I'm thinking of making a 3 phase converter to using an auto
transformer to step up the voltage.
The Jim Cox book on Electric Motors in the Home Workshop describes
rejigging an old welding transformer - and then tells you that you can
no longer take this approach 'cos transformers are welded now together
and not bolted!
Does anyone know of a source of smallish transformers to base a
converter around - (or alternatively a source of elderly welding
When I did this a few years back I got a large 240 to 240 volt isolation
transformer that had 120v taps on primary and secondary, wired the primary
and secondary in series and fed the 240 in at the two tap points (an idea
cribbed from seeing the circuit diagram of a Transwave converter). It
worked fine and I hope it still is (Tim Leech please confirm )
That would work - but having got a source of 480V single phase how
does one convert it to 3 phase?
I have seen 3 phase "converters" that are no more than applying single
phase to one of a motor's windings and then starting the motor by
applying the same voltage through a capacitor (to phase shift it a
bit) to another of the motor windings. Is that it?
Steve Randall G8KHW firstname.lastname@example.org
UKRA #1072 Level 2
Or you could use the 440ish volts to power one leg of a 415v 3 phase
in/out inverter. That should produce decent variable frequency 3 phase
at about 2/3 of the nominal inverter rating.
Inverters like that are cheap.
With care and an angle grinder it IS possible to disassemble
modern transformers by grinding away the weld metal. With laminated
E+I or T+U cores the weld metal is located in a low flux density
region of the core so the metal removed has little effect on the
I think I have some 440v to 240 v transformers out of a scrapped CNC.
I know I have one three phase one, three massive separate transformers but
can't remember what the output was, could be about 160 to 180 volt knowing
what they were for.
Just got back from Harrogate, let me check in the morning
That particular one came from Anchor Surplus, but it was HUGE - cannot
exactly remember the rating but the convertor was running my Moog Hydropoint
NC mill - just the hydraulic power pack was 10 HP iirc
You may have to be a little careful here, with a normal three phase input,
you use and have a 3 phase rectifier stack. When you try doing it with just
two legs of 415 Volts it may be worthwhile beefing up the bridge rectifier
and also upping the Smoothing caps a bit. Or down rate the converter by a
You would have less current per diode in true 3 phase compared to single
phase for the same load, and also you will get more ripple on the smoothing
caps due to only getting a charge every 180 degrees instead of 120 degrees.
More input volts may help to a degree.
Also be wary of the more sophisticated Inverters, they may well detect if a
phase is missing, or if the input volts are out of the 415 V +/- a bit shut
Just a few thoughts there. I have certainly seen it done with mains and a
110V site transformer. The 110 volts is isolated from mains, so as such was
connected to the live mains terminal and gave across the neutral and other
end of the transformer 350 to 360 volts output. It worked and was enough
for an old Jaguar inverter to run, but not a set-up I would like to try at
If you build your own converter with a transformer, motor and Caps, please
make sure you use volts rated caps at least 480 Volts ac if you are after
415 output levels. Unbalances can lead to some pretty high volts on the
When you have balanced the voltages under working load then it becomes quite
good fun sticking caps across the mains input to improve the power factor!
What ever you choice, take great care.
building site stransformers...
you want one in a metal case. you can get at it and pull some windings
out to lower the voltage if you need to. well i got a pair of largish
yellow things and they were both potted and i didn't want 480 volts.
but the ones i had had remarkably low rating so weren't any good to me
you use the 415 volts as two of your phases, run a motor from these
and the motor will generate from the third set of windings. The
capacitor to start it is just to give some direction to it[like you
said]. the winding will be generating less than 400 volt's on the
generated phase but if you were to excite it a bit you could get some
more, hence the capacitors from the other two phases to the generated
one. withthe caps you can balance it so you have equal voltages under
Just to clarify a few points:
a/ The transformer was originally a 240 - 240 volt isolation transformer
with 120 volt tappings on both primary and secondary. I wired the primary
and secondary in series and fed the 240 volts in at the 120 volt tapping
b/ I reduced the voltage from 480 to about 415 by winding a few turns
'antiphase' onto the transformer legs as it was easier than removing turns
that were varninshed in.
c/ The three phases are generated thus: Phase one is the 'low leg' of the
transformer, phase two is the 'high leg' of the transformer, and phase three
is connected to phase two via the start / run capacitor network. (This is
why mains input has to be at the tapping points, so mains neutral is nearer
to the 'star point' voltage of the motor)
d/ With no motor connected the capacitor bank gives approx 90 degrees phase
shift producing a second phase that will get the motor to start turning when
connected, and as the motor spins up its internal generator effect with its
coils spaced at 120 degrees drag the phase shift to nearer to the correct
120 degree relationship. Most commercial units monitor the voltage on the
third phase with a voltage sensitive relay which drops the start capacitor
out when at a reasonable level.
e/ Anyone reading this who is not experienced using high voltages and
doesn't understand to forgoing and is not confident in their own abilites to
stay safe SHOULD NOT experiment as i did.
You may well find that the invertor detects the presence of all three
phases and shuts down if it detects either the loss of one or more
phases or an imbalance in the phase input voltages. The 415V 3-phase
input invertor that I have does this.