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
Phone: 0121 708 4511
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..