3ph vs 1ph, or transformer + converter for lathe

[laurencen]

have a similar setup, use a 15hp rotary, my mill is 220v3phase, compressor is 220v3phase, I use a step up transformer for both 440 and 575, the 440 is mt small tool mill and 575 the TOSS lathe, we run any 3 machnes at the same time on a regular basis

something to keep in mind, on my lathe the DRO is wired into the panel, there is a coolant pump which is high voltage then all the starter relays even though they run 220volt single phase run off the internal transformer, if you drop the motor voltage it is not as simple as feeding the panel with 220volt

 

[Sawyer Rob]

I would keep it simple and just get a single phase 240 motor. A 5hp motor drawing peak is only going to draw 15.5amps, so you can plug it into any 240vac 20amp receptacle. Makes for relocating in the shop pretty easy. I知 assuming this is just hobby stuff, given that youæ±*e humming and hawing over a few hundred dollars.

That would be a very weak 5hp motor!!

MY 5hp 240v single phase draws 25.5 amps on 240, now that's a "properly built" 5hp motor!

SR

 

[marchanna]

I just did a quick calculation: 5hp = 3.73kW
3.73kW / 240vac = 15.5 amps

I understand there are also efficiency losses there and peaks.

 

[Mendonsy]

My handy Square D motor data card says a typical 5 hp 3 phase is 17.5 amps and 5 hp single phase is 28 amps. I'm sure some of the "premium efficiency" motors would be less, but not that much less.

 

[crazyal]

I would go with a single phase motor. Simply put if you find the lathe doesn't work out well for your needs (or your needs change) then you can sell it much easier.

 

[the old grind]

3 PH motor(s) assist (stabilize) RPC when idling & not loaded. Most don't need another RPC for that reason. Balancing legs can be tricky if one RPC is used by itself for multiple machines of different hp, thus running several machines in parallel and 'no load' can be done. (min amp draw w/o load) A 1:2 3 PH transformer 'after' the RPC shouldn't be a prob. Go to Practical Machinist and ask, where many of us are also members. Start here, and ... don't use my log-in. :laughing:

Transformers, Phase Converters and VFD

 

[GLyford]

Next silly question:

How often are you going to be doing big hogging production cuts on that lathe like it was originally designed for?

If you are doing regular run-of-the-mill dabbling and hobbyist work, could you get away with a 3 horse motor? Might take some of the sting out of buying a different one, as well as being easier to find?

I had an ancient flat belt 12" Hendy that the previous owner had a 1-1/2 horse motor on, which he wanted to keep...but he had a spare 3/4 horse motor which he threw in. I never stalled the motor. I slipped the belt a few times, but the smaller motor was more than enough for anything I wanted to do.

 

[the old grind]

^^^ This!!! You don't always need the power it'd take to challenge the rigidity of your tool-post. :laughing:

Be aware that hp of some SP (split phase) labelled 'compressor' are rated at their 'stall' torque vs continuous, so a '5 hp SP' may be 3 hp if rated for continuous duty. My 2 hp (8.5 amp, 1725rpm) 12"x 36" import breaks tooling, never stalls or slips, and the motor is connected by a single 'b' belt, neither doubled or a toothed 'Gilmer' as other versions. (Enco, Grizzly)

I'd put a 5 hp on that Monarch in a minute, given the right one. t o g,

still spinnin' handwheels ..

 

[the old grind]

Uh, regarding what I said about 5hp motors (and what others said about 15a vs 25a) ... Surplus Center catalog came today and examples are on p. 150.

Not entirely apples to apples, as the 'compressor duty' is 2-pole (3450 rpm) vs 2 & 4-pole above but the text explains the hp/duty diff.
View attachment 543752

 

[LD1]

Yep, well aware of the misleading "compressor duty" ratings.

I had an old motor that was a 5HP that was maybe 20 pounds....and either 120/240:laughing:

Unlikely the belts will ever slip....even with 5HP. First, its only a 1750rpm motor. And it has a ~8" diameter pulley on the motor....and ~11" on the head. And its driven with THREE BX belts. We have 40HP applications at work that only run 3 b-series belts.

I dont want to go down on power. 3HP vs 5hp is not much cost difference. And doing so would probably render the heaters useless.

I have decided to keep it 3 phase and run off the rotary. The old furnace electric reversing contactors + heaters already wired....didnt really want to have to redo all that for a single phase reversing motor. And come up with some other sort of O/L protection...all adding to cost on already a more expensive motor.

3-phase stuff is cheap in comparison. The control voltage xfmr was dual voltage. So changing a couple of wires and its fine with 240v input. just the motor that couldnt be swapped.