• More on fans and voltages.

    From The Natural Philosopher@3:770/3 to All on Sat Aug 26 10:19:20 2023
    DC fans brushless or not, are limited in speed by the simple formula of
    what back EMF per unit RPM they generate. Once their back EMF equals the applied voltage, they won't go any faster.

    In general the power they take will be proportional to the square of the
    rpm, plus any fixed losses.

    Dropping the voltage from 5 to 3, should in fact drop the power a LOT.
    Probably by a factor of 0.6, and the current should drop similarly, so
    overall with a perfect motor around 36% of power draw.

    I've been attaching propellors to electric motors brushed and brushless,
    since 2004 and measuring current, power, and RPM. And then flying them
    in planes.

    Of course there are some fixed losses with all motors, so there is
    always a current they will draw when no fan is attached, but it's hard
    to make a motor so bad that that dominates the equation.

    --
    “Some people like to travel by train because it combines the slowness of
    a car with the cramped public exposure of 
an airplane.”

    Dennis Miller

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  • From Andrew Smallshaw@3:770/3 to The Natural Philosopher on Sat Aug 26 23:14:14 2023
    On 2023-08-26, The Natural Philosopher <tnp@invalid.invalid> wrote:

    DC fans brushless or not, are limited in speed by the simple formula of
    what back EMF per unit RPM they generate. Once their back EMF equals the applied voltage, they won't go any faster.

    For a regular DC brushed motor this is fundamental, albeit a
    reduction to absurdity. If the applied voltage equals the back
    EMF the motor is stationary and both speed and voltage are zero.
    In operation there is _always_ a net applied voltage, it is that
    voltage which powers the motor. The only theoretical time the two
    could be equal in motion if if the motor is itself 100% efficient
    - no resistive or frictional losses - and has no load, in which
    case it is also doing no work.

    This is basic high school level stuff - if the two voltages are
    equal no current flows. Therefore no power is consumed and in turn
    that means no work is done.

    For a BLDC - basically the only game in town for computer fans -
    the two are unconnected. BLDCs are synchronous motors and as such
    the speed is directly controlled by the pulse train from the (on-fan) controller. The fans can't exceed that speed (unless externally
    driven) as there is nothng to accelerate it above that speed. If
    the applied torque is such the fan cannot maintain that programmed
    speed, it does not slow down but stall, as the phase relationship
    between the rotor and stator is lost.

    Dropping the voltage from 5 to 3, should in fact drop the power a LOT. Probably by a factor of 0.6, and the current should drop similarly, so overall with a perfect motor around 36% of power draw.

    This is utterly meaningless. If you drop the voltage to 60% original
    and current remains constant then power is also 60% of the original.
    If the current did drop by the same proportion then at that point
    you end up at 36% power. The high school level error is in the
    assumption that a motor remotely obeys Ohm's law - it's not true
    even to a first approximation.

    To cut a long story short a computer fan is a BLDC motor and its
    operation is governed in its entirety by the behaviour of the
    controller, and that is resistant to naive analysis. As I said I
    haven't actually measured this on more than a few samples over the
    years - perhaps half a dozen - but assuming a constant current draw
    has generally proven a good starting point.

    --
    Andrew Smallshaw
    andrews@sdf.org

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  • From The Natural Philosopher@3:770/3 to Andrew Smallshaw on Sun Aug 27 08:08:44 2023
    On 27/08/2023 00:14, Andrew Smallshaw wrote:
    On 2023-08-26, The Natural Philosopher <tnp@invalid.invalid> wrote:

    DC fans brushless or not, are limited in speed by the simple formula of
    what back EMF per unit RPM they generate. Once their back EMF equals the
    applied voltage, they won't go any faster.

    For a regular DC brushed motor this is fundamental, albeit a
    reduction to absurdity. If the applied voltage equals the back
    EMF the motor is stationary and both speed and voltage are zero.

    Incorrect

    In operation there is _always_ a net applied voltage, it is that
    voltage which powers the motor. The only theoretical time the two
    could be equal in motion if if the motor is itself 100% efficient
    - no resistive or frictional losses - and has no load, in which
    case it is also doing no work.

    You havent understood anything have you?

    This is basic high school level stuff - if the two voltages are
    equal no current flows. Therefore no power is consumed and in turn
    that means no work is done.

    No work is done if a perfect motor is spinning at a constant speed.


    For a BLDC - basically the only game in town for computer fans -
    the two are unconnected. BLDCs are synchronous motors and as such
    the speed is directly controlled by the pulse train from the (on-fan) controller. ]

    You really dont understand it at all do you?

    The fans can't exceed that speed (unless externally
    driven) as there is nothng to accelerate it above that speed. If
    the applied torque is such the fan cannot maintain that programmed
    speed, it does not slow down but stall, as the phase relationship
    between the rotor and stator is lost.

    No, it doesn't. Once again you have no idea how the controller works.

    Dropping the voltage from 5 to 3, should in fact drop the power a LOT.
    Probably by a factor of 0.6, and the current should drop similarly, so
    overall with a perfect motor around 36% of power draw.

    This is utterly meaningless. If you drop the voltage to 60% original
    and current remains constant then power is also 60% of the original.
    If the current did drop by the same proportion then at that point
    you end up at 36% power. The high school level error is in the
    assumption that a motor remotely obeys Ohm's law - it's not true
    even to a first approximation.

    It is hopeless to try and educate someone who has absorbed so little
    knowledge that he thinks he understands everything.

    To cut a long story short a computer fan is a BLDC motor and its
    operation is governed in its entirety by the behaviour of the
    controller, and that is resistant to naive analysis.

    It certainly is in your case.

    As I said I
    haven't actually measured this on more than a few samples over the
    years - perhaps half a dozen - but assuming a constant current draw
    has generally proven a good starting point.

    I have measured it on dozens of samples over the years. As I said I used
    to design model aircraft with BLDC motors and occasionally still do. I
    have an MA in electrical engineering.

    Grandmothers, eggs, suck.

    --
    "Socialist governments traditionally do make a financial mess. They
    always run out of other people's money. It's quite a characteristic of them"

    Margaret Thatcher

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  • From Andrew Smallshaw@3:770/3 to The Natural Philosopher on Sun Aug 27 13:21:54 2023
    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:
    On 27/08/2023 00:14, Andrew Smallshaw wrote:

    This is basic high school level stuff - if the two voltages are
    equal no current flows. Therefore no power is consumed and in turn
    that means no work is done.

    No work is done if a perfect motor is spinning at a constant speed.

    I'm not going to waste too much time with this, I gave some concrete
    arguments. Lack counter evidence you respond with insults.

    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by
    lifting a moderate amount - a metre say, enough to get it to constant
    speed. I now need apply no more energy to continue to lift that
    weight to an infinite height.

    --
    Andrew Smallshaw
    andrews@sdf.org

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  • From Ahem A Rivet's Shot@3:770/3 to Andrew Smallshaw on Sun Aug 27 15:08:40 2023
    On Sun, 27 Aug 2023 13:21:54 -0000 (UTC)
    Andrew Smallshaw <andrews@sdf.org> wrote:

    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:

    No work is done if a perfect motor is spinning at a constant speed.

    True but not very interesting since there is no such thing.

    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by

    Sigh - he hadn't until you added that weight before then it was
    just conservation of energy and momentum in a friction free environment.

    Why don't you two stop trying to one-down each other ? Nobody is
    right all the time, and very few people are wrong all the time so working together tends to be more productive than antagonism and point scoring.

    --
    Steve O'Hara-Smith
    Odds and Ends at http://www.sohara.org/
    Host: Beautiful Theory meet Inconvenient Fact
    Obit: Beautiful Theory died today of factual inconsistency

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  • From Jim H@3:770/3 to All on Mon Aug 28 00:14:08 2023
    On Sun, 27 Aug 2023 13:21:54 -0000 (UTC), in
    <slrnuemjfh.92m.andrews@sdf.org>, Andrew Smallshaw <andrews@sdf.org>
    wrote:

    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:
    On 27/08/2023 00:14, Andrew Smallshaw wrote:

    This is basic high school level stuff - if the two voltages are
    equal no current flows. Therefore no power is consumed and in turn
    that means no work is done.

    No work is done if a perfect motor is spinning at a constant speed.

    I'm not going to waste too much time with this, I gave some concrete >arguments. Lack counter evidence you respond with insults.

    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by
    lifting a moderate amount - a metre say, enough to get it to constant
    speed. I now need apply no more energy to continue to lift that
    weight to an infinite height.


    Exactly! With the air moved by the fan we were originally talking
    about being the load equivalent to the weight in your example.
    Continuous moving air for no expenditure of energy. Oddly enough, if
    the power source (that isn't delivering any power) is turned off, the
    fan stops. Hmmm... ;-)

    And even if no external work were being done, just an unloaded motor
    spinning at constant speed, frictionless for the sake of argument,
    flux in the ferromagnetics (or whatever if the materials are exotic)
    armature and field cores are still changing and that requires power.

    All in all I'd say there is no "perfect" motor possible even in theory
    - perfect meaning drawing no power when operating - because even
    absent friction there are going to be core losses from the changing
    magnetic field.

    To be way overly generous here, I'm guessing some sort of ChatGPT
    output was requested and misinterpreted and no explanation will
    suffice for NP since the knowledge required to understand it seems to
    be lacking.
    --
    Jim H

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  • From Jim H@3:770/3 to Shot on Mon Aug 28 00:21:46 2023
    On Sun, 27 Aug 2023 15:08:40 +0100, in <20230827150840.a1fafde1ad91627588d7dd32@eircom.net>, Ahem A Rivet's
    Shot <steveo@eircom.net> wrote:

    On Sun, 27 Aug 2023 13:21:54 -0000 (UTC)
    Andrew Smallshaw <andrews@sdf.org> wrote:

    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:

    No work is done if a perfect motor is spinning at a constant speed.

    True but not very interesting since there is no such thing.

    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by

    Sigh - he hadn't until you added that weight before then it was
    just conservation of energy and momentum in a friction free environment.

    Actually he had... tho he didn't realize it, and neither do you...
    since core losses occur when flux is changed, a requirement for a
    motor (or transformer), work is done to change the flux and heat the
    core as a result, even if there is no friction or no load.

    Why don't you two stop trying to one-down each other ? Nobody is
    right all the time, and very few people are wrong all the time so working >together tends to be more productive than antagonism and point scoring.

    Sometimes nonsense is too much to keep quiet about.

    --
    Jim H

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  • From The Natural Philosopher@3:770/3 to Andrew Smallshaw on Mon Aug 28 10:17:24 2023
    On 27/08/2023 14:21, Andrew Smallshaw wrote:
    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:
    On 27/08/2023 00:14, Andrew Smallshaw wrote:

    This is basic high school level stuff - if the two voltages are
    equal no current flows. Therefore no power is consumed and in turn
    that means no work is done.

    No work is done if a perfect motor is spinning at a constant speed.

    I'm not going to waste too much time with this, I gave some concrete arguments. Lack counter evidence you respond with insults.

    I'm not going to waste too much time with this, I gave some concrete
    arguments. Lack counter evidence you respond with insults.

    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by
    lifting a moderate amount - a metre say, enough to get it to constant
    speed. I now need apply no more energy to continue to lift that
    weight to an infinite height.


    Oh dear. Really scraping the barrel aren't we? Where in my post hqd I a
    pulley connected to a weight?



    --
    “The urge to save humanity is almost always only a false face for the
    urge to rule it.”
    – H. L. Mencken

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  • From The Natural Philosopher@3:770/3 to Jim H on Mon Aug 28 10:34:46 2023
    On 28/08/2023 01:21, Jim H wrote:
    On Sun, 27 Aug 2023 15:08:40 +0100, in <20230827150840.a1fafde1ad91627588d7dd32@eircom.net>, Ahem A Rivet's
    Shot <steveo@eircom.net> wrote:

    On Sun, 27 Aug 2023 13:21:54 -0000 (UTC)
    Andrew Smallshaw <andrews@sdf.org> wrote:

    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:

    No work is done if a perfect motor is spinning at a constant speed.

    True but not very interesting since there is no such thing.

    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by

    Sigh - he hadn't until you added that weight before then it was
    just conservation of energy and momentum in a friction free environment.

    Actually he had... tho he didn't realize it, and neither do you...
    since core losses occur when flux is changed, a requirement for a
    motor (or transformer), work is done to change the flux and heat the
    core as a result, even if there is no friction or no load.

    Indeed. In a perfect motor we assume the losses do not exist, and then
    to get to grips with the real world, we add them in.

    The point beong it would be pretty damn stupid to design a motor where
    such loses dominate the performance of the motor, as the OP claimed (implicitly) that they did, when he said the current didnt change under
    applied voltage.


    Sometimes nonsense is too much to keep quiet about.

    Yup.

    Its fine to go around believing in gender dysphoria until you start
    slicing off the genitals of people too young to understand.

    Its fine to believe in' climate change' until you destroy a nations
    economy by 'investing' in power generation that whilst wholly virtuous,
    simply doesnt work in a holistic situation. And makes absolutely no
    measurable impact on 'carbon emissions' anyway

    Sometimes you just have to yell 'bullshit'

    --
    I would rather have questions that cannot be answered...
    ...than to have answers that cannot be questioned

    Richard Feynman

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  • From The Natural Philosopher@3:770/3 to Ahem A Rivet's Shot on Mon Aug 28 10:27:06 2023
    On 27/08/2023 15:08, Ahem A Rivet's Shot wrote:
    On Sun, 27 Aug 2023 13:21:54 -0000 (UTC)
    Andrew Smallshaw <andrews@sdf.org> wrote:

    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:

    No work is done if a perfect motor is spinning at a constant speed.

    True but not very interesting since there is no such thing.

    The issue is that under no load a motor is best analysed as 'perfect'
    with a few quantifiable 'imperfections'.

    That is how it is done.. for very good reason. We add in 'idle current'
    which represents fixed losses due to magnetic hysteresis and friction,
    and we add in resistive loses due the the resistance of the windings.
    That gives a very accurate pitcure of moror behavior, and along with the
    Kv - the back EMF constant, defines the motor behaviour theoretically
    and experimentally. Kv, Io and Ro - those are the motors characteristic. Nowhere is it speed defined by frequency, for the simple reason that the frequency is controlled by the motors rotational speed. A brushless DC
    motor is simple DC motor whose commutator has been replaced by an H
    bridge of power FETs, and a sensor circuit that senses the position of
    the rotor either by sensors or by monitoring the unenergised pole(s) of
    the motor.
    To confuse it with a synchronous motor is akin to saying that you
    control a cars speed by varying the ignition timing.



    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by

    Sigh - he hadn't until you added that weight before then it was
    just conservation of energy and momentum in a friction free environment.

    Why don't you two stop trying to one-down each other ? Nobody is
    right all the time, and very few people are wrong all the time so working together tends to be more productive than antagonism and point scoring.

    People want to know if their fans will run safely on 3V. I have tried to explain why this is (probably) true. RPM and current draw is deeply
    sensitive to applied voltage.


    --
    “The urge to save humanity is almost always only a false face for the
    urge to rule it.”
    – H. L. Mencken

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  • From The Natural Philosopher@3:770/3 to Ahem A Rivet's Shot on Mon Aug 28 10:40:28 2023
    On 28/08/2023 08:04, Ahem A Rivet's Shot wrote:
    On Mon, 28 Aug 2023 00:21:46 +0000
    Jim H <invalid@invalid.invalid> wrote:

    On Sun, 27 Aug 2023 15:08:40 +0100, in
    <20230827150840.a1fafde1ad91627588d7dd32@eircom.net>, Ahem A Rivet's
    Shot <steveo@eircom.net> wrote:

    Sigh - he hadn't until you added that weight before then it was
    just conservation of energy and momentum in a friction free environment.

    Actually he had... tho he didn't realize it, and neither do you...
    since core losses occur when flux is changed, a requirement for a

    He specified "perfect" so no core losses.

    I had assumed since the OP claimed at least high school physics that he understood the term.

    The ideal motor has frictionless bearings, runs in a vacuum and has superconducting windings and no hysteresis losses in its core - in fact
    many motors do not use iron at all for that reasons.

    It only draws current when it has to drive a load. Its speed will never
    vary under load. It runs at the speed where the back emf equals the
    applied voltage. Adding more load simply increases the current it draws.

    Same as a 'perfect' transformer. The output voltage doesn't vary under
    load, only the input current does.



    --
    Ideas are more powerful than guns. We would not let our enemies have
    guns, why should we let them have ideas?

    Josef Stalin

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  • From The Natural Philosopher@3:770/3 to Jim H on Mon Aug 28 10:43:24 2023
    On 28/08/2023 01:14, Jim H wrote:
    On Sun, 27 Aug 2023 13:21:54 -0000 (UTC), in <slrnuemjfh.92m.andrews@sdf.org>, Andrew Smallshaw <andrews@sdf.org>
    wrote:

    On 2023-08-27, The Natural Philosopher <tnp@invalid.invalid> wrote:
    On 27/08/2023 00:14, Andrew Smallshaw wrote:

    This is basic high school level stuff - if the two voltages are
    equal no current flows. Therefore no power is consumed and in turn
    that means no work is done.

    No work is done if a perfect motor is spinning at a constant speed.

    I'm not going to waste too much time with this, I gave some concrete
    arguments. Lack counter evidence you respond with insults.

    Instead let me be the first to congratulate you in advance on your
    Nobel prize. You've just broken the conservation of energy. I
    have a motor connected to a pully lifting a weight. I begin by
    lifting a moderate amount - a metre say, enough to get it to constant
    speed. I now need apply no more energy to continue to lift that
    weight to an infinite height.


    Exactly! With the air moved by the fan we were originally talking
    about being the load equivalent to the weight in your example.
    Continuous moving air for no expenditure of energy. Oddly enough, if
    the power source (that isn't delivering any power) is turned off, the
    fan stops. Hmmm... ;-)

    And even if no external work were being done, just an unloaded motor
    spinning at constant speed, frictionless for the sake of argument,
    flux in the ferromagnetics (or whatever if the materials are exotic)
    armature and field cores are still changing and that requires power.

    All in all I'd say there is no "perfect" motor possible even in theory
    - perfect meaning drawing no power when operating - because even
    absent friction there are going to be core losses from the changing
    magnetic field.

    To be way overly generous here, I'm guessing some sort of ChatGPT
    output was requested and misinterpreted and no explanation will
    suffice for NP since the knowledge required to understand it seems to
    be lacking.

    I have no idea where the OP got his so called 'knowledge' from. Mine
    comes from absolutely standard high school electrical motor theory. At
    least, in my day, it was high school. These days its probably PhD
    standard material that the student 'rediscovers' after reading 20th
    century text books.


    --
    "I am inclined to tell the truth and dislike people who lie consistently.
    This makes me unfit for the company of people of a Left persuasion, and
    all women"

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