Dynamo no go

[Bob Culver]

For any that may be curious, if the field is connected in reverse, when an output begins to be produced it produces a field current which cancels instead of reinforcing the residual magnetism.... so no net output. Reversing the residual magnestism reverses everything and the situation is unchanged.

Sometimes after long standing , dismantling etc the residual magnetism becomes so reduced that the dynamo will not get under way. Operating it as a motor (or trying to if on the car) will restore any residual magnetism, polarised depending on the connection. (field and output terminals joined and connected to battery, dynamo frame to battery earth, whichever pole is earth in the car. ) Although motoring is a test it is not necessary to do to restore or reverse residual magnetism (polarise). Just connect the isolated field wire briefly to battery, frame to battery earth (as in car). Should draw enough current to spark. In the car prodding the cutout is fine but it must not be allowed to stay locked down.

To test the dynamo output connect field and output wires togehter and drive in normal direction .Between linked terminals and frame, should produce well over 6v as revs rise. The dynamo body must be negative if the car is negative earth.
An old 6 or 12v headlamp bulb is a good test load.

[Dave Wortley]

I had similar problems with 5 dynamos I had rebuilt a few years ago. They would run as motors but no go when connected to my electric drill rig. I think I got about 1v on all. A few weeks later I tried them on my rig again, this time for longer and on each one the drill would come under load suddenly and wind up my hosepipe drive coupling producing 8v or so. (Can’t remember exact volts.)

[Ruairidh Dunford]

Perhaps the brushes were wearing in a better fit Dave, once fully surface contacted it all kicks in properly?

[Tiger]

I only have a small 6volt battery and some larger 12 volts
Are either appropriate for polarising ?

[Tiger]

Just tried flashing the field coils .
1/2 volt now produced
Does run very well as motor on 6volt
Wondering whethe to reconnect the field coils ?
Just reconnect brush lead to field post and original field post to the brush ?
Internal cables will need lengthening so only want to do this if their is responsible certainly it
wil work
comments really appreciated
Regards
Tim

[Bob Culver]

Motoring is not a 100% test. I assume it motors with reasonable vigour. When motored, irrespective of pos or neg to earth, should rotate in same direction as operates in car. If rotating in reverse the field is reversed!
(without giving it a lot of thought I presume that if just one field was reversed, it would not seriously motor) 

I dunno which car you have but John Cornforths Dorset club site circuit diagram for a Ruby is also very close to RP and is probably the best aid to electrical puzzles for anyone.

[Tiger]

Field wiring reversed.
NO EFFECT !
I am guessing that not polarised correctly
I am using a 6volt motorcycle battery
Could the small battery capacity be the problem???

[Bob Culver]

You have me beat. There still seems to be doubt about the field connections. The dynamo should motor in the direction of normal operation.
To get a larger polarising current through the field the 3rd brush can be temporarily moved until it contacts the earth brush, or be linked to it. Whilst briefly applying battery between the external F terminal and the body bang the dynamo body a few times with a hammer.
To avoid current through the armature, the D output terminal can be left unconnected.

[Tiger]

Thanks Bob
It does rotate fast with both field connections.
Also always in the same direction indicated on the case.
I will try a higher current using the hammer.

[Dave Wortley]

Let it run for longer on a drill powered rig and make sure brushes are bedded in?