Talking lathe power

[Roger Chandler]

It seems problematic if one does not have a degree in electrical engineering, to make apples to apples comparisons on lathes offered on the market because the different companies put out specs in different ways.........one example is the horsepower rating.

I have been doing some research on my own trying to discern/understand electrical terminology, but I find myself lacking in understanding some basic symbols and formulas used to derive certain information to calculate properly the power on a motor for a woodworking machine.

One thing I do understand is the standard definition of "full load current" [I think ] At any rate, a reliable definition seems to be the following:

"The "full load current" is the current taken by a motor when it is delivering its full rated output power."

Now, some questions to see if we have enough collective knowledge on this forum about electrical issues.

1. The full load current being the current taken by a motor when delivering its full rated output power would speak to efficiency of the motor..... correct? For example if one has 2 motors rated at 5 hp, and one has a full load current of 20 amps and the other one has a full load current of 12 amps.........does that mean the 5 hp motor that only draws 12 amps is a more efficient motor than the 5 hp motor that draws 20 amps?

2. Is full load current rating a reliable way to do an apples to apples comparison between motors?

3. How does a 3 phase motor stack up against a single phase motor relating to efficiency and the way power is delivered? I realize the use of a VFD will enter into the picture here, but how does the inverter break down the single phase power and turn it into usable mechanical energy and does the use of a VFD automatically mean loss of power from the horsepower rating of the motor?

[Mike Goetzke]

Make some popcorn and grab your favorite beverage then look at this :

http://www.engineeringtoolbox.com/el...wer-d_653.html



(edited: I recently restored a 1950 Uni with a single phase RI 1-HP motor. From my usage experience cutting 8/4 hardwood this saw has as much power as my 2009 vintage 3-HP Uni)

[Thom Sturgill]

Motors are generally designed to run between 50 and 100% of their rated load with max efficiency at 75%. Efficiency drops off quickly below the rated load. So a 3hp motor is rated to run between 1.5 and 3 hp load and is most efficient at 2.25hp load. Another term is motor power factor which describes how much power is generated vs percentage of full load current. Typically the larger the motor the more efficient. Both generate graphs that are needed to fully understand the motor's performance.

Since I turn smaller items, I worry that this motor is just way over-powered for my needs. If you are operating at the lower end of the power spectrum - say turning boxes and ornaments - then efficiency is way down on a large motor. You really need to see the power graphs and understand what the loads being applied are to make use of the numbers.

This all reminds me of the issues with dust collectors and needing to see their performance curves to understand how a system would perform in your shop.

So, how do you compute the hp needed. Obviously the size (Mass) of the piece being turned is a factor, but so is the density and type of cut. More power is needed for coring than for finish cuts, for example. Also obviously, the power requirements go down as the piece has unbalanced mass removed, but goes back up as you increase speed.

I think other than to determine that the motor delivers enough power for worst case scenarios , the main considerations are the controls, does the headstock move, the quality of the machining and steel in the spindle, and the quality and number of bearings in the headstock. Secondary (to me) are the mass, length, and height of the bed. In the case of my miniatures, I may buy a small dedicated lathe to provide me with the high RPM needed or give up some throw and bed length and get a OneWay 1220 which gives about the best high rpm and is probably big enough for almost all of my turnings. If I go that route, I will keep the 1642 as a backup for larger turnings.

[Michael Mills]

All I go by (with my limited knowledge) is rule of thumb due to the complexity.
1 HP = 746 watts no matter where you look. (Volts X Amps)
My thumb: an AC motor is 60-70% efficient and a DC motor is 80-90% efficient.
The links are to a short article on AC motors and another link to a chart for normal HP with various types or phases.
So for a 1 HP AC I am looking for about 10 amps, for a DC about 8 amps.

http://www.kevinsbrady.net/motors.pdf
http://www.hvacwebtech.com/motoramps.htm

This Sears is about the worst example I have seen. 6.5 HP on 120V.
That would require a 40 amps circuit on a 120V line at 100% efficiency. (Sears states it is 12A)
http://www.sears.com/shop-vac-6.5-pe...1&blockType=G1

[Paul Williams]

Be careful with your apples to apples comparison. Never believe a rating of "peak horsepower" and compare it to amperage draw or a reliable horsepower rating. Peak horsepower is basically how many amps they can force through the windings while destroying the motor. You could calculate the power necessary to preform a woodturning task and then add 15% to 25% and calculate the amperage. Then use that to size your motor. Unfortunately you are still at the mercy of the manufacture's HP rating system. The only true way to see what HP you are using is to take an amperage reading while turning or coring. Of course that will only apply to that particular task and time.

Your last question is interesting. The three phase motor has current being delivered at three places and three points of time. You do not have a direct comparison to your single phase motor. The VFD converts single phase to three phase and in doing so has some loss. So measuring the current drawn by the VFD when running at full load would give you a horsepower rating for the system as a whole but not for the motor by itself.

The same concern is relevant for DC motors. The motor has a HP rating but the amperage draw from your AC outlet includes the conversion from AC to DC.

[Roger Chandler]

Perhaps we laymen do not really understand all the jargon.........I read the article on kevinsbrady.net/motors.pdf........my question about efficiency is simple........not answered directly by anyone yet.........

The full load current being the current taken by a motor when delivering its full rated output power would speak to efficiency of the motor..... correct? For example if one has 2 motors rated at 5 hp, and one has a full load current of 20 amps and the other one has a full load current of 12 amps.........does that mean the 5 hp motor that only draws 12 amps is a more efficient motor than the 5 hp motor that draws 20 amps?

I appreciate the pointing to the different sites, but I had done that on my previous efforts before this thread.......all the terminology and formulas for figuring is over my head because the nomenclature is for the person who has some training in these areas, which I do not.

Does my example [not that the exact amps/hp examples are accurate] but does it hold validity relating to efficiency? Does 3 phase motor power automatically get reduced by a vfd? Or, are the very premises of these two questions incorrect and make them null and not valid?

Got to be some simple answers to those questions from somebody........perhaps it is not so simple? I would think there has to be some sort of indicators that are common without having to know a specific load for a specific task, but a general rule?

[Dennis Ford]

Re your question about efficiency; unfortunately it is not as simple as volts X amps X efficiency with AC motors. An additional term "Power Factor" comes into play. My attempt at explaining this: during each cycle, the peak voltage and the peak amperage do not occur at the same time so multiplying them together does not equal power delivered.

2: full load amp rating is NOT enough information for a true apples to apples comparison (but it is useful information). I don't believe it is possible to do a true apples to apples comparison with specifications supplied by vendors.

3: I am not sure I understand what you are wanting here but will offer some thoughts:
3 phase motors are inherently more efficient than 1 phase motors (but only a little)
The VFD converts single phase AC into DC before converting it to 3 phase, a little power is lost in the process
When using a VFD, you get full HP only when the drive is putting out 60hz or higher. At lower frequencies, you get full torque at reduced speed. HP is torque X speed, so
full torque at lower speed is lower HP.

[Roger Chandler]

Re your question about efficiency; unfortunately it is not as simple as volts X amps X efficiency with AC motors. An additional term "Power Factor" comes into play. My attempt at explaining this: during each cycle, the peak voltage and the peak amperage do not occur at the same time so multiplying them together does not equal power delivered.

2: full load amp rating is NOT enough information for a true apples to apples comparison (but it is useful information). I don't believe it is possible to do a true apples to apples comparison with specifications supplied by vendors.

3: I am not sure I understand what you are wanting here but will offer some thoughts:
3 phase motors are inherently more efficient than 1 phase motors (but only a little)
The VFD converts single phase AC into DC before converting it to 3 phase, a little power is lost in the process
When using a VFD, you get full HP only when the drive is putting out 60hz or higher. At lower frequencies, you get full torque at reduced speed. HP is torque X speed, so
full torque at lower speed is lower HP.

Dennis........thank you! That information is some of the easiest I have had to digest, and understand in principle. I suppose the whole point of a vfd on a lathe is so that one gets full torque at reduced speed..........what you said makes perfect sense to me. Your point that you think it is not possible to do an apples to apples comparrison with specs supplied by vendors is helpful as well.

Perhaps the only true way is to measure with a dynometer.........hummm.......wonder if I can find somebody local with one?!

[Thom Sturgill]

What I was trying to get to earlier is that the question of efficiency depends on the load. It has to be measured at steps and graphed. Then the two graphs could be compared meaningfully.

Those two motors - the 20 amp motor could be more efficient at everything below 80% and the 12Amp motor beats it at full load. If you normally operate between 50-75% load then the 20 amp motor would be better. You NEED the graphs to determine which is actually more efficient across its entire power range, or more importantly, at the range you will typically use it.

[John Keeton]

It would seem, very generally, there are two categories of turners - production turners that push the lathe with coring, heavy roughing, etc., and those that are turning more artsy stuff that is smaller and doesn't require heavy cuts. To Reed Gray, aka Robo Hippy, power is very important. For someone like me, it doesn't mean a lot. With a few exceptions related to diameter, everything I have turned to date I could have turned on my former Delta midi, 1 HP lathe. When I moved up to the Jet 1642, I did so for the additional swing and more stability - not necessarily for power. I think this speaks to what Thom is saying. One needs to consider their turning style in deciding whether a bigger motor is going to make a difference.

Don't take me to task for over simplifying the categories. I am speaking very generally and realize some folks may do both heavy turning as well as more delicate work. For those folks, power is important.

[Roger Chandler]

What I was trying to get to earlier is that the question of efficiency depends on the load. It has to be measured at steps and graphed. Then the two graphs could be compared meaningfully.

Those two motors - the 20 amp motor could be more efficient at everything below 80% and the 12Amp motor beats it at full load. If you normally operate between 50-75% load then the 20 amp motor would be better. You NEED the graphs to determine which is actually more efficient across its entire power range, or more importantly, at the range you will typically use it.

Thanks Thom........I understand what you meant by this explanation.......I am really getting an education, and am most appreciative to all who have replied!

[Roger Chandler]

It would seem, very generally, there are two categories of turners - production turners that push the lathe with coring, heavy roughing, etc., and those that are turning more artsy stuff that is smaller and doesn't require heavy cuts. To Reed Gray, aka Robo Hippy, power is very important. For someone like me, it doesn't mean a lot. With a few exceptions related to diameter, everything I have turned to date I could have turned on my former Delta midi, 1 HP lathe. When I moved up to the Jet 1642, I did so for the additional swing and more stability - not necessarily for power. I think this speaks to what Thom is saying. One needs to consider their turning style in deciding whether a bigger motor is going to make a difference.

Don't take me to task for over simplifying the categories. I am speaking very generally and realize some folks may do both heavy turning as well as more delicate work. For those folks, power is important.

Thanks John.......for me personally........I do everything from pens to artsy stuff like hollow forms to large bowls, like the last one I posted.....a large ogee bowl that is nearly 14" diameter and 9" deep with steep walls. Part of what sparks my energy is to see all the forms I can do and I will probably narrow the scope at some point, but I like stretching myself.

I also plan to get a coring rig when I get the new lathe, so that is partly why this power issue interests me. Until it arrives I will have to settle for what that Delta midi will allow me to do. I need to make a few boxes and smaller bowls in the 8" range anyway to put into the galleries I am in, so when I can get this tax season behind me, the Delta will be put through its paces.

[Jeffrey J Smith]

Roger - I don't know beans about electrical engineering, but...If it's coring you're concerned with - learn to core with your Delta. It will require a light hand and patience, maybe a little fishtailing. If you can pull it off, by the time you get your new lathe you'll be ready to put some of that new power to good use. I learned to core with the Macnaughton rig on a 1.5hp jet 1642. Stalled it a few times early on, but got to the point that I knew why and could keep it from stalling pretty well. When I got the 3hp AB I found it so much smoother, but still used a light touch. I'm one who believes that there are times when it just doesn't pay to be a hurry - coring's one...

ps: you can still stall out a well made 3hp lathe with Kelton's rig on a 23" dia black acacia (incredibly heavy) very green blank - DAMHIK.

[Steve Schlumpf]

Roger, you can spend days going over specs for motors but unless you are having a custom lathe built, you are pretty much stuck with the one or two options that a manufacturer offers for any particular lathe. These days, when talking about AC motors, you have a choice of 120 or 220 VAC with the 220 VAC versions offering more low end torque. Depending on the size of the pieces you intend to turn, that may or may not be needed.

One thing that has changed over the years is the actual electric motor efficiency. Used to be that most motors were in the 70% to 85% range. Efficiency, like you already noted is simply the ratio of input power to realized output power. When a motor has a low efficiency rating you will be able to tell simply by touching the motor because that inefficiency, or lost power, is given off in heat. Ever touch a motor that has been running for awhile and burned your finger?

A lot of motor manufacturers these days are offering motors with high efficiency ratings or in some cases even ultra-high ratings. The 3 hp motor that is on my Robust AB is an ultra-high motor and is rated at 98%. Neat thing about it is I can turn a 100# chunk of wood for a couple of hours and the motor is still just as cool to the touch as it was before I turned the lathe on!

[Reed Gray]

If I remember my stories correctly, John likes lots of horses in his cars.... It has always confused me that my router has a 3 hp motor, and it would take 10 at least of them, to equal the weight of my 3 hp motor on my Robust. I guess the same is true for the 3/4 hp motor on my old blue no name grinder from Woodcraft, and my 3/4 hp motor Baldor grinder. Huge differences. That's advertising for you...

robo hippy