Bobh, do you want to reverse the polarity, or the rotation of the dynamo. Ian S C

Bob if this dynamo is for a motor vehicle then try contacting Pete at http://www.dynamoregulatorconversions.com

Hope this helps.

Oh, forgot the bit about lead and lag of brushes wrt commutator. Normally the brush set was set to lag behind the crossing of the field magnetic lines by a few degrees. So if you reverse the rotation then in theory you have to shift the brush set (not always easy). Also from limited experience I found the brushes object to reversal at least until they bed back in and get used to the new direction.

From memories of trying to reverse a 3 brush dynamo many years ago- the 3rd brush is for regulation (before the modern regulator ) this has to be physically moved to the other side, the whole mounting and adjustment gubbins. This was not easily possible on the one I had so was abandoned. Some makes may be easier.

Hi I think you need to look at a vintage motorcycle forum, they will have the knowledge to do what you need

Can I suggest that unless you can reposition the third brush so that it is 'the other side' you may just have found a quick way to burn out your windings. The field in a 3 brush system is fed from the output but not earthed. Instead it is connected to a section of the commutator which produces a low back-voltage that increases a lot with speed – thus compensating for the increase in output volts as the speed rises. So, the field coils see a reducing voltage and the output is steadier. Moving the third brush a small amount (2-3 degrees) will vary the output a lot – so having it in quite the wrong place may be a disaster.

You can run a 3 brush dynamo in the 2-brush manner, but unless you play with the output control the field coils (which are designed to work on lower than output voltage) will be likely to overheat and so overheat the armature as well (ie melt the commutator soldering).

The bearing type is only a problem if you drive by belt etc – with a side load at that end. If you can drive without a side load – by a quill shaft, for instance – your bronze bearing should be OK.

Most 3 brush car dynamos should not be set to produce more than about 8 amps, or you will have overheating problems. This is for a 12 volt system and a 4-5 inch diameter dynamo. Some big cars (and trucks) had bigger than this – but still nowhere near as much as the same sized alternator, and about half as much as a plastic insulated, ventilated, post-war (2 brush) version.

Changing the polarity is easy, and needs no rewiring – let me know if you need to know about this.

Cheers, Tim

Edited By Tim Stevens on 07/05/2014 21:21:25

My experience is limited to converting a 3 brush Miller dynamo to a 2 brush. The conversion is well documented, but as far as I can remember once converted all you have to do to reverse direction is simply disconnect the brushes and reconnect the earthed brush to the D terminal and earth the previously D connected brush. Test by "motoring" the dynamo.

Rob