Torque vs Power - cutting ability

Not sure if this is the appropriate forum. If not feel free to move.

I understand that power = torque x speed.

But what determines a machines ability to take a particular cut? Is it torque or power?

In reading about VFD’s and induction motors, i think that at 60 hz (on a motor rated for 60 hz) a motor delivers its rated power. As frequency is decreased below 60 speed is decreased, torque remains constant, therefore power decreases. As frequency is increased above 60 hz, speed increases, torque decreases, and power remains constant?

If what is required to make a particular cut is power instead of torque, it would reason that slowing the speed/frequency would reduce the cutting capability. Is this correct when speaking of an induction motor on a VFD.

And how does this compare to reducing the speed of a servo motor? Does a HP rated servo motor deliver constant power throughout its speed range?

I see servos generally have a peak and RMS torque curve vs speed. But not sure how power would plot on that graph.

Why don’t induction motors have similar graphs?

I’m just doing some comparisons of induction motors vs servos.


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TravisR100 said:

Not sure if this is the appropriate forum. If not feel free to move.

I understand that power = torque x speed.

But what determines a machines ability to take a particular cut? Is it torque or power?

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RPM is going to be determined by material SFM and tool diameter, so the question is whether the appropriate torque can be provided at that speed. The appropriate torque is obviously going to be a function of the tool as well, so it's a big ol' equation with lots of variables.

In reading about VFD’s and induction motors, i think that at 60 hz (on a motor rated for 60 hz) a motor delivers its rated power. As frequency is decreased below 60 speed is decreased, torque remains constant, therefore power decreases. As frequency is increased above 60 hz, speed increases, torque decreases, and power remains constant?

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Yes. Usually torque at twice rated frequency is half that at the rated frequency. Check the motor data sheet to confirm.

If what is required to make a particular cut is power instead of torque, it would reason that slowing the speed/frequency would reduce the cutting capability. Is this correct when speaking of an induction motor on a VFD.

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See my point above about SFM. Unless your material SFM requires rated RPM or higher, the answer's no. If it does, the answer is yes.

And how does this compare to reducing the speed of a servo motor? Does a HP rated servo motor deliver constant power throughout its speed range?

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No, usually servos deliver roughly constant torque (it's usually slightly higher at lower RPM) up to their rated frequency, so output power is going to be RPM_x/RPM_rated x P_rated.

I see servos generally have a peak and RMS torque curve vs speed. But not sure how power would plot on that graph.

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As you said before, RPM x torque. In general, unless you're doing professional machine design, I would suggest you ignore the peak torque values for machining. Any machine will likely be primarily limited by continuous torque required, especially given HSM type strategies most people are using these days.

Why don’t induction motors have similar graphs?

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Because it's just a straight line for torque up to 60Hz. Note that many servo manufacturers provide data sheets with straight line torque values up to rated frequency as well. Very rare that the slight difference at lower RPMs is the critical factor in motor selection (at least in my experience).

This whole thing sounds like an XY issue. What's the actual problem you're trying to solve? I can only assume that since you're talking about AC versus servo it's for a spindle motor, but maybe I'm totally off base.

Just to add some simplifications - Power gets you higher speed cutting capacity, for instance good remaining torque at the cutting edge when running a 1/4" endmill at 25K RPM. Torque gets you the ability to plow a large face mill through a steel block at low RPM. But torque alone is useless without a machine 'system" that's stiff enough dynamically to maintain cutting edge to material relationships.

So a stiff set of castings, properly sized and adjusted ways or linear guides, good workholding, good spindle bearings and spindle housing design, stiff cutter body - all and more are needed to take advantage of that torque.

So if someone asks your question, I answer with neither - give me stiffness as the number one priority when it comes to machine design or selection, then power or torque depending on what my cutting goals are.

TravisR100 said:

Not sure if this is the appropriate forum. If not feel free to move.

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Not sure there is any better one - partly because there is already a good deal of discussion and "the math" already HERE w/r VFD's , Volts/Hz, what happens at very high or very low Hz relative to mains/nominal of 50 or 60 Hz

I understand that power = torque x speed.

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There's actually a constant in there as well to get the "units" we use most-often to work out:
HP = Torque x RPM ÷ 5252

But what determines a machines ability to take a particular cut? Is it torque or power?

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Neither one "directly".

It is the derivative of Torque @ some working arm length translated to the pressure applied to force the material against the cutting-edge of a tool so as to cause "work-hardening" to the point of catastrophic failure.

For a lathe, that "arm" is the radius of the material being turned into the tool. For a mill or drill, the radius of the cutter being turned into the material. For convenience, Machinery's Handbook & other sources publish tables we can use for common materials.

I see servos generally have a peak and RMS torque curve vs speed. But not sure how power would plot on that graph.

Why don’t induction motors have similar graphs?

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They do have. Lots of them. All motors do. Even grouped by amount of "slip", with synchronous at one end and "torque motors" at the other end. Those last can tolerate full stall and act as if they were an "infinite spring". You have met them any time an elevator door tried to close on yer body but stopped short of harming you until a sensor had time to cycle it for another "bite".



Those among us who had to do original design work used those curves often. Those who bought our final products didn't have to care.

Or may not be able to AFFORD to care?

Take a 10EE lathe, for example. ANY of the factory OEM DC drives are hard to beat from a cold start at a brand-new design.

Those blessed to HAVE any of the large frame (low RPM, very high torque), small frame 3 HP motors (medium-medium), or the 5 HP (higher RPM, lower Torque) motors have learned it is advantageous to keep them and repair the OEM DC drive. Or at least keep the OEM DC motor and use a third-party solid-state DC drive.

So why did the VFD + 3-Phase motor take-over so many other NEW machine-tools?

Simple enough. Just go and price a DC motor or either "Type T" or "Special machine tool duty".

About $12,000 USD MSRP for a present-day Reliance 5 HP DC motor.

Roughly ten times the price of a premium-grade 5 HP 3-Phase motor, and 40 times the cost of an economy one. I have a 10 HP 3-Phase Weg that was something like $387 brand-new with free shipping!

Those among us who have ANY of the OEM Monarch DC motors and any drive that fully powers it are the ones with the big s**t-eating grins, too.

They just work better.... and we didn't even have to go find twelve thousand bucks nor have to fab a gearbox adapter, either!



Servos?

Well. IF my 10EE was a CNC-critter, AND It had to position the spindle exactly at a certain angle so a live-tool, sch as a milling-cutter - could cut round stock into a hex.. or there was a tool-changer magazine that wanted toolholder drive-lugs to line up...then a Servo and its control system would be of use.

For just making the spindle on a MANUAL lathe go 'round and 'round? Not a feature much in demand.

Even so, some among us who had economical access to servo goods and the technical skill and experience as to how to use them well have DONE it.

Search You Tube, 10EE servo drive, Jerry Biehler, or here on PM member macona, for one of mebbe 4 or 5 who - in his case - also documented the project and then made You Tube vids.

YouTube

Kinda neat. Just not essential for what a manual lathe is asked to do.

Pariel, you are correct. It's for a spindle motor on a lathe. Not designing a machine, but looking at replacing the current AC induction motor with a servo motor. I'm trying to figure out if the machine will perform similarly to its original configuration if I do so.

And thank you for the explanations above.

The situation is I have a Rivett 1020S. It has the original 5 hp induction motor and then a Reeves drive before going to the gear box. I'm looking at the possibility of removing the 5 hp motor and Reeves drive and replacing it with a 4 hp servo.

The machine could originally be had with a 3 or 5 hp motor. So the power of the 4 hp servo isn't as much of a question as what the result of removing the Reeves drive and using the variable speed of the servo will have on the ability to make cuts.

If you just want variable speed, a VFD and an AC motor are the best way to go, from a cost/performance standpoint. A servo is unlikely to get you much value on a manual lathe.

The only time I would expect a servo spindle in a lathe is a CNC lathe with rigid tapping.

TravisR100 said:

Pariel, you are correct. It's for a spindle motor on a lathe. Not designing a machine, but looking at replacing the current AC induction motor with a servo motor. I'm trying to figure out if the machine will perform similarly to its original configuration if I do so.

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Already been done. Many times. By the makers even more than by those of us with "Old Iron"

Examples abound.

Historically Monarch Machine Tool hydraulic drive (Sundstrand) -> Ward-Leonard MG, then "hollow State" thyratron WiAD, then Module Drive then "Monarch Sidney" solid-state DC drive.
Currently Monarch Lathe LP with industrial-duty VFD + 3-Phase motor.

Lodge & Shipley, AVS DC drive => AC motor + VFD

Cazeneuve HBX 3-Phase + Variator similar to your Rivett, current "Optica", mostly same castings & weldments is a "Teach-in" Siemens CNC hybrid with servo-drive spindle. See also Okuma 'Teach in".

Hendey Tool & Gage used more than one drive system as well - some with PIV-Werner-Reimers clones, far more expensive than Reeves-types, but better, too.

IF.. you seek to replace what you have NOW with a modern "inverter duty" motor, such as one often seen among Monarch community who have taken the VFD Kool Aid? A Marathon Black Max is well-regarded and often found cheaply.

If you go that route, it is wise to either KEEP the Worthington Allspeed drive OR up the motor HP by fifty percent, MINIMUM.

With NO mechanical ratio-change help, doubling the nameplate HP would be wiser yet.

Servo is not a free-lunch, BTW.

ISTR Jerry's second-go at servo spindle drive on his 10EE used a TEN HP Servo motor and amp where OEM Dinosaur Current motors had been 3 HP to 5 HP.

That's all in PM's archives from a few years back.

Unfortunately the particular servo I’m looking at maxes out at 4 hp.


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And the servo I’m looking at has no positioning capability.


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Not an electronics guy but you would definitely be loosing all of the torque multiplication that the reeves drive would provide at lower speeds.

TravisR100 said:

Unfortunately the particular servo I’m looking at maxes out at 4 hp.

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Anemic. Go find a decent used 10 HP as jerry did for the 10EE. Or don't bother.

War Two was already underway when Reliance & Monarch had to adapt an existing ELEVATOR motor and Ward-Leonard drive to the 10EE ... so Sundstrand could go away and make higher-priority hydraulic goods for combat aircraft & such.

They cheated.

Several among us had measured them driving their nominal 230 VDC 3 HP motor at 245 to 265 VDC. I run mine around 270-275 VDC, "peak only", then over-drive the Field to 140 VDC vs nominal 115 VDC as well. About 4.5 HP, IOW. Mind - I keep a spare motor to-hand.

But.. remember that HP/Torque formula?

KEY is to know at what ***RPM*** the HP was nameplated.

A mere 670 RPM for the large-frame 10EE motor. Old warhorse has Torque that is damned good vs a 5 HP stated at a much higher RPM - 1150 tot 1750 and such.

I did mention "s**t-eating grins?

TravisR100 said:

And the servo I’m looking at has no positioning capability.

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Well, the PWM does give you some 'stepping' capability, but without an encoder and controller it's not much use. I guess 90% of the time today, 'servo' really includes both of those things. Note that the only lathes that use that kind of servo motor are low HP imports (I think because they can take advantage of pricing on motors used in treadmills).

thermite said:

KEY is to know at what ***RPM*** the HP was nameplated.

A mere 670 RPM for the large-frame 10EE motor. Old warhorse has Torque that is damned good vs a 5 HP stated at a much higher RPM - 1150 tot 1750 and such.

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Yeah, that's a surprisingly low RPM and I'm not surprised that you need a much higher rated modern AC motor to get the required horsepower. Maybe leave the Reeve's drive on there, and modify things to get the newer AC motor working with it. Frankly, given the cost of good condition used motors, just going to 10HP is likely way less work for minimal increased expense.

Torque.
Two examples of younger day fails:
I have 3HP grinders made to run at 3250 with VFDs attached.
Need to cut a form in a 6 inch 4140 steel blank for a custom plated wheel.
Since the machine is 4 axis cnc just mount up a good carbide bit on the chuck and use the VFD to get down to 350 RPM.
A nail file would have been quicker. Stalled if you looked at it.

B-port, well let's leave in on the highest pulley and use the VFD.
Go way down on the VFD and try to drill a 1 inch plus hole. Same sort of 10:1. Same sort of progress.

The force to make a chip is a certain number of pounds, ounces or pick your units. All based on tensile, geometry and others. You have to make the material yield.
Then there is the arm length this force is applied over which makes gearing your friend.
The forum is packed with posts calling out "Hass-power" when in fact their claims are correct.

There are so many different flavors in motors and drives. A given cutting tool does not care as it has to exceed material yield.
Bob

thermite said:

Several among us had measured them driving their nominal 230 VDC 3 HP motor at 245 to 265 VDC. I run mine around 270-275 VDC, "peak only", then over-drive the Field to 140 VDC vs nominal 115 VDC as well. About 4.5 HP, IOW. Mind - I keep a spare motor to-hand.

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Ah but are they quick-change? Visions of NASCAR rattle guns in operation.....

CarbideBob said:

Torque.
Two examples of younger day fails:
I have 3HP grinders made to run at 3250 with VFDs attached.
Need to cut a form in a 6 inch 4140 steel blank for a custom plated wheel.
Since the machine is 4 axis cnc just mount up a good carbide bit on the chuck and use the VFD to get down to 350 RPM.
A nail file would have been quicker. Stalled if you looked at it.

B-port, well let's leave in on the highest pulley and use the VFD.
Go way down on the VFD and try to drill a 1 inch plus hole. Same sort of 10:1. Same sort of progress.

The force to make a chip is a certain number of pounds, ounces or pick your units. All based on tensile, geometry and others. You have to make the material yield.
Then there is the arm length this force is applied over which makes gearing your friend.
The forum is packed with posts calling out "Hass-power" when in fact their claims are correct.

There are so many different flavors in motors and drives. A given cutting tool does not care as it has to exceed material yield.
Bob

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I'd be using a servo, not induction with VFD. But I imagine the same principles apply.

Define servo to me , high pole count, AC, DC, steeped DC?
Just go ahead and try it if your head says maybe a good idea. It may be perfect or a new way to get things done.
Good or bad things learned and I sure as heck learned by doing not the best ideas.
There is a definite downside to listening to old farts and old ways when you go exploring.....
Bob

TravisR100 said:

I'd be using a servo, not induction with VFD. But I imagine the same principles apply.

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Low RPM = roughly constant torque for both, so you're correct, same principles apply.

If you're sticking with the DC motor, it sounds like you're going to need a gearbox -- can you modify the motor mounting so it still works with the Reeves drive?

This motor...

CPM-MCVC-N0563A-RLN_Fan | Shaft diameter = 0.6250in | Not Including Shaft Seal | Teknic

TravisR100 said:

I'd be using a servo, not induction with VFD. But I imagine the same principles apply.

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More tnan you might think. But maybe not the WAY you'd expect.

One of our members, 9100 Bill, took the time to count the poles on a large-frame Reliance 3 HP, Type T as the MG-era 10EE used. Type T have the sort of winding pattern meant for smooth operation over a wide RPM band, two sets of brushes, and LOTS of commutator segments, four field poles with four "interpoles".

Observation? Didn't have the exotic "servo Amplifier" as control, nor even a simple "H" bridge as Crydom illustrates for clever use of their packaged Solid State Relays as poor-boy servo control.

Even so, off the back of lots of Iron and Copper, War One era technology, it is near-as-dammit a "servo motor".

Now .. AC or DC, electromagnetism is wot it is, AC motor or DC, no nevermind in the air gap. Each pole only generates but so much pull.

But.. an external "clock" that passes a low-Hz current to the motor keeps moving that sine-wave.. whether the rotor could grab tight enough and hang-on to follow it or not. "Slip" ensues. It HAS to do.

Low-end - even locked-rotor is where the different nature shows.

DC motor cannot slip. its brushes and commutaotr segments to the coils are integral, have a fixed relationship. It will draw all the current one has provided Voltage enough to drive up to the point a protective device stops that suicidal silliness.

No such protection?

Move the load.

Or die trying, "wedging" the suffering commutator segment(s) under the brush from serious thermal expansion.

One Reliance "white paper" measured torque of 4, 6 - even NINE TIMES the nameplate figures. A 3-Phase motor, DOUBLE is about the upper bound.

Rule of thumb?

- High RPM, advantage to the VFD. If nothing else, they regulate RPM better with varying load.

- Low RPM, advantage to DC.

- Base RPM or close-to, above OR below? Makes no real difference.

TravisR100 said:

This motor...

CPM-MCVC-N0563A-RLN_Fan | Shaft diameter = 0.6250in | Not Including Shaft Seal | Teknic

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If you're going to run this straight off a potentiometer, getting it hooked to the Reeve's drive is quite likely the easiest and cheapest method. I'm not sure what level of control is integrated on that motor, so you'll have to figure out the total cost.

You can probably get a brand new 10 HP motor and VFD for <$1,500, as a comparison. If you're willing to go used (which is the direction I would go, given your machine), probably 1/3-1/2 that.