Section 3 Engine Systems
200
The condenser makes the breaking of the primary
current as instantaneous as possible by absorbing
the surge of primary current to prevent arcing
between the breaker points.
As the magnetic fi eld collapses through the
secondary winding of the coil, high voltage is
induced in the secondary winding. At exactly the
same time, the charge stored in the condenser
surges back into the primary winding, Figure 10-20,
and reverses the direction of current in the primary
windings. This change in direction sets up a
reversal in direction of the magnetic fi eld cutting
through the secondary and helps increase the
voltage in the secondary circuit. The high-voltage
potential causes secondary current to arc across
the spark plug gap.
Figure 10-18 shows the fi eld reversal taking place
in the center leg of the core and coil. The reversal
induces low-voltage current in the primary circuit
through the breaker points. Current fl owing in
the primary winding of the coil creates a primary
magnetic fi eld of its own, which reinforces and
helps maintain the direction of the lines of force in
the center leg of the lamination. It does this until
the magnets’ poles move into a position where
they can force the existing lines of force to change
direction in the center leg of the lamination. Just
before this happens, the breaker points are opened
by the cam.
Opening of the points breaks the primary circuit,
and the primary magnetic fi eld collapses through the
turns of the secondary winding. See Figure 10-19.
Goodheart-Willcox Publisher
Figure 10-18.
The change in the magnetic field through the core
induces a voltage in the primary circuit. Since the
breaker points are closed, the current is able to flow
through the circuit. The current through the primary
windings builds a magnetic field that passes through
the secondary windings.
Primary coil
Soft iron
laminated core
Ground
Ground
Cam
Ground
Breaker points
N S
Magnetic field
created by current
through the
primary windings
Flywheel
direction
Secondary coil
Breaker point
spring
Stop switch
Goodheart-Willcox Publisher
Figure 10-19.
When the breaker points open, the magnetic field
around the primary windings collapses quickly through
the secondary windings. This induces high voltage in
the secondary windings, which is required to fire the
spark plug. The field collapse also cuts through the
primary windings, where it induces a moderate voltage
that is absorbed by the condenser.
S
N
Flywheel
direction
Magnetic field
begins to collapse
Primary coil
Ground
Condenser
Soft iron
laminated core
Ground
Breaker points
open
Ground
Breaker point
spring
Ground
Secondary coil
Cam
Stop switch
Copyright Goodheart-Willcox Co., Inc.
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