In what position is the magnet in a magneto when the greatest change in flux density in the coil core takes place?

In what position is the magnet in a magneto when the greatest change in flux density in the coil core takes place?

• A. At neutral position
• B. A few degrees beyond neutral
• C. In the E-gap position
• D. Completely rotated to maximum

Answer: (B)

The key idea is that the spark energy in a magneto comes from how quickly the magnetic flux through the coil core changes as the magnet rotates. According to Faraday’s law, the induced voltage is proportional to the rate of change of flux. In a magneto, as the rotor magnet passes by the pole pieces, the flux through the core swings from one polarity toward the opposite. That change happens most rapidly just after the neutral position, when the magnet has moved a few degrees beyond the center alignment and the flux path through the core is changing direction fastest. At this point the slope of the flux vs angle is steepest, so dΦ/dt is at its maximum and the induced voltage is greatest. If the magnet is at neutral, or further beyond neutral toward the extreme rotation, the rate of flux change is not as large, so the induced voltage is not at its peak. The E-gap position is a specific alignment related to the magneto’s timing, but the maximum rate of flux change for the coil occurs a short distance past the neutral position.

The key idea is that the spark energy in a magneto comes from how quickly the magnetic flux through the coil core changes as the magnet rotates. According to Faraday’s law, the induced voltage is proportional to the rate of change of flux. In a magneto, as the rotor magnet passes by the pole pieces, the flux through the core swings from one polarity toward the opposite.

That change happens most rapidly just after the neutral position, when the magnet has moved a few degrees beyond the center alignment and the flux path through the core is changing direction fastest. At this point the slope of the flux vs angle is steepest, so dΦ/dt is at its maximum and the induced voltage is greatest.

If the magnet is at neutral, or further beyond neutral toward the extreme rotation, the rate of flux change is not as large, so the induced voltage is not at its peak. The E-gap position is a specific alignment related to the magneto’s timing, but the maximum rate of flux change for the coil occurs a short distance past the neutral position.