Magneto (Engine Generator) Symbol
Definition: The Magneto (Engine Generator) symbol represents a self-contained permanent-magnet alternator that generates its own ignition voltage without an external battery, used in circuit and wiring diagrams for small engines (chainsaws, lawn mowers, motorcycles, aircraft engines) to indicate the combined ignition and charging source.
Also known as: magneto ignition, permanent magnet alternator, PMA, magneto generator, engine magneto, flywheel magneto.
What the Magneto (Engine Generator) symbol means
The Magneto symbol denotes an electromechanical device in which a rotating permanent magnet assembly induces a high-voltage AC pulse in a stationary ignition coil winding, and optionally a lower-voltage AC output in a separate charging winding, all without requiring a battery or external excitation. The symbol's two pins represent the positive (+) and negative (−) output terminals of the device.
In engine wiring diagrams the magneto symbol marks the primary source of both ignition energy and battery-charging current in small-engine systems. It indicates that the ignition system is self-contained — the engine can start and run without a battery, unlike automotive systems that rely on a separate alternator and battery.
How to identify the Magneto (Engine Generator) symbol
The symbol is drawn as a rectangular block or stylised generator symbol labelled 'Magneto' or 'Magneto (Engine Generator)'. Two output pins emerge from the block: + (positive, typically the AC or rectified output hot terminal) and − (negative, the return/ground terminal). Some renderings use the standard generator circle symbol (a circle with the letter G or the sine-wave AC symbol inside) to indicate the generating function, with 'MAG' or 'MAGNETO' labelling.
Function in a circuit
As the engine crankshaft rotates, the permanent magnet rotor sweeps past the stationary coil (stator) laminations, inducing an AC voltage by electromagnetic induction. The primary winding produces a low-voltage AC signal that is collapsed by the breaker points or electronic trigger module to create a high-voltage spike in the secondary winding — sufficient to fire the spark plug (10,000–40,000 V). A separate stator winding (lighting/charging coil) generates AC output, typically 6–14 V AC at a few watts, for battery charging or lighting circuits in small motorcycles and outdoor power equipment.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-10 defines symbols for rotating machines; a magneto is represented as a generator symbol (circle with AC wave) with appropriate labels. There is no dedicated IEC glyph for a small-engine magneto distinct from a general AC generator. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 Section 16 defines the rotating-machine symbol (circle with designator letters). The magneto uses the generator (G) symbol with a PM (permanent magnet) qualifier. NEC does not govern magneto ignition circuits; SAE and FAA standards (e.g. FAA AC 43.13-1B) cover aircraft magneto maintenance. |
| Key difference | Both IEC and ANSI use a circle-based generator symbol for the magneto; the primary difference is the PM or MAGNETO qualifier label rather than a distinct glyph shape. |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + |
| neg | - |
Typical values
Primary ignition coil output: 200–300 V AC peak (before point collapse). Secondary (spark plug) voltage: 10,000–40,000 V. Charging coil output: 6–14 V AC at 20–60 W (typical for small motorcycles and ATVs). Engine speed range: 500–10,000 RPM. Spark timing: fixed or variable (advance mechanism).
Where the Magneto (Engine Generator) symbol is used
- Chainsaw and string-trimmer ignition systems: self-contained magneto fires spark plug without battery
- Lawn mower and garden tractor engines: single-cylinder magneto ignition provides reliable start without battery maintenance
- Small motorcycle and moped charging systems: magneto stator winding charges the 12 V battery via a rectifier
- Light aircraft piston engines: redundant dual magneto system ensures ignition without electrical system dependency
- Marine outboard engine ignition: magneto-based system tolerates salt-water corrosion better than battery-dependent electronics
- Emergency generator sets: magneto ignition ensures the generator can start and run during power outages without a charged battery
Example
In a small motorcycle wiring diagram, the magneto symbol shows the + pin connected through a rectifier/regulator block to the 12 V battery positive terminal, and the − pin connected to chassis ground. A separate ignition winding is shown internal to the magneto block, connecting to the CDI (capacitor discharge ignition) unit. The CDI triggers the spark plug coil from the magneto's trigger pulse, while the charging winding keeps the battery at 12.5–14.5 V during riding.
Key facts
- The Magneto symbol represents a permanent-magnet alternator that generates ignition voltage and optionally a charging current directly from engine rotation, without requiring an external battery.
- Two pins define the symbol: + (positive output terminal) and − (negative/ground terminal), representing the AC or rectified DC output of the magneto's charging winding.
- The magneto primary coil produces a low-voltage AC signal; the secondary coil (ignition coil transformer) steps this up to 10,000–40,000 V to fire the spark plug.
- In small engines (chainsaws, lawn mowers), the magneto is the only electrical source — the engine runs entirely on magneto energy with no battery required.
- Aircraft reciprocating engines use dual independent magneto systems for redundancy; the ignition system continues to function even with complete electrical system failure.
- IEC 60617-10 and ANSI Y32.2 / IEEE 315-1975 represent the magneto as a generator circle symbol (G) with a PM (permanent magnet) or MAGNETO qualifier label.
- The magneto charging winding output is AC; a separate rectifier and regulator are required to convert it to regulated DC for battery charging in motorcycle and ATV systems.
Frequently asked questions
What does the magneto symbol mean in a circuit diagram?
The magneto symbol represents a permanent-magnet alternator driven by the engine crankshaft that generates both ignition voltage (for spark plug firing) and optionally a low-voltage AC charging output, all without requiring an external battery. In small-engine schematics it marks the sole electrical power source for the ignition and charging systems.
What is the difference between a magneto and an alternator?
A magneto uses a permanent magnet rotor and requires no external excitation or battery — it generates voltage solely from rotation. An automotive alternator uses an electromagnet rotor (field winding) that requires battery excitation to begin generating; it cannot self-start without a battery. A magneto is therefore the correct choice for battery-free small engines.
What do the + and − pins mean on the magneto symbol?
The + pin represents the positive (hot) AC or rectified DC output terminal of the magneto's charging winding, which connects to the battery positive terminal through a rectifier-regulator. The − pin represents the return/ground terminal, connected to the engine block and chassis ground.
How does a magneto generate spark plug voltage?
As the engine turns the permanent magnet rotor, flux change through the primary stator winding induces a low-voltage AC signal (200–300 V peak). Breaker points or an electronic trigger module interrupt this primary current, causing a rapid flux collapse that induces a high-voltage pulse (10,000–40,000 V) in the secondary winding. This pulse travels to the spark plug, causing ignition.
Why do aircraft use dual magneto systems?
Aircraft piston engines use two independent magnetos — each firing one spark plug per cylinder — for redundancy and combustion efficiency. If one magneto fails, the engine continues running on the other. Additionally, dual ignition per cylinder improves flame propagation and power output. This is mandated by FAA regulations (FAR Part 23) for certified aircraft engines.
What standard defines the magneto symbol in electrical diagrams?
IEC 60617-10 defines rotating-machine symbols including generators; a magneto is represented as a generator circle (G symbol) with a PM (permanent magnet) qualifier. ANSI Y32.2 / IEEE 315-1975 uses the same generator circle symbol. Aircraft magneto maintenance is governed by FAA Advisory Circular AC 43.13-1B.
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