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Thread: Question about motors 1ph,3ph or DC

  1. #1
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    Default Question about motors 1ph,3ph or DC

    I have been following RB211 Monarch 10EE Threads and it seems DC is top of charts for finish when machining.I wonder why all Lathes and Mills are not DC powered,being mass produced should not increase price much.It seems 3ph finish is better than 1ph or is there more variables I'm not seeing.I wonder how finish was when line shafts were used in the old days being powered by Steam or Water Wheel.

  2. #2
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    Three phase motors should provide a great finish because there is very little torque ripple. As long as the voltages and phase angles are balanced, the total RMS current, and thus torque, is constant. The assertion that DC motors are better than 3 phase AC machines probably dates back to the days before technologically advanced VFDs. DC motors are actually just mechanically commutated AC machines, and the only true DC motor is the homopolar type, which is really not practical for most purposes.

    That being said, not all three phase motors are equal, and torque ripple depends on the construction of the stator and the rotor, which will have a number of internal windings, pole pieces, and "skew", that can greatly affect smoothness of operation.

    Steam power plants usually have a very massive flywheel which will smooth out fluctuations due to the operation of the piston, but that is so slow that it should not affect finish. Waterwheels are also effectively large flywheels. But lathe spindles operate at much higher speed than the prime mover, and finish will be mostly affected by the overall rotational inertia and slippage of drive belts and gears.

  3. #3
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    the lathe should have enough intertia in the spindle drive system to completely remove the torque ripple intrinsic to single phase ac motors.

    3 phase motors and dc motors will both provide substantially negligible torque ripple if driven properly. dc needs dc.. not rectified single phase ac. single phase scr drives are worse than a rectifier and a variac, they produce a 120hz ripple that is very nasty, along with higher harmonics. enough to make my south bend 9 into a tuning fork and you could feel the 120hz vibration everywhere in the machine.
    (adding an inductor stopped it. The inductor will need to weigh about 2-4 pounds per hp as a general rule for single phase scr drives.)

    3 phase motors need balanced 3 phase ac (better than 2% voltage balance). running a 3 phase machine from a rotary phase converter producing 240/230/230volts under load is as good as combining a 5 hp 3 phase motor with a 1 hp single phase motor. you're still going to get a 60hz torque ripple because the voltages aren't balanced.

    single phase ac motors produce a substantial torque ripple.. but they also have a 60hz vibration problem that is different from a 3 phase motor. if you can imagine the stator of the motor being distorted into an oval shape, the 3 phase motor, that distortion is constant but rotates round and round in a circle around the motor. in a single phase motor it doesn't rotate it just oscillates in a single location from )( to ()
    (unless the motor is a good capacitor run motor in which case the motor produces maybe half the torque ripple a standard single phase motor does.)


    a very good quality capacitor run motor, operated at partial load will produce a substantially negligible torque ripple similar to that of a 3 phase motor when run off an RPC. the reason why is because a proper 2 phase motor also has no torque ripple, but, you have to optimize the capacitor for the load. above or below that opimum value the motor is relying on the run winding at a far greater proportion as the auxiliary winding.

  4. #4

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    I'd imagine the clatter from the straight cut gears in most lathes is much more significant than the variation caused by a motor. I'd imagine they don't do DC because it's a pain in the ass to power it and 3 phase is plenty smooth. Also, if my knowledge of motors is correct, all (true) DC motors are brushed, and brushes suck.

    I'd also imagine that most of us in our home machine shops don't do work that requires a turned finish quality such that we need a 10EE.

  5. #5
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    Servo motors are great for drive motors. Just very expensive.

    A nice inexpensive 3ph motor with VFD is the teets these days.

    DC? Think about Thomas Edison and Mr. Tesla (the real one). DC is old and also expensive.

    Its the controls is what kills you. Spensive. JR
    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

  6. #6
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    Quote Originally Posted by Tundra Twin Track View Post
    I have been following RB211 Monarch 10EE Threads and it seems DC is top of charts for finish when machining
    I think there is another similar Q on one of RB's topics where someone asked about why?

    Why is the short, fat, heavy, no limbs, big headed lathe make such a nice finish?

    I think that is the reason. She is definitely not as svelte looking as the HLV-H which is in its class.

    So. I dont know. I think its her fat azz an not so much the drive system. JR
    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

  7. #7
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    Quote Originally Posted by JRouche View Post

    So. I dont know. I think its her fat azz an not so much the drive system. JR

    When I got my Lathe it was 10hp 3ph,I converted to 5hp 1ph.The Tool&Die shop it came from had a Shop Foreman that offered to come out and give some tips on running it.He said he noticed no differance on finish with 1ph motor.

  8. #8
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    In my next life my lathe will have a gas turbine drive.

  9. #9
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    Mass, mass, mass. 3000 lbs for a 12.5" lathe. Smooth drive that is further smoothed by a very long belt, and is direct drive. The gear train for feeds is further isolated by a flat belt off the spindle. As Abom79 said, Monarch set out to make the best possible lathe money could buy. As old as my 10ee is, it still shows it.

    Sent from my SM-G950U1 using Tapatalk

  10. #10
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    Agreed, almost as good as a Holbrook Mdel C - take a 10ee, add another half ton or more, 5 drive belts to a pulley in its own bearings with a 3-speed motor and 2-speed clutch/brake box in the base. Helical back-gear too, when you need it.

    Dave H. (the other one)
    Rules are for the obedience of fools, and the guidance of wise men.

    Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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