If you have any doubts about your ability to undertake work connected with electrical installations, you should employ an electrician, qualified to work on machine tools, to do the job for you; you will find the money well spent.

Books:
Do some homework - for excellent advice on how to use electric motors in the small workshop a copy of  the books: "Electric Motors in the Home Workshop" and "Electric Motors" will be invaluable. Both show how to identify and use motors from industrial and domestic sources and how to operate 3-phase industrial motors from a 1-phase domestic supply. For more advanced instruction the book "Electric Motors" is recommended, this being written with the amateur user in mind and covers motor types, characteristics, operation, installation, speed control, braking and a host of other matters - as well as advice on using and adapting surplus motors from industrial sources. For specific information on using 3-phase motors on a 1-phase supply see the book "Three-Phase Conversion" and for information on how to prepare the electrical system for a home-based workshop try "Workshop Electrics"

Switches:
Machine-tool switches are often a problem, especially on older examples where the parts are obsolete or don't meet current regulations. We can supply a high-quality UK or EU manufactured reversing and on/off switch that will meet the majority of requirements for motors up to 5 h.p. It can be easily wired to replace built-in switch gear that has become unreliable. Other switches to control 2-speed motors and various kinds of push-button starter can also be provided. Just email your requirements and we'll try to help

The Basics of 1-phase to 3-phase Inverters and Converters
Nearly all large machine tools are equipped with industrial-type 3-phase motors and to use them without a 3-phase supply requires either the motor to be changed for a 1-phase type - or a phase converter or inverter connected. The latter is now often the preferred choice: plug an inverter into the single-phase household supply and it will produce a current capable of running a three-phase motor with an output able to be varied that can vary the motor speed. However, "Converter" units still have a place for many applications as they have a distinct advantage: the supply from the inverter to the machine tool usually allows the original electrical control system to be used and accessories such as coolant pumps and light units to continue in use. An Inverter usually requires connection direct to the motor and subsequent control from the inverter itself - or a remote "pot" itself.
With an Inverter various other useful things can be arranged, including:
· "Soft" start
· Electronically controlled, safety quick stop
· A small remote-control unit placed where it can be reached with the greatest ease and safety.
In addition to getting the machine to work with the minimum of effort, using an inverter in conjunction with the machine's own belt or gear-driven spindle drive will give an even greater spread of speeds. For example, an inverter conversion recently seen on a Boxford lathe altered the bottom speed from 40 to 5 r.p.m. and the top from 1450 to 2000 r.p.m. - the same as the now very hard-to-find version fitted with the maker's original "high-speed" pulley set.

A word of caution: many inverters have an "over-speed" function than can allow spindles to be run at revolutions far in excess of the maker's original intentions. In addition, chucks mounted on the headstock spindle are always limited in how fast they can be spun, especially large ones as these, often made of cast iron, will eventually burst - and the clamping force of the jaws also weakens as the speed rises.

Some practical aspects of using an inverter
Normally the output from an inverter is connected directly to the machine's motor and not to the normal power-in box on the side of the machine. Control of stop, start, reverse (and jog) passes from the machine to a panel on the inverter - or a remote supplied with it. However, it is important to both follow the maker's instructions on how the connection is made - and one should also to be aware that, in connecting directly to the motor, the action of safety features will be overridden - for example, when opening a door or moving a handle that would normally have triggered a micro-switch to cut the current.
Once installed an inverter be used to run other three-phase machine tools within its power limits - drills, milling machines, saws and shapers, for example.
If you intend having several 3-phase machines in your workshop an inverter is almost certainly a more economical way of running them than changing the motor on each. Additionally, some machine tools have motors so tightly integrated into their construction that finding a single-phase motor to replace them is impossible and, unless you take out a bank loan and have the original motor reconstructed as a single-phase unit, using an 'inverter' is the only really practical solution.

Important facts about the conversion
Most 3-phase motors of recent years are dual-voltage marked with something like 220/240-380/440).  An ordinary 1-phase to 3-phase inverters produces an output voltage equal to the input, i.e. 220/240V and so you must make sure that the motor is set correctly. Following the simple instructions printed inside the lid of the terminal case and simply arranging links between the wiring terminals will normally allow this setting to be made.

Advanced Vector  240V Inverters
This latest development offers all the functions of the basic unit and are known as "Advanced Vector Inverters" They have the advantage of allowing control of the motor speed without loss of power. On a "basic" inverter the power of the motor reduces somewhat as the speed falls, but with the latest vector type this does not happen - and that's good news for applications like lathes etc., where torque will be maintained down to very slow speeds. In addition the Advanced type is able to interface to 3-wire control systems, for example normal green "on" and red "off" push button switches - and this also makes it possible to connect the inverter to a machine's existing controls. Advanced inverters are also suitable for use with computer control on CNC machines and can be bought in versions from 0.5 to 5 h.p. and are available with a 240V 1-phase input and a 415V 3-phase output. 415V Inverters  for Motors marked as high voltage only and dual-speed 3-phase motors If your 3-phase motor is marked only for 400+ volts operation, or is a multi-speed type, you will need an inverter with an output voltage to suit. These are now available, and offer the same functions as the basic and advanced unit making them especially useful for machines such as the Bridgeport miller where a 2-speed motor is built into the upper frame of the vertical head.
Models from 1 to 20 h.p. are available - as well as a 440V Bi-phase input type (as often found on farms) that will power motors up to 50HP.

Generally there is no disadvantage in using an inverter with higher capacity than the motor - indeed buying one like that is an advantage for, if in the future you buy a larger, more powerful machine tool, the existing inverter may well be large enough to run it. A high-quality VFD is "intelligent" and won't over-power the motor. It's also usually possible - depending upon the make and model - to program motor's service factor amps and set it up switch off if this is exceeded. Another way of setting up the drive is to experiment with pulley sizes, that on the motor being easiest to change. The inverter, combined with either belt or gear changes of speed can be fine tuned for either maximum torque at low speeds for heavy work or set to run very fast for small diameter and finishing work. One tip is to employ a slower motor, a 6-pole one running at 960 r.p.m. (or even an 8-pole at 720 r.p.m.) rather than the more common 4-pole 1420 r.p.m.. These speeds assume a European 50 Hz supply, motors in the US, being fed with a 60 Hz current, run faster, their "normal" 4-pole motor speed is 1700 r.p.m. Oddly, against what would seem to be common sense, a slower motor helps because the inverter (at least with vector drive) will hold a nearly constant torque from minimum speed up to its normally rated top speed. Beyond the marked maximum power remains constant but, unfortunately, the all-important torque falls off. Hence, doubling the speed of, say, a 0.5 h.p. 720 r.p.m. motor lets your set up act just like 1 h.p. 1420 r.p.m. motor. A disadvantage of the 1 h.p. 1420 r.p.m. motor is that slowing it down reduces the power (power = speed X torque) and the motor has no way of increasing torque to compensate for this. The result is that, with inverter control a 3-phase 720 r.p.m. motor will always have the same or better torque than a 1420 r.p.m. type together with the same or better power output. It's best to avoid cheaper 2-pole 2800 r.p.m. motors (3600 r.p.m. in the US) as they have little torque and slowing them down means a significant loss of power.

Inverters marked as high voltage for multi-speed and high-voltage only motors
If your 3-phase motor is not a dual-voltage type and is marked only for 380/440 + volts operation, or is a 2, 3 or 4-speed inverters to run these now available - and can be useful for machines such as the 2-speed Bridgeport miller and various models of Arboga milling and drilling machines where some models have a 2-speed motor is built into the upper frame of the vertical head. However, there is a downside to these units - no inverter manufacturer on earth produces an inverter that varies the voltage from low to high. If you wish to vary the frequency of a 380/415v 3-phase motor you can purchase an inverter that connects to a 380/415v 3-phase supply and gives a 380/415v 3-phase variable frequency output. If you  purchase an inverter designed to connect to a single-phase 220/240v supply, the output from the inverter will be 3-phase 220/240v, not 3-phase 380/415v. Any inverter offering a 380/415v 3-phase output from  a 220/240v single-phase supply is, in reality, a modified version of a standard 3-phase 380/415v input type. Retail companies offering this type of product take a standard inverter designed to operate on a 3-phase 380/415v supply and modify it for use on a 220/240v supply. The modification creates two issues, namely:
1) any warranty offered by the inverter manufacturer is now null and void because the inverter has been modified away from its original specification and 2) the inverter no longer complies with European Power Quality  Standards such as the EMC directive, BSEN 61000-3-2:2006 and BS EN 61000-3-12:2005 so cannot be CE marked without further approval. Original labels often appear on the modified products suggesting compliance  so exercise caution before considering these products. There are no cosmetic changes to the inverter, so the only way you know the inverter has  been modified is because has a label (usually stuck over the manufacturer's label) confirming the modified input and output voltages.

Where to buy your inverter
eBay is, of course, awash with sellers of low-cost units - but take care. Are these people just "box shifters" who offer no advice or a back-up service? Will they answer your questions when your units does not perform as expected? Are they familiar with the common makes and model of lathe and milling machine and know what advice to give regarding the ideal unit to run them. Best I suppose to check if they are long-established, have a contact phone number and are prepared to answer a few sample questions.
A firm that can be recommended is:

Power Capacitors Ltd.
30 Redfern Road
Tyseley
BIRMINGHAM
B11 2BH
Phone: 0121 708 4511

http://www.powercapacitors.co.uk/
sales@powercapacitors.co.uk

Most motor-repair shops in your local area will sell converters and  inverters and usually have experience of industrial applications - and may well be able to offer a delivery and installation service. Obviously, having overheads, their prices may well not be as competitive as an online-only seller but at least you know where they are should things not work out. My advice is to buy a high quality unit - such as those by IMO -  from a reputable and experienced dealer. Further PDF inverter documentation can be downloaded here

Installing a 1-phase motor in place of a 3-phase
Should you decide to change a 3-phase motor for a 1-phase one some thought is necessary to get the best out of the conversion. It is as well to bear in mind that a single-phase motor is not the direct equivalent of a three-phase type. A 1-phase motor is best run continuously near its rated capacity (i.e. worked nearly flat out); if the motor is switched on and off frequently against "no load" the windings will be damaged and, if run through a cycle where it is started, worked briefly, stopped and started again, the capacitor will fail.
A machine tool with a 1 h.p. 3-phase motor should, in theory, run just as well with a 1 h.p. 1-phase motor - but, life's not like that. If you need your machine to perform as well as it did with its original motor - and run to the same top speed without struggling (and it is by no means certain that you will) - you may well find that, for power for power, for ease of starting and long-term reliability, a successful and reliable conversion to 1-phase requires a motor which is marked as being some 30% to 50% more powerful - but do take care and see the notes below**
When changing the motor(s) leave the original 3-phase wiring and switchgear as intact as possible and store the old motor safely inside the machine's stand - i.e. where you can't loose it. Do not waste time trying to modify any of the original electrics; it is much simpler, and a lot safer, to fit new wiring, a new switch and a "no-volt" safety cut-out. Should the machine ever be put back on 3-phase, or wired with an inverter to give a variable-speed drive all you (or the next owner) needs to do replace the original motor - and hook up the wiring.
If you have any doubts about your ability to undertake work connected with electrical installations, you should employ an electrician, qualified to work on machine tools, to do the job for you; you will find the money well spent.

**Motor Power - a Word of Warning:
A note of caution might be appropriate here for the owners of smaller machine tools. If you are going to replace the electric motor on your lathe, miller or drill, etc. - think carefully about on how powerful it needs to be. Modern motors are very compact - and it is now all too easy to fit a massively powerful unit in a tiny space.
If you have a "dig in" when turning, milling or drilling - or other accident - instead of coming to a dead stop as the load overcomes the motor power, the machine may continue running and do itself, and you, serious damage.
I remember a little Grayson lathe that the owner had converted to chain drive via a motorcycle gearbox. He hooked it up to a 1 h.p. motor from a Bendix washing machine and revelled, for a time, in the slip-free, powerful drive he had constructed. When, as was inevitable one day, he allowed the cutting tool to run into the chuck the lathe failed to stop. Indeed, the irresistible forces being so efficiently transmitted by the chain ripped the headstock assembly from the bed and proceeded to smash it - and the bed - to pieces as it hurtled, at great speed, round and round the countershaft unit.
For lathes up to 3.5" centre height it's surprising what a 0.25 h.p. (one-quarter horse power) motor can achieve. 0.33 hp (one-third horse power) is probably a safe compromise (that's the size Myford fitted for many years to their ML7). Anything over 0.5 h.p. (half-horse power) and you need to be aware of the fact that you are driving a powerful little beast - that can have a savage bite.
Lathes between 3.5" and 5" centre height often have more complex drive systems - with the motor inside the cabinet stand - or with variable-speed drive, and so require more power to overcome the frictional losses in the transmission. Unfortunately, modern motors do not seem nearly (subjectively) as powerful as their rating would suggest, especially the more "affordable" ones, and the starting characteristics of single-phase motors often demand an excess power rating to get the spindle turning - especially in a cold workshop, on top speed and when no clutch is fitted. Thus, it becomes difficult to say exactly what size of motor you should fit to obtain the best compromise between starting, turning performance and safety. The maker's original advertising or maintenance literature will provide a guide or, if that is missing or unobtainable, there is often a plate on the machine that lists the original electrical specification. If the only clue is the existing 3-phase motor, replacing it with a single-phase one of the same nominal horse power will almost certainly leave the machine underpowered; instead, as previously advised, something 30% to 40% more powerful would be a good idea - but no more, or as recounted above, you may stray beyond the machine's design limits..

Single-phase motors - a few basic facts
The two basic types most commonly encountered are "split-phase" and "cap-start-induction-run". The former often use a combination of a start capacitor and a mechanical switch that disconnects the start winding from the run winding when the motor reaches around about 75% of its rated speed. Although less expensive than other types they do have some compromises including a limited starting torque and high starting currents where, if the motor is stopped and started frequently it can cause the start windings - and capacitor -  to fail. "Split phase" motors are typically to run things like air blowers, fans, washing machines and machine tool (small lathes and grinders for example) where a lowish starting torque will work well enough. However, if your machine needs a high starting torque to get running (typically it would have a mechanical variable-speed drive, an oil-filled gearbox or multiply drive belt runs with lots of friction), avoid this type.
"Cap-start-induction-run" motors have several advantages over the split-phase version: a capacitor is wired in series with the start circuit and not only creates a much greater starting torque but manages this with a lower starting current draw together with the facility to include an overload protection circuit.
"Cap-start-cap-run" employ a "start-type" capacitor for high starting torque and an additional "run-type" capacitor that only comes into operation after the start cap is switched out of the circuit. There are several advantages to this (more expensive) type including drawing a lower current at full load and running at a lower temperatures than an equal-sized, 1-phase motor of any other type. If your application requires a greater stating torque and higher, more reliable power, this is the type to go for. However, motors with capacitors do not like to be stopped and started frequently - say two or three times a minutes - if this is done regularly, the capacitors will fail.
There are other varieties of 1-phase motor as well (permanent split capacitor, shaded pole) but the ones just mentioned are easily obtainable, modestly priced and usually reliable.
Has your 1-phase motor failed? If so, it might just be the capacitor. If the one fitted is not marked with its rating the this site will provide a guide as to the correct size.
Can a single-phase motor be run in reverse? Yes, most can, by swapping over the connections between the start and run windings e.g. motors are usual found with four terminals - let's call then A and B for the two start windings and X and Y for the run. As delivered A will be connected to X and B to Y - thus making a single pair to which the live and neutral connections are made. To reverse the motor A is connected Y and B connected to X. This can be controlled by an external reversing switch of the type sold here.

Belts
If a motor is changed, or the drive system otherwise modified, ensure that the pulleys and belts receive proper attention. Pulleys, preferably in cast iron (for grip), should be a close fit on their shaft, secured with a key, locked securely and run without wobbling. If the pulley has to be changed, and the correct replacement cannot be found, the solution lies in the industry-standard but more expensive "Taper lock" type. These are made in two parts: a split centre, tapered on the outside, and an interchangeable outer rim held on by four bolts. The tightening action of the bolts compresses the inner section to grip the shaft. The locking force to the shaft is so great that, usually on lower-power installations, no key is required.
Belts need to run in line - and the easiest way to check this is to lay a long steel straight edge across the full width of both pulleys. Assuming the pulleys to be of the same type - and thickness - the edge should make contact at all four points on the rims. The drive belts also need to be in good condition: V-belts often wear in patches (or develop hard and soft spots through standing) and when the different sections run over the pulleys they either  "fall into the groove" or "climb the hill" and so continuously alter the drive ratio. This causes a cyclic speeding up and slowing down of the drive and can result in the most alarming vibrations - especially at higher speeds, with the result that marks are passed to the workpiece and show as vibration bands. New belts are inexpensive and can make a remarkable difference to the smooth running of a machine tool.. A supply of belts - V, flat and round, can be found here. If in doubt about the type of belt to employ, or other enquiries about drives, you are welcome to phone: 01298-871633 from 09:00 to 23:00.