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

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

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

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

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

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

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

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.

--
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

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

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

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

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

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"