DY­NAMO

Classic Bike (UK) - - Workshop -

1 Miller 3 brush dy­namos are rare, so where could I get a re­place­ment for this bro­ken cast­ing? Ebay was the an­swer – Paul Dunn (01782 856839) sells these ex­cel­lent re­pros, CNC ma­chined from solid alu­minium. It’s bet­ter than orig­i­nal for a pretty rea­son­able £50.

4 But clean-up first. It’s amaz­ing it worked at all with all this oil and gunge in­side. I’ll put some heat-shrink on the field coil wires to pre­vent a short cir­cuit.

7 To con­vert, re­move the un­nec­es­sary stuff and con­nect one brush and one field wire to earth. The sec­ond field wire and other brush go to the out­put ter­mi­nals.

5 Check the ar­ma­ture with a me­ter set to re­sis­tance (Ohms). There should be no con­ti­nu­ity be­tween any of the cop­per com­mu­ta­tor seg­ments and the main spin­dle.

8 Good – it works con­nected to a bat­tery. If the di­rec­tion is wrong, swap the two field wire con­nec­tions over which will re­verse the ro­ta­tion.

2 This ‘third’ brush picks up cur­rent and feeds it to the field coil. By mov­ing its po­si­tion, cur­rent can be re­duced, pro­vid­ing prim­i­tive reg­u­la­tion.

3 The cut-out pre­vents dis­charge when static. It’s neat and self-con­tained but we’ll con­vert to sim­pler 2 brush type.

6 But there should be con­ti­nu­ity be­tween each seg­ment and the all the oth­ers. You can check the field coil by at­tach­ing a bat­tery and see­ing if it mag­ne­tises.

9 All that re­mains is to po­larise the field for the bike’s elec­tri­cal sys­tem. Take a live wire from the bat­tery and touch it to the ter­mi­nal con­nected to the field wire.

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