ST conversion to Alternator

[veggie]

Hi All,

I have seen others make modifications to an ST head in order to get a wild AC current which can then be rectified to DC. I have also seen things like big teroidal coils screwed to the wall to assist in the conversion. I don't know what they do and I don't have access to those kind of coils.

So my question is...
How difficult is it to convert an ST-3 head into a variable speed DC battery charger?
A dc generator that does not care bout synchronous speeds.

Step one: How to convert the ST to a wild 3 phase AC output
Step two: How to tame the wild AC current. What rectifier to use?
Step three: Can this power then be fed straight into a DC solar charge controller ?

I have an ST-3 in storage and it would be nice to convert it to a heavy duty DC alternator system.
Of course the usual ST mods will have to be done for reliability. Cleaning, new bearings etc...

Basically, whats the process for converting?

[mobile_bob]

i assume the st3 is just a smaller version of the st5 and others?

that being 120/240 output?

if so...

there will be 4 coils on the stator, and for whatever reason they are spaced in pairs 90 degrees apart and then either parallel connected for 120 output, or in series for 240vac operation.

if one is crafty he/she could separate each of these coils, which
at rated output would put out ~60vac each, that at 1800rpm and rated field current (whatever that is).

each of these coils could then be rectified with preferrably a full wave rectifier, and then parallel the resultant DC output for around 3kwatts of DC output, the voltage of which would be whatever the field drive current would be and/or whatever the drive rpm and of course load would be.

i don't think this would be difficult at all, and i have postulated this possibility for at least 15 years now, but have never done it.

i have been working to clear out my shop, a huge task any time of the year, and only made worse with the low temps of late.  one of the projects i want to finish is the tear down and final welding of the frame of the trigenerator i built 13 plus years ago now.

it was only tack welded together to prove out the concept and i never got around to taking it all apart to finish the welding, so i want to finish that.

in my collection of stuff i have 2 sd-5 welding heads which have 240vac output and welding output, i am pretty sure they have the same stator layout as the regular st-5.  i have no real use for them as welding heads, so i might well find the time to take one and do the above mentioned modifications to convert it to a dc charger,  and set it up to charge a 48v battery bank.

i think one of these st or in my case sd heads would make a nice heavy duty battery charger, and with a bit of relay control, maybe a dual purpose unit that could do both dc charging and ac 120/240vac output, just not at the same time.

then i am going to throw this out for consideration

the apc ups systems use some really nice heavy transformers, and these units can be found sometimes pretty cheap to free, with dead batteries.  95% of which can be brought back to life with new batteries, and of which in my experience all can be powered by much larger capacity battery banks. most also are pure sine wave units and are really clean output with low distortion.

so you could couple one or more of these to a system and have battery charging, and AC sine wave output, or...

you can strip them down and use the transformers as step down and rectify for 24vdc nominal battery charging, drive them with 120vac off the st3 and off you go.

of course there is the hit in efficiency for the added level of conversion, so maybe the modification of the st3 to rectify the stator coils for dc output would be preferrable?

babbling on here.

bob g

[Tom Reed]

The ST3 is a single phase gen head. No way to get 3 phase from it. IIRC the ST5C is 3 phase. Or a 3 phase induction motor with a cap across two phases will work too.

[mobile_bob]

we don't need 3 phase from the st3 to rectify each of the 4 coils on the stator, and then parallel them for higher current DC charging.  the ripple will be decidedly higher than a three phase stator, but that might be smoothed a bit with a large bank of capacitors, many DC welders have large banks of electrolytics.

the sd heads are single phase, but were setup to be welders, with a reostat to control the field current and by extension the output of the genhead.

i don't have a schematic for the sd heads, they are lost to time, but might be available online?

the stc genheads are three phase, and as such would make for a better charger, likely more efficient too.

i have a 12kva stc-12, but have no plans to modify it for charger use,

my thinking is this, the project might be quite interesting to work out, especially if it were to be switchable from charger to AC output, might make for something useful

having said that, the overall efficiency of the head as a charger  might be as high as the low 70's. the st7.5 in my testing works out to be about 80% efficient making 120 vac power being direct driven. the rectifier losses for a charger would have to be accounted for and would lower the overall efficiency.

the best efficiency would like be with a 48volt nominal system, as that voltage would be close to what each of the coil output would be, under load.

if you try to pull it down to 24 or 12 volt charging, one would either have to dramatically reduce the engine speed and/or the field current, and the rectifier losses would have a larger portion of the losses, otherwise the stator losses will be higher  than if used in a 48volt system, or so i would think.

fwiw
bob g

[veggie]

Here's an ST3 wiring diagram and a look inside the j-Box.

 

[mobile_bob]

the schematic shows two stator winding, unless the st 3 is a 3600rpm head, which would have only two stator windinds, the schematic does not show that the two windings are actually internally connected four windings, grouped as coils 1 and 2 as one in the schematic along with 3 and 4 as the other group shown as one in the schematic.

so basically what i am saying is this, one would have to take the head apart and find the jumper in the windings, where one coil is wound and then jumps over to the next coil which is 90 degree from the first (something the chinese do, most everyone else spaces them 180 degrees apart.)  that jumper is likely buried in the lacing and should have some sort of insulating sleeve which will make it easier to find.

once found that jumper would be split so that each coil is then independent of the other, leads attached to each end and brought out to the junction box. one would then have 4 separate coils and  8 leads, rectify each coil and then parallel connect the DC output of each rectifier for the maximum power output.

in theory the stator should then be capable of twice the output power of the original design, as a maximum output.  reason being each of the original coils as connected in series internally is limited to 1.5kw and pairing both externally in parallel gives you 3kw output.  however breaking each coil and paralleling all 4 coils would give you 6kw AC if you didn't rectify them (and phased them properly of course), but at only ~60vac output.

so you could in theory parallel connect all 4 coils, bring them out as AC at 60 volts and rectify the sum with one rectifier bridge for DC output.  however the bridge would have to be large enought to handle the total output of 6kwatts of DC power and i would suspect one would want to design and use ~200 piv (peak inverse voltage) diodes to form the rectifier bridge.

you could possibly use something like these to rectify the sum of all 4 coils, these are used, but you get the idea of what to look for.  i have in years past found nos heavy duty diodes such as these at really good prices on ebay, and maybe the chinese also have something that could be used for reasonable prices too.

https://www.ebay.com/itm/304415057563?hash=item46e08d269b:g:dKwAAOSwI-RiP-o4&amdata=enc%3AAQAHAAAA4IalDKOIBhm1Sd0LRV1HPAdUR0U5Q2gCd3hexRxiDq70OWVths073eebowxA5ROPEMismhPYa2WgiXSPnzCAnz37AWohsgoj34bLzzvkcnmTt%2Fry8%2BSj%2FKRwiFU%2BUkcuEAQSxMeji0ilJRkWu3Aly4vvnLHdP0cNUDpjnUNeqSxxkZvIWekwLTz70wXAiMqteaJzA1weCXBqXJEXwN6pZEAIUHoISW4Oiz%2FyE2nW%2BKiA4Z8MmpExAPymWeydDNwjjuVJTIJ%2BZQQ5R5%2FxT8eBTuhZiOCjqTgcevxZrnFNbVC2%7Ctkp%3ABk9SR86ujeWrYQ

let me know if the link does not work.

also i thank you for this topic
as looking for the diodes above, i came across high frequency rated rectifiers, made for the modern inverter welders from chinese suppliers on ebay.  these would likely solve the problem with rectifying modified automotive alternators running in 48vdc nominal operation, where the AC stator frequency can be ~600hz or more, making the higher piv welder diodes which are designed to operate at around 60hz useless as the avalanche and cannot recover.  so having the ability to get fast recovery high current  rectifiers at low cost is something i could only dream about 13 or 14 years ago.  try as i might i could not find anything that would fit the bill back then, but now??? they are readily available on ebay, and likely alibaba, aliexpress, bangood and the corner convenience store in china.

this topic has opened more doors

thanks
bob g

[veggie]

Well .... I'm glad it helped you find the high frequency rated rectifiers.
What I learned is that it's too complicated for me to easily make an alternator from my ST-3.
...As compared to wiring it as a single phase 120VAC unit and attaching an IOTA battery charger to it :-)

By the way, I really look forward to seeing that Tri-Gen unit on youtube in the future !
The 195 Changfa's are such great engines.

By the way, I'm not surprised if that schematic does not match a 4 winding ST-3. Almost every manual that came with any of my Chinese products had bad translation and diagrams copied from other unrelated documents. Terrible.

cheers