Alternator Help

[Bob_Matthews]

Hi Guys

Below is a picture of a 6 coil Motoplat stator that has come from my Mototrans Elite.
Obviously someone has rewired it in the past but has used the wrong coloured wires!
I wondered if any of you know if it has been wired correctly, then I can just change the wires to the correct colours?
I am going to be using a 5 wire Reg/Rec that Ducwiz suggested to convert it to 12 volts and would like maximum output.

The red wire on the left is connected to a centre tapping of a coil at the 11 o'clock position.
The red wire in the middle is connected to an end tapping of the same coil at the 11 o'clock position.
The black wire on the right is connected to the centre tapping of a coil at the 1 o'clock position.

Any help greatly appreciated.

PXL_20210126_131349254 (Large).jpg

PXL_20210126_131412547 (Large).jpg

[Jordan]

Do you want to change to correct colours for aesthetics or a practical reason?
Some coloured sleeving, heat-shrink for example, could be added to the existing wrong coloured wires so you know what goes where.
I think the red wires should be yellow, and your black wire should be red.

But if going to 12V, you won't need the red (your black) wire anyway, and the other two wires can be interchanged.
So if you deleted the un-needed wire, you wouldn't need to know how to connect the other two as they can be either way.

[veloduke]

The soldering on those wires looks decidedly dodgy .
Those joints need remaking. Ensure the wire is scrupulously clean and free from oil before soldering, otherwise the joint will be contaminated and the solder won't flow properly.
Once done, I would cover the exposed joint with heatshrink sleeving to prevent contact with the windings. you don't want to chafe through the enamel on the coil wire.
Personally I would not install this alternator in its current state. Not unless you enjoy removing the clutch and flywheel .

[Bob_Matthews]

Jordan, I would like to change the wire colours for practical reasons and so as they are easily identifiable in the future.
When I ran the engine with the two red wires connected into the reg/rec, there was enough current coming out of the black wire to brightly light a halogen bulb - that's why I don't trust the connections that have been made by a previous owner. So, off with the clutch and flywheel to try and get to the bottom of this.

Max, yes the soldering is horrendous and so another reason for re-doing the wires. My soldering skills are pretty good - as a sideline I do a lot of guitar repairs and often have to change pickups and resolder volume and tone controls, jack plugs etc.

What I really need to know is how to connect to this generator from scratch - imagine there are no wires connected at all - so, which terminals do I connect the two yellow wires to? From what I have read, the coils could be connected all in series, all in parallel or many other configurations. And, what are the centre tappings for?

Single phase.png

4coil 2 coil.jpg

[Jordan]

I never checked, but it's not surprising some current is available at your red wire.
If the wire is still there but not being used, good idea to make sure the end is well insulated.

Note that the term "centre tapping" has a specific meaning, describing a type of coil connection that can provide a full wave output.

To understand how your Ducati/Motoplat alternator stator was designed to work, look up "single phase alternator, full-wave centre tapped, rectified".
It only needs two diodes.

Compare with "single phase alternator, full wave, bridge rectified" that uses 4 diodes and is more common on motorbikes from UK and Japan.

LIke a Ducati-made stator (made by its subsiduary company, Elettrotecnica) your Motoplat evidently has two windings on each bobbin, so there are four wire ends emerging from each coil. The best circuit diagram I've seen was contributed by Hans I think (sorry if I got that wrong).
It shows a 6 pole stator with only 5 of them having coils for battery charging. The other one is for a CDI charging coil in a completely separate circuit (and not shown in the drawing).
It is shown split for clarity, and it should be recognised that it is all on the same stator.

5 charging coils winding layout.jpg

Yours is similar in principle, but uses all 6 poles for battery charging. Must be for a points ignition bike.

Here's a Ducati Elettrotecnica 6 pole stator (with a brown wire where the red one was).
Looks like a mirror image of the Motoplat, but is logically the same.

Elettrotecnica-6coil-stator.jpg

Your last drawing shows switchable dual current output, used on some alternators (but not on Ducatis).
On these, ignition current demand can be catered for with 2 coils, the other 4 coming into circuit via the "lights on" switch.

[ducwiz]

Jordan,

you gave a perfect explanation of the winding scheme I once posted here. Also, I agree with your estimation about Bob's 2nd figure.

For clarity about the differences of the center-tapped/3wire stators and the 2wire version, I uploaded a page from the Ducati Pantah WSM. Both versions were used in the Pantah 500, and both work as so called "series" regulators.

page173.jpg

In the top figure, one can see the center tap wire (red) is connected to Bat+, while two "phases" or branch ends (yellow) are connected to one SCR (silicon contollabel rectifier) each. The winding branches act in antiphase fashion, i. e. one carries positive voltage while the other is negative, and vice versa. In turn, only one branch carries current at a time, the other is idling because the rectifier/diode blocks.
The bottom figure shows the single phase version, where the 2wire stator winding is connected to a rectifier bridge circuit (2 diodes, 2 SCRs). The winding carries current all the time, because it is commutated by the bridge, producing pulsating DC continuously.

The trick with the 12V-conversion of a 6V alternator is (as was already explained several times) to leave the center tap wire unconnected, and connect 2 yellow wires to a regulator containing a rectifier bridge. By this, we double the active AC voltage, and due to the constant maximum power output of the alternator, cut the current in half.
It's no wonder that an AC voltage is still present on the center tap, which can source AC power against ground. During the commutating action, either end of the stator wdg. is bound to ground or Bat+ dy the diodes in the bridge. So, the (virtual) center of the winding is not at a constant reference potential/voltage anymore. One should avoid to use the center tap (red) as a power source! Safely tuck it away and insulate its AMP faston terminal.

If one of the old 12V bridge regulators (750GT/S/SS, 860GT/GTS, early Pantahs) is a leftover in your workshop, it can be used to do the conversion, you need no further modifications.
A second possibility was described by me and others - utilisation af a cheap chinese 5wire scooter R-R. This contains also a diode bridge, but is a so called "shunt" regulator. When the max. Bat voltage is reched, it short-circuits the stator winding, hence cutting out AC power as long as the battery voltage is below a certain threshold. the wdg. idles for a cerain time, until the battery voltage again reaches the threshold for re-connection.

Hans

[Bob_Matthews]

Thank you all for your replies.

The bike is a coil and points ignition.

I have now re-wired the stator with the correct colour wires and some much better soldered connections, the engine is back together and running.

I have measured the output by connecting the output from the reg/rec into a low charged car battery to give me a chance of obtaining the max output from the alternator.

The output at tick-over is a max of 2.0 amps and at high revs the max is 5.5 amps (66 watts) - which is a little disappointing, but I guess fairly normal for this alternator?

Normally I would like to run a Philips Racing Vision H4 halogen bulb (60/55W) but I guess I'll be looking at a B1235P43 45/45W H4 with an led tail light - which will probably slightly drain the battery with ignition circuit taken into consideration.

[Harvey]

Can you fit a LED headlight blub?

[Elitepete]

Hi all, sorry to jump in on this topic but looks like guys here can help me. I would like to upgrade and simplify the Elite electrics using a modern Rectifier Regulator, and have acquired one for 6 volts which will handle up to 140 watts, and has 4 wire connection, obviously 2 for the generator connection and 2 for the battery connection. The original generator [40 watts I think] has 3 wires, and I was wondering if I can re-wire to provide a suitable output for the Rectifier Regulator. Hopefully this will avoid the complex wiring which switches in the additional coils when the lighting is turned on. I guess I can use LED bulbs to reduce the current requirement and have something better than half candle power lighting!

[ducwiz]

Is it a Mototrans Elite or an italian model?

Hans