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Adventures in 3-Phase
No matter how old the mill, the original motor is very likely OK - three phase motors seem to run forever. However, unless you live in an industrial park, you probably won't have three phase available. An obvious mechanical option is to replace the three phase motor with a single phase one. Alternatively there are many electrical options, not all of which involve expensive boxes of mystery electronics. One is to make ersatz three phase with a rotary converter. I tried this, and it seems to be working pretty well. Made In USA-type houses like mine have 230 volt single phase service - that is, two live legs with 230 volts between them, and a common at ground potential. Either of these 230 volt lines and common powers the normal household 115 volt circuits. (It's sometimes useful to think of household service as 115 volt 2-phase, with the two phases at 180 degrees.) From this 230 volt single phase power, we can generate a reasonable sort of 3-phase with the help of an idler motor. Try these sites (all links good as of Dec. 2003) - Building a Phase Converter Phase Converters and Starting Circuits Phase Converters Building an Auto-Start Rotary Three Phase Converter Home Made 3 Phase Self Starting Phase Converter 3 Phase Rotary Convertor Useful information (no title) And these commercial sites - Phase Converter Standards Build your own rotary phase converter! Below is the circuit I wired up. The starting procedure is important. First the capacitor switch S2 is switched on, then the motor starter switch S1 is switched on. S2 is switched off as soon as the idler gets up to speed (much less than a second). Shutdown procedure is simple - just turn off S1. 
S1 - Allen-Bradley 3-pole manual starter switch with thermal overloads.
This is an old item I salvaged. It was attached to the mill when I acquired it, although not enough wires were still hooked up for me to be able to tell what it was doing there. In any case, it's basically a 3PST switch with relay heater elements in series with two of the poles. My circuit uses only two of the three switch poles. I'm using the center pole only as a convenient terminal, not as a switch. T1, T2 - relay heater elements. These are the "thermal overloads". They open the circuit if the motors draw too much current, for whatever reason. The heating business is just a way to provide a time delay so they aren't tripped immediately by the motor's starting current. The ones which happened to be in the switch already are sized for a full-load motor current of 3.01 amps, just right for the mill's 2.9 amp motor. However, they are not big enough for my idler motor, which will trip them in about a minute even though it's just idling. So, I'm not using the overloads. Ideally I'd get a pair with higher ratings. S2 - DPDT toggle switch. This switches the starting capacitor into the circuit. The switch is rated at ten amps, but as I expect the starting capacitor to draw more than that, I wired up the two poles in parallel. So my circuit uses it as a 20 amp SPST switch. A high-current momentary pushbutton switch would be better. C1 - 130-156 uf, 250 V motor start capacitor. This provides a phase shift in the current to one of the motor connections, which is all we need to start rotation. It doesn't matter which direction the idler motor rotates. 130-156 microfarads (a strange way to specify it - usually one would expect to see 143 +/- 10%, which is actually pretty good precision for a cheapo capacitor - but that's how it's marked) seems to be just right for the 2 hp idler, bringing it up to speed in less than a second without dimming all the lights in the house. Get one for about $5.00 from McMaster-Carr (stock number 7245K39). F - 7 amp slow-blow fuses. These are glass 3AG type 1 1/4" x 1/4" fuses, easily obtained at Radio Shack and hardware stores. Fusing two lines would be enough in normal operation, but I fused all three to guard against damage on startup in case of wiring errors. There are two sets of three fuses, as I left provision for another 3-phase line should I want one in future. M1 - idler motor, Dayton 2 hp TEFC ("totally enclosed fan cooled"), 208-230/450 volt, 6.20 full-load amps at 230 volts. This motor was new in the box, just taking up space in somebody's stockroom before they unloaded it on eBay. A new motor is nice, you don't have the annoyances you sometimes get with used motors, like somebody painting over the plate which tells you how to wire it for 230 volt operation. The motor typically costs less than the shipping costs. In the US, be prepared for a minimum of $30 to ship a motor in the 2 to 5 HP range. Insure it if it's not in its factory packaging. The motor itself tends to be very sturdy but if it's dropped on its aft end - where the fan and cover are - it can be wrecked. (Ask me how I know ... @$#%&* UPS). My wiring is 12 gauge 2 conductor plus ground or 3 conductor plus ground. The 12 gauge is actually far heavier than needed - the motor and idler don't draw anything like the 20 amps that 12 gauge can comfortably carry, even at startup. I estimate that the motor start capacitor might draw 12 amps. Well, better too heavy than too light. All the boxes, panels, cabinets, whatever are grounded, eventually, to a good earth ground at the house's main service breaker panels. This circuit isn't ideal. A magnetic contactor would be nice instead of the manual starter, as that would disconnect in case of a momentary power failure. In my circuit, if power was to be interrupted long enough for the motors to stop rotating, they wouldn't start rotating again after power was restored, as the start capacitor would be switched out. So they would sit there drawing their starting currents and would overheat within a few seconds. Maybe the fuses would come to the rescue, maybe not. Fortunately I haven't had any trouble with momentary outages around here lately, so there's no need to worry about that stuff just yet. Another improvement would be a timer relay to switch off the start capacitor after, say, a second. Then no operator coordination would be required on startup. Window dressing, really - if I don't have my act sufficiently together to work two switches, I should be kept away from sharp objects, like machine tools. If I had balancing capacitors in the circuit, one would connect between A and B in my schematic, the other between A and C. Properly-sized capacitors could equalize, more or less, the currents in the three phases. However the price paid for that improvement is phase shifts, so these ideal currents would no longer be at the ideal phase angles of 120 degrees. On my setup here, without any balancing capacitors, I have less than +/-10% variation between the phase voltages. That doesn't strike me as all that bad, so I'm in no rush about balancing them. Here's what the hardware looks like -
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