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DC or 3 phase motor for a Lathe

piedmontg

Aluminum
Joined
Dec 10, 2007
Location
NW Illinois
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
 
The VFD is well-known and works well too. And the 3phase AC motor is probably the cheapest of all types, available in all mounting styles. DC is the most expensive (other than surplus treadmill motors) and is much more available in 120V (90V DC ) than 240V.

Yes, DC can be used, and has been. Still is, by some. But they tend to be folks with a tolerance for "fiddling". A VFD and 3 phase motor will be smooth, very usable, "standard", and the least hassle once it is set up and working. It can take a bit more setting up, but it is all putting in parameter values, not hardware part swapping.
 
Use whatever motor you can get that has a lot of rotational inertia . . . this will give the best finish whether 3-phase induction motor or DC brush motor.
 
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?

:D

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?
 
DC motors are actually AC motors with a built in mechanical inverter.

Single phase AC motors only get two kicks per cycle with spaces in between them. They are inherently rough. Three phase motors get six pulses per cycle, making them smoother. DC motors can have many more, depending on the number of bars on their commutators. DC motors can be smoother as long as they are fed from a smooth DC source. The motors on thyratron controlled 10EEs are an example of a good DC motor fed poor unfiltered rectified AC. If you slow one way down in high gear, the pulsation becomes very marked. The motor generator 10EEs are an example of a good DC motor fed essentially clean DC and do not suffer from the problem.

A good servo motor will give smooth full torque down to 0 RPM although on most motors you cannot hold it there for long without damaging the armature windings.

If you want the maximum smoothness, use a servo type DC motor with a well filtered power supply.

Bill
 
I see where my note was confusing, not putting a 3/4HP on 15” lathe. My 15” Triumph 2000 has a single phase 3hp motor on it (really need to change it to 3ph). My 12” Clausing has 2HP 3 phase with a TECO VFD real nice. Have a Bridgeport and a Clausing mill both with VFD’s real nice. Other tools run off the 10HP American Rotary.

As I mentioned I have a smaller lathe that I use and was just thinking of making the speed changes a little easier. It is a very well equipped Myford Super 7B, rather light weight but nice for small stuff or when there is something I do not want to move in the other lathes. It calls for a 1725 if I want the plate speed numbers to match. It has an easy belt change system and nice clutch system.

Based on the comments I have received I think I will just stick to the VFD solution.

I did some homework as I did not know what “rotational inertia” meant. Looked it up and kind of got it, now how to know that a motor has it? Found a document by WEG that has 40 pages of motor calculations, at this point I filed that away for reading. Tried various combinations of searches and really got no where finding any way to tell if a motor had it. Obviously I need to be better educated.

I found that 6 pole motors have an RPM of 1200. If I can run the VFD up to 88Hz ie approx. 1725 I would have the equivalent. However most of the work on this machine would be in the 300 to 900 RPM the 1200 speed would really not be an issue. Looks like a Baldor M3543 would be an option.

Any additional information would be appreciated.

Thank You

Bob
 
- 15” Triumph 2000 has a single phase 3hp motor on it (really need to change it to 3ph).

- 12” Clausing has 2HP 3 phase with a TECO VFD real nice.

- Myford Super 7B, .... calls for a 1725 if I want the plate speed numbers to match. It has an easy belt change system and nice clutch system.
All decent value-for money, not junk, and - to put it bluntly - "are actually WORTH a bit of f**king around with"

. Thanks for that clarification.

Worried you were trying to get 10EE class surface finshes off a Whorer-Fright grade Lathe Shaped Object by crazy-gluing a bustid treadmill motor to it!

:)

Based on the comments I have received I think I will just stick to the VFD solution.
.
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However most of the work on this machine would be in the 300 to 900 RPM the 1200 speed would really not be an issue. Looks like a Baldor M3543 would be an option.

8-poles can get gnarly. The ranges you use, I do b'lieve a 6-pole on a decent VFD will best hit the sweet spot inexpensively.

Even IF.. the motor is "Inverter Duty" rated?

I'd also grab a TCL "Sine Gard" 3-phase filter ELSE a GE/FANUC, dv/dt since both are cheap and common, used-but-good or even "NOS"....as-are a coupl of other respected brands.

... and put it between the VFD output and the load motor. VFD manual will have the value, usually 3% and 5% - which isn't even all that critical.

I am old enough to remember when Baldor still made good motors.

Nowadays, if I cannot find a Reliance in-budget, I just keep looking.

Or use a Weg ...if it is an emergency.
 
DC motors are actually AC motors with a built in mechanical inverter.

Single phase AC motors only get two kicks per cycle with spaces in between them. They are inherently rough. Three phase motors get six pulses per cycle, making them smoother.

........
Bill

Actually, the power flow and torque is just about perfectly smooth with 3 phase, marred only by any portions of the motor or supply that are not ideal.

You can calculate it out, there is NO torque variation with 3 phase in a perfect system.

Single phase has 100% torque variation, max to zero to max, and DC motors have whatever their design and commutator allow. The "mechanical inverter" can be good, "OK", or crummy.
 
Actually, the power flow and torque is just about perfectly smooth with 3 phase, marred only by any portions of the motor or supply that are not ideal.

You can calculate it out, there is NO torque variation with 3 phase in a perfect system.
Not until you calculate it out, anyway.

:D

Three 60 Hz (or 50 Hz.. or 15 Hz.. or 400 Hz,, or...) Sine waves, evenly distributed are surely better than one, but when the need is more demanding yet, technology simply fills that more demanding need. BFD.

And I mean polyphase AC. Not DC.

3-Phase is nice. A "sweet spot", even. Most "goodness for the buck" for the most economical use of conductors.

But it surely ain't the end to technology. Nor are the most common of servos.

What mankind is CAPABLE of gets used only where we must.

Otherwise, EVERYTHING has to meet the harsh reality of economics.

What we can actually AFFORD, not just WISH we could afford.
 
Bill

Thank you for the added information on a DC motor. Maybe I will still think about it. As all of this is somewhat an experiment and not a necessity I will add it back into the hunt. I will have to check out servo motors, know nothing about them but time to learn.

Thermite

Thanks for the additional info, OK the filter will go on the list for the VFD approach did some searches not much luck with a Sinegard, did find a dv/dt for 3A that weighs 13# and is about 7 x 8 x7 its big.

Have not purchased many motors in the last 20 years, mostly used ones. I had been told to watch out for Baldor, reason for the number given was I had an old Baldor catalog and it was the easiest to find one for an example.

Bob
 
DC motors are actually AC motors with a built in mechanical inverter.

Single phase AC motors only get two kicks per cycle with spaces in between them. They are inherently rough. Three phase motors get six pulses per cycle, making them smoother.

You have been saying that forever and you have been wrong at least as long.

First, it isn't a square-wave pulse or "kick". Just change the time-base on your oscilloscope. Please.

That would be a Ledex rotary solenoid or a stepper motor, not an AC induction motor.

What we get is a smoothly rising and falling Sine Wave. Neat stuff, Sine waves are.

Secondly, the approach and departure alignment of the poles and/or the effect of inductance in stretching-out rise and fall time of an electromagnetic field will not even support transitions all that abrupt.

The motor itself will have an effect on integrating the "packets of energy" under the operating curve. And then it also has MECHANICAL momentum & inertia. Add-in the drivetrain, workholder, mass of the work itself.

The forces are distributed "smoothly enough" over time for most real-world applications.

The result is that when a designer builds a motor for single-phase, it will be as smooth as single phase can be without sacrificing too much efficiency.

The proof is all AROUND us that they do NOT operate at 60 "BANGS" per second at all.

Their shortcomings are more basic:

- First, for machine-tool use, they are duecedly gnarly to reverse on the fly, if even it can be done at all. Read; NOT.. for MOST. Also no fun to wire for reversing, even when done at rest.

-Secondly, they tend to fall off their best behaviour further below max load rating than 3-phase goods, making their fewer sine waves obvious. Whereas a 3-Phase stays smooth right up to max nameplate loading or even a bit beyond.

Points to the 3-Phase, surely, but not because a single-phase motor mistakes itself for a tire mechanic's air-powered wrench.

They are not "bad".

Just "less good".

:D
 
You folks can compromise on "kicks".... they are kicks, but with soft rubber soled shoes.....:D

But 3 phase really does have constant power/torque transmission, and single phase really does have pulsations. So while that is not too important with vacuum cleaners and other household stuff, it is not too great with some industrial stuff.

The pulsing torque of single phase really does tend to input a vibration to the machine, leading to increased chatter on lathes and similar issues with mills and grinders. Result is worse finish quality.
 
You folks can compromise on "kicks".... they are kicks, but with soft rubber soled shoes.....:D

But 3 phase really does have constant power/torque transmission, and single phase really does have pulsations. So while that is not too important with vacuum cleaners and other household stuff, it is not too great with some industrial stuff.

The pulsing torque of single phase really does tend to input a vibration to the machine, leading to increased chatter on lathes and similar issues with mills and grinders. Result is worse finish quality.

Folks who spout pre-conceived notions rather than basic grocery-store arthmetic analysis "really does tend to input vibration" ...into the information space.

3-Phase is still the same Sine wave. Three not one. Same wavelength and periodicity, too, not at a faster pace.

But if it is a "kick"? It is still a "kick" times-three, same shape.

Except of course it is NOT a "kick". Sine wave, integrated in the current realm is much more like pulling the oars on a boat. The greatest power is at a certain range of points along the full stroke of the oars, but the entire "wet" portion of the stroke is conveying "some" power.

Tell me, please:

Which is it as has what count of "rubber soled shoe" kicks per revolution.

A TWO pole three-Phase motor?

Or a SIXTEEN pole single-phase motor?

Not airy-fairy theoretical. Common as dirt and almost as old... as electrical goods go, anyway.

Ceiling fans, Pilgrims. Ceiling fans... decent ones are quiet as can be. Even cheap-ass Chinese ones ain't half bad.

And there went your pre-conceived notion, as to vibration being solely the product of phase count, yah?

There was a reason I suggested the OP use a SIX pole motor to replace a four-pole after all.

OK. I do cheat. Reading thing. I don't "invent" this s**t. I go and look it up!

I did say "cheat?"

:D
 
More poles will give you more pulses per rotation. It will not give you more pulses per cycle of the 50/60Hz mains. So while you might get more pulses per revolution of the motor, gearing it back up to the desired speed will separate them out again.

With a full-wave three phase rectifier, the output voltage is the highest line-to-line voltage at any time. Because of this, you still have a pulsing effect because you are essentially running off single phase power, just switching between which legs you use. You get the characteristic six pulses. Obviously further smoothing can help this, and it is already far better than rectified single phase. Very large rectifiers (in the MW class) use dedicated transformers with extra taps to get 12 or 24 pulses per cycle.

Three phase motors... in theory the delivered power is continuous as the sum of the squares is always continuous. Full power on one phase corresponds to 0.25 on each of the other two. One phase crossing through zero corresponds to 0.75 on each of the other two.
 
More poles will give you more pulses per rotation.

That IS the goal, yes.

It's a lathe, after all.

The choice of RPM will be dictated by diameter, alloy, nature of the cut, and the tooling. The motor is meant to deliver to that need.

Not the other way around.

Why would one seeking a smoother surface finish NOT want more "impulses" and more closely spaced per EACH revolution of the workpiece?

That wasn't really all that hard was it?
 
More pulses per rotation of the motor.

If you are using a fix frequency supply, then you will need to set the gearing appropriately to still get the same spindle speed.

Considering single phase motors, you will get 120 pulses per second regardless of how many poles the motor has. You get more pulses per rotation because you have fewer rotations.

Gear/belt that through to the spindle, and the number of pulses per second is still 120. Given that the spindle RPM is now the same and the pulses per second are the same, the pulses per spindle rotation will be the same.

If you're using a VFD, and overspeeding the motor, this is different. I didn't think we were considering that given single-phase motors were being thrown about.

If you want the added benefits of more rotor mass, nothing is stopping you putting a flywheel on there.
 
You can smooth things out a lot with a flywheel. I built a shock dyno, used a 20hp 3 phase motor with a vfd. 5' 3 groove pulley driving a 17" pulley on the jack shaft driving a scotch yoke with 1", 1.5" and 2" of shock travel. you could see the 3 ph sine wave in the data until I added a 15" diameter 125 lb flywheel to the jack shaft.
 
….as the discussion wanders way off the track from the OP...

Put a treadmill motor on it*, comes with a flywheel, surplus center sells/sold these....:D

* I'm just joking, go with 3 phase and a standard inverter.
 
The real answer to the original question needs to have another aspect added to the engineering discussion. It's not an engineering equation unless one of the variables has units of "$." The person needs to prioritize his goals - smoothness, variable speed, supply power, fit to the machine in question and so on, and determine what he has on hand to these goals, and how easy it and how costly, to get the parts needed.

If he has time, the story is 'target of opportunity:' wait around until what you want comes available cheap, near where you are.

1) the demon of 'funny marks on the workpiece' from a single phase motor, is always overstated. Typically by those who use dc drives. IMO.

2) if he is going to have other machines with three phase motors, a rotary converter can be considered, if variable speed is not paramount.
 








 
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