This simple, mechanically driven power source is nearly 200 years old and still has a well-defined, albeit limited, role in today’s systems.
What or who is a magneto? No, a “magneto” is not only the powerful mutant who has the ability to generate and control magnetic fields and first appeared in the debut issue of The X-Men in 1963.
Instead, the engineering magneto is a basic electromechanical way to convert motion – usually rotary, but can be linear – into electrical power. Despite its age, it is a very old technique and is still used in modern versions ranging from relatively mundane applications to higher-end ones where safety is critical. In some cases, you could even consider the use of the magneto as a form of energy harvesting, as it takes or “siphons” some available mechanical energy and converts it to electrical energy.
Note: the terms and parameters energy and power are distinct entities. Energy is the ability to do work, and power is the rate at which work is done. For our purposes in this FAQ, however, the two terms are roughly interchangeable, and there is no need here to make a distinction between them.
This FAQ will explore the magneto’s history, construction, and applications, as well as its advantages and limitations.
Q: Start with basics: what does a magneto do?
A: A magneto consists of a permanent magnet and a wound electrical (copper) coil). It is a source of “unsteady” non-DC electrical current, functioning as a crude, basic, yet effective electrical generator. The changing or reversing flux generated by a magnet passing through or across the coil produces a voltage difference across the primary coil (Faraday’s Law) (Figure 1). This voltage difference also causes a current to pass through the coil and to a load, thus creating electrical energy from mechanical motion.
Q: Is there more than one basic magneto structure?
A: Yes. A shuttle magneto has a fixed magnet while the coil rotates or moves; an inductor magneto keeps the coil fixed while rotating or moving the magnet. The inductor design is usually preferred as there is no need to connect to wires to a moving or rotating coil assembly, so the design is more reliable. Either way, the underlying physics principle and use of Faraday’s Law is the same.
Q: What does the raw output of a magneto look like? What is its amplitude?
A: The output is a very non-sine-like AC waveform (Figure 2). The aptitude is a function of design, size, and speed with which the magnet passes the coil and is topically between ±15 and ±40 V; some larger magnetos can deliver as much as ±100 V.
Q: What is the difference between a magneto and a dynamo?
A: In general, a dynamo produces a clean, low-harmonic sinusoidal AC current, which is very different from the distorted, “spikey” waveform output of the magneto.
Q: How old is the use of the magneto as an electrical source?
A: Very old. French instrument maker Hippolyte Pixii built it in 1832 — just one year after Michael Faraday’s discovery of the principles of electromagnetic induction. Operated by a hand crank, his magneto was the first practical generator of electrical current (Figure 3).
Turning the crank caused the rectangular magnets (relatively weak, in those days) to go into and out of the wound coils via a simple rotary-to-linear motion mechanism, thus generating the electrical current. This was the first non-battery scheme for generating a reliable and somewhat continuous flow of current. Even if it was not continuous as it would be from a battery, it was a “clean” source that did not involve chemicals or suffer depletion.
Q: This all sounds like ancient and somewhat irrelevant history. Why should we look at it now?
A: First, magnetos were used for early cars, such as the Model T, and led to many developments in the technology of modern spark ignitions and electromagnetism. More importantly, they are still widely and more effective due to better materials, mechanical components, and magnets.
Q: Where are they used? What is their role?
A: They are used in both “low-end” applications, such as gasoline-powered lawnmowers or chainsaws, and in small aircraft. The low-voltage electrical output of the magneto is transformed into a voltage/current suitable for firing the spark plug of the internal combustion piston-based gasoline engine. These spark-related applications are the dominant ones for magnetos, where they do not need to provide a continuous flow of power but only periodic bursts.
They were also used in the early days of telephony, where the user would turn a crank on the side of the phone to generate the higher voltage (50 to 100 V) needed for the ringer (bell), while a low-voltage battery would be used for the actual talk circuit (Figure 4).
Q: Is the magneto output voltage used directly?
A: Generally, it is not useful directly. It may be used “as is” if it is only powering a lower-voltage circuit such as the telephone ringer (around 20-40 V, sometimes as high as 80-100 V). However, the voltage is much too low to ignite a spark in a gas-fueled engine where around 10 kV is needed. In these cases, various voltage step-up schemes using a dual-winding transformer are employed where the magneto voltage is the primary, and the transformer steps that up by two orders of magnitude (10 V to 1000 V).
Q: Why are magnetos even used these days? After all, we now have powerful, lightweight batteries.
A: One of the advantages of a magneto ignition system is that it does not require any external power source, such as a battery; it’s a totally self-contained, self-powering system. This makes it a good choice for applications where a battery as a primary DC power source is not available or feasible, such as small engines (lawnmowers, chainsaws). Additionally, magneto ignition systems are relatively simple and have few moving parts, which makes them durable and easy to maintain.
Q: That makes sense, but what is their use in aircraft?
A: There are several reasons that even modern propeller/piston engine aircraft with batteries still use magnetos to provide the cylinders with high-voltage sparks; they are as follows:
- Magnetos do not depend on the airplane’s electrical system, on batteries, or other sources of onboard electricity the airplane’s electrical system goes offline and completely loses power, the magneto-driven airplane’s engines will continue to run.
- Magnetos are reliable. As previously mentioned, they are one of the oldest ignition technologies in the aerospace industry, used by the first airplanes. They are time-tested and reliable
- Pilots may have to deliberately shut down the airplane’s entire electrical system under some drastic circumstances, such as in case of an electrical fire, to prevent it from spreading. By using magnetos for its ignition system, the engines will continue to run even when the electrical system is shut down.
The next part goes into more detail on the use of magnetos in ignition systems.
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Hemmings Motor News, “Magneto Ignition Systems”
National MagLab Magnet Academy, “Magneto – 1832”
Mechanical Jungle, “What Is Magneto Ignition System?”
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