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Advice powering 3-phase 440v 7.5hp lathe

jbruning

Plastic
Joined
Dec 12, 2021
If the stock motor cannot be rewired for 240VAC, not sure the practicality on upsizing the RPC, buying a step-up transformer and all the ancillary costs of the install. A new 5-7.5Hp 3 phase 230/260VAC 1750 RPM replacement motor can be had for $500-600, a lot less than the cost of a step-up transformer. If the lathe is not going to be used for heavy turning, use a 5 Hp motor and this will reduce the cost of the RPC or VFD used. A VFD would be less than a new RPC, but you could make a 10Hp RPC at about the same price point if you can find a local motor and add an RPC control box. With a 5Hp motor you can either go with a native single phase input VFD like the (Yaskawa GA50UB018ABA) or use a 3 phase input VFD in a derated mode (Hitachi WJ200-075LF, FUji, etc.) which run around $700.

Thanks. This may be the way I would have to proceed, unless I could find the right RPC/transformer equipment used at a good price.....and that could be challenging.
 

scsmith42

Aluminum
Joined
Jul 28, 2020
Location
New Hill, NC
I don't have an image and it is located some distance from where I am, unfortunately. However, I was able to get the information that is on the plate....below.

Type AEEF
Frame 132S
Poles 4/8
Output 7.5/3.75 HP 5.5/2.8 KW
Volts 440 (note there is no indication of dual voltage)
HZ 60
Insulation 2
Rating Continuous
Amps 10/7
RPM 1720/860

If it's a 12 wire motor you should be able to reconfigure it for 240 3 phase.
 

DaveKamp

Titanium
Joined
Oct 3, 2004
Location
LeClaire, Ia
If it only specified single voltage, but two-speed it's probably not a circumstance where it's a Y-D motor, it's a consequent-pole... I've had some that would, but most would not...

Without changing the motor, the best suggestion is to come into a single phase 480:240v transformer backwards (240 in on the secondary, 480 out on the primary) into a 480v VFD, then into the motor. You'll have the ability to limit the incoming current, so that your 100A panel won't take a beating on startup. You will also only wind up pulling as much current as you need to make desired power at the SPINDLE... if you're only taking a 1/4hp cut, it won't pull current of a 7.5hp cut. You'll have full variable speed, direction control, and dynamic braking at your fingertips, and it will be pretty darned quiet.

Don't put an RPC idler in the attic... put it on rubber feet on a concrete pad outside, in a baffled/ventilated/insulated cover (airflow, but sound-deadening). Mounting the motor to your attic will turn the ceiling and walls into a big speaker. ;-)
 

Local631

Plastic
Joined
Jun 12, 2022
Location
Lost Wages, Nevada
Lots of good ideas......This would be a nice way to go since it wouldn't require any additional messing around with the wiring on the lathe itself. Additionally, it would be easy to add any additional 240v 3 phase machines too. I am in a rural area so it may take a bit of looking around, but the used industrial electric supply is a good suggestion. Thank you.
i have a 7.5/3.75hp also..it was a Cornwell when i got it but that motor was smoked .. i wanted to keep it as close to original working order and i happened across a Baldor Relience brand new delivered for 1000 bucks and i thought the same way about the rpc but as luck would have it American rotary had a sale a little while back and i went with the 20hp .. i havent looked into the step up transformer but i am about to.. i still have a little time mine is taken apart for its 70th year i felt she deserved a good cleaning and fluid change and might as well paint her while i got her clean and apart something differant then all the instatutional shades of grey...
 

DDoug

Diamond
Joined
Oct 18, 2005
Location
NW Pa
Are you really going to be running your lathe at full 7.5 hp all the time ?
It's not an air compressor.
 

Local631

Plastic
Joined
Jun 12, 2022
Location
Lost Wages, Nevada
I doubt I will be running it 1/2 that most of the time, but that's the beauty of the 7.5/3.75 you can run it on the low side till something comes up that requires more... I got the AR-20 rpc because for 1300 delivered with the control box that gives me plenty of room for a Bridgeport or whatever else I decide to play with. as far as the 7.5/3.75 yes I could have gotten something cheaper.. but that is what was in it from the factory 3 phase, 2 speed, 1 voltage... when new circa 1953 Warner & Swasey No.3 M-1200 originally ordered and owned by Uncle Sam, She Served in the Navy for The Northern Ordinance Division.. at least until 1993 I know because one of Her Maintenance cards was in the electric control box that came mounted on the back of the headstock along with the Original signed, Dated, Serialized Matching My Machines Numbers set of Electrical blueprints. i got the Baldor Reliance M1709T TEFC Constant Torque Motor brand new still in the box screwed to the pallet delivered for 1000 bucks i paid 500 for the lathe with Hydraulic Collet and 3 trays worth 3 x 24 collets.. round, square, Hex, it also came with the very rare Screw & Threading Gear Box and Apron with its own feed shaft.. and of course the factory cooling pump system This Beautiful Lathe has Served our Country for 40 yrs. and did another 30 yrs. Parts Unknown. That to Me deserves some time, attention, fresh fluids, and a New Non-Governmental paint job. i dont count the rpc in the total price of the Lathe because i wanted some other equipment that would need it also.. I am not a Machinist by any Trade Persons Definition .. This Machine will never work a hard week again as long as i own it.. I will use it to help Me build Custom Motorcycles, and anything else i could possibly find a reason for.. like the old man doing 60 in a 65 zone in his new Porsche, Ferrari whatever... while everyone else flies bye wondering WHY.... He Thinks To Himself..... I Did It.. I Finally Got What I Always Wanted... A Metallic Candy Apple Red with Gold Leaf Accents Lathe
 








 
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