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Les P Offline OP
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Hello electrics wizards.
I need to order some electrical parts and this seems like a good place and time to ask.

I picked up one of the mini permanent magnet alternators as used on Kubota engines last year and proceeded to modify it for the Sunbeam S7, these units are 12 volt but have doubts about it at lower rpm's since it will be turning at crankshaft speed.

[Linked Image]

Any suggestions to convert its output to 6 volts which should boost the low speed charging ?
I guess a 6 volt regulator is needed but which one of good quality unless there are other suggestions ?
Any help appreciated.

[Linked Image]

[Linked Image]

[Linked Image]

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Hi Les,

Originally Posted by Les P
mini permanent magnet alternators as used on Kubota engines
units are 12 volt but have doubts about it at lower rpm's since it will be turning at crankshaft speed.

Uh-uh.

Firstly, as you say, it's an alternator, so what comes out of the alternator itself is AC (Alternating Current).

Then, even on a Kubota engine, for that alternator output to charge a battery, say, the AC must be both rectified to DC and regulated to a particular DC Voltage.

The AC Volts out of the (any) alternator vary wildly with rpm; however, the regulator will regulate the DC Volts to the value chosen - usually around 7V for a nominally-6V regulator or around 14V for a nominally-12V regulator.

The (regulated) DC Volts won't vary significantly with engine rpm, between the nominal (6V or 12V) at tickover and the top of the chosen regulator's range at around 3,000 to 3,500 rpm - certainly not between 6V at low rpm and 12V at high rpm.

The DC that will vary with rpm is the Amps; however, it isn't linear; while DC Amps are obviously low at, say, tickover, they will rise relatively rapidly with only small increases in rpm at low rpm, tailing off fairly rapidly so that, at high rpm, even with large rpm increases, the DC Amps increase only a little. As an example, Lucas rated its alternators for Amps @ 5,000 rpm but, @ 2,500 rpm, you can expect about 75% of rated from a single-phase and about 85% of rated from a 3-phase; the Kubota alternator is likely to be similar.

Originally Posted by Les P
suggestions to convert its output to 6 volts which should boost the low speed charging ?

I'm only familiar with Lucas alternators but, while there will be more DC Amps if the output is regulated to 6V than if it's regulated to 12V, the difference isn't significant, and not worth the other aggravation of 6V electrics, ime and mho. Although "Semper Gumby" will likely disagree. grin What would make the 6V/12V decision easier is actual DC Amps figures from the Kubota alternator, rather than 'wet finger in the breeze' guesses. smile

Hth.

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Sounds like you have a solution ( the kubota alternator.)
In search of a problem ( 6 volts)
... what makes you think this alternator has limited low speed output ?

( without any specs at hand ) these utility motors typically
are run at lower RPMs.

There is no benefit in running a lower voltage , other than staying historically correct .
... Funky , irritating wiring problems go up , as line voltage go down .

You can/could manipulate the stators output , somewhat , to a give RPM , with the wire gauge and number of turns , but I'd hook up what you have ,
regulate to 12 volts and test it against your power needs
... before trying to clamp it's output down to 6 volts .

.

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If you double the voltage, you halve the amps.
If you double the amps, you halve the voltage.

The power (watts) the alternator puts out is actually entirely controlled by the rpms,
what you choose to rectify it and regulate it to is entirely up to you.
(If you wanted it for a 24v aircraft, the amps would halve again...).

British bikes were mostly pretty crude with the voltage control - they simply used zener diode(s), and dumped any excess power into heat. With almost nothing to prevent the battery being boiled, other than choosing a large enough battery this didn't occur too much.

Modern regulators are a little more sophisticated = your choice....


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Originally Posted by NickL

Don't go down the 6 volt route, why do you think the manufacturers changed?


Because starter motors and headlamps weren't too strong back then ?

Modern halogen lamps are available these days in 6V, and likely LED lights too.
And last time we looked, 'beams didn't have leccy starters...

Originally Posted by NickL

Don't go down the 6 volt route, why do you think the manufacturers changed?


And they went +ve earth because selenium rectifiers (remember those) were +ve earth....

6 volts was original, no reason why it can't remain 6v....

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With a dynamo (DC generator), as originally fitted to the Sunbeam, a 12V conversion, by simply using a 12V regulator on the 6V dynamo, causes you to need more rpm for charging to start. It is a well known, easily observed phenomenon.

I've never heard anyone mention a significant similar effect with permanent magnet alternators on old bikes.

So, regulate it to whatever you like, but 12V generally works better.


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Originally Posted by Rohan

And they went +ve earth because selenium rectifiers (remember those) were +ve earth....

...


Why did post-War cars, with no rectifiers, go to positive earth?


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i think running it as either 12v neg or 12v pos along side a SRM or wassell power module that have the capability of running either way and maintaining a steady 14 - 14.5 v to the battery
if given the choice i would defiantly go with 12v if the bikes to be used, better starting and brighter lights

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Quote
6 volts was original, no reason why it can't remain 6v....

Wrong .
Using a (common) hydraulic model ,
Voltage is like pressure .

6 volts of pressure is less able to overcome
(Simple wiring resistances)
than 12 volts of pressure .

there is more likelihood for error , in any circut , as voltage drops .
Starting with a higher voltage baseline
is a simple way of increasing reliability .


.

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Originally Posted by Rohan
If you double the voltage, you halve the amps.
If you double the amps, you halve the voltage.

The power (watts) the alternator puts out is actually entirely controlled by the rpms,
what you choose to rectify it and regulate it to is entirely up to you.
(If you wanted it for a 24v aircraft, the amps would halve again...).

British bikes were mostly pretty crude with the voltage control - they simply used zener diode(s), and dumped any excess power into heat. With almost nothing to prevent the battery being boiled, other than choosing a large enough battery this didn't occur too much.

Modern regulators are a little more sophisticated = your choice....

Sorry, but this is utter, total and complete rubbish.

I have copies of original Lucas test graphs - which anyone can get - from the Lucas Heritage Archive at the British Heritage Motor Centre. Where an alternator - e.g. the RM19 fitted to most British bikes from the early to the late 1960's - was tested regulated to both 6V and 12V, the graphs show the alternator producing a few less 12V DC Amps than 6V DC Amps. Nothing special about Lucas alternators that makes them unique in this respect.

If you think "modern regulators" don't dump "excess power into heat", I suggest you read past posts by "NickL"; he has explained it very simply several times.

"With almost nothing to prevent the battery being boiled" displays a simple inablility to read certainly any 1960's pre-12V wiring diagram.

I have recently been looking at a modern Kawasaki regulator that uses a Zener diode.

Les the o.p. asked reasonable questions. There are enough people posting in this Forum who know about the subject to give him reasonable answers. Your posts just confuse.

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This is the original story about this conversion

http://impbarn.blogspot.co.uk/2011/12/sunbeam-s7-s8-permanent-magnet.html


His first problem after fitting it was taming the output which fried his first regulator, I would be going for 12V based on this.



Quote
I’m quite pleased that the mechanics of my alternator conversion work well. The next challenge was to rectify and regulate the alternating current into direct current. Normal people would just have converted to 12 volts and put the Kubota tractor regulator in place of the Lucas MCR2 dynamo regulator. I had purchased a Rooster Booster 6 volt positive ground ignition module, and already had my 6 volt battery and lamps, so I was determined to stay with 6 volts for the present time. I know it’s not ideal, but I knew I could make it work fine. The 6 volt scooter rectifier/regulator that I bought was quickly overwhelmed by the 14 amp capacity (and probably the open circuit voltage capability) of the stator coil, and burned out. I ended up with a commercial shunt regulator from Electrex World LTD.

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Use the 12V tractor regulator, that is made for the job!

How much slower than a tractor engine does your Sunbeam turn?

If you want to experiment, that's fine. If you want a bike that works, keep things simple.


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Originally Posted by Stuart

Sorry, but this is utter, total and complete rubbish.


The fly in the ointment here, which you have chosen to ignore,
is the grade of wiring used in most 6v alternators.
Trying to extract double the watts out of thin wiring is a good excuse for a meltdown.
Even 6v generators, driven at 12v, are not immune from this.

I have several 3 phase alternators, and they regulate the output by limiting the input.
No heat loss from dumping power, thats for sure.
Sophistication beats crude, every day....

Under your scheme, can a 6v alternator be driven to output 240v ?? If not, why not...

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Originally Posted by Rohan
Originally Posted by Stuart

Sorry, but this is utter, total and complete rubbish.


The fly in the ointment here, which you have chosen to ignore,
is the grade of wiring used in most 6v alternators.
Trying to extract double the watts out of thin wiring is a good excuse for a meltdown.
Even 6v generators, driven at 12v, are not immune from this.

I have several 3 phase alternators, and they regulate the output by limiting the input.
No heat loss from dumping power, thats for sure.
Sophistication beats crude, every day....

Under your scheme, can a 6v alternator be driven to output 240v ?? If not, why not...


It's not a 6V alternator.


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Originally Posted by triton thrasher

It's not a 6V alternator.


So can a 12v alternator be driven to produce 240v then ?

Someone, somewhere, is going to have to quote flux densities,
and saturated flux densities....

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Yes if you spin it fast enough and no load to drag the voltage down, also its 240v AC not DC. Forget all you know about dynamo's, alternators are different and their advantages are why the dynamo is no longer used. Its an alternator so its voltage is set by the regulator, when Lucas changed from 6V to 12V the alternator they supplied was initially the same and only changed as the loads increased later.

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When Brit cars changed back to negative earth in the 1960s, it was purported to be an anti-corrosion measure.

The cars, of course, rusted very fast with either polarity.


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Originally Posted by Stuart
Sorry, but this is utter, total and complete rubbish.

Originally Posted by Rohan
the grade of wiring used in most 6v alternators.

As posted already there is no such thing as "6v alternators".

Then we aren't talking about "most 6v alternators"; we're talking specifically about the Kubota alternator that Les posted pictures of in his first post, that we can see "the grade of wiring".

Originally Posted by Rohan
Trying to extract double

You cannot "extract" anything from a permanent-magnet alternator; it supplies what it supplies based on the strength of the magnets, the distance of those from the coils, the number of those coils, the diameter of the coil wire and how much is exposed to the magnetism.

Originally Posted by Rohan
I have several 3 phase alternators, and they regulate the output by limiting the input.

But they aren't permanent-magnet alternators.

Les the o.p. was absolutely clear about what he wanted to know. Why do you continue to post, at best, irrelevance; at worst, simple nonsense?

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If you read your Lucas auto electrical history, the change to +ve earth was in anticipation of selenium diodes in alternators only being made in +ve earth varieties.

Which doesn't explain what the americans did ?

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Originally Posted by Rohan
If you read your Lucas auto electrical history, the change to +ve earth was in anticipation of selenium diodes in alternators only being made in +ve earth varieties.

Which doesn't explain what the americans did ?


What did the Americans do?


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Originally Posted by Stuart
Why do you continue to post, at best, irrelevance; at worst, simple nonsense?


Because there is more to this than the simpletons version ?

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Originally Posted by kommando
Yes if you spin it fast enough


Can we enquire what rpms this is going to require ?

And getting 500 watts at 12v is not likely to produce the same amps at 240 v ??

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P.S. Les suggested he wanted to stick to 6v,
and I suggested similar.

How is this not keeping to the topic ?

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i think its great what you have done so far les

go 12v give the purists something to moan about when there at the side of the road with a flat battery

3 phase power module £20 or solid state rectifier £3.50 and 12v + or - zener diode to regulate jobs a good one

the simpler solution is usually the best and most reliable,sometimes better not to know how it works ...just that it works

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Hi Les,

Originally Posted by kommando
This is the original story about this conversion
http://impbarn.blogspot.co.uk/2011/12/sunbeam-s7-s8-permanent-magnet.html

Originally Posted by triton thrasher
Use the 12V tractor regulator, that is made for the job

Originally Posted by NickL
Be careful using a zener on that alternator, you may need 2 in parallel because of the power.

In the ImpBarn post, you mention "the 14 amp capacity"; where did this figure come from?

Just based on that figure, Nick is correct(ish):-

. If you were considering regulating the DC output of the rectifier, two Zeners in parallel certainly would be A Good Idea.

. In theory, and depending what the Amps rise to at, say, the max. engine rpm, because you posted you "want to run the lights all the time", if the alternator output remaining after running ignition and lights will never exceed ~12A, you could get away with a single Zener.

. However, if say, the headlamp bulb went phut and you didn't notice, if the alternator output remaining then rose significantly above ~12A, a single Zener might also go phut. frown

. However2, two Zeners in parallel can't be any old two Zeners, they must be 'matched' for trigger Volts and capacity; these used to be available but they aren't any more. frown

. One alternative is to connect a Zener to each alternator cable - i.e. regulate the AC before it gets to the rectifier - these Zeners don't have to be 'matched'; Norton did this when they fitted the high-output single-phase Lucas RM23 alternator, but the bikes were 12V.

... Or you could use a combined electronic reg./rect. smile Just ensure its Amp capacity exceeds that of the alternator when spun to max. engine rpm.; the "tractor regulator" will only be suitable if its capacity exceeds that of the alternator when spun to your chosen maximum engine rpm.

Hth.

Regards,

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