TheDurk said: (quoted from post at 17:50:01 11/21/08)
John G. Hasler said: (quoted from post at 17:25:42 11/21/08) > if you have a 12 volt system a resistor is
> required in some form or points will have a
> short life.
The coil will also have a short life. It will overheat at low rpm due to the long dwell time.
I agree with everything except the last eight words. The dwell time as a percentage of each second or minute does not change with RPM, unless you count zero, when it 'dwells' as long as the points are closed to start with. The length of each power cycle is greater, but so is the cool-down time, both being in milliseconds. I would think it would overheat equally at any RPM.
At high rpm & thus short dwell time, the current may never reach the coil's maximum, since it rises exponentially and at such rpms, the dwell time is shorter than the time dictated by the L/R ratio. HOWEVER, at low rpm, the dwell time being longer, coil current reaches it's maximum & then just sits there, at that maximum, making heat until the dwell period ends (current is lower trace). Even without an oscilloscope (using a simple ammeter) you can observe the AVERAGE current decrease into the coil as rpm increases. The product of this average current times voltage does heat the coil more at the lower rpm/longer dwell time. Because of this, heating increases by a factor greater than just the time, but cooling is just proportional to time.
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