ST head modification for 48 volt battery charging

[mobile_bob]

there seems to be more than a passing interest amongst the membership in regard to 48volt battery charging

in an effort to reduce the number of conversion steps and to simplify things a bit, i figured i might pass this along
with a sketch to explain how a typical st head might be altered to produce power for charging a 48 volt bank.

the modification could also be coupled with a couple contactors so that the st head could do double duty, that is
charge batteries as needed and be switched back from the control panel with a flip of the switch to its original design
of 120/240 operation.

one would of course need to add rectification, and some means of regulation, but the result would be nearly the full
kva rating of the head, rather than half its rating based on simply rectifying its output in its standard configuration.

this modification should be fairly easy to do for a diy'er, or if he is not comfortable in doing it, he could take the stator core
to any motor shop for the reconnection and addition of leads, when the unit is torn down for brgs or whatever,, you could even
have them dip the stator at that time as well.

please see attached

hopefully this provides another option for folks contemplating a charging system for 48volts,  the result should be somewhere around 10% or better in efficiency over that of simply plugging in a charger to the st head.

bob g

[mbryner]

hey bob g, 

Great idea and quite logical!   And not too hard to implement.   The trick would be getting a switch heavy duty enough, but I guess a breaker would work.    You just saved me the expense of a transformer!

Marcus

[Apogee]

Can this also be done on the STC 3 phase head?

[mobile_bob]

Apogee:

yes it could also be done to an stc three phase head, the resulting dc after rectification would be much lower ripple as well.

bob g

[BruceM]

On my own AC battery charging setup I opted to use a toroidal transformer (240VAC down to nominal 120VDC battery bank), so that I could carry other loads (washing machine) while battery charging.  (High quality toroids have 93% efficiency.)

But at the higher currents for charging a big 48V battery bank, Bob's direct off the ST approach has real merit, and cost savings, if you can live with mutually exclusive AC or battery charging functions.


If someone needs it, my basic AVR design (PC boards can be ordered from expressPCB) can be used as a regulator for this approach.  The present design can be set to any fixed output voltage.  It's a simple one op amp design, and it would not be hard to add some special features to make a better 48V battery charger out of it. I can help, but if you don't know the basics of op amps, that's more help than I can give at this time. (I can help with circuit changes but am not willing to develop and test for you.)

An inductive primary filter after rectification (well selected surplus transformer secondary) would give you 60VDC, which is almost perfect;  the regulator could bump that up a bit for cyclic peak charging. (You need 15.6V per 12V battery in cold temps, or 62.4V for a nominal 48V bank) 

Texas Instruments has a  "smart chip" for 3 step battery charging that might be incorporated...the AVR could then become a state of the art, 48V smart charger.  Battery bank voltage, net charge current and temperature feedback are needed for this sort of advanced charger.  Battery "full" is detected by a drop in charge current while maintaining the proper temperature compensated cyclic charge voltage.

The other thought that comes to mind is to buy a 48V battery bank charger...these are switching supplies which will operate with pretty high efficiency off of your standard 120/240VAC.  Not cheap, but no development work at all.

[rl71459]

Thank You Bob

[mbryner]

One problem,  my Hobbs meter runs on 120 volts.

[mobile_bob]

place a relay into the system that will direct 120vac from the st head when it is produceing AC
and redirect 120 AC from the inverter system when the ST head is produceing 48vdc nominal for charging.

bob g

[BruceM]

There are some digital lithium battery powered units now that clock engine time based on vibration.  I've been using one for the last few years, as I sometimes run only the air compressor.  It's about $20.

[bschwartz]

OK Bob, my ST-5 is in pieces on the floor.  Where do I find the jumpers connecting the windings?    I understand they are just connections, but how do I identify them?  Do I have to cut the string holding everything together and pull stuff apart to see?  If I post a picture of the stator, could you somehow point out where to start?

[mobile_bob]

look very closely at each leg pair, they will be wound 180 degree's from one another, typically

between each of the pairs there will be a jumper that may very well be just a single wire of one of the
coils jumping over to the other position where the coil is then wound.

there are so many ways of providing for this connection, but i suspect it is just an extension of the same
wire from one coil end crossing over to start the other coil.

probably best to trace down to one coil with one of the legs of one 120 group, and then trace down the
other leg of the same 120 leg, and see if it enters a coil group 180 degree's from the first group, if so
then look for a long wire wrapping around between the two coils, it should be fairly easy to find.

if on the other hand the two coils are displaced by 90 degree's they will then be placed basically side by side
and locating the jumper might be a bit tougher, but it will be there if you look hard enough.

if you have any doubt, take the stator to a motor rewind shop and ask them to find the jumpers for you
and they can also cut and install additional leads for you, with hi temp wire, followed by relace and dipping for you.

it really should not cost much because for a qualified motor guy it really doesn't take much time to do this sort of
modification.

bob g

[bschwartz]

I'm kind of middle of nowhere, so the nearest motor rebuilder is probably over 75 miles away.  As this isn't my primary power source, I'd rather chance an oops and dig in myself.  It looks like I'll need to cut the cloth (?) string holding things together to get at the wires to see what is connected to what, and check continuity.  Sooooo, what can I use to tie things back up again when I'm done?  If it isn't available at a small hardware store, I'll need to find it online and order it.
Thanks for all your help! 

[mobile_bob]

because the stator is insulated at class B, i would imagine just about any type of cord that is appropriate size
would be just fine, basically it is just there to hold everything in tight while the resin they use to dip the stator
in has a chance to dry.

maybe a fishing line, not the monofilament but the old woven cord stuff, whatever it is called?
it is very strong and should work fine.

we need to find a motor/generator rewind guy to join the group, so he could supply some of the needed and otherwise
hard to get materials for the group, or at least tell us where to get it.

i usually end up down at the rewind shop mooching off of them, they generally give me what i want just to get me out
of their hair.



i always offer to pay them, but because what i need usually comes out of their scrap, and is of small quantity they just
give it to me rather than go to the trouble to invoice me.  i usually give em something to put in the donut/coffee fund
anyway,, trying to keep them happy.

bob g

[bschwartz]

After nervously and carefully cutting away the cloth wrapping, I've exposed many wires.
4 go to the 120v terminals U1-U5  U2-U6 I think.  They are connected to the thicker winding wire towards the outside of the stator.  The windings look like they are in 4 sections.  Between two pairs of windings are three winding wires connecting the sections.  Strange.  On the inner windings that are thinner wire,there are 2 more wires that connect to the Z pair I believe and another 2 that look like yet another winding.  They connect to the stupid switch and light in the doghouse.

[bschwartz]

http://s828.photobucket.com/albums/zz204/brettschwartz/?action=view&current=stator.jpg
http://s828.photobucket.com/albums/zz204/brettschwartz/?action=view&current=jumpers.jpg