What is the working principle of a motorcycle magneto?

[Raymond]

Motorcycle magneto works by generating a magnetic field after being electrified, then the rotor starts to rotate, the motor drives the wheels, and controlling the speed is essentially controlling the size of the resistance. Overview: The function of a motorcycle magneto is to start the motorcycle. It is a small alternator that uses permanent magnets to generate a magnetic field and serves as the ignition power source in the gasoline engine's ignition system. The magneto ignition system consists of a switch, ignition coil, circuit breaker, capacitor, distributor, and safety discharge device. It generates high-voltage electricity and distributes it to the spark plugs of each cylinder. Others: Most magnetos adopt a rotating magnet type and can be divided into unitary type and flywheel type according to the overall design. The unitary type integrates all the above components into one unit, which is installed on the internal combustion engine. The flywheel type mounts the magnet on the flywheel of the internal combustion engine, with other components installed separately on the engine, mostly used in single-cylinder engines.

[PaisleyLee]

The magneto is that power-generating device next to the motorcycle's heart—the engine, and its working principle is quite fascinating. You see, when the engine starts running, the crankshaft drives the rotor inside the magneto to spin along. That rotor is a disc embedded with magnets, surrounded by a stator made of copper coils. As the rotor spins, its magnetic field rotates too, cutting through the magnetic lines of the coils, which then generates alternating current (AC). This electricity is stored in the battery to power the headlights, horn, ignition system, and various electronic devices. I always enjoy dismantling my vintage bike to observe this setup—the copper coils wrapped like a honeycomb and the magnet plates gleaming brightly. If the power generation becomes unstable, it's usually due to one of three issues: demagnetized magnets, aged coils, or poor contact at the coil terminals. However, older motorcycles use an AC system, unlike many modern bikes that rely on DC systems. The former is simpler in structure and cheaper to maintain, while the latter is more stable and efficient but costlier to repair.

[OLane]

The power generation after a motorcycle starts is handled by the magneto. Simply put, it utilizes electromagnetic induction—when the engine drives the internal magnets to rotate, the magnetic field lines continuously cut through the outer copper windings. During this process, each change in the magnetic field induces an electric current in the windings. This portion of electrical energy is converted into direct current by the rectifier and stored in the battery. I remember last time when repairing my 125cc bike, I found that abnormal power generation from the magneto usually stems from three causes: demagnetization of the magnets leading to a weakened magnetic field; cracked insulation on the windings causing short circuits; or bearing wear causing friction between the rotor and stator. By the way, most motorcycle magnetos use a flywheel-type structure, with over a dozen arc-shaped magnets arranged radially inside. Compared to the belt-driven structure of car alternators, this design, which is directly coupled to the engine, may produce more vibration but is much easier to maintain.

[McLucia]

The magneto is that round metal casing next to the motorcycle engine. When opened, you can see magnets fixed on the rotor disc. As the engine crankshaft rotates, it spins along, causing the copper coils in the outer stator to continuously cut through the magnetic field, generating alternating current. This current is then converted into direct current by the rectifier-regulator and stored in the battery, powering ignition systems, lights, and other electrical devices. Having ridden motorcycles for over a decade, I know that if the battery keeps losing charge, it's usually a magneto issue. You need to check if the magnets have demagnetized, whether the coils are short-circuited, or if the connectors have rusted. Structurally, they come in single-phase and three-phase types. Older bikes mostly use single-phase systems, which generate less power but are cheaper, while newer high-displacement models typically feature three-phase systems for more stable power supply. They are much lighter than car alternators, but their poor heat dissipation makes them prone to coil burnout.