Im looking at mays of having a 3 phase supply in the garage, to run a lathe and ultimtely a mill and a compressor (all circa 2.5kW.

Ive been reading about rotary converters developed from old 3 phase motors online, but a lot of a sites are American.

Has anyone on here built one? Any links to guides or circuit diagrams or suchlike?

Cheers!

Be careful with American 3-phase converter designs - they have a different way of supplying electrickery to houses.

Now this may be wrong, but I believe that most of their appliances run on 110v, but they use 220v for heavy current devices such as ovens. To achieve this they combine two 110v phases - so a house gets 2 phases as standard. Many of the US circuits for generating 3 phases are based on having 2 phases to start with, which makes life much easier. Unfortunately they won't work with our single-phase supplies.

There are ways of making a rotary converter, including using single-phase 240v to run a 3-phase motor, and taking the power off the 3-phase motor's terminals (you end up with a rough approximation of the 3 phases). The trouble is getting the 3-phase motor to start (I saw an old book that recommended a rope round the pulley - turn on, then pull the cord!).

I looked at all these things - then eventually went out and bought a Transwave converter that makes life SO much easier!

Rotary converters were very common before the advent of high power solid state electronics, particularly for use a frequency converter/speed controller.

There are several problems for a diy version assuming you mean to drive it with a single pahse motor

1 Losses will be quite high and you should use a conversion efficiency of 80%.
2) The primary side currents will be very high (6.5amps per kw output)
3)You could have difficulty finding a single phase drive motor at the size you would need.
4) Making an alternator from an ac motor is only possible if you have a wound rotor. These are not common in the sizes you are talking about.
5) single to 3 phase inverters are not that expensive in the size you need

I'd be more inclined to uprate the garage supply to 10mm and run single phase, safer and less faffing about. As for Yank wiring it looks decidedly dodgy to me.
Cheers,
Bob

I looked into it before buying my TIG but read that they weren't good or didn't work with 3 phase TIGs. Something along those lines but the most basic ones seemed to be a motor turning an old alternator with a power connectd to the field coil and then feeding off the alternator connections.

Yes i've done it, initially to power a 3 phase harrison lathe. Then i piggybacked a drill and milling machine.
I used an american circuit diagram and it worked a treat. The American diagrams show 2 lives, 110v but that's simply live and neutral over here.

I now have a workshop with 3 phase power and the lathe runs exactly the same as it did on the rotary convertor.

have to say though i wouldn't do it again, inverter drives are so cheap now its not worth the bother, and as a bonus you get stepless speed control.

Al.



Originally posted by alistairolsen
Im looking at mays of having a 3 phase supply in the garage, to run a lathe and ultimtely a mill and a compressor (all circa 2.5kW.

Ive been reading about rotary converters developed from old 3 phase motors online, but a lot of a sites are American.

Has anyone on here built one? Any links to guides or circuit diagrams or suchlike?

Cheers!

I made one, but in the end just resorted to some ex military 10uF capacitors grouped up to make the third phase.

Ran a 3hp saw quite happily, just had to add a few more UF when starting due to initial load.

The motor got a bit hotter but its not like you are running commercially so all's good.

3 phase stuff is really cheap and usually better quality.

You can get resonably powerful 3ph convertors for about £300-£500 depending on size....

One of those would be more efficient and safer... and it also gives you the bonus of being a proper speed controler for your lathe/mill etc

I've no idea on the design, but if you do decide to give it a go, I've got some huge (old) single phase motors in my workshop taking up space.

No idea how powerful, but the housings have got to be a foot in diameter.

Chris

hmm, sounds like being a pikey in this instance has too many disadvantages and saving up for the proper digital converter will be the best way!

its to power this:

My eventual opinion at the time I was considering making my own converter was "Life's too short, why make it shorter by electrocuting myself."

Also, a lot of these DIY converters are a bit of a faff to operate, especially as the generated phases can alter according to the amount of power taken by the load. My Transwave converter has two rotary switches on the front - On-Off and a multi-position switch to adjust the phases. The latter one was used once to get the correct value on the unit's voltmeter while the lathe is running, and I haven't felt the need to adjust it again - it really is that simple. If you're using it on 2 different machines (not at the same time!) then you may need to change the settings for each one.

You do have to buy the right size unit for the expected load - too small is obvious, but too large can cause problems as well (the unit has trouble generating the phases as it uses the motor windings as part of the phase-conversion process).

To make these converters truly excellent you can run a 'pilot motor' - this is a good-sized 3-phase motor that sits under the bench running continuously with no load. This makes it easier to use the converter to drive different-sized loads, as the pilot motor forces the 3 phases into good order, minimising the effect of stopping and starting, etc. It also allows you to run a coolant pump, if that is also 3-phase. The pilot motor isn't essential though.

Note that you CANNOT run a 3-phase coolant pump without the main motor running (or running a pilot motor) as its small motor is not big enough to generate the extra phases from the converter.

To summarise: phase converters are very good, but even they have limitations - in the end it's hard to beat a proper 3-phase power supply from the grid.