When I tried researching this I came up with the is some trial and error involved starting with an educated quess.
I was trying to sort out what control gear was required for a submersible bore hole pump with no idea of what was down the bore hole, it got really interesting when I was on the phone to a guy who knows about such things and emailed him some photos of the control gear that was in place, he went quiet then said I’m surprised it hasn’t gone up in smoke as whoever wired it used a start capacitor as a run capacitor.
Anyway I sent for a expert who retired the controls, then when he had it running smoothly I had to rewire his rewiring to make it safe.
Just remember that someone who can make things work aren’t always neat and tid, also as I realised afterwards he had phoned his dad to ask what to do!
Its a bit of a kludge to use a single value capacitor, as during start, or heavy load, the motor will present a lower impedance, and the phase shift is too high, (as it gets nearer the 90 degees you get with a capacitor and a small R in series ) but if you get it right for a decent start up, then it is too much ( and the phase shift too little) once up to speed (so it never gets up to speed).
Posh units switch caps in and out to get more or less equal voltages accross the two "flying" phase voltages.
on 50 Hz and 250V mains you need circa 50-100uF per horse power, and be prepared to switch some C in and out if you have trouble starting, and do realise it will never be as good as a real 3 phase PSU.
I remember wiring motors on the bench at college, but have never put what I was taught into practice on a regular basis, it’s another of those things I know that I can fo, because I have done it, but would have to refresh my knowledge before proceeding.
That borehole pump job job taught me I know enough to be able to tell the people who really know what they are doing from those who are winging it.
if its a bench top demo you wont be loading it hard, it'll be fine - if it was 1 HP or so, get 3 or 4 22uF caps for example some light switches will allow you to vary the C value and bolt it all securely to a block of wood- actually for any motor demo clamping it down is by far the most important part - avoids it rolling off, taking the mains leads with it and hurting someone. Its also a good idea not to use anything too big on a bench any way - imagine it going for a spin at near synchronous speed if the supply is accidentally shorted so the rotor locks up. ?
If you can muster an oscilloscope and some mains rated probes you can also look at the phase voltage varying as it spins up. An isolation transformer may be a good idea, as well as a supply side RCD.
totally unloaded, you may be able to run it single phase and decide it which way to start by pulling on the belt to give it some pre-spin one way or the other before energising.
Don't do so many starts and stops in a short time that it smells of burning though.
You don't mention the motor rating, but again I caution demos of anything too big unmounted.
Circa 30 years ago I was head of The Electrical Training Department for a region of the CEGB (became National Power). We conducted many “experiments” with AC & DC Motors for training purposes, including using some equipment expensively produced specifically for training purposes, plus motor and motor-generator sets salvaged or surplus. The first thing to point out, is that the whole area was purpose built, RCD and emergency stop button protected. Have you carried out a RAMS process, because if one of your apprentices came to harm then you will be skinned alive!
At that time, there seemed to be many surplus three-phase lathes and other machine tools available on the market and I was asked on several occasions how to run these three-phase equipped machines in a home garage on single phase. I won’t go over the ground already discussed, but a significant part of the issue was; what capacitors of suitable voltage rating were actually available? Naturally the person concerned didn’t want to spend money on new capacitors. Therefore in practice they often found it easier to source a salvaged single-phase motor and adapt the mechanical drive. By coincidence, I was clearing a shed last week and came across and old Washing Machine Motor, which went to my local recycling centre
As has already been mentioned, lash-ups like this will never produce a smooth running full power motor. Small variable frequency drives were also available then, but at prohibitive cost for the person concerned.
Just to reiterate – please take care with possible demo’s and experiments like this. In an era when there was a little more leeway, I had to be very careful ,although I didn’t issue a Sanction for Test to myself, because this was a carefully controlled “training environment”.
