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
Reply to
Loading thread data ...
Lefty, i am very confused. is you main service single phase and all your equipment 3 phase running on rotary converter(s) ?
Reply to
Yes, my main is a 200 Amp regular residential service coming off a dedicated 25,000 kva transformer. I have a 7.5 HP three phase motor wired out of the main through a disconnect on a 40 amp breaker. The 7.5 motor is wired with a switch and starting capacitors as a self starter, then feeds the various three phase machines in the shop. I'm pretty sure the 7.5 HP motor will not provide the amperage required for the welder. Hope this makes my questions more lucid Regards
TimPerry wrote:
Reply to
ok, i think i see what you have, however i have no particular expertise or experience with this. i would use a true rotary converter (motor/generator set) for this application. i have been at one mountain top location that had several running simultaneously.
Reply to
25,000 kVA??? Wow, that transformer must be as big as your house! You probably meant 25kVA which is 25,000VA, but I couldn't resist poking fun at your mistake.
If you go to
formatting link
and enter the "Transformers and Phase Converters" forum, there are a lot of very knowledeable people on there with a world of experience with RPCs.
Reply to
Bob Ferapples
You're right. I had way too many zeros in the equation.
Bob Ferapples wrote:
Reply to

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.