240AC (Generator) Alternator rewind to 12V ????

[(unknown)]

Hi All,

I have a shed load of 240V petrol gen-Sets which have Brushless, self exciting 240 AC alternators. Does anyone know the details for rewinding this type of alternator for 12V DC (or more acuratley 12V AC which is then rectified)?

These already have a 12V dc outlet, but it is only 8Amp max and I expect that it is from a coil 'Tapped' into the 240 coils? . The Alternators are rated for 650W continuous, 720W max and 950w peak (running from the engine at either 1800 or 3600 rpm presumably). So any Ideas what kind of output they would have if rewound to 12V.

If used in a wind turbine, Would the field coils in the stator need rewinding to account for the reduced rotor speed? (to provide enough voltage to excite the rotor coils)

Any help or advise would be very, very helpful.

Thanks

AL

[Ungrounded Lightning Rod]

These already have a 12V dc outlet, but it is only 8Amp max and I expect that it is from a coil 'Tapped' into the 240 coils? .

That's typical, yes.

The Alternators are rated for 650W continuous, 720W max and 950w peak (running from the engine at either 1800 or 3600 rpm presumably). So any Ideas what kind of output they would have if rewound to 12V

Exactly the same wattage.  Current will be multiplied by 20, voltage divided ditto.  (Somebody else can tell you what RMS voltage you'll actually want for your application - it won't be exactly 12, so the factor won't be exactly 20.)

To rewind them, unwind the output coils, counting the turns.  Rewind them with 1/20th (or whatever) the number of turns using wire with 20 times the cross-section.

Of course that would mean you're winding with bar stock and you'd have extra eddy-current losses from the increased wire thickness.  So you'll want to do with something considerably less than 20 times as thick going several-in-hand so the product of the thickness increase and the number of paralleled strands is 20.

In principle you could wind 20-in-hand with the original wire, making 20 paralleled coils each with 1/20th the number of turns of the original.  In practice the original wire will probably be too dinged up to use, and 20-in-hand is a bitch.  So you'll want to use fewer in hand of something somewhat thicker.  Probably the thickest wire you can easily get to fit the slots and bend into shape when winding.

Wire cross-section doubles for a count-of-three decrease in wire number, i.e. #16 is four times the cross-section of #22.

[Ungrounded Lightning Rod]

Now that I think about it:

Typically the 12v battery-charging output is full-wave rectified from a tap on the main coil near the neutral end - or a separate coil.  (The one I've got does it from a tap - leaving the battery floating at about 6VAC above ground.  B-(  )

When you unwind the old output coils, you can count the turns on that part of the winding and use it for the number of turns you want for your thicker (and thus higher current) coil.

You'll need to consult the wiring diagram to see exactly how that's rectified.  If it's half-wave you may need to adjust your winding for the extra diode drop of a bridge rectifier.  (And it might be two-diode full-wave with a center tap, in which case you'll want to adjust it AND use the turns count from one end to the center tap.)

= = = =

One last point:  DON'T unwind the excitation winding (the one connected to the capacitor).  You'll want to leave that just as it is.

Also:  All the above assumes the gennies are the simple, brushless, capacitor-excited, saturation-regulated types.  If they have an active regulator it may take feedback from the output coil.  If that's the case you'll have to arrange a replacement for that feedback (such as a 240v:12v transformer run backward, or an extra fine-wire winding of the original turns count.)  Details will vary with the generator.

[scottsAI]

Hello Dark AL,

Not to discourage what your trying to do, much easier to step down the voltage with a transformer?

Would add small losses, considering the power source nothing noticeable.

Had no trouble finding a 250w transformer for $25 at a surplus store. Use couple.

Would take hours to rewind, and need to buy the wire at near the cost.

This would leave you the option to use the 240vac later if needed.

Have fun,

Scott

[Flux]

The advice given is correct for winding to 12v. It will probably require several wires in hand or converting to several parallel circuits with separate rectifiers.

The output after rectification will be way below the original rating, these things make poor battery chargers as you will find out if you try the transformer option someone suggested ( mains battery charger).

Forget it for wind, you will get nowhere.

Flux

[Dark AL]

Thanks everyone for the useful info. I think I will try to rewind just one of them to see what the end result is. I'm not looking forward to it, but I think its an excercise that needs to be done just for the experience is nothing else. (And maybe a warning to other not to try the same ! LOL).

One thing that has crossed my mind regarding the existing 12V tap on the 240 coil.

Would it be possible rather than to rewind the 240 coil, to just short the coil at the same interval/spacing as the tap?

I imagine that the 240 coil is offset in its spacing in some way as it crosses itself on the next circuit of the stator (circuit as in around the stator, when it passes over itself as it is wound into the stator channels, not circuit in the electrical sense)

I am thinking in terms of cutting or delaminating the 240 coil at the loops and resoldering the ends to create a new coil spacing.

Does this sound more feasible than a complete rewind?

Obviously I would need to isolate to the excitor coil for the rotor and leave it alone (it is a simple Coil+capacitor type).

Could this be a simple solution to the problem of rewinding a 240v alternator, or would there be unforseen effects and losses that I haven't foreseen?

Or just plain 'beyond calculation'?

Cheers

AL

[Flux]

Don't under estimate the problems facing you. The 12v winding will almost certainly have only occupied a small part of the main winding space and it is unlikely that it will be the best winding arrangement for the complete replacement of the power winding.

I would think that reconnecting the 240v winding would be a major nightmare even if you could find the relevant places to cut the windings. You need to copy the power winding with about 1/10 the the turns with 10 times the wire csa.

If you are very lucky the exciting winding may be wound on first, if it was wound after the power winding then there is no way you are going to remove the main winding and leave the exciting coils. Also the prospect of removing coils from a machine without the usual heating and burning them out is pretty daunting and I wouldn't be too happy about your chances of not damaging the exciter winding.

I would regard this as a total rewind rather than a modification,  In view of the other uncertainties of how it will behave with a single phase rectifier on it then I think you had better consider it a labour of love and an experiment rather than a serious project.

Flux

 

[Dark AL]

I have decided to give it a go and rewind one of the alternators to 12V. Just to see what the output is.

I also have a quick follow up question relating to using a 240V AC generator alternator, not sure if this should be a new thread or not. Perhaps it has already been answered somewhere.

Rather than rewinding a 240 AC alternator. could it be utilised by stepping down the speed rather than the voltage. Eg drive it at 1/20th the speed (180rpm for a 3600rpm alternator).

In theory this would put out the required 12V which could be rectified. (as for Amps???)

My only problem with this is that I can see the excitor coils only getting 1/20th the power, so this would further reduce the output voltage. If this is the case then the drive speed would need to be increased until 12V is achieved ? so it wouldn't be a straight linear equation.

Does anyone know of a way to calculate what the out put voltage and Amps would be for a self exciting alternator run at different speeds?

I can see this being a trial and error type of thing rather than hard figures.

I am determined to get some usable power out of these alternators, and preferably without rewinding them all.

Any Ideas? or am I peeing in the wind? (like many before)

[thyristor]

No, you cannot step down the speed as the turns per volt were designed for a frequency of either 50hz or 60hz. The slower the speed, the more turns per volt would be required, which also means a rewind! If you leave the turns as they are and reduce the speed, you'll get your lower voltage, but at a lower frequency (hell of a ripple). Also, if you then load the output, you are going to burn your machine.

[Flux]

No you can't significantly drop the speed, for minor drops in speed you will need to increase the excitation capacitors. You can't take this very far before it will fail to excite. The things are designed as cheap and cheerful power sources to be coupled to high speed engines. They are rather inflexible in their operation.

You can't mess much with the excitation winding, capacitance or operating speed. All you can hope to do is rewind the power winding but even then you will probably find lots of limitations. It's your choice, I am trying to save you wasting your time, it will never work for wind but if you want to engine drive it at rated speed then it will work in some manner.

Flux