[Homestead] Lawnmower generator/welders
tvoivozd at infionline.net
Fri Sep 10 23:53:03 EDT 2004
This is supplementary to previous postings I made on engine-generator
welders (not sure they made it into the Homestead Archives)
http://www.huv.com/jon/jeep/Welder/portable-welder.html (Jon's Jeep
website, schematic, materials list to make a rotary lawnmower into a
welder, also refer to the old Mother Earth archives which had an article
doing the same thing.)
http://theepicenter.com/tow02077.html (epicenter horizontal shaft
engine generator, they also sell a metal plate upon which to mount both
engine and generator, easier than trying to fabricate your own.
See the easier-to-come by vertical shaft engine option, and the bracket
to direct-couple (no belts) engine and generator---it's not a
quill-mount where a tapered shaft fits into a tapered hole, uses
something like a Lord coupling to couple two, hopefully same-diameter
Back in the mid 1950's I used 7/8 inch tempered masonite as a base for
one of my engine-generator models---more rigid than plywood and a lot
more rigid than cast-aluminum. Also used a Lord coupling which didn't
pound out under two-stroke power impulses as some other brands did.
http://www.qsl.net/ns8o/Induction_Generator.html (some info on
induction motors as generators---for resistance loads, don't use them to
start motor loads)
The above are intended for stick-welders, one battery is really
insufficient, as is 12 volts---better to put two batteries in series to
the welder cables, and charge the batteries via 12-volt parallel hookup.
And you should buy the Homepower archive CD's---Hp #77 describes a
pulsing desulfator circuit that is the only way to restore a battery so
dead and sulfated it has reversed polarity.
And you would get better performance from a wire-feed than from welding
rods. Two 12-volt batteries in series will suffice for most wire-feed work
Using a wire-feed welder will produce better welds. You can easily
modify a "wal-mart" wire-feed welder to run on 24v DC.
Use a heavy-current parallel mosfet bank in series with the high-power
inductor on the positive side (electrode).
The mosfets can be wired with all common leads in parallel. The
current-handling capability of the bank is the sum of all the mosfet's
current ratings. It can be built from almost any kind of mosfet as long
as you have enough total current handling capability and you have plenty
of heat-sinking capability.
Place a HEAVY CURRENT rectifier in series between the mosfet bank and
the inductor. Place another HEAVY CURRENT rectifier in REVERSE BIAS mode
between the GROUND and mosfet side of the inductor.
Drive the mosfet bank's common gate lead with a pulse-width modulator
built from two 555 timer IC's.
One 555 is in ASTABLE mode, set for a 50% duty-cycle (important), set to
the harmonic frequency of the heavy inductor. It's output trigger's the
second 555 timer chip. This frequency is adjusted ONCE and then FIXED in
The second 555 is in MONOSTABLE mode and it's output pulse width should
be adjustable from 1% to 100% of the FIRST 555's output pulse-width.
This output goes to the mosfet bank gate lead. This 555's pulse width is
your WELD-POWER adjustment. Narrowing the pulse-width reduces the amount
of time the mosfet bank in on.
Here's what happens, in termes of "positive going" current flow. Don't
This is ONE complete cycle.
1. The mosfet bank turns on, allowing current to flow through the
rectifer, through the inductor and then through the electrode and
workpiece and back to ground.
2. The mosfet bank shuts off. The inductor's field collapses,
releasing it's EMF as a second current pulse that goes through the
electrode and workpiece. This field collapse pulse is routed through the
REVERSE BIAS rectifier.
This is the MOST EFFICIENT way to drive your wire-feed welder from a
pure DC source.
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