George
Yup an RPC will be pretty much plug and play providing its correctly sized and has enough starting punch to get the machine going with a decent size job in the chuck. I'd think a 5 HP commercial brand, Transwave or similar, should be up for it but a 3 HP one would struggle at times.
That said I do know of one guy whose 5 HP RPC went down and killed its internal motor, a 1024 VSL was what it was mostly used on. However this was one of the "clever" RPC units with an internal tuning box said to dynamically adjust the internal capacitor banks to keep the phase currents stable. Think very low rent version of the PhasePerfect units common in America. On strip down the owner was less than impressed with the internal arrangements and suspects that this tuning box went down leading to the demise of the internal motor. I sold him a 5HP motor I had lying around doing nowt and he re-jigged the internals to the standard configuration as used by Transwave and most other makers with entirely satisfactory results. Looks like the clever ones are off the market now which may mean something!
Obviously any advice folks give you on this topic will reflect their personal experiences and priorities so its best to get a good cross section of views and go with the guy who's situation was nearest to yours.
In my view the options can be ranked in descending order of (new) cost and increasing hassle / DIY effort factor as follows :-
1) Utility three phase, £ ?? thousand :- Most expensive, need an electrician to install it and sign off but you just have to put the plug in. Safe and just works for however many machines you have.
2) Plug and Play 220 in 440 out VFD box, £1,500 - £3,000 (ish) :- Not cheap but you can wire it yourself. A 10 HP one is known to work on a 1204 VSL and should supply any sane persons workshop needs, a 5 HP one should do the deed but will be working hard on start up and may not quite be big enough for the future shop contents.
3) Commercial RPC, £1,500 - £3,000 :- Plug and play but has to run all the time which may be distracting. A 5HP one ought to be enough to run a VSL and should be up for future requirements but many be close to the edge at start up. I've known a Transwave 5 HP having issues with the 3 HP motor on a hydrovane rotary compressor in cold weather. (Unlike piston compressors Hydrovane is not generally considered a heavy start load.) I'd go for a 7.5 HP RPC, maybe 10 HP if I had serious ambitions for extra machines but larger commercial RPC boxes have minimum motor power specifications and may require adjustment of tuning switches. Three phase output is almost invariably imperfect, unless you luckily hit the sweet spot, but usually more than good enough for machines. Bigger ones may have issues with RCD and MCB breakers in the mains input. Best supplied via a dedicated connection and checked by an electrician.
4)Static converter with (large) permanent idler motor, £400 - £600 :- Clarke (Machine Mart) and other makers do properly packaged 5.5 HP 220 V in 440 V out static phase converters which can readily be made into pseudo rotary converters by adding a 3 phase idler motor of nameplate rated output power. Free running 3 phase motor doesn't take much power, probably turns a 5.5 HP rated box into true 5 HP one. I've done this with MotoRun static converters in the past and it works about as well as a proper RPC but you need to fiddle with the switches to get the phases balanced. Back in the day only practical way was by ear, significant out of balance made things buzz loudly. These days you can buy clamp on current meters for under £10. Not objectively particularly accurate but one hooked permanently on each line would make it easy to see that the phase currents were adequately balanced. Three of same brand should read pretty similar.
5) VFD boxes, one per motor £300 - £500 for the three :- Needs motors converted to 220 V delta and, in the case of the 1024, a certain amount of control system rejigging. Once done just works. Big advantage is that the VFD units will be bog standard commercial units so it one dies its easily changed. Needs another VFD if you buy another machine tho'.
6) Self build RPC, £200 up depending on how nice a job you make and what can be got cheaply :- Considerable amount of work, significant issues with finding good UK specific circuit details with all the vital information on how to set it up. First time I did one was with the "help" of a single page A4 sheet circuit drawing. I knew electrical and electronics but it wasn't easy! Couple of commercial suppliers of diagrams and parts lists for around £15- £20. If you are seriously considering going this route, invest in a set. Read-up on whats involved and add up the costs to you of getting all the components. Spending £20 to discover that it is not the way for you or that the promised cost savings evaporate if you haven't got a degree in dumpster diving is not wasted money. I'm deeply suspicious of the "How I did it for thruppence three farthing and got it to work on my machine" folks. I suspect that if you have to buy significant amounts of new-new parts, a transformer and motor will make serious holes in the budget, converting a Clarke or similar commercial static converter will come out at similar cost or maybe less. And it will be neat. Do budget for a triplet of clamp on current meters. Balancing up a home build can be a right pain in the butt (how do I know?).
7) Static converter, £ how much money do you want to waste? :- A static converter will not run the main motor but home built static units should be OK on the speed change and coolant pump motors. I've built enough that it would be no bother for me but frankly, not worth the effort as a small VFD will cost little more.
Thats my three ha'pence worth.
In your position I'd go the basic VFD route and accept the work of re-configuring the controls. For plug and play 220 V in - 440 V out from a converted static looks to be good balance of bang per buck and not too much work. Maybe use the DIY plans to do a better job of converting the static to a true rotary. But this particular elderly penguin has more than enough electrical knowledge and experience to make such jobs pretty easy. Different if its your first rodeo.
Clive