What does your magneto voltage tell you? Learn how to use a voltmeter to accurately measure a low-tension rotary magneto’s condition.
It is common practice to try to determine the health of a low-tension rotary magneto by putting a voltmeter on the magneto and spinning it up. Typical readings range from 5v to 12v. What does the value of that magneto voltage tell you? The short answer is — nothing. It turns out that your voltmeter does not tell the truth.
Let’s say the reading was 10v. If 10v will run an engine, why not just hook a 12v battery and a resistor to the igniter and forget the magneto? Also, when the engine is running, the igniter is closed (shorted) for all but a brief period of time, so the actual running magneto voltage is 0v. Those thoughts alone should raise some questions. Let’s try to understand those issues and find a way for how to use a voltmeter to accurately measure a magneto’s condition.
We measure voltage as a difference between two points. One of those points is a reference, often a stake driven in the ground, aptly called ground. Generally, we make the block on our engine the reference, or ground. When we say the magneto voltage is 10v, we are saying its voltage is 10v higher than the engine block voltage. But as the bird sitting on the 50kv power line would tell you, voltage by itself isn’t worth much. A single turn of wire will deliver several hundred volts to an igniter — but no spark. Shuffling across the carpet on a dry day will provide several thousand volts. Rather than thinking of voltage as the answer to all our problems, think of it as a facilitator that will enable things to happen.
A few hundred volts across your igniter is enough voltage for an electron to jump across the open points of your igniter. It takes about 70v per 0.001-inch of gap for the arc to start. If the voltage is consumed and disappears when that electron jumps, you won’t have combustion. There must be enough voltage to drive enough electrons (the current is measured in Amps) across the points’ gap to create a hot spark channel for combustion. The simultaneous delivery of voltage and current is power (measured in Watts). You may have a 75Watt heating blanket (120v using 0.63A of current) that will keep you nice and warm long term. It won’t do much if a fuse blows the instant you turn on the blanket.
Here is where it gets tough. For the magneto, it isn’t enough to deliver voltage (a few 100v) and current (1/2 Amp or so) to create a hot spark channel. It must do so long enough to cause combustion, about 0.002 of a second. Voltage plus current plus time is energy measured as Watts per seconds (kWh by your utility company) or Joules over in the physics department. We need to know if the magneto is capable of delivering enough energy to cause combustion. Almost any magneto can deliver the voltage and current, but doing so long enough to create combustion is the stumbling block for most weak magnetos. You’re probably thinking, “there’s no way I can to measure voltage, current, and time simultaneously.” It is hoped that you understand energy is the key and voltage is just one component.
Rotary low-tension magneto ignition
These magnetos are simply dressed-up battery and coil systems. The magneto steals energy from the flywheel and converts it to electrical energy thus replacing the battery. The magneto armature windings act as the coil for the battery and coil ignition as well as the windings for the generator action.
Figure 1a is a simplified schematic during the first half of rotation. When the igniter is closed, the generator action (represented here as a battery) pushes current through the coil and its wire resistance (often around 5 Ohm) through the igniter to ground. During the second half of the magneto’s rotation, the generated voltage pulls current out of ground, through the igniter, and through the coil resistance (see Figure 1b). The voltage and current alternate in and out of the magneto. That’s what we call AC or alternating current.
Figure 2 is key to understanding the low-tension magneto. Anytime current is flowing in a coil, the coil develops a magnetic field around itself. That field is pure energy. You can’t feel it or see it, but it’s real. Just as real as the energy in a spinning flywheel.
The physics department has given us an easy way to measure the stored magnetic energy, although we can’t touch or feel it.
L is the coil’s inductance, a measure of how much the coil acts like a coil. It is determined by the number of wire turns, core material that the coil is wound around (air, steel, etc.), length and diameter of the coil, and other physical terms. Once the armature coil is wound, L doesn’t change. A magneto model will have the same L value over the years of manufacturing. The L value is around 0.1 for many magnetos. I is the current in Amps and is squared. Current-squared means that the energy goes up rapidly with current. The energy goes up four times if the current, I, is doubled.
Physical analogy
Let’s use a rope to pull a 50-pound rock up a rafter (the magneto is building current in the armature coil). It is storing energy while the rock hangs there. This is called potential energy; you can’t see or feel the magneto’s magnetic field energy. When the rope breaks (igniter trips), the rock won’t sit there storing energy (the coil is not going to continue storing energy). Energy is always conserved. It might move or change form, but it will not evaporate. The rock’s potential energy is converted to damage on the ground (magnetic energy is converted to a spark).
Measuring magneto voltage
Using the energy equation: If the current is zero, the stored energy in the armature coil is zero; if there is current in the armature coil, there is stored energy. The magneto acts as a battery and coil for half a rotation of the magneto and pushes current through the coil building energy in the coil’s magnetic field. Suddenly, the igniter trips and the points pop open creating an open circuit. The coil looks at that and says, “They want the current to be zero and my energy to be zero.” The coil must dump all the stored energy, but the open points are blocking its only path. The only way out is to jump the igniter and dissipate the energy as heat in a spark. The coil will find a way to dump its stored energy, hopefully across the igniter rather than a spark across the mica washers, or other undesired places.
How does the coil do that? The amount of energy is the product of voltage, current, and time. The time goes down if the required voltage goes up. The coil takes that low-voltage, high-current energy it has collected over a long period of time (1/2 of an armature rotation) and trades time for voltage and dumps its energy as high-voltage high-current for a very short time. The energy in your spark is the energy stored in the armature coil the moment the igniter trips. Period. There is no voltage in that equation, only current. Want a hotter spark? Store more energy in the armature coil. Increase the current at the time the igniter trips to store more energy and get a hotter spark. Measure short-circuit AC current to measure the health of a magneto.
How to use a voltmeter to measure magneto voltage per early magneto manuals
When magnetos were being developed in the late 1800s and early 1900s, the only meters capable of measuring AC were voltmeters. AC current meters, AC power meters, and scopes were developed later. Using a voltmeter is easy: Stick one lead on the point of interest and the other lead on the engine block. Measuring current is a bit more complicated. The meter must be inserted inline. To make matters worse, many inexpensive multimeters will not measure AC current.
The L value would double and the spark would be twice as hot if we were to double the number of wire turns in the armature coil, but the armature would grow to twice its size. The spark would be 4X hotter if we found a way to raise the current from 1 Amp to 2 Amps. The early magneto developers looked at the energy equation and realized only one thing was important: get the most voltage possible to drive the most current possible at the exact time the igniter trips. Any time the igniter is not tripping, voltage and current are irrelevant. The irrelevant periods of time can be sacrificed to enable maximum voltage (and maximum current) when the igniter trips. Early engineers added moon-shaped pieces of steel (pole pieces) inside the magneto body, rounded plates on the end of the armature core, and shaped the magnets like horseshoes (see Figure 3). Those design changes created a severely distorted voltage wave form that pushes the current to a peak just as the igniter trips. Figure 4 is the voltage developed by the magneto in Figure 3. That is the voltage you are trying to measure when you hang a voltmeter on a magneto and spin it up.
Time moves from left to right while voltage is up and down in a scope trace. In Figure 4, 0v is a horizontal line across the middle of the screen, the vertical is 20v per box. As time progresses, the armature is rotating clockwise and we move along the scope trace to the right. In Figure 4, the key in the armature shaft is at the 6 o’clock position starting at the left negative peak at -55V. As we move in time toward the right, the armature rotates and, by 7 o’clock, the voltage has abruptly risen to near zero and stays there until near 11 o’clock where it rapidly shoots up to +60v (12 o’clock) when the igniter trips. By 1 o’clock, the voltage is down near 0v again and hangs around that value until near 5 o’clock. Remember, the only time that counts is the 11 o’clock to 1 o’clock value during which the igniter trips, if timed properly. The relevant region represents less than 10 percent of the total time. No meter in your arsenal can read the voltage at this time. Voltmeters are all designed to read a smooth looking sinusoidal wave form, similar to Figure 5.
Your voltmeter will be happily reading 5v to 15v in the irrelevant region when it gets hit with a short 60v ping. The ping is too short, timewise, to register. Basically, you are left reading the voltage in the irrelevant region and ignoring the voltage in the relevant region. To make matters worse, most voltmeters are designed to read voltages with a frequency above 50Hz (cycles per second). The 600rpm you may have spinning the magneto is 10Hz, well below your meter’s range. Your voltmeter doesn’t have a prayer and can’t read both the wave form and frequency. The voltage wave form of Figure 4 was read on the four meters shown in Figure 6. All four meters have been shown to be accurate on their intended sinusoidal voltages. The meters range from relatively inexpensive to a professional grade meter on the right. The readings were 5.9v, 8.6v, 5.7v and 8.2v. Had I told you I read four magnetos with voltages of 5.9V, 8.6V, 5.7V and 8.2V, you would likely have told me to send the 5.7V and 5.9V units to the shop, but the other two would be OK. The lesson there is, if you don’t like the reading you get, try a different meter. But keep in mind, any reading is pretty meaningless.
The current you are interested in that develops from this distorted voltage is rather well behaved, nearly sinusoidal and readable with an AC Amp meter. At 600rpm, the magneto in Figure 3 produced 1.0 Amp peaks (shown in Figure 7). If the igniter is timed properly and trips at one of those peaks, a healthy, high energy spark will be delivered.
Looking again at Figure 7, if your magneto and igniter are not timed properly, things can quickly go bad. If you have your igniter tripping at exactly where your engine manufacturer suggests, maybe 20 degrees B.T.D.C., the magneto must also be timed correctly. If the magneto is as little as one gear tooth too late or early, the current at the time of igniter trip can be 1/2 the peak current producing a spark 1/4 as hot.
The takeaways here are: The energy in your spark is the energy being stored in the armature coil when the igniter trips and that energy is dependent on the coil current. The only meaningful measure of the magnetos condition is to measure its AC output current. Measuring an armature output voltage does have some value — if the voltage is zero, the magneto is dead; if the voltage is not zero, the magneto is not dead. Not dead, however, could mean it needs intensive care or it could mean it’s capable of running a marathon.
The voltage waveform in Figure 4 came from the John Deere magneto in Figure 3. As a comparison, the output of an IHC Type R is shown in Figure 8a, a Sumpter No. 12 in Figure 8b, and a Webster (springs removed and spun up) in Figure 8c.
You are interested in the peak current, always trying to have the igniter trip at the very top of the wave in Figure 7. The current spends only a short time at the peak. Your meter will give you the R.M.S. (root mean square) value that represents how much the current could get done (over all time). Your reading will always be around 0.7 times the peak. So, what should the AC current be for a good magneto? I cannot speak for all, but in the magnetos I have measured (Iowa Dairy, John Deere, Associated, IHC, Sumpter, Webster, etc.) a reading of 0.5 Amp and above on your AC current meter is a good magneto, when turning 300rpm or higher.
Addendum
As pointed out above (due to meter short comings), reading the output voltage of a magneto while spinning it up provides no useful information on the health of the magneto. The output of the magneto is short pulses of voltage, in the range of 70v to 100v, that are too narrow for any meter to read. Additionally the frequency is well below what is readable by standard meters. The 5v to 12v typical meter reading is simply the meter’s “stab in the dark” at a voltage it cannot read. It was pointed out that different meters will likely give varying results. Also stated, the energy in your spark is determined by the current in the coil at the time the igniter trips, not the voltage you are trying to measure.
Fortunately, coil action converts these narrow voltage pulses to a nice AC coil current. The function of the voltage, that your meter cannot read, is to get the current rolling in the coil. The useful measure of a low-tension magneto’s health is its AC short circuit current (see Figure 1). Although the output varies across magneto brands, 0.7A is typically a good magneto. Most magnetos that produce only 0.5A or less are likely in trouble.
But, as pointed out in the previous article, many low-cost analog meters don’t have an AC current setting. Fortunately, any analog AC voltmeter can be converted to an AC current meter by simply adding a resistor (see Figure 2).
First, short the output of the magneto through a 1 Ohm resistor. When the resistor is in place, connect the AC volt meter across it. Figure 3 is a completed set up.
By Ohms Law, the voltage across the resistor will be exactly the AC current in Amps. When the magneto is spun up, at least 500rpm, a 0.75v voltage reading (for example) means the magneto is putting out 0.75A. This test should be conducted using an analog meter. A digital meter is often bothered by the low frequency. The resistor should have a power rating of at least 1 Watt and is readily available on Amazon and eBay.
Above addendum originally published as “Addendum to Voltage Value” in the June/July 2023 issue of Gas Engine Magazine.
Dr. David Cave is a regular contributor to Gas Engine Magazine and can be reached at jdengines@cox.net.
Originally published as “Voltage value” in the February/March issue of Gas Engine Magazine.
