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IndustryArena Forum > CNC Electronics > Spindles / VFD > Is there any actual upside to gear-free DC spindle motors?
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  1. #1
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    Is there any actual upside to gear-free DC spindle motors?

    Just curious why people buy these seemingly over-priced DC spindle motors like this:

    https://www.automationtechnologiesin...or-and-driver/

    They seem to be on a large portion of mini / benchtop mills and stores like Little Machine Shop seem to think that they offer "tremendous low end torque". They essentially imply that they offer the same low speed torque as motors with a gear reduction but without the gears.

    Obviously, no gears are better than plastic gears if you get the same torque but is this really the case? LMS offers no info of what sort of torque they deliver which seems odd if it's such a selling point.

    The 1.5hp variant sold by Automation technologies only offers 1.7ft lb of torque which suggests they offer no more torque than any other type of motor with equivalent power. So why would anyone buy one when they are at least 2 to 3 times more expensive than your average ebay 4 pole 1725rpm induction motor plus a VFD?

  2. #2
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    Re: Is there any actual upside to gear-free DC spindle motors?

    I believe that they are a lot smaller than the equivalent power induction motor. I used a DMM 1.8kw AC servo so that I could do rigid tapping. I'm still geared by toothed belt to get more RPM at the spindle.

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    Re: Is there any actual upside to gear-free DC spindle motors?

    Quote Originally Posted by rcheli View Post
    I believe that they are a lot smaller than the equivalent power induction motor. I used a DMM 1.8kw AC servo so that I could do rigid tapping. I'm still geared by toothed belt to get more RPM at the spindle.
    I initially thought that too until I looked at the specs in detail. The Automation technology ones have a rated speed of 6000 rpm. I.e. They don't hit 3hp until they reach 6000rpm so they would deliver a constant 2.6ft lb of torque. A typical 3hp 4 pole induction motor has a base speed of 1725 rpm giving you a constant 9.1ft lb of torque before any gearing. At 1725 rpm, the DC motor from AT only reaches 0.85hp so we are not comparing like with like.

    My guess is that, if we found a DC spindle motor capable of reaching 3hp at 1725rpm and /or 9.1ft lb of torque, it would be equally big and heavy next to a 3hp 4 pole induction motor.

    It is kinda hard to prove this either way though as only AT provided any meaningful specs on their DC drives. If anyone here has a 3hp dc drive that is capable of delivering 9.1ft lb of torque (before gearing), please tell us how much it weighs?

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    Re: Is there any actual upside to gear-free DC spindle motors?

    For reference, the 2.2kw 6000rpm induction motor spindle made by GMT is about the same size as the 2.2kw 6000rpm dc motor sold by AT once you have made a small adjustment for the integrated toolholder on the GMT spindle:

    GMT Air Cooled CNC Spindle 2.2 kW 220V / 380V 6000RPM S | Air Cooled Spindles

  5. #5
    Super Moderator Al_The_Man's Avatar
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    Re: Is there any actual upside to gear-free DC spindle motors?

    Typically quality DC and BLDC motors exhibit maxim torque at zero RPM, with the continuous torque curve fairly flat up to max RPM.
    See the Manuf. torque specs if published.
    There is a sometimes a confusion between torque and HP.
    DC brushed motors are typically used in treadmills and run at quite a low rpm with High torque.
    Al.
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    Re: Is there any actual upside to gear-free DC spindle motors?

    Quote Originally Posted by Al_The_Man View Post
    Typically quality DC and BLDC motors exhibit maxim torque at zero RPM, with the continuous torque curve fairly flat up to max RPM.
    See the Manuf. torque specs if published.
    There is a sometimes a confusion between torque and HP.
    DC brushed motors are typically used in treadmills and run at quite a low rpm with High torque.
    Al.
    The specs on the automation technologies dc spindle motors don't support that. Their 1.5hp and 3hp motors have base speeds of 8000rpm and 6000rpm respectively. They deliver no more torque (at any speed) than an equivalent spec induction motor.

    Similarly, Little Machine shop advertises that their DC spindle motors deliver constant torque throughout their range which means their base speed is the same as their stated top speed. In other words, there is no additional low speed torque (without adding gears) there either.

    There is no confusion on torque VS horsepower here. They are linked though. Torque is just horsepower at a given speed. 3hp at 6000rpm is always 2.6 ft lbs. 3hp at 2000rpm is always 7.9ft lb. With motor speed, torque and horsepower, if you know any two of these then you can work out the 3rd using a calc like this:

    https://spicerparts.com/calculators/...que-calculator


    In order to achieve the same sort of low end torque you get on a geared milling head but without any gearing, you would need a motor that hit peak horsepower at a super low speed (probably around 100rpm). Then torque would decline as speed increased (as it does with geared heads). I can't find any DC spindle motors that actually do that though. A 3hp motor with a base speed of 100rpm (158 ft lb) would be enormous.

  7. #7
    Community Moderator ger21's Avatar
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    Re: Is there any actual upside to gear-free DC spindle motors?

    Their 1.5hp and 3hp motors have base speeds of 8000rpm and 6000rpm respectively
    These are the speeds at the rated voltage.

    These motors are like servo motors, where torque is constant across all rpm.
    Gerry

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  8. #8

    Re: Is there any actual upside to gear-free DC spindle motors?

    The difference is in limited duty and continuous duty. A DC motor or Servo will exhibit much higher limited duty starting torque, something like a 50% increase without the gear reduction. It will however create a constant continuous duty torque. Motors are generally rated in continuous duty for the obvious reasons.

    Automation Tech does a pretty poor job of posting documents, it would be nice if they had curves. But if you look up other motors you will find graphs where limited duty is shown. For a mill spindle, limited duty is extremely useful right up until you have crazy long run times. For example I used to over heat my 1300w continuous spindle when trying to use too large of tools. The VFD would shut it down when overheat was detected, I just had to learn to keep it in the continuous zone, only using limited duty for special cases.

  9. #9
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    Re: Is there any actual upside to gear-free DC spindle motors?

    Just curious why people buy these seemingly over-priced DC spindle motors like this:

    https://www.automationtechnologiesin...or-and-driver/

    They seem to be on a large portion of mini / benchtop mills and stores like Little Machine Shop seem to think that they offer "tremendous low end torque". They essentially imply that they offer the same low speed torque as motors with a gear reduction but without the gears.
    Are you specifically referring to the brushless DC motors like the one you linked? I mostly see those type brushless motors on machines that either previously came with brushed variable speed DC motors, or similar designs. Mini-mills, mini-lathes, PM-25/G0704 type machines, etc.. I don't generally see them on machines that had induction motors and gearboxes.

    Obviously, no gears are better than plastic gears if you get the same torque but is this really the case? LMS offers no info of what sort of torque they deliver which seems odd if it's such a selling point.

    The 1.5hp variant sold by Automation technologies only offers 1.7ft lb of torque which suggests they offer no more torque than any other type of motor with equivalent power. So why would anyone buy one when they are at least 2 to 3 times more expensive than your average ebay 4 pole 1725rpm induction motor plus a VFD?
    A DC motor, brushless or brushed provides maximum torque at 0 rpm decreasing about linearly as rpm increases. A VFD controlled AC motor offers constant torque below the base speed. So if the brushless and AC motors have the same torque at say 1800 rpm, the AC motor will have less torque at lower rpms. So, for low rpm use you would need a much larger AC motor to have the required torque, or gear it down to preserve rpm and thus torque. So the benefit is the high torque at low rpm and less complexity by not needing a geartrain.

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    Re: Is there any actual upside to gear-free DC spindle motors?

    Quote Originally Posted by ger21 View Post
    These are the speeds at the rated voltage.

    These motors are like servo motors, where torque is constant across all rpm.

    All motors produce constant torque up to the rated voltage / speed / hp. It's just the speed at which the motor hits maximum power. The specs for the 1.5hp motor on the AT site state a torque of 1.7nm (or 1.3 ft lb) which means it hits peak hp at 6000rpm. If hp does not rise past 1.5 then the torque would obviously decline from 6000rpm to 9000rpm as it switches from constant torque to constant power.

    The key point I am making here is that, if you are buying these motors for a hard metal milling machine (the purpose they are sold for), you would still need a gear reduction to deliver low speed torque. I.e. there seems to be no advantage over buying a regular motor plus a vfd, and yet, they cost a good bit more.

    You would get over 3 times more low speed torque with a 1.5hp 4 pole 1750rpm induction motor (with no sacrifice in higher speed torque) and they sell new for a fraction of the price. So why do people buy DC motor drives?

    - - - Updated - - -

    Quote Originally Posted by ger21 View Post
    These are the speeds at the rated voltage.

    These motors are like servo motors, where torque is constant across all rpm.

    All motors produce constant torque up to the rated voltage / speed / hp. It's just the speed at which the motor hits maximum power. The specs for the 1.5hp motor on the AT site state a torque of 1.7nm (or 1.3 ft lb) which means it hits peak hp at 6000rpm. If hp does not rise past 1.5 then the torque would obviously decline from 6000rpm to 9000rpm as it switches from constant torque to constant power.

    The key point I am making here is that, if you are buying these motors for a hard metal milling machine (the purpose they are sold for), you would still need a gear reduction to deliver low speed torque. I.e. there seems to be no advantage over buying a regular motor plus a vfd, and yet, they cost a good bit more.

    You would get over 3 times more low speed torque with a 1.5hp 4 pole 1750rpm induction motor (with no sacrifice in higher speed torque) and they sell new for a fraction of the price. So why do people buy DC motor drives?

  11. #11
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    Re: Is there any actual upside to gear-free DC spindle motors?

    Quote Originally Posted by CL_MotoTech View Post
    The difference is in limited duty and continuous duty. A DC motor or Servo will exhibit much higher limited duty starting torque, something like a 50% increase without the gear reduction. It will however create a constant continuous duty torque. Motors are generally rated in continuous duty for the obvious reasons.

    Automation Tech does a pretty poor job of posting documents, it would be nice if they had curves. But if you look up other motors you will find graphs where limited duty is shown. For a mill spindle, limited duty is extremely useful right up until you have crazy long run times. For example I used to over heat my 1300w continuous spindle when trying to use too large of tools. The VFD would shut it down when overheat was detected, I just had to learn to keep it in the continuous zone, only using limited duty for special cases.
    The poor documentation from AT and little Machine Shop certainly does not help here.

    I'm not really clear on what you are saying about how these DC motors differ from induction motors. When I cut something on my machine, the spindle is running at the speed I want well before the end mill touches metal. So, how does additional starting torque help? I.e. Why does anything except the constant torque figure matter?

    In order to produce an additional burst of torque (above the stated constant torque number), the motors would need to draw more power. I assume that you only have access to the same limited specs as I do. Where does this info come from? I.e what makes you think that the 1.5hp DC spindle motor from AT is capable of rising meaningfully above the stated constant 1.7nm?

    Motors overheating is a different issue. I am going to assume that, with any type of motor, the appropriate cooling method is necessary for the type of work. If you are going to run a shaft fan motor at low speeds, cooling will be an issue.

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    Re: Is there any actual upside to gear-free DC spindle motors?

    Quote Originally Posted by 109jb View Post
    Are you specifically referring to the brushless DC motors like the one you linked? I mostly see those type brushless motors on machines that either previously came with brushed variable speed DC motors, or similar designs. Mini-mills, mini-lathes, PM-25/G0704 type machines, etc.. I don't generally see them on machines that had induction motors and gearboxes.


    A DC motor, brushless or brushed provides maximum torque at 0 rpm decreasing about linearly as rpm increases. A VFD controlled AC motor offers constant torque below the base speed. So if the brushless and AC motors have the same torque at say 1800 rpm, the AC motor will have less torque at lower rpms. So, for low rpm use you would need a much larger AC motor to have the required torque, or gear it down to preserve rpm and thus torque. So the benefit is the high torque at low rpm and less complexity by not needing a geartrain.

    The specs on the AT and LMS site don't support this claim. Both show constant torque up to the base speed and, in both cases, the base speed on their DC spindle motors is a good bit higher than your average 4 pole induction so they produce a fraction of the torque.

    Constant torque means just that. Without gearing, they produce the same torque from zero to the base speed. It's a nice story but does anyone really believe these Dc motors are capable of hitting 1.5hp at 1rpm and therefore producing 7878 ft lb of torque without any gearing?

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