1974 GT Alternator Mod

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Bern
Mach 3
Posts: 98
Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

1974 GT Alternator Mod

Post by Bern » Thu Jan 12, 2012 12:51 am

As per normal the winter has given me a little time to tinker with the bike. I took it upon myself to fix this wiring/ charging thing once and for all.

I cleaned up the wiring, changed a few things around and then I started thinking about the charging system. I have a new rotor that I will be installing but I am also thinking about the stator. The stator from the bevel is centre tapped due to the old rectifier circuit used. That means that only half the output voltage is used when rectifying in the original design. I found some comments by a guy name James de Raeve who wrote about this fix.

When using a modern bridge rectifier that only requires two inputs from a single phase stator. The excess voltage will just get turned into heat by the regulator. So why not modify the stator to use the coils from the center tap in parallel rather than in series. This will half the output voltage but it will double the current available.


Stator Before:
____|--------yellow(a)
/
\
/
\____
____|--------red
/
\
/
\____|--------yellow(b)


The following would be the mod to the stator

1.Disconnect one copper wire from the windings from the center tap.
2.Determine which yellow wire(a) and the disconnected copper from the windings form an open circuit.
3 Now attach that copper wire to the alternate yellow(b).
4. There should now be one copper wire left at the center tap from the windings this can be attached to the other yellow(a) 5. the center tap red can be ignored and that wire discarded
6. use these two yellow in a two wire bridge rectifier.

Stator After:

_______|--------yellow (a)
/ /
\ \ ________ center tap is no longer used
/ /
\_\______
-----------|--------yellow (b)


Thoughts? Downside other than wrecking my stator when I do the soldering.
Last edited by Bern on Thu Jan 12, 2012 7:18 am, edited 1 time in total.

eternally_troubled
Cucciolo - the Lil Pup
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Location: Cambridge, UK.

Post by eternally_troubled » Fri Jan 13, 2012 11:01 am

Will you not have problems by the halved voltage, or have I misunderstood?

Bern
Mach 3
Posts: 98
Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

Post by Bern » Fri Jan 13, 2012 11:32 am

eternally_troubled wrote:Will you not have problems by the halved voltage, or have I misunderstood?
I wouldn't think so since the the previous center tap rectifier was using only the voltage across half the coil anyhow. Keep in mind the center tap rectifier and a bridge rectifier work differently. The center tap was to provide a wave
that was 180 degrees out of phase.

I will test the output voltage before and after and let you know what I find.

Bern
Mach 3
Posts: 98
Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

Re: 1974 GT Alternator Mod

Post by Bern » Thu Mar 22, 2012 9:46 pm

We had record high temps here in Canada and I finally have had the Ducati out for a few long rides 'AT NIGHT' after the winter electrical mods.. The alternator mods were successful! I can run lights with no loss at about 2500 rpm with just the taillight(headlight off ) there is no loss at about 1500 rpm. This means city driving is a non issue now with the Ducati. Previously I would have to constantly recharge the battery. I have used up 3 tanks of gas of mixed city and highway, and absolutely no need to charge the battery so far. This was not remotely possible before. Battery voltage is a respectable 12.75 after a night time city ride using my headlight the entire time.


Mods performed

New rotor magnet.
Modifying the stator removing the center tap and setting up both sets of windings in parallel
Installing a modern two wire bridge rectifier/regulator

I will post pics and a detailed set of instructions and pictures and where to source the rotor and rectifier regulator.

The one thing I didn't do was swap out the old aprilia fuse box. The headlight circuit (15 amp fuse??) fuseholder has lost a fair bit of tension required
to hold the fuse in place tightly. Consequently after hitting a bump I was occasionally losing my headlight. I solved this issue temporarily by using a little dielectric grease on the contacts and then securing the fuse in place with a small plastic cable tie. A new fuse box may be necessary.

I noticed that the terminals for the headlight fuse were heating up more than I would care for. Is this typical when running the headlight? I'm wondering if the headlight bulb I am using may draw too much current for this poor old italian wiring?

joepagejr
Mariana
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Location: Tacoma, WA, USA
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Re: 1974 GT Alternator Mod

Post by joepagejr » Wed Nov 28, 2012 7:46 pm

Any new developments with this mod? Were you using the stock stator? Is it still functioning well for you? Thanks.

Joe
1975 Ducati 860 GT - 850589

Bern
Mach 3
Posts: 98
Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

Re: 1974 GT Alternator Mod

Post by Bern » Thu Nov 29, 2012 5:13 am

Working great still with 2,500 miles on the bike this summer.

I used the stock stator however I modified it changing it from a centre tapped stator to two parallel sets of coils, as described in the first post.
Be very careful when doing the mods, if you scratch the coatings or break the leads you will need to get the stator rewound. The reason for the change in the stator was to use a more modern bridge rectifier.

Before
Image

After
Image

Bernie
Last edited by Bern on Sun Aug 23, 2015 7:18 am, edited 1 time in total.

machten
MHR / S2
Posts: 481
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Location: Perth, Western Australia

Re: 1974 GT Alternator Mod

Post by machten » Fri Nov 30, 2012 4:50 am

Bern,

What was "the new rotor" that you installed?

Kev

Bern
Mach 3
Posts: 98
Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

Re: 1974 GT Alternator Mod

Post by Bern » Fri Nov 30, 2012 6:30 am

I was able to find this rotor it fits perfectly. The old rotor had a recess for a washer this one doesn't. It makes no difference.

Image
Last edited by Bern on Sun Aug 23, 2015 7:19 am, edited 1 time in total.

enforcer
Cucciolo - the Lil Pup
Posts: 24
Joined: Sun Feb 12, 2012 8:26 am
Location: N.S.W

Re: 1974 GT Alternator Mod

Post by enforcer » Tue May 14, 2013 11:09 pm

sounds great but where has your pictures gone ?

Bern
Mach 3
Posts: 98
Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

Re: 1974 GT Alternator Mod

Post by Bern » Sun Aug 23, 2015 7:32 am

Sorry, my flckr account disappeared with my old email. Silly Yahoo. Here is the new rotor I used.

Image

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