High-Tension Magneto System Theory of Operation – The Primary Electrical Circuit

in Engine Ignition and Electrical Systems

The primary electrical circuit consists of a set of breaker contact points, a condenser, and an insulated coil. [Figure 4-5] The coil is made up of a few turns of heavy copper wire, one end is grounded to the coil core and the other end to the ungrounded side of the breaker points. [Figure 4-5] The primary circuit is complete only when the ungrounded breaker point contacts the grounded breaker point. The third unit in the circuit, the condenser (capacitor), is wired in parallel with the breaker points. The condenser prevents arcing at the points when the circuit is opened and hastens the collapse of the magnetic field about the primary coil.

Figure 4-5. Primary electrical circuit of a high-tension magneto.

Figure 4-5. Primary electrical circuit of a high-tension magneto.

The primary breaker closes at approximately full register position. When the breaker points are closed, the primary electrical circuit is completed and the rotating magnet induces current flow in the primary circuit. This current flow generates its own magnetic field, which is in such a direction that it opposes any change in the magnetic flux of the permanent magnet’s circuit.


While the induced current is flowing in the primary circuit, it opposes any decrease in the magnetic flux in the core. This is in accordance with Lenz’s Law that states: “An induced current always flows in such a direction that its magnetism opposes the motion or the change that induced it.” (For a review of Lenz’s Law, refer to the Aviation Maintenance Technician—General Handbook, FAA-H-8083-30). Thus, the current flowing in the primary circuit holds the flux in the core at a high value in one direction until the rotating magnet has time to rotate through the neutral position to a point a few degrees beyond neutral. This position is called the E-gap position (E stands for efficiency).

With the magnetic rotor in E-gap position and the primary coil holding the magnetic field of the magnetic circuit in the opposite polarity, a very high rate of flux change can be obtained by opening the primary breaker points. Opening the breaker points stops the flow of current in the primary circuit and allows the magnetic rotor to quickly reverse the field through the coil core. This sudden flux reversal produces a high rate of flux change in the core, that cuts across the secondary coil of the magneto (wound over and insulated from the primary coil), inducing the pulse of high-voltage electricity in the secondary needed to fire a spark plug. As the rotor continues to rotate to approximately full register position, the primary breaker points close again and the cycle is repeated to fire the next spark plug in firing order.

The sequence of events can now be reviewed in greater detail to explain how the state of extreme magnetic stress occurs.

Figure 4-6. Components of a high-tension magneto circuit.

Figure 4-6. Components of a high-tension magneto circuit.

With the breaker points, cam, and condenser connected in the circuit as shown in Figure 4-6, the action that takes place as the magnetic rotor turns is depicted by the graph curve in Figure 4-7. At the top (A) of Figure 4-7, the original static flux curve of the magnets is shown. Shown below the static flux curve is the sequence of opening and closing the magneto breaker points. Note that opening and closing the breaker points is timed by the breaker cam. The points close when a maximum amount of flux is passing through the coil core and open at a position after neutral. Since there are four lobes on the cam, the breaker points close and open in the same relation to each of the four neutral positions of the rotor magnet. Also, the point opening and point closing intervals are approximately equal.

Figure 4-7. Magneto flux curves.

Figure 4-7. Magneto flux curves.

Starting at the maximum flux position marked 0° at the top of Figure 4-7, the sequence of events in the following paragraphs occurs.

As the magnet rotor is turned toward the neutral position, the amount of flux through the core starts to decrease. [Figure 4-7D] This change in flux linkages induces a current in the primary winding. [Figure 4-7C] This induced current creates a magnetic field of its own that opposes the change of flux linkages inducing the current. Without current flowing in the primary coil, the flux in the coil core decreases to zero as the magnet rotor turns to neutral and starts to increase in the opposite direction (dotted static flux curve in Figure 4-7D). But, the electromagnetic action of the primary current prevents the flux from changing and temporarily holds the field instead of allowing it to change (resultant flux line in Figure 4-7D).

As a result of the holding process, there is a very high stress in the magnetic circuit by the time the magnet rotor has reached the position where the breaker points are about to open. The breaker points, when opened, function with the condenser to interrupt the flow of current in the primary coil, causing an extremely rapid change in flux linkages. The high-voltage in the secondary winding discharges across the gap in the spark plug to ignite the fuel/air mixture in the engine cylinder. Each spark actually consists of one peak discharge, after which a series of small oscillations takes place.

They continue to occur until the voltage becomes too low to maintain the discharge. Current flows in the secondary winding during the time that it takes for the spark to completely discharge. The energy or stress in the magnetic circuit is completely dissipated by the time the contacts close for the production of the next spark. Breaker assemblies, used in high-tension magneto-ignition systems, automatically open and close the primary circuit at the proper time in relation to piston position in the cylinder to which an ignition spark is being furnished. The interruption of the primary current flow is accomplished through a pair of breaker contact points made of an alloy that resists pitting and burning.

Most breaker points used in aircraft ignition systems are of the pivotless type in which one of the breaker points is movable and the other stationary. [Figure 4-8] The movable breaker point attached to the leaf spring is insulated from the magneto housing and is connected to the primary coil. [Figure 4-8] The stationary breaker point is grounded to the magneto housing to complete the primary circuit when the points are closed and can be adjusted so that the points can open at the proper time.

Figure 4-8. Pivotless type breaker assembly and cam.

Figure 4-8. Pivotless type breaker assembly and cam.

Another part of the breaker assembly is the cam follower, which is spring-loaded against the cam by the metal leaf spring. The cam follower is a Micarta block or similar material that rides the cam and moves upward to force the movable breaker contact away from the stationary breaker contact each time a lobe of the cam passes beneath the follower. A felt oiler pad is located on the underside of the metal spring leaf to lubricate and prevent corrosion of the cam.

The breaker-actuating cam may be directly driven by the magneto rotor shaft or through a gear train from the rotor shaft. Most large radial engines use a compensated cam that is designed to operate with a specific engine and has one lobe for each cylinder to be fired by the magneto. The cam lobes are machine ground at unequal intervals to compensate for the elliptical path of the articulated connecting rods. This path causes the pistons top dead center position to vary from cylinder to cylinder with regard to crankshaft rotation. A compensated 14-lobe cam, together with a two-, four-, and eight-lobe uncompensated cam, is shown in Figure 4-9.

Figure 4-9. Typical breaker assemblies.

Figure 4-9. Typical breaker assemblies.

The unequal spacing of the compensated cam lobes, although it provides the same relative piston position for ignition to occur, causes a slight variation of the E-gap position of the rotating magnet and thus a slight variation in the highvoltage impulses generated by the magneto. Since the spacing between each lobe is tailored to a particular cylinder of a particular engine, compensated cams are marked to show the series of the engine, the location of the master rods, the lobe used for magneto timing, the direction of cam rotation, and the E-gap specification in degrees past neutral of magnet rotation. In addition to these markings, a step is cut across the face of the cam, that, when aligned with scribed marks on the magneto housing, places the rotating magnet in the E-gap position for the timing cylinder. Since the breaker points should begin to open when the rotating magnet moves into the E-gap position, alignment of the step on the cam with marks in the housing provides a quick and easy method of establishing the exact E-gap position to check and adjust the breaker points.