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View Full Version : Electrical help - Amps to run 7.5 hp lathe



BadDog
06-28-2008, 11:34 PM
I'm looking strongly at a very nice 16x60 lathe to replace my 11x37. I'm reluctant to start the "re-tool process" all over, but other than space (I have none even with the 11x37), my biggest concern is power. My shop currently is run off a single 70 Amp sub panel. By my rough (and ignorant) calcs, that's going to be well shy of handling the in-rush of a loaded 7.5 hp lathe (big chucks, potentially 100hp work pieces).

So, other than expensive rewire or dedicated shop service ($$$$), what are my options? First off, assuming only the lathe runs and all other big machines are off (only lights/fans need be on, plus maybe coolant pump if I keep it), will it handle full load 7.5 UL hp? Calcs I found indicate barely/maybe. Then again, how often would I *use* 7.5 hp? Probably never. My biggest concern is "in-rush" at startup with a 100+ lb chuck and potentially 100+ lb work piece.

One possibility is a VFD so I can "soft start" as needed. But VFDs to handle 7.5hp are not cheap OR small. Still...

Another possibility, since I don't run production or really NEED massive hogging cuts with carbide speeds, I could replace the 7.5 hp with a 5 hp Baldor I happen to have lying around. That should make things more sane, electrically speaking. We won't start on evaluating whether I *need* a lathe that big... ;)

Speaking of which, it's not really the total spin capacity I'm after. The spindle bore is a bit part, as is the higher rpm top speed and ability to turn metric and diametral pitches. And I actually wish this lathe was shorter, that 60" c2c makes for a near 12' long footprint. I would be far better off with a 16x40 or so. But as they say, you can never have TOO much length. Who knows, I may need to do a drive shaft again some day...

doctor demo
06-28-2008, 11:44 PM
Single or three phase, and what voltage?
My Linclon Electric 7.5 hp motor is 230V 21A 3phase and 11 amp at idle (no load)
Steve

wierdscience
06-29-2008, 12:18 AM
I have a 10hp 3~ woodworking planer that I start and run off a 50 amp service with no problem.I just make sure my compressor isn't about to kick on at the same time I start it.

The planer is a heavier load really than a lathe would be since there is no clutch and the 24" cutterhead weighs more than a 10" 3 jaw chuck would.

7.5 on lathe isn't so bad you'll prolly never use more than 5 of it anyway.

quasi
06-29-2008, 12:50 AM
At 7.5 hp I assume your lathe has a clutch? If it does you should have no problems. Inrush currents are very short time wise under a virtual no load start.

Fasttrack
06-29-2008, 12:56 AM
Yep - I've got a 16" Pacemaker with a 7.5 horse GE motor. Idle current is 7.5 amps at 208 volts and with the clutch, start-up issues are essentially non-existent. The reactance in the motor responds very quickly at 60Hz so the amount of time that you would theoretically pull alot of amps is very very tiny.

The rated current is 21 amps at 208 volts so a 70 amp panel should be no problem.

For reference there is about 746 watts to the HP and, of course, 1 watt equalls 1 volt times 1 amp. Of course, motors are not perfectly efficient and three phase changes things a bit, but it will give you a ball park.

For instance, 7.5 HP = 5595 watts = 26 amps at 208 volts.

mcassill
06-29-2008, 01:33 AM
Does the lathe have a clutch? I am running a 16" Pacemaker (with clutch, startup involves the motor only) with 15 HP motor (rated to pull 36 amps full load) from a 60 amp panel - no problem. If lathe doesn't have a clutch, that changes things...
Mark

BadDog
06-29-2008, 02:21 AM
Oops! Sorry, forgot the obvious. It's a 220V/440V 3 ph that will be run as 220V 3ph.

Clutch? Hmm, good question, don't know. If I can beat down this crud (BAD Summer Cold I can't seem to shake) I'll be going town to look it over again tomorrow. I've never seen/used one like this before, so I can't say right now.

Obviously my cold med befuddled brain screwed up the calcs. I came up with something near 40 amps at full 7.5hp, and guessed that startup could be at least twice that (not figuring on a clutch on a 16").

Anyway, seems like my concerns were unfounded. Based on what I'm reading here, sounds like I would be fine with a 7.5-10hp RPC (as I originally said, I can't imagine I'll ever actually use 7.5hp).

It's not the Mori I've been dreaming of, but even with the new Newal DRO it's about half what a comparable Mori will cost bare. If I can manage to get this thing into my shop, it will sure be a huge improvement from my Rockwell. And I've even got a line on a 4" tracer attachment that would work well on it. Who needs a tapper attachment when you've got a tracer? ;) I see some fun projects on that lathe, not to mention making a few bucks doing suspension/steering links for my rock buggy friends...

Ian B
06-29-2008, 03:41 AM
BD,

As the others say, a clutch will save you in most cases. I'd be surprised if a lathe with that size motor didn't have a clutch, in which case the motor will only be starting under no load and not pull too much current doing this.

I'm running a lathe with a 10Hp 3 phase 380v motor on a 380v 16 amp fused power supply. Starting the motor is no problem. If I set the lathe to highest RPM (2,000) and drop the clutch too quickly, even with only an empty 10" 3 jaw fitted, I blow the fuses. Any speed less than this is ok.

Ian

Forrest Addy
06-29-2008, 08:27 AM
The lathe is three phase? Is your shop single or three phase? A threee HP single phase motor will draw 31 to 35 Amps full load from a single phase service. A three phase 7 1/2 HP motor will draw 21 Amps per leg.

A 70 Amp single phase residential branch circuit will start it providing there are no other major loads connected at the time. Since the motor is three phase and if you use a VFD as a phase converter set for 3 seconds accel time the starting surge will all but disapperar and idle surrent will drop to 1/10 the full load current or less.

An RPC increases your electrical demand even when if you size it properly. They are not electrically efficient if you intend to run the machine tol its ratings.

Otherwise a 70 Amp single phase service run a 7 1/2 HP motor quite handily provided the compressor doesn't kick on when the lathe motor is started and there are no heating or A/C loads when you take full HP cuts. But you may experience nuiscance trips particularly if the breaker in your 70 Amp service is not motor rated..

BadDog
06-29-2008, 01:23 PM
The lathe is three phase? Is your shop single or three phase? A threee HP single phase motor will draw 31 to 35 Amps full load from a single phase service. A three phase 7 1/2 HP motor will draw 21 Amps per leg.

A 70 Amp single phase residential branch circuit will start it providing there are no other major loads connected at the time. Since the motor is three phase and if you use a VFD as a phase converter set for 3 seconds accel time the starting surge will all but disapperar and idle surrent will drop to 1/10 the full load current or less.

Ah, that sounds a lot more like what I thought I read.

Yes, my shop is single phase. Hence, the mention of VFDs and RPCs in my earlier posts. What Forrest describes is exactly what I was finding (and concerned about).

I'm going to try to get down and look at it again today. I'll also see about getting to the motor to read the data plate for verification. Sure hope it has a clutch...

BadDog
06-29-2008, 01:28 PM
And now that I think about it, I should have realized things were going wrong when folks asked about the 1 or 3 ph motor. The calcs I did were based on 7.5hp on 1ph, and including estimated loss from conversion. No wonder my calcs and all others were so far off. Sorry for not giving clear and complete info up front. And thanks for all the great responses!

japcas
06-29-2008, 01:59 PM
You need to make sure and find out if this lathe has a clutch or not when you go and check it out. I have a friend that has an older lathe, not sure of the make, that has a 3 phase, 220 volt, 7.5 hp motor on it that doesn't have a clutch. He says it pulls quite a lot of power on start up. I believe it is a 16 inch lathe also. Lathes that size may not be very common without a clutch, but there are some out there.

lazlo
06-29-2008, 03:21 PM
The lathe is three phase? Is your shop single or three phase? A threee HP single phase motor will draw 31 to 35 Amps full load from a single phase service. A three phase 7 1/2 HP motor will draw 21 Amps per leg.

Forrest is right: 7.5 Horsepower * 746 Watts = 5600 Watts / 200 V (worse-case) is 28 Amps.

You'll be fine with a VFD on a 50 amp circuit.

lazlo
06-29-2008, 03:25 PM
You need to make sure and find out if this lathe has a clutch or not when you go and check it out. He says it pulls quite a lot of power on start up.

At startup, an AC induction motor can pull 250% torque at low RPM, but it's still a 7.5 HP motor. It can't draw more than 7.5 plus it's service factor (say, 1.2 for a good NEMA motor).

Remember, Horsepower = Torque * RPM / 5252

You can't change one without the other...

lazlo
06-29-2008, 03:33 PM
I have a 10hp 3~ woodworking planer that I start and run off a 50 amp service with no problem.

Sure, that sounds right Wierd. Just for fun, on a 50A circuit:
50A * 200V (worst-case line droop) = 10 KW / 746 = 13.4 Horsepower.

So you should be able to run up to 13.4 Horsepower on a conventional 50A circuit with a service factor of 1.0. The NEC derates everything by a 25% safety factor, so the table below shows 10 HP on a 50A circuit.

Note that NEMA's definition of heavy industrial conditions corresponds to a service factor of .89, which would be even lower stress on the 50A circuit, but most NEMA nameplates are rated for 1.15 service factor.

Edit: here's a copy of the NEC FLC (Full Load Current) table for various motors. Note that NEC cable ampacity is 25% more than the worst-case motor draw:

http://i164.photobucket.com/albums/u15/rtgeorge_album/FLC.gif

rdfeil
06-29-2008, 03:36 PM
BadDog,

I agree with Forrest. Look into a VFD, the cost is higher than a RPC but you will gain a lot of features that you will like. The inrush (starting) current will be reduced greatly with a slower startup. The variable speed is a great feature and with the right VFD you also can get resistive breaking to stop the spindle very fast. Also, the VFD's will allow you to add start/stop controls at both ends of the lathe, A stop button at the tail stock end is a nice safety feature. The RPC creates a lot of noise and is much less efficient on your power bill. Normally, unless the VFD is specifically rated for 1 phase input the drive will need to be up-sized for the application, I normally go twice the rating, in your case 15 HP. Check with the drive suppliers and they will give you the right answer. I have used 10 HP drives on 7.5 HP motors but it is a little thin, but a lathe very seldom sees the rated horsepower for extended times. If you want to save money you could put the 5 HP motor you mentioned on the lathe and then use a 7.5 or 10 HP VFD. I have a 17 inch LeBlond lathe with a puny 1 HP motor on it and I can still hog a lot of metal, .200 DOC is no problem. It would be nice to have more HP and I will do that some day but for now it works fine with 1 HP. I checked around and found VFD's in the $655 to $835 range for 10 and 15 HP respectively, these are the GS3 drives from www.automationdirect.com and they have free shipping on orders over $300 :D . There are other suppliers that are great also so check around and ask questions. I am sure cost is a BIG consideration but bang for the buck VFD's are nice.

Good Luck,
Robin

Edit: Check out Lazlo's info above.

MickeyD
06-29-2008, 03:50 PM
I have an American Rotary 15HP phase converter hanging on the end of a 50 amp circuit (it is derated from 70 because I had a 50 amp breaker in my electrical box). I have run my cnc mill, 7.5 hp lathe, and 1hp bandsaw all at the same time and have not seen a problem. I was curious one day and used a clamp on amp meter and it showed 36 amps on 1 leg and 37 on the other on the single phase feeder, and this was while the machines were all working, but not grunting real hard. It did spike up and down when the cnc changed tools and started/stopped the spindle, but I was impressed by how little power was being used. I did try to run the lathe on a 5hp rotary converter on a 20amp single phase circuit and it sorta worked. It ran fine with a 5C spindle nose up to 2000 rpm, but if I had a real chuck on it (10" 4 jaw) it would kick the breaker if I tried to run it over about 600 rpm (engage clutch and everything would stop).

lazlo
06-29-2008, 04:03 PM
I checked around and found VFD's in the $655 to $835 range for 10 and 15 HP respectively, these are the GS3 drives from www.automationdirect.com

I have one of the 2 HP GS3 drives on my mill -- it's a re-branded Samsung drive -- very nice!

Out of curiosity, how much does a 7.5 HP rotary phase converter cost?
MickeyD's is kinda loud (sorry Mike) :D

japcas
06-29-2008, 04:09 PM
At startup, an AC induction motor can pull 250% torque at low RPM, but it's still a 7.5 HP motor. It can't draw more than 7.5 plus it's service factor (say, 1.2 for a good NEMA motor).

Remember, Horsepower = Torque * RPM / 5252

You can't change one without the other...

Lazlo, I'm no electrician and I don't play one on tv. My point was that there are some bigger lathes out there that don't have a clutch on them. Personally, on a lathe that size I would want a clutch or at least have it on a vfd so that I could soft start it. I don't like to spool up a heavy part real quick if I don't have too. Baddog, if you go and check out the machine how about getting us some pics? Someone here may have one like it and be able to share some info with you about them.

BadDog
06-29-2008, 07:07 PM
Thanks for the great info guys.

Update on looking at the machine: I was going down there today, but my friend (who owns the shop where it's currently stored) had an AC failure at one of the other shops he rents out. So he's been tied up there all day and it looks like he won't be getting back to the main shop today. So, maybe tomorrow or Tuesday before I see it again. But I'll definitely be getting pictures.

BadDog
06-29-2008, 10:16 PM
Ok, first off, let me say that my memory was further off than what I expected. And some "assumptions" by me and my friend were wrong...

I couldn't get to the motor because a leveling screw was in the way and it was too much trouble to get it OUT of the way and remove the panel. However, there is a "soft start" box on the back that is rated at 5hp. Seems safe to assume that it is (or was changed to?) 5hp.

It is an SJ-1760G "Osama Sr" produced by Shen Jey in Taiwan.

While the basic shape, particularly of the headstock and carriage, is (to me) Mori, the bed is quite different. It is not nearly as wide or heavy as the Mori located nearby (at my friend's shop). My friend says it looks similar to some late Leblonds, but I've never seen one like that. <shrug>

Anyway, requested pics are below.

http://img4.pictiger.com/43b/15836722_th.jpg (http://baddog.pictiger.com/albums/30606/15836722/)

http://img4.pictiger.com/476/15836712_th.jpg (http://baddog.pictiger.com/albums/30606/15836712/)

http://img4.pictiger.com/733/15836715_th.jpg (http://baddog.pictiger.com/albums/30606/15836715/)

http://img4.pictiger.com/a41/15836725_th.jpg (http://baddog.pictiger.com/albums/30606/15836725/)

BadDog
06-29-2008, 10:17 PM
The rest of the pics.

http://img4.pictiger.com/90c/15836711_th.jpg (http://baddog.pictiger.com/albums/30606/15836711/)

http://img4.pictiger.com/6a3/15836719_th.jpg (http://baddog.pictiger.com/albums/30606/15836719/)

lazlo
06-29-2008, 10:30 PM
Looks like a nice machine Russ! Build date is November, 1988, which is pretty young for a manual lathe.

I love the chip shield for your hand in the second to the last picture -- why don't they have that on other lathes? :)

gt2ride
06-29-2008, 10:55 PM
http://i175.photobucket.com/albums/w147/gt2ride/HPIM0037.jpg

BadDog I have an Osama Sr. It is a nice lathe. There is no clutch. There is a two speed motor. I run my lathe on a 7.5 RPC. It will not start the lathe on hi speed. The RPC is not big enough. If I want to run the lathe on high speed I start it on the low speed and then switch it to high. I use it most of the time for big work so that is no problem.

BadDog
06-29-2008, 11:17 PM
Great to see someone here has one. And a nice one at that! Now you've done it! If I get this thing, I'll surely be trying to pick your brain! :D

This lathe has the "soft start" box (as seen in the pics), so maybe that will help with the startup. With suitable sized VFDs costing so much, I would likely go with a 7.5 to 10hp RPC, at least to start with. Keeping an eye out for a "deal". VFDs are always falling in price, so maybe the used big fellas'll get a bit more affordable in the near future.