Cost of running: 5HP vs. 10HP?

Kind of a theoretical question, but math is not my strong suit.

Say one has a big lathe, with a 10HP 3-phase motor. It's largely used in a home shop setting, so for the most part relatively lightly loaded- no one's peeling off 1/2" of alloy steel at a pass with it.

Would there be any significant cost savings in swapping the 10HP for a 5HP? Even the smaller motor is likely more HP than needed, so chances are there would be little or no performance difference.

Presume both motors are 3-phase, and run through a VFD.

Doc.

DocsMachine said:

Kind of a theoretical question, but math is not my strong suit.

Say one has a big lathe, with a 10HP 3-phase motor. It's largely used in a home shop setting, so for the most part relatively lightly loaded- no one's peeling off 1/2" of alloy steel at a pass with it.

Would there be any significant cost savings in swapping the 10HP for a 5HP? Even the smaller motor is likely more HP than needed, so chances are there would be little or no performance difference.

Presume both motors are 3-phase, and run through a VFD.

Doc.

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Just to throw together some very rough, round number estimates.
motors use 1kW per HP at full load (remember I said round numbers)
Motors us 10% of full load power at idle, think of this as a background number
10HP idle power is 1kw, 5hp motor is 0.5 kW
Assuming power costs of $0.10 per kWH, the 5 hp motor will save you a nickel an hour.
How many nickels do you need to save to pay for your 5hp motor and the time to put it in?
I may be off by factor of 2 or 3, how much does that change things?

Merry Christmass
CarlBoyd

Cost difference on a power consumption is minimal doing the same workload, cost of the VFD installation and service requirements would be 2-3X more expensive for a 10Hp VFD setup/motor vs. 5Hp. I am assuming single phase power, if 3 phase then the difference would be a bit less. Lathe installations of VFDs tend to require a bit more thought/detail vs. say a mill, but cost of the control functions would not differ much.

Edit: mksj beat me to it...

The electricity cost savings is likely not worth going after as Carl points out. But in a home shop setting I'm assuming there is no 3 phase power available and the question simply becomes how does one run a 5 or 10HP motor 3 phase motor. I'm under the impression that almost any way you slice it it will be cheaper to get the 5hp motor running, likely enough cheaper to pay for the 5hp motor.

It sounds like the machine is already running, so no VFD costs associated with the change. If the energy cost difference is that negligible it's probably not worth the time and effort.

The machine is already set up with a 10HP and a monstrously expensive 10HP VFD. I only asked because I'd considered setting it up with a 5HP instead, back when I was rebuilding it, mainly because a 5HP VFD was going to be less than half the cost of the 10HP.

But I also wondered what, if any, per-hour power savings there would be. I may have asked this same question several years ago, but a recent discussion on another board brought it back to mind, so I figured I'd ask again.

And no, a nickel an hour isn't enough to worry about.

Thanks, guys.

Doc.

You are going to change motors? Despite the frame size problem isn't that step expensive?
While a 10 will idle more than a 5 that is not a lot of it's time.
In a light cut yes that idle is added so maybe?
What about when you need or want the ten? This is a concept I have never considered. Most people want more power.
If living off the grid every watt counts so would you downsize a machine tool to give yourself more hours?

Take living in the great white north with a lathe and B-port for repairs to your stuff with solar and your maybe windmill for electric.
Could one way downsize the motors to give you more run time? How low could you go?
Bob

The effort to start a 5Hp vs 10Hp motor is less in parts and energy.
Whether the parts equate to more capactiors, a larger idler, or a pony motor for a RPC. Or a larger VFD. That's the savings.

CarbideBob said:

You are going to change motors?

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-No. I was thinking about it, when I was in the middle of rebuilding it. The machine is currently back apart to have the bed reground, and if it were to decide to make the change, this would be the time to do it.

Despite the frame size problem isn't that step expensive?

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-Frame size wouldn't be a big issue. The mount in this particular machine is basically a big flat cast-iron plate. I'd have to make a sort of sub-plate with risers to fit the smaller motor to the same shaft centerline for the belt pulley, but that's a sheet of steel plate and some aluminum slugs. No biggie.

And "expense" was the reason I was wondering- the primary expense was that Polyspede VFD for the 10HP motor- pretty much the only one I could find that'd run 10HP off single phase- was going to run $1,200 plus shipping on an 80-lb crate to Alaska.

Whereas a 5HP model would be roughly half that. And I already had a 5HP motor, along with the 10 that was in it. So it was a question of spending $600 plus shipping or $1,200 plus shipping.

As noted, I went with the 10HP in part because it was already there, but I still wondered if there was a significant difference in day-to-day operating costs.

What about when you need or want the ten? This is a concept I have never considered. Most people want more power.

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-I honestly have no idea. The most powerful lathe I ever ran was 7.5HP, and I'm not sure I ever used even a quarter of that capacity.

If living off the grid every watt counts so would you downsize a machine tool to give yourself more hours?

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-No, no "off grid" nonsense. Trying to have a small production machine shop "off grid" is like having a new-car dealership in Barrow, Alaska. I was simply curious, since I've never done the math and aren't any good at it anyway, if there might be enough of a monthly bill difference to make it worthwhile.

Doc.

Re: using all the horsepower; 10 HP really isn't all that much if you're doing any larger work. The larger the diameter you're cutting on and the higher the SFM you're running, the more horsepower it takes for an equivalent DOC and feed. I have been running at 110% on the load meter while doing heavy roughing on a 40HP lathe many times.

DocsMachine said:

-No. I was thinking about it, when I was in the middle of rebuilding it. The machine is currently back apart to have the bed reground, and if it were to decide to make the change, this would be the time to do it.

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I just read that 10 pager on your Springfield lathe and wondered if you ever got the bed ground.

Did you sent it to Schaeffer in LA?

Doug W said:

I just read that 10 pager on your Springfield lathe and wondered if you ever got the bed ground.

Did you sent it to Schaeffer in LA?

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-No, but I damn well should have. Or Commerce, or just about anywhere else.

Without making this too big a story, as shipping such a heavy unit from up here in Alaska was going to be so expensive, I made arrangements with a friend to haul it. He was driving down with an empty trailer, and was going to bring a car back up.

For a few hundred in gas money, He'd swing the bed (and some other parts) by the grinder. He was already heading for California, but was going to wind up some 300 miles short of LA. He didn't want to try and drive the truck and trailer into LA and back, let alone twice- once to drop it off, and again to pick it back up.

So we made arrangements to drop it off at Carr's in Vancouver- the driver had to make a slight detour, but it wasn't bad.

As the page notes, I got everything packed up, he picked 'em up, and we had arrangements for him to meet Carr.

And then he didn't leave for another four months. By the time he dropped the bed off, our window of opportunity had passed, and Carr couldn't get to it right away- the immediate issue being that the driver couldn't pick it back up on his return trip. So instead of a few hundred in gas money, now I'm looking at $2,500 to $3,500 just in return shipping.

And that's IF I ever get the goddam things back. Carr, as many might know, is now closing down and selling off his shop. You can actually see the corner of my bed in a couple of his sales photos.

He's assured me he'll finish off my parts and get them shipped out before he has to vacate the building, but that's something like only two months away, and the parts have already been sitting in his shop for sixteen.

I still have confidence, waning though it may be, in Carr- many PM'ers recommended him and his work rather heartily- but I may well also wind up a victim of circumstance. A best-case scenario may well wind up with me just paying to have it all shipped to Schaffer or Commerce, paying them to have the work done, and then paying whatever it costs to get it all shipped back up here.

Which I can see pushing $10-$12K or more.

We'll see. I haven't given up hope yet, but it's getting pretty thin by this point.

Doc.

Doc, I have PMed you.
Not sure if you got my last PM??

What's Sealand want for a 20 footer from Tacoma?

Doug W said:

Doc, I have PMed you.
Not sure if you got my last PM??

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-Replied via Email. I don't always check my PMs on a regular basis over here.

What's Sealand want for a 20 footer from Tacoma?

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-I have a regular consolidator in Kent I've used in the past to bring heavier items up. Last year I got a thousand-pound shipment for about $250, so if that holds, all the parts together (Carr has pieces of three of my machines, probably totalling about 3K lb) should theoretically get up here for around $750.

BUT... I still need to get at the very least from Vancouver to Kent, or at worst from Vancouver, to LA, then LA back up to Kent, then up to me. Which I can see easily pushing $3-$4K all together.

Doc.

DocsMachine said:

-I have a regular consolidator in Kent I've used in the past to bring heavier items up. Last year I got a thousand-pound shipment for about $250, so if that holds, all the parts together (Carr has pieces of three of my machines, probably totalling about 3K lb) should theoretically get up here for around $750.

BUT... I still need to get at the very least from Vancouver to Kent, or at worst from Vancouver, to LA, then LA back up to Kent, then up to me. Which I can see easily pushing $3-$4K all together.

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I think we usually used Nelson for Seattle-Portland runs. Since it sounds like you have a little bit of time, it might be worth seeing if they can fit you in on a northbound deadhead run. It won't be as convenient as being able to schedule a pickup on the day of your choice, but it might be cheaper.

Boiled down: since you ALREADY OWN the 10HP drive and motor, the ONLY difference will be in the fixed energy cost difference in the two motors. Had you known at the initial install that you needed less than 5HP, that would have been the time to change.

Losses in motors can be divided into two main categories: those that are associated with the load, which account for about 75% of the losses, and those associated with turning the steel and copper into a motor, the other 25% of the losses. Your load based losses remain the same regardless of which motor you use. The energization losses increase in the 10HP motor. But assuming an older motor with 80% efficiency, the total fixed losses will be 25% of 20%, so 5% of the total energy, then the difference between them would represent 1/2 of that. It would be a significant energy cost issue in a motor that runs 24/7, but for a job shop, you would be hard pressed to justify having to buy just a 5HP motor (you could re-use the VFD). If you buy a new VFD too, I doubt you would live long enough to see the ROI.

DocsMachine said:

-Replied via Email. I don't always check my PMs on a regular basis over here.

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Never got an email, nothing in the spam box.

An engine lathe with 5HP would probably piss me off on a regular basis.

I spose since the lathe has really never been used by you, you may not have a frame of reference?

You can say you'll never use 5HP, but then what happens when you need it?

In my shop all my manual machines have 15HP spindle motors- My two lathes, my 4" HBM, heck even my 250 ton stamping press is 15HP. When I'm making a part and trying to make a living- not die of old age standing in front of a machine- I make sure the machine is lubed up and run the roughing tools that I have the most inserts stockpiled for.

I got a holder for that outside corner on the CNMG inserts and you bet your ass it gets used.

I bet my manual machines don't account for $20 of my monthly electric bill. Even the seven CNC's from 10 to 80HP probably don't cost $150 a month to run.

Doug W said:

Never got an email, nothing in the spam box.

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-Tried sending again. Please let me know if you get it.

An engine lathe with 5HP would probably piss me off on a regular basis.

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-I'm sure it would, as it would other people too. I have no current frame of reference. I know I rarely use much of the HP for either of my small lathes- one's a 2HP and the other a 3HP. The 3 is my daily-use machine, and I'm not sure I've ever even bogged it down, outside of accidentally jamming the tool in too fast or something.

And, as noted above, I already have the 10HP and the fabulously expensive VFD. This question was more academic than anything- even if there was a significant difference between the two motors, chances are this machine will see so little day-to-day use that it'd make little real change in the bill.

And since there isn't a noteworthy difference, I'm no longer worried about it. Question asked and answered.

Doc.