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Tech. Advice: Industrial Engines
V-Twin Magnetos
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<blockquote data-quote="Magnetoman" data-source="post: 90064" data-attributes="member: 2806"><p>Er, um, not really. The current in the coil is created by the time rate of change of the magnetic flux (d(flux)/dt). It's not that the current in the coil tries to stay constant, it's that a current is induced in the coil to try to keep the magnetic flux in the coil constant (known as Lenz's Law). </p><p></p><p>If the flux ceased changing the current would decay in a time ~L/R, where L is the inductance of the coil and R is its resistance. If there were no arcing at all when the points opened the resistance of the primary would instantaneously increase to infinity from ~0.5 Ohms so the current would decay instantaneously. This in turn would create an infinite voltage across the coil (again, Lenz's Law). Thanks to arcing nothing reaches infinity in actual operation. </p><p></p><p>Anyway, two things take place simultaneously in our magnetos that create our sparks. Once is that the coil on the armature spins in the magnetic field of the permanent magnet, creating a current in the coil that, when interrupted by the points opening, decays extremely rapidly and induces a large voltage across the secondary. The other is the pole pieces in the magneto body along with the shape of the armature cause the flux through the armature to abruptly reverse direction at a certain, um, point. That represents a large d(flux)/dt, which induces a large voltage across the secondary. So, if the points are arranged to open at just the right moment the output of the magneto is the sum of these two processes.</p><p></p><p>Early Bosch magnetos (e.g. the ZEV) had asymmetric armatures to have the point of flux reversal maximum at two orientations ~45-deg. apart of engine rotation. Later it was realized this wasn't strictly necessary even though a symmetric armature supplies one cylinder with a weaker spark.</p></blockquote><p></p>
[QUOTE="Magnetoman, post: 90064, member: 2806"] Er, um, not really. The current in the coil is created by the time rate of change of the magnetic flux (d(flux)/dt). It's not that the current in the coil tries to stay constant, it's that a current is induced in the coil to try to keep the magnetic flux in the coil constant (known as Lenz's Law). If the flux ceased changing the current would decay in a time ~L/R, where L is the inductance of the coil and R is its resistance. If there were no arcing at all when the points opened the resistance of the primary would instantaneously increase to infinity from ~0.5 Ohms so the current would decay instantaneously. This in turn would create an infinite voltage across the coil (again, Lenz's Law). Thanks to arcing nothing reaches infinity in actual operation. Anyway, two things take place simultaneously in our magnetos that create our sparks. Once is that the coil on the armature spins in the magnetic field of the permanent magnet, creating a current in the coil that, when interrupted by the points opening, decays extremely rapidly and induces a large voltage across the secondary. The other is the pole pieces in the magneto body along with the shape of the armature cause the flux through the armature to abruptly reverse direction at a certain, um, point. That represents a large d(flux)/dt, which induces a large voltage across the secondary. So, if the points are arranged to open at just the right moment the output of the magneto is the sum of these two processes. Early Bosch magnetos (e.g. the ZEV) had asymmetric armatures to have the point of flux reversal maximum at two orientations ~45-deg. apart of engine rotation. Later it was realized this wasn't strictly necessary even though a symmetric armature supplies one cylinder with a weaker spark. [/QUOTE]
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