The Home Machinist!

Heya,

I'm working on trying to figure the best way to power a lathe (3 phase, 3hp, 380/220v) -- I've ruled out VFD's as it'd be a significant amount of rewiring internally - I know a rotary phase converter would work and be a simple plug and play (but it's 1500€) -- I'm looking for a way to plug the lathe in upstream of everything where the existing plug is. I've come across an option I don't understand -- a "digital phase converter." I wrote the fella whose selling them and he sent this back

"The converters I have are digital not static they have more starting torque are more efficient and run multiple machines at once with out selecting hp etc, they are plug and play type. The 4kw would suit you as you have to oversize them slightly."

This seems all too good to be true -- anyone have any experience running a lathe off of one of these?

Jonah

as per all sales descriptions:

Money back guarantee if you are not happy?

I started out using a rotary phase converter and switched to a "Phase Perfect" digital converter. I've been using it for many rears now to run numerous machines and would never use anything else again.

which is good to hear.

in reality...their product looks like a VFD without the "V" -> which makes sense...

I don't understand how a vfd would require rewiring. Single phase comes in. 3 phase goes out.

A standard VFD gets very unhappy if you disconnect the motor while it is running. This means that you cannot use the installed motor controller to control the motor. You could run the power through it, but you'd have to be careful never to hit STOP. You have to use the controls on the VFD. This means that even if you never vary the frequency (which you must not do if anything but the spindle motor is on the VFD) you would be shutting down everything on the machine every time you stopped the spindle. This probably does not apply to the fixed frequency digital phase converter.

Any phase to neutral loads have to be rewired (though these are rare on machine tools).

Some electronics runs poorly from a VFD due the high harmonic content.

What else could a lathe have on it besides a coolant pump that might be 3 phase ? Otherwise just use the forward and reverse contactors to enable the VFD into forward or reverse, that takes 3 wires. yes you will want a master on/off/e-stop but that is simple as well and just a couple 2 or 3 wires. Those wires can be small and in a shielded cable. If I needed the coolant pump to run and it was 3 phase I would just get a small vfd and have flow control via speed.

I don't recall the brand but I recently looked at a fairly new large (125 amp rated) commercial type add a phase that is already having issues. The incoming power capacitors have to be checked and replaced if out of spec every year, the manual suggests 6 month intervals is even better depending on the quality of utility power. The owner has several of these units in various locations and so far this is the only one to have issues. It is using electronics to make 3 phase, and you would not want it in your shop close to you as it makes a whiny Locust at night sound.

I had a long chat with the electrician selling the digital phase converters and I've decided to go ahead and buy one. It's half the cost of the same rotary phase converter, so that was the big selling point He said that he mainly sells them for folks to run three phase lathes and hasn't had any issues in the 5+ years he's been selling, seems honest enough. Thanks for the all responses -- i'll let ye know how i get on with it..

What is this “digital phase converter?” It does not sound like a Phase Perfect, which costs about three times as much as a commercial rotary phase converter (RPC). You can build a fine RPC for a couple hundred $$ based on a used three-phase motor. Google is your friend for DIY RPCs. PracticalMachinist.com has a sticky post with plans.


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Well the digital phase converter has arrived. It seems to be a vfd with a transformer to step it from 220v to 440v. I have a question about something in the manual -- (The electrician whose selling these ensured me a million times over that they are "plug and play," can simply be wired upstream of the whole lathe and he's loads of people running lathes off of them.

but the manual says...

"If switching devices like contactor are installed between the output end of the inverter and the motor, it should ensure that the on/off operation is conducted when the inverter has no output. Otherwise the modules in the inverter may be damaged."

Am I correct that a lathe would have contactors that are used to switch the motor on and off? So it would be an issue to wire it upstream of the whole machine and not bypass the contactors and go direct to the spindle motor? Think I might have gotten talked into buying a very over priced VFD

Yes, your lathe's contactors will switch the motor power on and off, which will be contrary to the phase converter's instructions. I can't say for sure whether you'll fry the phase converter if you do this, but I would not expect warranty coverage.

Any idea on how one could ensure "that the on/off operation is conducted when the inverter has no output"? I'm grasping at straws here but could one just hold the lathe on button on when starting the phase converter?