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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. #13
    Community Moderator Al_The_Man's Avatar
    Join Date
    Dec 2003

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

    This months Electrical Business
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  2. #14

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

    There are two great truths in life 1) There ain't no free lunch, and 2) there ain't no substitute for cubic inches.

    As stated above a 3 HP, 4 pole induction motor produces about 9 ft/lb of torque up to its base speed of 1800 RPM (@60 Hz), a brushed DC motor has similar specs, both weigh in at about 95 lbs. Some standard 4 pole motors have an absolute maximum speed of 6000 RPM (Baldor for one) Torque drops off above the base speed. Using a sensorless vector VFD, near full torque can be achieved from near 0 RPM up to the base speed. Brushed DC motors normally have a much lower max RPM rating and constant torque through the RPM range when using a good DC motor controller.

    The ''3HP'' BLDC motors advertized would not come close to the torque performance of a standard 4 pole motor. If you want a real 6000 RPM BLDC spindle motor with the torque performance to match a 3 HP induction motor then you need to get at least a 5.5 KW to 7.5 KW high speed BLDC motor. Parker makes some nice spindle motors, but $$$$$
    Jim Dawson
    Sandy, Oregon, USA

  3. #15
    Join Date
    Jan 2005

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

    Quote Originally Posted by Goemon View Post
    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?
    I don't see anywhere on either the AT or LMS sites where they show any torque curves or make any claims about constant torque. Where do you see this documented?

    Also, nobody said anything about constant power. In general DC motors are neither constant power or constant torque. They have higher torque at 0 rpm than at higher rpms but nobody every said it would equal 1.5 hp at 1 rpm, only that they would have more torque at lower rpm than at higher rpm.

    Attached is a motor parameter graph for a brushless DC motor. As you can see this motor is neither constant power or constant torque but is typical of brushed or brushless DC motors in that the torque is higest at 0 rpm and drops linearly to 0 torque at the motors max no-load rpm. This particular motor produces about 110 oz-in at 0 rpm, 42 oz-in at about 4300 rpm (rated torque), and drops to 0 oz-in at 7200 rpm (no-load). This shows that a brushless DC motor is not constant torque but is inversely proportional to rpm. It is also not constant power since the power output at 0 rpm and the no-load rpm are both 0 and the max power is between these.

    Click image for larger version. 

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