Magneto turns ratio

aobp11 · Post by **aobp11** » Mon Aug 11, 2008 12:40 pm

I wonder if there is original Lucas information on the turns ratio primary:secondary of SR1, N1 and K2F magnetos. For SR1 approx 1:35 seems to be common, but I also encountered much higher figures up to 1:65. The SR1 was also used on stationary engines and the higher ratios seem to come from these.
A rewound and temperature-sensitive K2F showed 1:85 compared to an orginal(?) one roughly 1:45.
Albert

Don Madden · Post by **Don Madden** » Mon Aug 11, 2008 5:35 pm

I don't have exact figures available but think the primary windings generate approximately 6v & the ignition requires in the range of 25,000 for reliable spark. The Lucas service manual specified testing the armature using 4v, which will provide a long spark.

This would need a ratio of about 4,000:1.

Cheers, Don.

aobp11 · Post by **aobp11** » Mon Aug 11, 2008 10:04 pm

Hi Don, that's not the way the comparison works. In that test with an external interruptor, the battery voltage serves only to generate a certain primary current. When the contact points open, the primary induction voltage can rise to a few hundred volts; that voltage compared to the secondary voltage is roughly speaking equal to the turns ratio. Roughly, because reality is much more complicated e.g. due to superimposed parasitic oscillations via leakage induction. A 3-point spark gap at 5.5mm (specified by Lucas) sparks at about 9-10kV.
Albert

aobp11 · Post by **aobp11** » Wed Aug 13, 2008 2:28 pm

Maybe I'd better term the figures above something like EMF-ratio. They were obtained using a low voltage mains transformer as AC-supply for the primary. Because of losses and because of a less than 100% magnetic coupling between windings, the ratio of the numbers of turns will exceed the measured voltage ratio. In Technical Articles 200 turns primary and 12000 turns secundary are mentioned.
Albert