Over the years, there have been several papers published in Gas Engine Magazine explaining the low-tension ignition, but very little information is available on the magneto itself. First, a quick review of the low-tension system before getting into the magneto. Rotation of the magneto armature causes current to flow in the armature coil through the shorted igniter points. Any time current is flowing in a coil, there is a magnetic field around the coil, and that field is storing a lot of energy. You may not be able to see that energy, but put a piece of iron near the end of the coil and you’ll feel its electromagnetic pull. When the igniter points pop open, the igniter, in a way, is telling the coil that it wants the coil current to instantly go to zero. The coil in turn is thinking, “a bit ago I had a bunch of current flowing through me and I was storing a bunch of energy, suddenly the igniter wants my current and energy to be zero.” The coil has to get rid of all that energy.
Energy is conserved, it can change forms or move from place to place, but it can’t just disappear or evaporate. The only option for the coil is to convert its magnetic energy to heat energy by blowing current across the open points as a spark. From that discussion, it becomes clear that the energy in a spark, or how hot a spark is, is exactly the energy stored in the coil when the points pop open. “Hot” is a street term meaning strength not temperature. That energy is: E = 1/2 X L X I².
Mistake #1 is trying to determine the quality of a magneto by reading its output voltage. There is no V in that equation, only the current, I. Good low-tension magnetos will produce around 1 amp of peak current. A deeper dive can be found in The Low-Tension Ignition System.
So, how do low-tension magnetos do their magic and get the current rolling? To figure that out, it’s necessary to take a look at their structure. Figure 1 is the magneto as it’s most often seen. Its parts are a magnet, a gear and a housing. This magneto is an Iowa Dairy, better known as an Associated 2 bolt or a John Deere, but its general shape and construction are similar to all low-tension rotating magnetos.
After the gear, magnet, front plate, lead-out tower and armature are removed, the inner workings are more visible as in Figure 2. It appears that each inner wall of the magneto housing is half-moon shaped and not the same material as the housing itself. The armature core that the coil is wound on has some large chunks of metal that wrap partially around the armature.
Figure 3 is what’s left after stripping everything away that isn’t necessary to make a magneto function. All that’s needed is the armature core with wings shaped a bit like a Star Wars fighter, a pair of half-moon shaped pole pieces, about 250 feet of fine copper wire that wraps about 800 times around the armature core and a magnet. Everything else in Figure 1 is there to hold it together, keep it aligned or get the energy out. Figure 3 reveals the real shape of the pieces, using that information and again looking at Figure 2, it can be seen that the half-moon pieces, darker material in the housing, are separated almost exactly the width of the armature wings.
Figure 4 is what an X-ray view of Figure 1 would look like if the X-ray machine could only see iron-based metal and copper.
A normal generator used to make electrical power would not have the wings on the armature core nor the half-moon-shaped pole pieces. An armature with these wings that mate with half-moon pole pieces are technically called shuttle armatures. It seems that the engineers in the late 1800 must have had a reason to make those changes. Indeed, they did. Their thinking was the only time we need output is just as the igniter trips, all other output is a waste. They squeezed and bunched up a half turn of energy into a packet available just before the igniter trips. The wings and the pole pieces are used to create a massive pulse of energy just before and during the time the igniter trips but near zero at all other times during the armature’s rotation.
In paragraph 1, it was pointed out the energy in the spark depends only on the coil current when the igniter trips, but in order to get the current rolling, voltage is necessary. In the operating mode, with the igniter shorted, the generated voltage is consumed in building a magnetic field to store energy and to overcome the coil wire resistance.
Summarizing to this point: With the igniter shorted, the armature turns, creating an unmeasurable voltage that causes current to flow in the coil, creating a magnetic field that stores energy and which is used to create the spark. The armature wings and pole pieces are used to create voltage spikes which in turn create current spikes before and during the igniter trip.
Mistake #2 is believing that only a strong magnet can make a good magneto. Figure 5 is a thought experiment where a magnet creates a magnetic field that passes through an iron core with a coil wrapped around it. There is a voltmeter attached to the coil and some sort of magnetic valve in the magnetic field path. Not much is known about the magnetic valve other than the fact that it can turn on and off the magnetic field. Physics tells us that the voltage that the voltmeter will see depends on how fast the valve changes the magnetic field. For example, if the magnet has 100 magnetic lines and it takes the valve 1 second to shut off the field, the voltage is related to 100/1 = 100. But a weak magnet of only 50 magnetic lines, which has a fast valve that shuts down the field in 0.25 seconds, will generate a voltage related to 50/0.25 = 200. The weak magnet with a fast valve would be the better magneto. The important point is that, in order to get good voltage, the magnetic field must change quickly through the center of a coil. It’s easier to get rapid change by starting with a strong magnet, but rate of change is key. So, it’s not the rotation of the armature that creates spark energy, rather it’s the fact that as the armature rotates, the magnetic field passing through coil changes. Slow turning rotary magnetos, gummed up Websters and sluggish Wico’s (high tension) will all have slow-changing magnetic fields and create weak sparks. It’s likely then that the armature wings and the pole pieces in rotary magnetos are used to create a very fast change in the magnetic field passing through the armature core just before and during the time the igniter trips. The rapid change in magnetic field would create a large voltage, which, in turn, would create the necessary current.
The mock up of a magneto, seen in Figure 6, will be rotated while observing what happens to the magnetic field and therefore the expected voltage. First, a quick review of some rules of magnetism. Rule 1, magnetic lines always form a closed loop. If followed long enough it will always come back to where it started. Rule 2, some materials are harder to pass through than others. Iron can be thousands of times easier than air. When multiple paths are available, magnetic lines will take the easy path. More on that can be found in Evac: A Super-Charged Horseshoe Magnet. As an aside, the magnet has been welded in Figures 3 and 4. That article also explains why that is not a problem.
This demonstration will arbitrarily assume that the magnet has 100 magnetic lines and initially they all pass through the armature core and thus the coil. Further, the magneto is sitting at the 3 o’clock position and will rotate counterclockwise.
A magnetic line representing the 100 is added in Figure 7. The lines leave the N pole, pass into the half-moon-shaped pole piece, jump the 0.010-inch clearance air gap, down the armature core passing through the coil, jumping the second air gap into the pole piece, into the S pole, up and around the horseshoe to complete the closed loop. That path is mostly iron-based with just two small air gaps making other paths for the magnetic lines more difficult. Note the direction the lines are traveling as they pass down the armature core through the coil.
In Figure 8, the armature has rotated to 1 o’clock. As far as the coil is concerned, nothing much has changed. The magnetic lines now need to travel upward a bit in the pole piece but it’s still the easiest and preferred path. There are still 100 lines passing through the coil so no voltage is generated as the armature rotated from 3 o’clock to 1 o’clock.
The armature has progressed to about 12:30 in Figure 9. At 12:30, the lines need to travel even further in the pole piece and they will enter the armature on the end of a wing. Other paths will all have larger air gaps and, therefore, be more difficult.
In Figure 10, the rotor has progressed to 11:30. Wow, what happened? Now to avoid a large air gap at the top as the lines leave the N pole, , they travel downward to enter the armature. All 100 lines have flipped direction as they pass through the coil. The coil has gone from +100 lines to -100 lines, for a change of 200 lines. At 600 RPM, this would have happened in 0.008 second, resulting in a large voltage spike.
As the rotor continues to turn, the field remains at -100 and no voltage is developed until 6:30. Between 6:30 and 5:30, there will be another reversal. Figure 11 is a graph of what we just observed as the armature rotates through 360 degrees.
The wings on the armature and the half-moon pole pieces make the rotary low-tension magneto a bit of a magical machine. Most of the time, the magneto is generating no output, but just before the igniter points snap open, the magneto creates a pulse of high voltage. That pulse of voltage drives a large current, generally around 1 amp, which causes the coil to store a lot of energy in its magnetic field. When the igniter trips, the coil dumps that energy as a spark across the igniter points. The magneto then goes back to doing nothing for another 180 degrees of rotation.
Figure 12 is the actual voltage output of the magneto in Figure 1 at about 600 RPM. Each pulse of voltage is 70V tall but less than 0.010 second wide. Some magnetos will produce voltage pulses well over 100V that are only 0.005 of a second wide. But all low-tension rotary magnetos will have a voltage output similar to Figure 12. In this plot, as the armature gets near the 12 o’clock and 6 o’clock positions, a small voltage does begin to develop due to the extreme crowding on the edge of the pole piece and armature wing. The extreme crowding causes some lines to find other paths, not through the core, to complete their loop, resulting in less lines through the core and a smaller voltage.
Mistake #3 is ignoring mistake 1 and trying to read the output voltage. No voltmeter on Planet Earth can read those 70 to 100V pulses that are 0.01 of a second wide or narrower. All meters, analog or digital, ignore the all-important narrow pulses and simply read what they can–the longer unimportant time where the voltage is near zero. Therefore, the 8 volts to 12 volts voltmeter readings typically reported provide no useful information. As stated in paragraph one, the current created by these unreadable voltage pulses determines the quality of a magneto. A method of measuring the current can be found in Addendum to Voltage Value. Magnetos are sometimes referred to as special generators. Looking at Figure 12, it’s clear that the magneto’s pulsed output would make a poor-quality generator, while the smooth output of a generator would produce a weak spark.
Dr. David Cave is a regular contributor to Gas Engine Magazine and Farm Collector. He can be reached at jdengines@cox.net.
