Dynamos

Can you reverse the direction of a 12 volt dynamo and in particular a 3rd brush dynamo. I know its a little off ME, but there are some very clever people who read this forum.

From my enquiries, it seems that the commutator segments have an angular offset to the armature slots which when the direction is reversed limit the usefull output of the dynamo.

I believe a 3rd brush can be converted to a 2 brush one and by using an electronic control box to regulate the output.

So what does the team think?

Bobh

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

Definitely reverse direction (then perhaps polarity). The dynamo has a shaft both ends BUT if I turn the dynamo end for end, then the end nearest to the drive shaft just has a plain bearing, with the ball race at the non load bearing end - not as designed.

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

Hope this helps.

I have no practical hands-on experience of 3 brush dynamos so what I say may be rubbish. However if I understand the theory aright the 'third' brush picks up enough volts with respect to the 'ground' brush to drive the field coils and allow some regulation, which way round the volts are should not matter. But there is a snag, for a given direction of rotation there is a 'sustainable' and an 'unsustainable' way of connecting the field coils. If you change the direction of rotation then you must swap over the field coil - armature connections and reflash the field coils afterwards. It is the reflash that decides the polarity and the swapover that decides whether the dynamo will startup and generate. If you want +ve wrt to ground then that is the way you flash the coils - no mysterious reversals needed.

For a given direction of rotation a working shunt dynamo (what this is) can deliver +ve or -ve wrt ground depending on which way it is flashed - which is why jump leads and confusion over +ve and -ve earth cars caused so much trouble back in the old dynamo days.

Sounds like Pete may be the main man here.

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.

Roger has beaten me to it.

The other way is to reverse the polarity of the neutron flow...

Neil

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.

Thanks all, Info from this site + MadMikes reference encourages me to reverse the field wiring and have a go. The 3rd brush can be moved a few degrees, and so I will see what happens.

BobH

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

It may also depend on whether the brushes are mounted radial, or tangentally, if the latter maybe not reversible as far as rotation. There is quite a bit on Google, but no one talks of reversing the rotation. Ian S C

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

Was thinking more about this last night, my brain seams to have started up again, but Tim seems to have covered most of it. I can remember working night shift and needed lights all the time, dynamo set for max. amps, come saturday and a long run forgot to reset the dynamo ,result a boiled battery. It also occurs to me that it would be easier to make or modify the brush end cover to include a ball race.

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