Usefulness of DC Motor
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  1. #1
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    Default Usefulness of DC Motor

    I am trying to clear out some stuff.

    One of the items I have is a 1/2 HP variable speed, reversible DC motor. This probably has pretty limited application on old lathes, right? Doesn't DC lose torque as it slows down? Would be great on a jeweler's lathe or maybe something like a Levin.

    I only kept it around in case I needed it to power an old lathe or little mill.

    Should I sell it?

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    Quote Originally Posted by Frank R View Post
    I am trying to clear out some stuff.

    One of the items I have is a 1/2 HP variable speed, reversible DC motor. This probably has pretty limited application on old lathes, right? Doesn't DC lose torque as it slows down? Would be great on a jeweler's lathe or maybe something like a Levin.

    I only kept it around in case I needed it to power an old lathe or little mill.

    Should I sell it?
    "DC motor" is overly broad. There are several possibilities. The "main" diff is whether shunt wound or series (potentially "universal" EG: AC/DC) wound.

    "Straight" Shunt, compound shunt, and compensated shunt wound DC motors have good self-regulation and constant "reserve" torque at base RPM and below, clear down to locked-rotor. At (or near-as-dammit) locked rotor torque "CAN" go to double, potentially four to six - even NINE times rated torque, if only briefly. Around 60 to 90 seconds, per a Reliance "white paper", before thermal damage occurs.

    So no, they do not "lose" torque" as they are loaded down. The opposite, rather.

    Unless protected by a power source with inherent limits as to max allowed Current and Voltage, a thermal or current protective device, fuse or circuit-breaker, such a motor will attempt to "sustain or move the load, or DIE trying".

    Rare event. as fuses exist, etc., but a useful trait, even when limited. Think hoists, elevators, IC engine starter motors, where the initial load is really, really high, but drops-off once the load is set into even partial motion.

    Series-wound AC/DC motors are another horse altogether. Think "legacy" hand power tools for some of the most common applications.

    Latest tech, the cordless tribe most of all, may combine "smart" batteries, speed/torque sensing controllers and motor "systems" with active management that are sort of "hybrid" AC/DC tech.

    Small DC motor as described?

    Just fine for a small lathe. Implementing variable speed is cheaper and easier than a VFD. Simple Variac+rectifier, basic SCR PSU, more sophisticated "DC Drive", or even just a great fat rheostat.

    With access to a wound Field rather than permanent magnets, one can even cheaply dial-in a limit to torque so it stalls before it harms something fragile, and .. a DC motor CAN, in more than just a few situations, perform the work of an AC motor of significantly greater nameplate power - 3/4 to 1 HP in this case?

    This size is also useful for power traverse on a lathe or mill.

    3CW.. and "many" DC motors. KB-Penta Eurothem/Parker.

    3-P AC, too. Horses for courses. They all have their place...


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    Here is what I have. The previous owner used it to power a Halloween exhibit.

    Correction: Just noticed it is only 1/4 HP.
    Attached Thumbnails Attached Thumbnails img_20191002_180043224_hdr.jpg   img_20191002_175943683.jpg   img_20191002_180002352.jpg  

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    I will add that I have lots of watch lathes and a Levin 10 mm is the one I use most. I power it with a 1/4 HP 5000 RPM series-wound DC motor and use an SCR foot pedal controller. The round urethane final drive belt will slip before the motor could stall, so I could probably get by with a smaller motor.

    With series motors, an SCR controller will give variable speed with more or less constant torque down to very low RPM. In the old days, these motors were run with foot rheostats (same as old sewing machine controllers) and the motor will lose torque as the speed (voltage) is decreased. That was good enough, with 1/10 HP motors, to turn a pocket watch balance staff, but I do much larger work on my Levin and Derbyshire lathes.

    A 1725 RPM motor would be slower than ideal for a watch lathe, but pulley sizes could fix that.

    Larry

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    I had the exact same one on my 9" SBL. Got tired of not having enough power to get me thru threading full 8-pitch Acme threads on occasion. Pulled it off and upgraded to a 1/2 HP DC PM motor, have plenty of power now. The 1/4 HP size will eventually go on my feed motor on my Index 645 mill someday. They are pretty much limited in use today. The one I have came off of a right angle speed reducer that was going to be used on a welding positioner that never happen. Ken

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    Quote Originally Posted by 4GSR View Post
    The 1/4 HP size will eventually go on my feed motor on my Index 645 mill someday.
    Hmmmmm........ so I could use this as a table feed on a mill? Maybe I don't need to get rid of this after all (as he puts it back on his shelf).

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    Quote Originally Posted by Frank R View Post
    Hmmmmm........ so I could use this as a table feed on a mill? Maybe I don't need to get rid of this after all (as he puts it back on his shelf).
    My "light/medium" 40-taper // #9 B&S Quartet combo mill's OEM knee power comes off a 3/4 HP 3-P "pancake" motor through Gilmer belt to a box full of dogs, clutches, and gears, but "yeah, probably", so long as the mill is a light enough one, and presuming long-axis, only.

    Downside? A DC motor is basically a "consistent torque animal", will slow down to match the actual load.

    AC motors are essentially "frequency" animals, better at consistent SPEED. An attribute which neat and tidy milling doth greatly appreciate.

    IOW - you'll want plenty of reserve to avoid problems, even if only weird toolmark patterns..

    I'd be tempted .. to belt or gear it down for vertical knee travel - normally LOCKED once in-place, so stable rate of traverse is less important than either of the other two axis drives mought be.

    2CW

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    Slap a diamond wheel on the shaft and use it to sharpen carbide.

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    A variable speed motor is very handy for a welding positioner/rotator. Probably need some reduction between the
    motor and the rotator table but having the variable speed is almost an essential...

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    IIRC the motor on my Hamilton sensitive drill is about 1/4hp, maybe 1/3.. it drives a round urethane belt drive which slips long before the motor starts having to work hard. The arrangement could use a little more torque, or perhaps the operator needs to use smaller drill bits...


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