Yeah, "That" was a poor example. I searched for something in a hurry, but I mean the ones which start as a one-piece casting. Proper, old British stuff with 1/2" thick tool rests etc. Rather than the modern (mostly far eastern) flimsy rubbish.
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Hopefuldave
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Two main ways:
VFD (Inverter) drive rectifies incoming 240v power and charges the main "bus" capacitors, giving a 350ish volt DC bus that's then chopped and applied to the output terminals - each output is pulse-width modulated so the resulting DC (taking the average over successive pulses and gaps) varies from 0 to 350ish. this is done with 120 degree offsets between the PWM waveforms, replicating 3-phase power. Note that the outputs NEVER go negative relative to the incoming power's neutral, but they go negative RELATIVE to the other outputs, just like real 3-phase.
The 240-415 VFDs are slightly different, they use a voltage-doubler to the bus capacitors - alternate + and - peaks charge different capacitors (neutral is taken to the common point between them) to give a 700ish volt bus *with neutral in the middle) - from there on the process is the same, but the chopping is between + and - 350v to give what looks like 15 3-phase. This makes it fairly easy to hack 415v-input VFD to produce 415 magically from 240 - my lathe's powerd this way, the hack cost me about 20p...
Rotary converters - incoming power is applied to 2 phases of a 3-phase motor, and once startdd it generates the 3rd phase by magnetic induction in the motor - also called a rotary transformer - the motor on't start without help, and would sit there humming and overheating, so a starting circuit's usd, just like in a single-phase motor a capacitor phase shifts and supplies power temporarily to the third winding to set up a rotating field in the motor, which then spins up - once spinning the start circuit can be switched off as the 3rd phase "generates itself". Usually it's ncessary to have "balancing" run-type capacitors to correct the phase relationship and voltage - trial and error, pick the best!
This doesn't, though, give 415 out, only 240v 3-phase as used by the Truculent Colonies - there's a funky setup in a Youtube video that will, but it's a) very complicated and b) horribly inefficient! Instead, a 240-415 transformer can be used to boost the voltage, either at the single-phase input (mine's home-brewed using an old oil-cooled welder with a rewound transformer) or a 3-phase transformer (much more expensive!) at the output.
A 415v rotary has the advantage of being "just like mains" - I can plug in lathe, mill, bandsaw, pillar drill, TIG welder - all 3-phase and expecting 415v - and run them up to the rotary converter's capacity, very handy in the Shed Of Danger that only has 240 coming in!
Therre are also "static converters" which use capacitors only (often switched for different power ratings) to produce a phase-shifted 3rd leg, but they're naff, so I won't go into them...
Dave H. (the other one)
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When did that ever matter?
When did that ever matter?
To be fair after I wrote that I did contemplate most of my "projects" over the years and most fall into that category lol.
Screwdriver
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Oh right, you want a technical briefing on how 3ph is generated? You're a brave man. It is a relatively simple concept which is difficult to achieve in practice.
You just need to "power up" a circuit which acts a bit like a signal generator. Your stereo system does that all the time and you could call that a 2 phase system...
For three phase, you need three separate circuits which develop a sinusoidal voltage output relative to the same "ground" or "neutral". I won't go onto any detail since many other here are better placed. The common ground/neutral is a separate topic all of its own!
Suffice to say the problem in practice is not making these three sinusoidal signals 120 degrees out of phase, the problem is doing that with five or ten amps! Your solid state devices suddenly need to be able to handle a LOT of power. Even that is not so much of a problem until you suddenly stick a massive inductor onto the output. Weird things happen with the sudden inrush of power. It can affect the circuity and it can do weird things to your motor phases. Throw in an "imbalance" and the complexity of deriving 3ph from a single phase source can be an engineering headache.
Naturally, none of the professionals on here will have any problem with 3ph. It's easy (when you know how) and of course a professional user won't be buying consumer grade products. That is really where the problems lie: cheap solid state circuits. They're are great until you go slightly over their limits and with big 3ph motors, that is comically easy every time you switch them on and off.
Guineafowl
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Basic topology here (ignore one input phase - all goes to DC bus anyway):
Each transistor pair uses PWM to approximate a sine wave:
Each transistor pair uses PWM to approximate a sine wave:
premmington
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I would agree - allot of effort for a bench/pedestal grinder.
But this does raise a question - where do you buy a decent - 240v pedestal/bench grinder?
I been using Sealey 8in ones in the workshop - they not great - been picking the highest wattage ones I could find - but spindle bearings don't outlas teh original grinding wheels? - I would put my hand in my pocket for a decent one.
Any suggestions?
Seadog
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The other point is that 3-phase kit is generally quite cheap, so the additional cost of a vfd gives you a solid, capable machine. If you have to faff with the motor, so what? Isn't that what we're all about? 
Guineafowl
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My 230V 8” Wolf grinder has been very good.Any suggestions?
It was the find of the year - local scrapyard, on the heap. £10 and it only needed a switch. They do pop up for sale now and again.
The traditional "rotary" converter is basically just a Static Converter with a Pilot Motor (which smooths things out & means you don't have a minimum motor load & the need to switch settings for different motor sizes ike you do with a base Static).
I think the Static uses the single phase to feed a double-winding transformer - so the voltage induced into the transformer creates the 2nd phase.
The third artificial phase is created by putting ac through a bank of capacitors.
Because its not "real" 3ph though, the voltages Ph-Ph & Ph -N are all over the place (can't recall the numbers).
You need to make sure that any 1ph controls or motor start circuits are wired to run of L1 (as its the only genuine 230-240vac phase, being essentially the utility 1ph supply voltage).
Guineafowl
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I like faffing with motors. Plug and play is boring.The other point is that 3-phase kit is generally quite cheap, so the additional cost of a vfd gives you a solid, capable machine. If you have to faff with the motor, so what? Isn't that what we're all about?
In all honesty, just buy a VFD.
Boost (aka 240in 415out) ones the size required to run a grinder are about £70 delivered.
Non-boost can be had for sub£40, but it would mean you loose power, but on a bench grinder, that's not likely to be too much of an issue.
Something often overlooked with single phase motors, is they don't run as smooth.
The single phase means the motor goes from producing zero torque to full torque 100 times per second and relies on inertia to provide output during those zero torque periods, which can show up in the finished product.
Whereas a 3 phase motor always has coils powered and producing torque (there is a slight variation in torque output, but not something you're ever likely to notice)
Boost (aka 240in 415out) ones the size required to run a grinder are about £70 delivered.
Non-boost can be had for sub£40, but it would mean you loose power, but on a bench grinder, that's not likely to be too much of an issue.
Something often overlooked with single phase motors, is they don't run as smooth.
The single phase means the motor goes from producing zero torque to full torque 100 times per second and relies on inertia to provide output during those zero torque periods, which can show up in the finished product.
Whereas a 3 phase motor always has coils powered and producing torque (there is a slight variation in torque output, but not something you're ever likely to notice)
Hopefuldave
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I think I remember that thread - my natural scepticism faded into outright disbelief pretty quickly - as you say, defies theory and he'd never either explain how it worked nor show a diagram...
Dave H. (the other one)
A forum member claims just that. Or, in fact that you can create 3 phases using a cleverly wound transfomer.
It defies transformer theory if so.
Well, as long as you had a really long bit wire, it is theoretically possible.
Might suffer slightly from losses though
I must have missed that thread.
If I had that grinder and wanted it to run on 240v I would strip the motor, take the windings, fields and all wiring etc out, turn a Vee in the armature, cut a slot in the middle housing and drive with vee belt from a 240v motor bolted down in the stand.
But not everyone takes such drastic action.
But not everyone takes such drastic action.
Here is that thread:
3 Phase converter
Hi, Many thanks Moose & Richard for your replies. :thumbup: I'm pleased I asked before jumping in; I've been around the block a few times and know these days things are made down to a price not up to an high standard unless serious money is involved. I note heavy duty Murex TIG welders are...
www.mig-welding.co.uk
I used to deal with Transwave (Power Capacitors) & while I don't pretend to understand exactly how they work, the principle I explained (& the principle components) are what you'll find inside a static (& a rotary) converter.
The expensive bit is the transformer (as when a customer had a fault with one, the advice was to test the Caps & the Transformer.... & that if it was the transformer, then it would be uneconomical to repair.
Could it be that the single phase input provides L1, and also feeds the transformer which produces L2, while the AC through the Caps generates ph3?
Peter from Power Capacitors told me that the voltages on the L2 & L3 created phases were at very different voltages to L1 & the 1-phase utility input (as said) due to the way the Ph Converter worked. Apparently the variation wasn't an issue with actual motor operation (though as stated, and something to do with the motor load being critical to it all working - statics can't run sub 1hp motors or the motor will burn out).
So.... if its impossible, then someone needs to inform Transwave that their converters use Black Magic
