remagnetizing dynamo

[David Tinsley]

1955 GF
Finally got the bike running, starts very easily with rebuilt mag. Happy days.

Starting looking at the charging side and found that dynamo is not producing any output (used the shorting D and F terminals together and checking for voltage between there and earth). Digging through some old posts I find that maybe I need to remagnetize the dynamo as it has not been run for a long time. I need to flash the F terminal to the live side of battery? And so finally to my question!
I only have a 12V battery on hand, the bike is 6V. Is using a 12V battery going to cause any issues?

As an aside I want to get into the brushes and have a nosy at the commutator but it seems I have to remove the damn unit from the bike? I can see a clamping screw for the cover but cannot access it due to the engine/dynamo covers. Or am I missing something, which is not unusual for me!

Thanks in advance for any tips or tricks

[Jim S]

Hi David
Do you have a 6v battery installed on the bike? If yes,  just make up a lead and run it from the live side of the battery to the F (Field) side of the dynamo.

If you do this and still no output with D and F connected together, remove the timing cover and check that the dynamo drive side is working. (Chain not broken,  sprocket not spinning on shaft). If that is ok it looks like the dynamo needs to be removed since your access to the cover is limited.

Where are you located? I'm near Montreal.

[David Tinsley]

Hi David
Do you have a 6v battery installed on the bike? If yes,  just make up a lead and run it from the live side of the battery to the F (Field) side of the dynamo.

If you do this and still no output with D and F connected together, remove the timing cover and check that the dynamo drive side is working. (Chain not broken,  sprocket not spinning on shaft). If that is ok it looks like the dynamo needs to be removed since your access to the cover is limited.

Where are you located? I'm near Montreal.

Hi Jim,
I do not have a 6V battery to hand, but I do have a 12V one. I suspect 12V would be OK to flash the dynamo, but Just wanted to make sure.
Chain is fine and adjusted correctly when I had the timing side cover off.
I am in Fergus, just north of Guelph.
 

[trevinoz]

12V won't hurt to flash the field.

[David Tinsley]

12V won't hurt to flash the field.

Thanks, I guessed I would be OK with 12V, just wanted to confirm.
If that does not work then I will take dynamo off and investigate further.
Dave

[paulmbsa]

shorting the f&d and running it up will not be effected by polarity you should get power, however you do need to run it fast a lot of battery drills will not do it

Dynamos are not magnetized, i think you are talking about polarity

[Triton Thrasher]

shorting the f&d and running it up will not be effected[sic] by polarity you should get power…

…Dynamos are not magnetized, i think you are talking about polarity

No, that’s all wrong. 

Polarising is the process of magnetising the yoke (and probably other ferrous components). 

[Swarfcut]

  The test of joining F-D then to earth via a load eg headlamp bulb relies on the residual magnetism in the pole piece to produce a small output current which is fed back to the field coil. This induced field strength then becomes self sustaining and rises rapidly from nothing, hence the rising power output from the dynamo and the brightening of the test bulb. If the pole piece is suspected of losing  residual magnetism, you stand a better chance of an output by running the dynamo as normal and feeding voltage directly to the field coil for a few seconds, in the same way that the regulator does to control the power output.

  The dynamo needs to generate sufficient initial voltage to pull in the cutout, connecting the battery and regulator. Then the mechanical regulator controls the dynamo output by modulating the current in the field coil to increase or decrease the magnetic strength of the field and balance dynamo output to the electrical load. It is done in a series of on/off pulses, and small sparks can sometimes be seen on the regulator contacts as they go about their work.

 All this is detailed in previous posts, it's all there, just a matter of a good search.

 Back in the day a service exchange  E3L was dead as a doornail straight from the box. Down to the Lucas Service Department we went and that's the trick they used on new or recon units which had no output. On the test rig it performed as it should, and they made us feel like a couple of idiots. Polarising determines whether terminal D output is positive or negative. Flashing the dynamo as described elsewhere is the crude term for this.

 Swarfy.

[KiwiGF]

shorting the f&d and running it up will not be effected[sic] by polarity you should get power…

…Dynamos are not magnetized, i think you are talking about polarity

No, that’s all wrong. 

Polarising is the process of magnetising the yoke (and probably other ferrous components).

Paul may have meant the dynamos don’t have permanent magnets? (Which is correct)

The “yoke” is actually a chunk of cast iron (Or maybe steel?)  that fits inside the field coil and holds the field coil in place, there is a pic in this thread of one, next to the burnt out coil  (link below). It is this chunk of iron that needs to have some residual magnetism in order that the armature will generate some current when the dynamo is spun when the engine starts, and the regulator is not providing any current to the field coil.

I am a bit unclear if this residual magnetism is ALWAYS required to be present in sufficient strength whenever the motor drops below the speed required to generate 6v (or 12v in “converted” systems) eg idling, and not just when a motor is started up. However, I have observed current flowing “backwards” through the armature on a system with a mechanical regulator, when the motor rpm drops, which implies the need for residual magnetism is a “one off” thing at start up. Electronic regulators, or at least some, do not allow “backwards” current flow.  Maybe someone could explain more on that.

Pic of field coil

https://www.a7a10.net/forum/index.php?action=dlattach;topic=13999.0;attach=46647

Thread
https://www.a7a10.net/forum/index.php?topic=13999.msg115701#msg115701

[Rex]

The “yoke” is actually a chunk of cast iron (Or maybe steel?)  that fits inside the field coil and holds the field coil in place

Back when I were t'apprentice the "yoke" was basically the body or frame of the dynamo or motor, and it was the pole piece that had the field coil wound around it.

[Triton Thrasher]

The “yoke” is actually a chunk of cast iron (Or maybe steel?)  that fits inside the field coil and holds the field coil in place,

No it ain’t.

[trevinoz]

Kiwi, even if the current is flowing "backwards", it is the correct polarity. The reverse current occurs when the generator output voltage drops below the battery voltage but if the cut-out is adjusted correctly this will only be momentary.
By the way, I call the pole piece a pole shoe.

[groily]

Dave - if you are still having problems, this other thread on here from not long ago might give some more pointers / ideas. Sometimes, giving the field a boost with the thing running can get the thing to fire up.
Cheers, Bill
https://www.a7a10.net/forum/index.php?topic=16452.msg140631;topicseen#msg140631

[Swarfcut]

 Thank you Mr G, that's the thread that explains all.

    I stated in one of the posts there that field coils have a memory in an attempt to give a simple explanation of the difference between a brand new field coil and one that has been installed, polarised and working. I can only conclude there are far more succinct ways of approaching the problem as being shot down in flames by Trev, who takes these things seriously. Of course its not the wire in the coil, it's the soft iron pole shoe, as Lucas describe it, and the cylindrical body is referred to as the yoke, as above. The yoke and pole piece need to be in close physical contact so that the magnetic lines of force generated by the  field coil are spread  evenly through the dynamo body. The rotating armature coils cutting the magnetic field generates the output current. It's the trinity of Faraday, Fleming and Maxwell in action and the output varies with rotational speed and field strength. Give a little clap for the regulator, it's balancing all the plates in the air at the same time!

 Swarfy.

[David Tinsley]

Just to close this thread off.

I now have the dynamo working, at least on the bench. Resistance between D and earth was 45 Ohms. Took dyno off bike and cleaned commutator and all connections. Brushes seem in good condition and decent ones (could hardly draw a mark with them on paper).

Reassembled and motored it with 6V battery charger. All good. Then connected to a drill and it spun up to show over 12V.

Now to refit it to bike and wait for new loom and DVR box.

Onward and upwards.

As always thanks for all the tips and ideas.

Dave