of current, the
bottom of the core comes in contact range, leaving an open space which
breaks the current and changes the direction of flowage as shown in
Fig. 58A and 58B. This current is of a low tension nature, and will not
jump the gap at the spark plugs when the engine is running slow. The
secondary winding, shown in Fig. 58, is made up of many more windings
of a finer wire. The low tension or primary current is led through the
armature shaft to a contact breaker at the rear of the magneto.
Fig. 59 shows the contact breaker, which consists of a housing in which
two platinum points are arranged, one point stationary, the other
attached to an arm on a pivot. The points are held together by spring
tension.
[Illustration: Fig. 59. Breaker--Slip Ring--Distributor]
A cam on the armature shaft comes into contact with the arm on which
the second point is located, forcing it from the stationary point,
thus breaking the low tension current which returns to the secondary
coil, the magnetizing and demagnetizing caused by the break in the low
tension current, and sets up a rapid alternating current. One end of
the secondary is led to a collector ring on the front of the magneto.
Fig. 59A shows the collector ring. A carbon brush collects the current
from the ring and conducts it to the distributor’s centrally located
arm. Fig. 59B shows the distributor. The centrally located arm is timed
to deliver the current, or comes into contact with one of the segments
or brushes and allows the current to flow from the segment to the gap
at the spark plug, where it jumps the gaps and ignites the gas in the
cylinders at the proper time. Then it returns through the ground (the
engine and the frame) to the magneto, where it passes back into the
secondary coil, passing through an insulated condenser consisting of
small plates of steel insulated from one another. This regulates the
flowage of the returning current, by reducing it through resistance,
and prevents the armature from heating.
A safety spark gap is provided on some magnetos which causes the spark
to jump and lose some of its force through resistance when the plugs
become shorted. This also restricts the current and greatly aids the
condenser in performing its purpose.