I would like to put a variable speed motor setup on a lathe. It is a small one and a .75 Hp will be just fine.
My power source is 220 single phase. I can also tap a 10HP American Rotary.
I have a TECO N3 VFD , a Lesson Speedmaster , and a Danfos 150. I do not have a motor.
Which type of motor would give me the least amount of < for lack of a correct technical term > harmonic or resonance showing up in the turned part as a pattern. When I switched my 12” lathe from single phase to a VFD controlled 3 phase I noticed a dramatic improvement in my ability to get a real fine finish. My 15” is on a single phase motor and it is a real pain to get a nice finish.
Would a DC motor provide any advantage over a 3 phase motor (given the above power sources) relative to the “pattern issue”?
Thanks
Bob
"Depends".
DC motors built specifically FOR smooth, variable speeds. machine-tool applications, are as good as such things get. Monarch 10EE on "rotating" power MG, early. Last ones made on multi-pulse Solid State DC off 3-Phase-only DC Drives.
But FIRST you need that type of MOTOR!
"Type T". Those had interpoles and interleaved windings. Less efficient at power conversion, but turbine-smooth. It was what they were FOR.
Smallest ones of that type I am aware of were about two HP (I have two), about the physical size of a TEN HP AC 3-Phase motor, five times as heavy, and easily TEN times as costly.
And then? If no utility-mains-grade 3-Phase to power such a creature, one has to jump though another large chunk of change and space budget barrier to tame a single-phase DC Drive's rudely hammering 120 Hz pulse-train.
"In between" you might try to salvage an electric MHE battery pile and motor, charge it 20 hours a day, run it 4 or fewer?
Or have a look at a "servo"?
Otherwise, go the El Cheapo road with a VFD and a six-pole AC motor, add mechanical ratios to let it run at base to base X 2, (60 to 120 Hz).
Smooth-enough, torquey enough. They surely can DO "slow", 30 or 15 Hz. They just don't do it as well as DC.
Belted ratio choices can handle that well and rather cheaply.
You may have noticed multiple ratios, geared, if not belted, are really, really
common on lathes?
PS: Smooth variable speeds, Monarch 10EE, Hendey Tool & Gage, Rivett 10X0, Nebel Microturn, Axelson Tool & Gage
et al was only a PORTION of how they could all deliver incredibly smooth passes.
The other part is that each of those worthies was about as stiff and rigid as a lathe two full sizes larger from lesser makers - high-precision spindle bearings included.
NOTHING you can do with a motor will substitute for what may be the absence of 2,000 lbs avoir - or MORE- of solid Iron. The lightest of these - 9" and 10" - went over 3,000 lbs, Avoir. And every damned bit of it put into an advantageous place by experts with a hundred and more years of learning what mattered, why, and where.
3/4 HP on a 15" lathe? A Hendey tie-bar, we have a fighting chance, otherwise...
Tell the PM community what you HAVE?
Odds-are there are more of them out here. Many, usually.
And "many hands" running the same lathes ...for Donkey's Years - already - or have done, at some earlier time in a long life.
That can give you a simpler answer that is KNOWN to work well ...
with that specific lathe.
No "magic".
But
good results without wasting time or money, EITHER.
What have yah got?