single phase to three phase rotary converter

[boaty pete]

I am currently building an A.A.Leak 3&5x3 compound engine. I have recently purchased a 3 phase Beaverpal milling machine. I have sort of got my head around building a 240v to 3 phase rotary converter using start and run capacitors and a larger 3 phase motor[idler] as a generator.
My problem is that my mill has 5 wires [ie 3 active, an earth and a neutral.] By some miracle the single phase neutral becomes an active leg on the 3 phase idler motor. Because it is now active I am reluctant to connect the other fifth wire neutral to it.
A bit afraid of blowing myself up. It may not come as a surprise to reveal that I am not exactly an electrician.
There plenty of info around on the 5 wire US of A conversion but they don,t seem to have a neutral. The U.K. Kiwi,s and the Aussies all have similar systems. I know that plenty of members have bought mills [although most are 4 wire] and I am therefore wondering if they have solved the five wire/converter problem. I should add that VFD is not a real option as the mill has four other 3 ph motors including a slotting head plus a retrofitted 240v/110 X axis drive[hence the need for the additional neutral wire]. The motors and idler are all wired in delta. Would certainly appreciate any advice or relevant details,
thanking you Peter
Have sorted out the neutral wire problem by sourcing the 240v active and neutral from a separate terminal. Its taken awhile, had a few dramas, [ a few big puffs of smoke] but the mill is finally running smoothly off the rotary converter.
Thanks for all the advice Peter

[cyberbadger]

There plenty of info around on the 5 wire US of A conversion but they don,t seem to have a neutral.

Sounds like the difference between a Wye three phase motor and a delta three phase motor.

wye_delta.jpg (47.17 KiB) Viewed 13979 times

R1 should equal R2 which should equal R3, same for Ra Rb Rc (each is a coil or series of coil in the motor).

You can use your volt meter to figure out which configuration it is.

Some combinations of resistance should equal to R, and some equal to 2*R - that will help you find that common in the center if it's a wye configuration.

Hope this helps.

-CB

[marinesteam]

With the cost of VFD's as low as they are now they are usually a better solution than a rotary phase converter.

Ken

[Oilking]

A VFD also has the added advantage of infinite speed control across its range of operation.

Have fun with the new toys

Dave

[Bob Cleek]

I'm no electrician, either, and certainly not in any way familiar with anything other than US electrical codes, but I do have 3-phase 440 VAC in my shop building and managed to split off 220 and 110 VAC circuits from it without killing myself. That said, and this is just a thought, your mileage may vary, but it may be far easier and more practical to simply repower your mill with a 110 or 220 VAC motor and be done with it. That would, at most, cost only a few hundred bucks and when the time comes that you or your widow decides to sell the mill, it will be a lot more marketable... or at least that would be the case in the US, where 3-phase is only found in commercial and industrial applications and almost never in a residential application. For this reason, here in the US at least, high quality stationary machinery can often be had for much less than the equivalent machine running on 110 or 120 VAC. (Here, a 3-phase installation drop from the main requires two dedicated transformers at the pole for which the utility companies charge dearly. 3-phase industrial service is far less expensive per KW hour, but the huge installation costs to install the drop aren't justifiable for anything other than commercial applications that use a lot of KW hours.)

[cyberbadger]

Boaty pete,

Ok - so it's a Delta.

By some miracle the single phase neutral becomes an active leg on the 3 phase idler motor. Because it is now active I am reluctant to connect the other fifth wire neutral to it.

Well the way I see it there are two potential issues.

First is ... Can you wire it to work as desired with no shorts. Second is can you wire it so that is safe to operate, so that anything you touch on the mill is earth.

If you can assure those two things with your multimeter (using the resistance setting again) I think it would safe enough to try.

You want to make sure none of the 3 phases have a very small resistance to each other- #1 and that none of the 3 phases has a very small resistance to earth - #2.

If either of those two things aren't true I'd get professional help, otherwise I'd carefully try it.

-CB

[johngriffiths]

Using a 230/240 supply and a variable speed controller you need to have the windings as per the left side of the drawing eg delta. Most motors have the terminals and six wires present in the cable joint box so you can swap between the two arrangements. Some motors only have three wires being either delta or star and unless you are really determined and can find the joint/joints on the coils and rewire them it is best to leave them well alone.

If you use an auto transformer single to 3 phase converter which delivers 415/440 then the motor needs to be wired as per the right part of the diagram, that is star and you ignore the neutral unless the machine has control gear which may require wiring from one phase to neutral. Read the plate on the frame/side of the motor first to check voltages and the instructions which come with the converter.

In case you are wondering, in my shop the Bridgeport works off a variable speed controller whilst the Harrison mill, with its control gear, and a couple of other things run off an 8hp single to 3 phase converter which weighs about 1 cwt and has a 30A supply.

If in doubt, get it sorted by someone who knows what goes where and why.