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Brush Or Brushless Motor For A Mini/midi Lathe?

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by TonyL, May 29, 2016.

  1. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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    I was wondering if it matters. I like the HiTorque 8.5 X, Jet 920, and the PM. They will all cost me the same once they are tooled. (Enco has 25% off and free shipping on the Jet920).

    Thank you for any feedback.
     
  2. Ulma Doctor

    Ulma Doctor Infinitely Curious Active Member

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    they are both good machines.
    i personally, would get the largest swing and bed that could fit into my workspace.

    the brushless motor and brushed motors will both work for the purpose of driving a lathe.
    a brushless motor will be maintenance free, they are usually AC powered.
    whereas as brushed motor may require the changing of carbon brushes and the cleaning of the motor's commutator when the brushes wear out.
    i may add that, in my experience, a brushless motor in- the same given specification envelope- will wear out or fail before the brushless motor
    brushed motors are either AC, DC or universal
    most brushed motors may be a little less efficient than their brushless counterparts.
    variable speed control is a lot easier to accomplish with a brushed motor, but can be accomplished on small AC motors with a PWM board (Pulse Width Modulation).
     
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  3. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Wow! That's an excellent explanation; I even understood it. Thank you.
     
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  4. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    A bit of information about brushed and brushless motors may help.

    Alternating current, especially 3 phase will produce a rotating magnetic field causing a motor to rotate, the drawback is that it will always try and rotate at the same speed as the frequency dependent on the number of coils (or fields) in the motor, a VFD (Variable Frequency Drive) alters the frequency to control speed. An AC induction motors speed is generally not effected by voltage up to the point of very high or very low values.

    Direct current has no frequency as such so a switching device is employed to create a rotating field, many use a commutator on the rotor with brushes and commutator as switches. Changing the voltage will change the speed, the starter motor in a car is a perfect example, these are simple permanent magnet devices however.
    A brushless DC or AC motor is externally commutated which means that the switching is done by a control outside of the motor in place of a fixed commutator.
    The advantages of this are speed control and torque control at a given speed in a field wired motor.

    To answer your question a synchronous motor (commom AC induction motor) is by far the least expensive, as controls and other devices are added it becomes considerably more expensive so it is not a matter of which is better but what you are willing to spend to achieve the performance that you require.

    You may have noticed that your corded 110 VAC hand drill has brushes as well, it will also run on DC power. This is often called a universal motor
     
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  5. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    Brushless DC motors (which are actually synchronous AC motors with PM fields) are capable of very good speed control over a very wide range but require much more complex controllers than do DC motors. In a hobby lathe the brushes and commutator in a DC motor will last for decades.
     
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  6. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Thank you. I want to turn, cut tbread, bore, and part off 304L , 303, ss rod that is .75 inches in diameter I don't know it that helps. But I apprecite your comprehensive response.
     
  7. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    I also work in the automation industry and can not even count the number of times that I have had the same conversation with customers.

    Customer: We need to control the speed of this conveyor

    Wreck™: Easy, I will install a VFD linked to the input device and the speed will be perfectly coordinated

    Customer: $2500.00, you are crazy, I can buy an entire new conveyor for less

    Wreck™: Not with speed control linked to the previous process or device

    Six Months Later

    Customer: When can you install that control that you quoted $1500.00 for 6 months ago, we need it ASAP.
     
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  8. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    I like the way you quote them $2500 and six months later they remember it as $1500. That's real.

    Just recently I had a guy show interest in an old hay conveyor I had out behind the shed. I quoted him $500 and he said he'd think about it. Since he had shown interest and it is the season to sell such things I dragged it out, overhauled the motor, put air in the tires, put a drawbar and a stand on it, and greased and oiled it. The other day he came back and asked if I'd stll be willing to let it go for $300.
     
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  9. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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  10. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    If it's a properly specified good quality motor with a properly specified good quality controller, no. The simple induction motor will, of course, be more reliable because it is simpler but will also not be variable speed.
     
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  11. higgite

    higgite General Manger - Proofreading Dept. Active Member

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    If low speed torque matters to you, in general brushless DC (BLDC) motors have more torque at low speeds than their brushed counterparts. That's why brushed mini machines have hi-lo gear shifts or multi-sheaved pulleys with a set number of speeds available, like the Jet 920. That's to keep the motor up on its torque curve at low spindle rpm. BLDC belt drives, like the LMS, run min to max RPM (motor and spindle) with no gear/pulley shifting. I don't know how the PMs are set up. If you do most of your work at a single speed, it's no big deal one way or the other. Some may say no big deal, period. All 3 of the machines you listed require messing with change gears to do threading.

    That said, I had a 7x14 that had no trouble threading at 60-70 rpm in low gear. I now have an LMS 8.5x20. I have turned and faced a 4" aluminum "hockey puck" on it, but most of my work has been turning and threading aluminum, mild steel and brass up to 3/4" round. That is about the largest size that will fit through the spindle. (actually something like 20mm if I recall). It's a very well made machine, as I'm sure the others are, too. A lot of folks on the forum also speak highly of the PM machines.

    Tom
     
    Last edited: May 29, 2016
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  12. TOOLMASTER

    TOOLMASTER you don't want to know Active Member

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    i would take the mathews just to have the bigger bore
     
  13. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Thank you all again. It looks like both machines will serve my needs. I am surprise few talk about the Jet 920.
     
  14. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    Good point. It's a lot easier to upgrade the motor than to upgrade the bore.
     
  15. mksj

    mksj Active User H-M Supporter-Premium

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    The Jet 920 design has been around for a long time, but is a very basic lathe for the price. Even with the "discount" I do not think it is a wise purchase for the price. With the newer designs, variable speed, larger spindle, less gear changes, more feeds and thread options, etc..., there are better options these days in this size and price range. The other factor is the total package, options and support after the purchase. Many of these smaller lathes work but are very minimal in design and features. If you are spending 2-3K for a lathe, I feel there are a lot better options. Either the PM127 VF-LF or the PM1128, would offer a lot more range features and an flexibility whether you do small or larger work. Many of the newer smaller lathes with variable speed use a DC motor, usually brushless. Given there size, they have more than adequate power running them off of 120V, and should do anything you want them to do. If you want smaller, then maybe look at something like the Grizzly G0752 - 10" x 22" Variable-Speed Lathe. http://www.grizzly.com/products/10-x-22-Variable-Speed-Lathe/G0752
     
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  16. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    I live in a small Quaker founded town just across the river from Philadelphia, many wealthy business owners built large summer homes along the riverbank, to this day it is a dry town, one can not sell a glass of wine with dinner at a
    Yes, one can expect problems from any system that makes a dedicated effort at being the lowest cost alternative, however I suspect that if you bang on the vendor to keep sending you new products every week you will end up with something that works for you.
    Your mileage may vary, I am in no way disparaging any company linked above, I merely work in a jobshop 50 hours per week, that is more then enough machine work for me.
     
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  17. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Thanks again folks. It looks like I have beaten this subject to death. I do continue to look for an "old" non-overseas lathe, but I haven't seen any that I am equipped to fix/appraise its needs. Your guidance and experience is very much appreciated and highly respected.
     
  18. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Given I want to turn SS (304L, 303, 316): Is there a big difference between a 1 HP brushed motor, and a 1.36 HP brushless motor? Would I benefit from 33% more HP? Is HP simply HP, regardless of brushed or brushless?

    Thank you!
     
  19. mksj

    mksj Active User H-M Supporter-Premium

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    Short answer, most likely no, especially because you previously mentioned doing small diameter turning. It is unlikely you will see a difference, and even if you could, you could take a slightly smaller cut. It also depends on the belting or gear ratio. Brrushless motors are about 10% more efficient then a brushed motor.

    A DC Brushless Motor uses a permanent magnet external rotor, three phases of driving coils, one or more Hall effect devices to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from the Hall effect sensors, they act as three-phase synchronous motors containing their own variable frequency drive electronics.

    BLDC Motor Pros:
    Electronic commutation based on Hall position sensors
    Less required maintenance due to absence of brushes
    Speed/Torque- flat, enables operation at all speeds with rated load
    High efficiency, no voltage drop across brushes
    High output power/frame size.
    Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better
    Higher speed range - no mechanical limitation imposed by brushes/commutator
    Low electric noise generation

    BLDC Motor Cons:
    Higher cost of construction
    Control is complex and expensive
    Electric Controller is required to keep the motor running. It offers double the price of the motor.

    A Brushed Motor has a rotating set of wound wire coils called an armature which acts as an electromagnet with two poles. A mechanical rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During the instant of switching polarity, inertia keeps the classical motor going in the proper direction.

    Brushed Motor Pros:
    Two wire control
    Replaceable brushes for extended life
    Low cost of construction
    Simple and inexpensive control
    No controller is required for fixed speeds
    Operates in extreme environments due to lack of electronics

    Brushed Motor Cons:
    Periodic maintenance is required
    Speed/torque is moderately flat. At higher speeds, brush friction increases, thus reducing useful torque
    Poor heat dissipation due to internal rotor construction
    Higher rotor inertia which limits the dynamic characteristics
    Lower speed range due to mechanical limitations on the brushes
    Brush Arcing will generate noise causing EMI
     
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  20. TonyL

    TonyL United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Amazing! Thank you. I am looking forward to the when I can pay-it-forward with my knowledge. I got a feeling it is a long way off. Thanks again.
     
  21. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    Note that this is a simplified version of a DC motor. Practical ones have many poles and many commutator segments.
     
  22. mksj

    mksj Active User H-M Supporter-Premium

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    Correct, and so can BLDC motors. There are many variations on poles, wingdings, etc., and this can significantly affect their performance and characteristics of the DC motor. In this case it is doubtful one would see an appreciable difference in the context of this post/specific application. I would expect the BLDC motor to perform better over a wider range, but there are a lot of variables. Same applies for VFD driven 3 phase motors, there can be very wide performance variations between 3 phase motors and also the VFD used.
     
  23. gregc

    gregc United States Active Member Active Member

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    Maybe a few pictures would help. It seems that there may be some confusion and I hope this helps.
    Without getting into the physics and math on why these are true I'll provide a top level picture.

    In a conventional induction or brushed motor the torque decreases with speed and is nearly linear between the stall torque and the max rpm.
    The delivered power is the product of the speed and torque and the power peaks near the maximum unloaded speed.
    The maximum rpm is where the back emf equals the supply voltage.

    upload_2017-4-20_12-32-33.png

    Below is the characteristics of a BLDC motor with proper control.
    At low RPM, the torque is limited by the coil resistance power dissipation.
    Above some critical speed the torque starts to drop inversely with the speed but the delivered power is the
    maximum the motor can deliver for it size and is nearly constant over most of the possible operating range.
    The curves have been exaggerated some for illustration and does not correspond to a real motor.

    The net effect is that this is close to the characteristics of a conventional motor if the motor was kept at the maximum
    delivery power using a transmission or other speed conversion method.

    This is why most machines with BLDC's on have 1 or 2 gear ratios. The electronic controller effectively does the
    work of a transmission over most of the range.

    A more sophisticated controller could allow larger torque outputs at low speed as long as the average dissipated
    power or case temperature does not exceed the design limit. Many BLDC's specify the rms torque vs. speed similar what is below
    and another peak torque curve (maximum torque possible over short time spans).

    The next curve is from the data sheet of a real BLDC. (teknic clear path CPM-MCVC-3441S-RL)
    The curve is somewhat different as this motor spec includes the controller behavior, but is similar with the BLDC characteristics.
    The knee in the second figure near 700 rpm is where the back emf starts to limit the torque (vs. power dissipation) and at 925 rpm the torque is 0 and this
    is where the back emf equals the supply voltage limit. (not shown before). this 0 torque speed is present in the traditional motors
    and applies to all types.

    upload_2017-4-20_12-48-50.png

    upload_2017-4-20_12-56-42.png
     

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