Option 1 --STATIC CONVERTER-- is to use a few capacitors and wire up to get a  START and RUN  feeds to run the 3 phase motor.. Takes some type of start - momentary switch to KICK the motor over.

Second --ROTARY CONVETER--- way is to have an OLD 3 phase motor that you pull start or roll over with a 1/4 HP motor to get it going.. then use it as a GENERATOR to put out 3 phase for your motor..

THIRD--VFD--- way is the "NEW METHOD" that involves the VFD... Variable Frequency Drive.. Its a small shoe box with all the MAGIC inside to  start your motor, keep it running, convert 240v single phase to 3 phase 240v , and have the ability to change the FREQUENCY from 60 hz up or down to use as a SPEED CONTROL, without loss of horse power.  It can do WAY MORE than you will need... The one box could put out power to a "MILL" to run both of your 3 phase motors.....you can buy an el-cheap-o 2 HP converter on e-bay for under $100.

i made a static converter for my 3 HP lathe about 15 years ago.. if doing it today, i would buy a VFD and go that way.

once you have 3 phase 240 v power, you can use that as 240v 3 phase or use 2 of the wires to get 240v single phase. ... you can open the elect box on the second motor and look at the wires and determine what was used.

  Hey Steve,,I always thought that you would "lose" 1/3 of the HP that the 3 phase motor you were using,,,? I had located a nice 5HP  3 phase motor for my compressor and was told there would not be enough available HP for the compressor, by using the VFD,,,??

DJ, providing you've got enough electrons coming out of the wall... running a 5HP motor though a VFD will get 100% of the power it needs.

Now IF the VFD is rated for 2HP, you can't run a 5HP motor......

I could be wrong, but i would think a 3 HP VFD would run a 1.5 + 1 HP motors.... 

A VFD is RATED for a given HP, so if it says 3HP ... i should put out 3 HP.

I agree, i would not try to use a VFD if i had a 1940  5 HP motor running 1750 rpm with a   1990 1/2 HP motor running 3600 rpm ..... that dont work.

Just to get my 'new to me' machines started up, I use a homemade rotary converter.  Once I know their state-of-function, I fit the spindles with VFDs, and find an alternate method of powering any other 3-phase-necessary equipment.

In the long run, you WILL want to run your SPINDLE motor on a VFD, and you can feed MANY VFDs with single-phase power.  SOME will require a 'derate' of the VFD's output, because it is only able to handle so-much-power coming in on just two of it's three phases, but as others noted, upsize the VFD by a bit, and you're fine.  SOME VFDs will NOT run on single phase input.  OTHERS will have 'phase loss detection' which can often be disabled in software programming.  YET OTHERS can be 'fooled' into operating on 3 phase by connecting the unused phase, to one of the operating phases by a small capacitor (the power detection circuit sees an out-of-phase sine wave and THINKS it's live, so it allows run).  And there are many that just plain old designed for, work on, and advertised as allowing single-phase in, with no sweat.

"Joe, a STATIC or ROTARY phase converter does not give a 100% true third phase.. and there for they say you only get 60- 80% of the motor HP output."

Not true.  Static converters will fit into the 'not 100%' category simply because the phasing of the 'generated leg' is very heavily reliant on tuning with a stable load, so it will be assymetrical under most points, but yet, still functional, and you'll get right to 100% IF you have the capacitor values dialed-in properly for THAT loadpoint. 

The ROTARY converter WILL produce a 'true' three phase, with those who claim otherwise are basing their argument on a neutral-to-generated-leg measurement, which in case of most any rotary design, is inappropriate, because the 'center' of the sinusoidal rotation is offset from neutral.  Measurements must be done strictly from leg-to-leg.

FWIW, the design that most guys use for building their own rotary converters, comes from Fitch Williams's white paper of several decades ago.  I don't know if Fitch is still kickin' around, but last I checked (half a decade ago), he was... and he was still answering questions now'n then, but his work still lives on, and many 'OEM' outfits use teh same architecture in what they sell.

http://https://www.practicalmachinist.com/forum/threads/fitch-williams-converter-design-balancing-instructions.101882/" rel="nofollow - https://www.practicalmachinist.com/forum/threads/fitch-williams-converter-design-balancing-instructions.101882/

Now, the easy way to build a SIMPLE rotary (and one lots of guys use), is just a three-phase motor, a hand-operated 3-phase switch, and a piece of plywood.  Bolt the motor to the plywood, connect any TWO phases of the motor to one side of the knife switch, connect the other two to your mains.  As others noted, make certain your motor is wired to match your mains (ie. 240v single phase mains, wire the motor for 230v 3ph).  Throw the switch (the motor will growl) Now shut it off, spin the shaft with your foot, or a piece of string, then throw the switch, the motor will spin up.  At this point, there will be three-phase voltage at that motor... one at each of your 240v mains, and the third on the 'unused' wire.  Connect those three to your mill, leave the mill power switch off... repeat the Cro-Magnon starting procedure, and once the idler motor is spinning, throw the machine's power switch, and it's on.

If your mill's spindle motor is 2hp, use a 4hp idler motor.  A junked-up shaft is fine, just so long as the windings still hold electrons and the bearings aren't too wiped out, it'll do.

I ran my first Bridgeport, and my Monarch 10EE off a homemade rotary converter using a 7.5hp 3600rpm pump motor that I got for free.  After I had it all working, I added some capacitors (using Fitch's diagram) to help balance out voltages, and I added a capacitor-start system to kick it over automatically, and I wired in a couple of control relays and a timer to make sure it started safely, without the load accidentally engaged, and if something went wrong, it'd shut down and lock itself out for safety.  Handy for testing machines, and I still use a rotary for this today.

My Monarch 10EE has a 480v Allen-Bradley 1336 VFD and a 480-240v single phase transformer in it's belly, powering a 7.5hp Allis-Chalmers motor driving a 3.5:1 Gates Polychain belt to the spindle.  My Bridgeport J-mill uses it's original 'pancake' motor and spindle V-belts (basically permanently set to the center sheaves) fed by a 240v Allen-Bradley 1305 type AA12A inverter.  My Johannsen radial drill also runs an AA12A, powering a 3hp 1800rpm motor into a 3:1 toothed belt, to it's Bridgeport J-head spindle.  On all these, the upper frequency limit is around 210hz, so my motor speeds are up in the 6k range.  NONE of them have cooling fans on the motor shafts, instead, I've fitted constant-flow fans over the motors, so when they're turning slow, they have excellent cooling airflow, and when they're turning fast, they're not howling air-raid sirens in my workshop.

My Whiteman power trowel runs an Allen-Bradley 1336S, on 480v power.  I set a 480-240v transformer by a utility panel, feed 240v in at the secondary, take 480v out of the primary, through a 600v 3-wire drop-cord, hanging above a work area, and there's a 3hp electric motor driving the Whiteman's gearbox.  I can float concrete with ordinary blades at silly-slow speeds, with full torque, and when it's set up and ready to burnish, the blades can be tilted up, motor raced, to wail it down shiny.

I have a tiny Omron VFD on my HSS tool grinder... and I have an inexpensive analog (Minarik, I think) on the carbide tool grinder.

The table feed motors, you will NOT want to have on the VFD WITH a spindle motor.  Chances are, even if they're connected to 3-phase, that they're DC motors driving substantial gear reductions and have a variable speed circuit integrated into the drive electronics... it is also equally likely that they're single-phase fed off the machine's main power contactor.

Thad, take a look at the wiring to the second motor and determine if its running off 3 phase or a single leg of 240v .... also is the motor size "similar" to the main motor or much smaller HP.

 WHOOOeeeee,,,my head is about to explode and I run out of ink in my pen,,,,That right there is why I just LOVE all these "JUICETRICIANS" on here,,,but ask me bout the inner workins of a SBC or a weed eater,,,,,,

steve(ill) wrote: Long term, a VFD can be programmed to have SPEED CONTROL on the motors... if you ever want to get fancy..... the rotary converter is just a standard 240v 3 phase output..

This is #1 of 2 VERY BIG reasons to opt for a VFD on a SPINDLE...

(because a spindle is either a workholding, or toolholding device, where cut SURFACE SPEED is very important)

When using a VFD on a LATHE, it means that you can adjust the speed to maintain SFPM while going from a large diameter cut, to a smaller diameter cut, or vise versal... as diameter of the cut path has a very remarkable affect on SFPM (Circumference = Pi x D right?).  On a mill, the cutting radius won't change as RAPIDLY as a lathe (because the lathe crossfeed and compound move while working), but if you're running say... a Criterion-type boring head, and you've adjusted a boring bar to cut a 4" diameter, and you're in backgear, with the lowest ratio belt combination, and your boring bar is STILL going too fast for the material and feedrate, you can dial the VFD back to whatever RPM fits the boring bar's SFPM to the material feedrate and cutter type.  In essence, the VFD's ability to give an infinite variability, solves lots of cutting problems, just by turning a knob.

The #2 reason:  DYNAMIC BRAKING.  With exception of the power trowel, my machines all have VERY PROMPT dynamic braking curves programmed in... because STOP is the most important performance function when something bad is either about to go wrong, or is in the process of going wrong.

Running a VFD on 2 motors of two different types, at the same time, though... not wise... especially a table feed motor.  Fortunately, it is rarely an issue, As it usually isn't necessary to do so.

The VFD then has a small BRAIN (chip) inside that you can vary the frequency, startup ( rollup) time, etc ........... on the Phase converter, they leave out the chip and everything is preset at some constant.... That little chip does a lot, but dont cost much to add.. so the VFD and the Phase converter are basically the same cost. 

for 2 HP that is 6 amps ........... 6 amps is about a 16 gauge wire which is not even available for a house.. I would run a standard  12-2 with ground romex ( or 12-3)  for the power inlet from the breaker box.... then individual 12 gauge wires from the VFD to the motor... That would be PLENTY..  12 gauge is 20 amps.

https://www.banggood.com/4KW-220V-20A-Single-Phase-Input-3-Phase-Output-PWM-Frequency-Converter-Drive-Inverter-5HP-VFD-VSD-p-1286175.html?rmmds=detail-left-hotproducts&cur_warehouse=CN&trace_id=3fc41676117079201" rel="nofollow - https://www.banggood.com/4KW-220V-20A-Single-Phase-Input-3-Phase-Output-PWM-Frequency-Converter-Drive-Inverter-5HP-VFD-VSD-p-1286175.html?rmmds=detail-left-hotproducts&cur_warehouse=CN&trace_id=3fc41676117079201

yes, even these guys that I've bought lots of 'stuff' in the past.

 sadly prices are up..even the wireless,no abttery needed dingdong bells

   4;25 in the AM, Thad ,,,,?? Sounds like ole Joe pulling an All Nighter,,,,???   I believe they call it "Anticipation"

 Chit,,you'll be having that machine ready to make chips soon,,,,I best be getting my chit together and get down there quick,,,,,,

    OK,,I get it Thad,,,, The beauty of doing that is like a car or tractor restoration is even tho the doors, fenders, tires and all that extra stuff are still in the corner,,,you can get extreme pleasure in being able to walk up to it,,,put a spark in it and listen to the hum,,,,,

How does the rotary START UP ?  Do you have a momentary button and a start, or pull rope or does it have its own timer/ start circuit ?