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14/45 cut-out problem

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  • 14/45 cut-out problem

    For a little while I have been plagued by an intermittent fault in that sometimes the cut-out contacts weren't releasing when the dynamo output voltage fell below the battery voltage. It occasionally showed a discharge on the ammeter of up to 16 amps at below 600 rpm. Thanks to great support from Ian Potts I came to the conclusion that the fault must lie within the cutout assembly so this was removed carefully and closely examined. The large lead from one end of the bottom coil that goes to terminal 5A was found to have the insulation failed where it had been in contact to the earth post which itself showed evidence of electrical contact. The insulation on the wire to 5A was charred and when cleaned showed the breakdown was complete. The rest of the adjacent insulation was hard and probably brittle.
    Clearly the cut-out assembly requires an expert rebuild to ensure that it would operate reliably. So for the moment I have made up a replacement cut-out employing a Schottky diode capable of handling 30 amps at 100 volts so when in use it would be running, at worst, at no moe than 50% of its rating. The diode is mounted directly to a large heat sink all enclosed within the original Rotax cover. A picture is attached.
    This has cured the problem and the ignition light goes out at 850 rpm and comes on at about 600 rpm so nicely within Talbot spec. The hardware is cheap to buy but since I did not wish to cannibalise the old cut-but I made a new base plate to fit the Rotax cover correctly and this also aids heat dissipation.
    I think what had been happening was that intermittent partial or complete conic to the earth had been occurring and probably there ws some vibration in the lead duet the nature of the normal operation. This heavy gauge wire is a tight fit in the distribution panel and I had completely missed this danger. At least there seems to have been no damage to the dynamotor.
    Attached Files
    Last edited by gordon higginbotham; 05-06-2021, 12:37 PM.
 
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