Three Phase Converter Question

I've got a working 7.5 HP idler running a true 5 HP (24 amp draw) 120
gallon air compressor plus my Bridgeport and a couple of other smaller
motors. This motor runs on a 40 amp breaker out of a 200 amp main. I
need to increase my service to accommodate a three phase welder. Is it
possible to add another motor in the circuit rather than build a new
larger system from scratch. My idea is to break the circuit between the
existing 7.5 HP idler and the air compressor/ mill and drop each of the
three legs down to a disconnect. Then from the disconnect wire the
three legs into a larger motor which would feed the welder when needed
but would be dropped from the circuit when not using the welder. The
welder requires about 35 amps line input and outputs 250 amps at 100%
duty cycle.
Anyone with experience with this? I'm not sure this is feasible. I'm
unsure of how big of a 'secondary idler' I'll need and unsure of how
big of a motor the 7.5 HP 'primary idler' will start. What will this do
to my feed from the main to the 7.5 HP motor? Will I need to increase
the wire size and breaker size from the 7.5 HP motor to the 200 Amp
main service, or should I wire the larger motor as the primary Idler
straight from the 200 Amp Main and use the 7.5 to supply the additional
power for the welder?
All comments/suggestions welcome. Best regards
"Suppose you were an idiot. And suppose you were a member of Congress..
But I repeat myself."

--Mark Twain
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In a single phase driven three phase system ALL motors contribute to improve the regulation of the phantom phase so its perfectly OK to add additional idler power and not neccesary to start from scratch.
You can estimate the needed extra idler power requirement (if any!) by checking welder performance with;-
idler alone
idler + Bridgeport
idler + compressor
idler + compressor + Bridgeport
Each additional motor brought into circuit improves the system in relation to its nameplate HP rating. Maximum improvement occurrs with a motor at idle but almost all the improvement is still present if the added motor is operating at full load.
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What I had in Colorado was a two stage setup. For the mill and smaller stuff I had a 3HP rotary converter. For larger loads, I started a 7.5 HP idler from the running 3HP converter. Both converters ran when I had a large load, like the motor/generator arc welder. This worked great. The 3HP converter had capacitor start, etc. The 7.5HP idler had a mag starter and caps, but no additional start circuit, it started just fine off of the 3HP motor (no load).
The main trick here is you have to think carefully where the currents flow (in which wires) and how heavy those wires have to be. You need 10.5HP wiring from the breaker to this setup. Instead of running 10.5 HP heavy wire to the plugs in the shop, I ran 3HP wiring to the small plugs, and 10.5HP wiring to the big (50A welder) plugs. I suspect it would have given an inspector fits, but I'm pretty sure it was safe.
Lefty wrote:
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Steve Smith

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