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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Poor starting on new Amal and BTH Magneto
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<blockquote data-quote="Magnetoman" data-source="post: 43139" data-attributes="member: 2806"><p>A changing current in a coil induces a voltage in a direction that opposes that change. I'll come back to that, but first let's ignore it. </p><p></p><p>In the case where we can ignore the armature reaction (i.e. where we can ignore precisely the effect you're asking about), an "ideal" armature would be designed so the brush moved from being symmetrically across the gap between one segment to the next the moment the current through the coil connected to the two segments bridged by the brush had completely reversed direction. In this case, there would be no sparking at the brush because the current would be no longer changing.</p><p></p><p>OK, what if we don't ignore the voltage induced by the collapsing field (i.e. by the changing current)? Here the magnitude of the effect depends on the specifics of the armature design. A well designed armature will be made with the smallest number of large diameter wires that are necessary to generate the desired voltage (i.e. the 6V or 12V). This is done to keep the inductance of each coil in the dynamo as small as possible, since it is in the inductance that the problematic magnetic energy is stored: the smaller the inductance, the smaller the energy stored in the field, and thus the smaller the induced voltage to cause sparking when that field collapses. </p><p></p><p>What this means in practice is that although the very real effect you asked about does exist, the small diameter and short length of motorcycle armatures means the coils have a relatively few number of turns and thus the induced "sparking voltage" at the trailing edge of the brushes is small. This is in contrast with magneto armatures whose coils are designed with a large number of turns to maximize the effect of a collapsing field rather than to minimize it.</p></blockquote><p></p>
[QUOTE="Magnetoman, post: 43139, member: 2806"] A changing current in a coil induces a voltage in a direction that opposes that change. I'll come back to that, but first let's ignore it. In the case where we can ignore the armature reaction (i.e. where we can ignore precisely the effect you're asking about), an "ideal" armature would be designed so the brush moved from being symmetrically across the gap between one segment to the next the moment the current through the coil connected to the two segments bridged by the brush had completely reversed direction. In this case, there would be no sparking at the brush because the current would be no longer changing. OK, what if we don't ignore the voltage induced by the collapsing field (i.e. by the changing current)? Here the magnitude of the effect depends on the specifics of the armature design. A well designed armature will be made with the smallest number of large diameter wires that are necessary to generate the desired voltage (i.e. the 6V or 12V). This is done to keep the inductance of each coil in the dynamo as small as possible, since it is in the inductance that the problematic magnetic energy is stored: the smaller the inductance, the smaller the energy stored in the field, and thus the smaller the induced voltage to cause sparking when that field collapses. What this means in practice is that although the very real effect you asked about does exist, the small diameter and short length of motorcycle armatures means the coils have a relatively few number of turns and thus the induced "sparking voltage" at the trailing edge of the brushes is small. This is in contrast with magneto armatures whose coils are designed with a large number of turns to maximize the effect of a collapsing field rather than to minimize it. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Poor starting on new Amal and BTH Magneto
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