How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
On 08/09/2026 03:33, Don Y wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Will they be exposed to identical environments?
Differences in
air flow resistance and dirt buildup are going to cause variation.
It would be a very unusual environment that is exactly
symmetrical.˙ Even if they start very closely matched the bearings
will probably deteriorate at different rates.
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Will they be exposed to identical environments?
Yes.˙ Imagine them side by side on an infinite baffle.
Differences in
air flow resistance and dirt buildup are going to cause variation.
It would be a very unusual environment that is exactly
symmetrical.˙ Even if they start very closely matched the bearings
will probably deteriorate at different rates.
But this doesn't address the electrical issue.
E.g., if I provide the same PWM *control* signal to two
"same make/model" fans, will they rotate at the same rate?
I.e., is the PWM talking to a controller as anything other
than a "digital representation of an analog signal"?
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
What I see of fan specifications suggests the tach is
consistently designed ("two pulses per revolution" -- though
that doesn't mean they are displaced by 180 degrees).
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).˙ So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
produced from molds with reasonably tight tolerances so
swapping the *blades* from two fans will not alter their
actual airflow characteristics.˙ Things like bearings
should degrade at comparable rates.]
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Will they be exposed to identical environments?
Yes.˙ Imagine them side by side on an infinite baffle.
Differences in
air flow resistance and dirt buildup are going to cause variation.
It would be a very unusual environment that is exactly
symmetrical.˙ Even if they start very closely matched the bearings
will probably deteriorate at different rates.
But this doesn't address the electrical issue.
E.g., if I provide the same PWM *control* signal to two
"same make/model" fans, will they rotate at the same rate?
I.e., is the PWM talking to a controller as anything other
than a "digital representation of an analog signal"?
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
What I see of fan specifications suggests the tach is
consistently designed ("two pulses per revolution" -- though
that doesn't mean they are displaced by 180 degrees).
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).˙ So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
Yes, they will rotate at slightly different rates.
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
What I see of fan specifications suggests the tach is
consistently designed ("two pulses per revolution" -- though
that doesn't mean they are displaced by 180 degrees).
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).˙ So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
Yes, they will rotate at slightly different rates.
[I assume the blades and other air flow components are
produced from molds with reasonably tight tolerances so
swapping the *blades* from two fans will not alter their
actual airflow characteristics.˙ Things like bearings
should degrade at comparable rates.]
On 9/8/2026 4:09 AM, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Will they be exposed to identical environments?
Yes.? Imagine them side by side on an infinite baffle.
Differences in
air flow resistance and dirt buildup are going to cause variation.
It would be a very unusual environment that is exactly
symmetrical.? Even if they start very closely matched the bearings
will probably deteriorate at different rates.
But this doesn't address the electrical issue.
E.g., if I provide the same PWM *control* signal to two
"same make/model" fans, will they rotate at the same rate?
I.e., is the PWM talking to a controller as anything other
than a "digital representation of an analog signal"?
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
I don't believe I *claimed* they were used in a fan
(though many are BLDC -- which is a special case of
a stepper motor, electrically commutated) rather, to
illustrate a point: that the signal(s) directly
controlled the ROTATION (instead of acting as
a digitization of an *analog* control signal that
tries to control the RATE of rotation).
What I see of fan specifications suggests the tach is
consistently designed ("two pulses per revolution" -- though
that doesn't mean they are displaced by 180 degrees).
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).? So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
Yes, they will rotate at slightly different rates.
So, I should "close the loop" for each fan and then drive
the *setpoints* with the same signal.
On Tue, 8 Sep 2026 04:56:41 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:
On 9/8/2026 4:09 AM, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Will they be exposed to identical environments?
Yes.˙ Imagine them side by side on an infinite baffle.
Differences in
air flow resistance and dirt buildup are going to cause variation.
It would be a very unusual environment that is exactly
symmetrical.˙ Even if they start very closely matched the bearings
will probably deteriorate at different rates.
But this doesn't address the electrical issue.
E.g., if I provide the same PWM *control* signal to two
"same make/model" fans, will they rotate at the same rate?
I.e., is the PWM talking to a controller as anything other
than a "digital representation of an analog signal"?
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
I don't believe I *claimed* they were used in a fan
(though many are BLDC -- which is a special case of
a stepper motor, electrically commutated) rather, to
illustrate a point: that the signal(s) directly
controlled the ROTATION (instead of acting as
a digitization of an *analog* control signal that
tries to control the RATE of rotation).
What I see of fan specifications suggests the tach is
consistently designed ("two pulses per revolution" -- though
that doesn't mean they are displaced by 180 degrees).
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).˙ So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
Yes, they will rotate at slightly different rates.
So, I should "close the loop" for each fan and then drive
the *setpoints* with the same signal.
We often have a temperature sensor somewhere and control the fan
speeds from that. Big fans are noisy so we only want to go max when we
have to. We also limit the rate of change of fan speed, to not make
shocking sounds.
Air flow is entirely perverse. It doesn't do anything reasonable.
Even the flow direction can be a surprise.
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
On 9/09/2026 12:54 am, john larkin wrote:
On Tue, 8 Sep 2026 04:56:41 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:
On 9/8/2026 4:09 AM, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same >>>>>>> "airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Will they be exposed to identical environments?
Yes.? Imagine them side by side on an infinite baffle.
Differences in
air flow resistance and dirt buildup are going to cause variation. >>>>>> It would be a very unusual environment that is exactly
symmetrical.? Even if they start very closely matched the bearings >>>>>> will probably deteriorate at different rates.
But this doesn't address the electrical issue.
E.g., if I provide the same PWM *control* signal to two
"same make/model" fans, will they rotate at the same rate?
I.e., is the PWM talking to a controller as anything other
than a "digital representation of an analog signal"?
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
I don't believe I *claimed* they were used in a fan
(though many are BLDC -- which is a special case of
a stepper motor, electrically commutated) rather, to
illustrate a point: that the signal(s) directly
controlled the ROTATION (instead of acting as
a digitization of an *analog* control signal that
tries to control the RATE of rotation).
What I see of fan specifications suggests the tach is
consistently designed ("two pulses per revolution" -- though
that doesn't mean they are displaced by 180 degrees).
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).? So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
Yes, they will rotate at slightly different rates.
So, I should "close the loop" for each fan and then drive
the *setpoints* with the same signal.
We often have a temperature sensor somewhere and control the fan
speeds from that. Big fans are noisy so we only want to go max when we
have to. We also limit the rate of change of fan speed, to not make
shocking sounds.
Air flow is entirely perverse. It doesn't do anything reasonable.
There's no aviation industry. Planes would fall out of the sky at
random, if anybody had been silly enough to try and invent heavier than
air flight.
Even the flow direction can be a surprise.
To John Larkin. In fact air flow is merely complicated. It goes from
laminar at low air speeds (when the Reynolds number is less than 2400)
to turbulent at higher speeds. There's always a region of laminar flow
close to a surface.
At higher airspeeds the air flow behind a restriction to the flow can do >complicated stuff - I worked on a flow rate meter that counted the
number of von Karman vortices shed per second behind a bluff body.
https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street
The
https://en.wikipedia.org/wiki/Coand%C4%83_effect
is even more counter-intuitive, but predictable enough to be exploited.
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).˙ So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
On 8/09/2026 9:09 pm, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are talking about).
Synchronous motors are driven by alternating current, as are induction motors.
https://www.tlclam.net/synchronous-motor-vs-induction-motor/
Induction motors slip - the more heavily loaded they are the more the rotation rate lags the rotation rate of the applied magnetic field.
Synchronous motors don't. The rotation rate always follows that of the applied magnetic field, with a phase lag that gets larger as the load
gets heavier.
On Mon, 7 Sep 2026 19:33:34 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
If you want consistency, I suggest you use brushless drone motors and
ESC's (electronic speed controllers). I don't know what you're trying
to accomplish, but if it's stable air flow, methinks you'll do better
using existing drone technology.
For "airflow performance", it's customary to use a turbine drivenWith the tacho indicating actual rotation (and not just commutation),
anemometer, thermal anemometer or Pitot tube. These work well inside
a duct but badly in open air or around a tangle of airflow obstacles.
On Wed, 9 Sep 2026 01:48:20 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
On 9/09/2026 12:54 am, john larkin wrote:
On Tue, 8 Sep 2026 04:56:41 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:
On 9/8/2026 4:09 AM, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same >>>>>>>> "airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same >>>>>>>> PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Will they be exposed to identical environments?
Yes.˙ Imagine them side by side on an infinite baffle.
Differences in
air flow resistance and dirt buildup are going to cause variation. >>>>>>> It would be a very unusual environment that is exactly
symmetrical.˙ Even if they start very closely matched the bearings >>>>>>> will probably deteriorate at different rates.
But this doesn't address the electrical issue.
E.g., if I provide the same PWM *control* signal to two
"same make/model" fans, will they rotate at the same rate?
I.e., is the PWM talking to a controller as anything other
than a "digital representation of an analog signal"?
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
I don't believe I *claimed* they were used in a fan
(though many are BLDC -- which is a special case of
a stepper motor, electrically commutated) rather, to
illustrate a point: that the signal(s) directly
controlled the ROTATION (instead of acting as
a digitization of an *analog* control signal that
tries to control the RATE of rotation).
What I see of fan specifications suggests the tach is
consistently designed ("two pulses per revolution" -- though
that doesn't mean they are displaced by 180 degrees).
But, the PWM signal appears more of a suggestion, subject to
tolerances (10%!).˙ So, conceivably, identical fans could rotate
at different speeds when driven by the same signal.
Yes, they will rotate at slightly different rates.
So, I should "close the loop" for each fan and then drive
the *setpoints* with the same signal.
We often have a temperature sensor somewhere and control the fan
speeds from that. Big fans are noisy so we only want to go max when we
have to. We also limit the rate of change of fan speed, to not make
shocking sounds.
Air flow is entirely perverse. It doesn't do anything reasonable.
There's no aviation industry. Planes would fall out of the sky at
random, if anybody had been silly enough to try and invent heavier than
air flight.
The ultimate test of air flow theory is wind tunnels.
SF State University 3D prints their own wind tunnels. Cal Poly has a hypersonic wind tunnel.
Even the flow direction can be a surprise.
To John Larkin. In fact air flow is merely complicated. It goes from
laminar at low air speeds (when the Reynolds number is less than 2400)
to turbulent at higher speeds. There's always a region of laminar flow
close to a surface.
I've seen expensive card cages, with fan trays blowing air upward,
where some card slots have near zero or reverse air flow.
At higher airspeeds the air flow behind a restriction to the flow can do
complicated stuff - I worked on a flow rate meter that counted the
number of von Karman vortices shed per second behind a bluff body.
https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street
The
https://en.wikipedia.org/wiki/Coand%C4%83_effect
is even more counter-intuitive, but predictable enough to be exploited.
On 2026-09-08 16:12, Bill Sloman wrote:
On 8/09/2026 9:09 pm, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are
talking about).
Synchronous motors are driven by alternating current, as are induction
motors.
https://www.tlclam.net/synchronous-motor-vs-induction-motor/
Induction motors slip - the more heavily loaded they are the more the
rotation rate lags the rotation rate of the applied magnetic field.
Synchronous motors don't. The rotation rate always follows that of the
applied magnetic field, with a phase lag that gets larger as the load
gets heavier.
They can slip if the torque is big enough. I have seen it.
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are talking about).
On 9/09/2026 3:46 am, Carlos E.R. wrote:
On 2026-09-08 16:12, Bill Sloman wrote:
On 8/09/2026 9:09 pm, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are
talking about).
Synchronous motors are driven by alternating current, as are induction
motors.
https://www.tlclam.net/synchronous-motor-vs-induction-motor/
Induction motors slip - the more heavily loaded they are the more the
rotation rate lags the rotation rate of the applied magnetic field.
Synchronous motors don't. The rotation rate always follows that of the
applied magnetic field, with a phase lag that gets larger as the load
gets heavier.
They can slip if the torque is big enough. I have seen it.
They don't slip. They skip steps and eventually stop rotating at all.
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Bill Sloman <bill.sloman@ieee.org> wrote:
[...]
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are
talking about).
Whilst stepper motors are technically a type of synchronous motor,
people who know what they are talking about will not call them
"synchronous motors" so as to avoid confusion with sinewave-driven >synchronous motors.
Don Y <blockedofcourse@foo.invalid> wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Years ago I performed some rough-and-ready experiments on small fans to
see if the airflow was consistent in practical environments. The
general conclusion was that the more efficient the fan, the greater was
the drop in performance when the blades acquired a thin coating of dirt
and dust. We finished up using a very inefficient blade shape so as to
keep the airflow constant over a wet bulb thermometer.
Or rotate backwards - or oscillate. The magnetic field has compliance
and the rotor has inertia, so they will form an oscillating system at
certain stepping frequencies - this can happen even it there is no load.
A 'soggy' resilient coupling between the motor and the inertia of the
load is a convenient way of damping this out.
The designer may choose to to mount the motor resiliently because of the noise caused by the stepping; in combination with the mass of the motor,But most of these can be addressed in design.
this can also be excited into resonance by the torque reaction. Stepper motors have a lot of hidden snags.
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Bill Sloman <bill.sloman@ieee.org> wrote:
[...]
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are
talking about).
Whilst stepper motors are technically a type of synchronous motor,
people who know what they are talking about will not call them
"synchronous motors" so as to avoid confusion with sinewave-driven synchronous motors.
Bill Sloman <bill.sloman@ieee.org> wrote:
On 9/09/2026 3:46 am, Carlos E.R. wrote:
On 2026-09-08 16:12, Bill Sloman wrote:
On 8/09/2026 9:09 pm, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are >>>> talking about).
Synchronous motors are driven by alternating current, as are induction >>>> motors.
https://www.tlclam.net/synchronous-motor-vs-induction-motor/
Induction motors slip - the more heavily loaded they are the more the
rotation rate lags the rotation rate of the applied magnetic field.
Synchronous motors don't. The rotation rate always follows that of the >>>> applied magnetic field, with a phase lag that gets larger as the load
gets heavier.
They can slip if the torque is big enough. I have seen it.
They don't slip. They skip steps and eventually stop rotating at all.
Or rotate backwards - or oscillate. The magnetic field has compliance
and the rotor has inertia, so they will form an oscillating system at
certain stepping frequencies - this can happen even it there is no load.
A 'soggy' resilient coupling between the motor and the inertia of the
load is a convenient way of damping this out.
The designer may choose to to mount the motor resiliently because of the noise caused by the stepping; in combination with the mass of the motor,
this can also be excited into resonance by the torque reaction. Stepper motors have a lot of hidden snags.
On 9/8/2026 8:49 AM, Jeff Liebermann wrote:
On Mon, 7 Sep 2026 19:33:34 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
If you want consistency, I suggest you use brushless drone motors and
ESC's (electronic speed controllers).˙ I don't know what you're trying
to accomplish, but if it's stable air flow, methinks you'll do better
using existing drone technology.
I don't see why a regular 4 (or 3) wire fan can't give consistent
performance -- if you close the loop around it.˙ They're already designed
to be used to move air.
What I was wondering was how realistic using the tacho feedback
from *one* fan would be as an indicator of how N fans, driven from the
same control signal, would behave.
Like driving a set of lamps from one control signal based
on the sensed state of *one* of those lamps.
E.g., it is not uncommon to see a bunch of fans, side-by-side, in
a blade server, seemingly operating at the same setting.˙ *The* setting defined by *one* criteria in the box.
It seems the safer bet is to close the loop around *each* fan
and then drive the setpoints for all loops as desired, counting
on each controller to deliver consistent performance from the
individual fan controlled.
[This has the benefit of providing feedback as to the performance
of the individual fans, over time]
For "airflow performance", it's customary to use a turbine drivenWith the tacho indicating actual rotation (and not just commutation),
anemometer, thermal anemometer or Pitot tube.˙ These work well inside
a duct but badly in open air or around a tangle of airflow obstacles.
it should be a good enough indicator of the individual fan's operation.
And, I doubt the shape of the fan blade changes over time so how
the fan performs (at a given setting) should be consistent.
In the past, we've used individual sensors to monitor the airflow from
the individual fans to determine faults.˙ But, that's costly and adds
the potential for another class of failures to the design.
Bill Sloman <bill.sloman@ieee.org> wrote:
[...]
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are
talking about).
Whilst stepper motors are technically a type of synchronous motor,
people who know what they are talking about will not call them
"synchronous motors" so as to avoid confusion with sinewave-driven synchronous motors.
On 9/9/2026 2:53 AM, Liz Tuddenham wrote:
Don Y <blockedofcourse@foo.invalid> wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Years ago I performed some rough-and-ready experiments on small fans to see if the airflow was consistent in practical environments. The
general conclusion was that the more efficient the fan, the greater was
the drop in performance when the blades acquired a thin coating of dirt
and dust. We finished up using a very inefficient blade shape so as to keep the airflow constant over a wet bulb thermometer.
Interesting. But, if all the blades see the same environment, then
they will all "degrade" similarly (?).
I haven't sorted out how to keep fans (and other cooling mechanisms)
from being a continuous maintenance issue -- short of specifying operation
in a clean room!
OTOH, automobiles suck a lot of "dirty air: into their carburetors,
yet those filters don't see frequent replacement.
On 9/09/2026 3:46 am, Carlos E.R. wrote:
On 2026-09-08 16:12, Bill Sloman wrote:
On 8/09/2026 9:09 pm, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they
are talking about).
Synchronous motors are driven by alternating current, as are
induction motors.
https://www.tlclam.net/synchronous-motor-vs-induction-motor/
Induction motors slip - the more heavily loaded they are the more the
rotation rate lags the rotation rate of the applied magnetic field.
Synchronous motors don't. The rotation rate always follows that of
the applied magnetic field, with a phase lag that gets larger as the
load gets heavier.
They can slip if the torque is big enough. I have seen it.
They don't slip. They skip steps and eventually stop rotating at all.
Years ago I performed some rough-and-ready experiments on small fans to >>> see if the airflow was consistent in practical environments. The
general conclusion was that the more efficient the fan, the greater was
the drop in performance when the blades acquired a thin coating of dirt
and dust. We finished up using a very inefficient blade shape so as to
keep the airflow constant over a wet bulb thermometer.
Interesting. But, if all the blades see the same environment, then
they will all "degrade" similarly (?).
I think it is because the high-efficiency designs rely on laminar
airflow over the blades, which reduces rapidly as the blades become
dirty. A 'badly designed' blade will have turbulence to start with -
but this won't get much worse when it is dirty.
I haven't sorted out how to keep fans (and other cooling mechanisms)
from being a continuous maintenance issue -- short of specifying operation >> in a clean room!
Big, flat low-temperature-differential cooling plates.
OTOH, automobiles suck a lot of "dirty air: into their carburetors,
yet those filters don't see frequent replacement.
I had a horrible fault on a Volvo that would suddenly produce vast
clouds of blue smoke and drop to idling speed when I tried to accelerate (highly unpopular in traffic). After a few moments of roadside idling,
it would return to normal.
One day it did it in a place where I could pull off the road safely and
look under the bonnet. The corrugated PVC air hose from the air filter
to the carburettor was flat but slowly resumed its normal shape after a
few seconds. As it was connected to the sump breather, the corrugatios
were full of oily condensate, which had been sucked into the engine.
Although the air filter appeared to be perfectly clean, something must
have been blocking the pores sufficiently to cause enough suction under
heavy acceleration to collapse the PVC hose when it was hot and pliable.
On 9/9/26 10:53, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
[...]
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are
talking about).
Whilst stepper motors are technically a type of synchronous motor,
people who know what they are talking about will not call them
"synchronous motors" so as to avoid confusion with sinewave-driven
synchronous motors.
Stepper motors are not good at high speed, unless designed
for that class of service, as the rotors are usually solid,
eddy current losses, and will typically get quite hot.
The old synchronous motor driven clocks, 1950's vintage,
typically had a mechanism to ensure that they started
running in the right direction, not in reverse.
Even
earlier versions had a shaft that you had to spin to start
the motor. Have a 1930's deco instance, Temco, of such
design. restored many years ago, that is still running on
the wall here, and is accurate within a minute or so,
over months, but that shows how accurate the mains
frequency is here in the uk, averaged over time.
On 9/9/2026 2:53 AM, Liz Tuddenham wrote:
Don Y <blockedofcourse@foo.invalid> wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
Years ago I performed some rough-and-ready experiments on small fans˙ to
see if the airflow was consistent in practical environments.˙ The
general conclusion was that the more efficient the fan, the greater was
the drop in performance when the blades acquired a thin coating of dirt
and dust.˙ We finished up using a very inefficient blade shape so as to
keep the airflow constant over a wet bulb thermometer.
Interesting.˙ But, if all the blades see the same environment, then
they will all "degrade" similarly (?).
I haven't sorted out how to keep fans (and other cooling mechanisms)
from being a continuous maintenance issue -- short of specifying operation
in a clean room!
OTOH, automobiles suck a lot of "dirty air: into their carburetors,
yet˙ those filters don't see frequent replacement.
OToOH, residential HVAC systems seem to need monthy replaecments
(our homes are dirtier than the outdoor air our vehicles breathe?)
On 2026-09-09 12:20, Don Y wrote:Internal combustion engines are effectively air pumps.
On 9/9/2026 2:53 AM, Liz Tuddenham wrote:
OTOH, automobiles suck a lot of "dirty air: into their carburetors,
yet˙ those filters don't see frequent replacement.
OToOH, residential HVAC systems seem to need monthy replaecments
(our homes are dirtier than the outdoor air our vehicles breathe?)
The suction force of combustion motors is tremendous. It was actually used to
power the brakes.
On 9/9/2026 5:01 AM, Carlos E.R. wrote:
On 2026-09-09 12:20, Don Y wrote:Internal combustion engines are effectively air pumps.
On 9/9/2026 2:53 AM, Liz Tuddenham wrote:
OTOH, automobiles suck a lot of "dirty air: into their carburetors,
yet˙ those filters don't see frequent replacement.
OToOH, residential HVAC systems seem to need monthy replaecments
(our homes are dirtier than the outdoor air our vehicles breathe?)
The suction force of combustion motors is tremendous. It was actually used to
power the brakes.
It would be interesting to sort out how much air is processed
(per mile driven, per engine revolution, etc.)
But, they aren't operated 24/7/365 (as HVAC systems are).
And, the volume of air processed by an HVAC system is pretty
impressive (our cooler moves 6000 CFM -- continuously,
regardless of whether we are awake, asleep, etc.).
On 9/9/2026 5:31 AM, Don Y wrote:
On 9/9/2026 5:01 AM, Carlos E.R. wrote:
On 2026-09-09 12:20, Don Y wrote:Internal combustion engines are effectively air pumps.
On 9/9/2026 2:53 AM, Liz Tuddenham wrote:
OTOH, automobiles suck a lot of "dirty air: into their carburetors,
yet˙ those filters don't see frequent replacement.
OToOH, residential HVAC systems seem to need monthy replaecments
(our homes are dirtier than the outdoor air our vehicles breathe?)
The suction force of combustion motors is tremendous. It was actually used >>> to power the brakes.
It would be interesting to sort out how much air is processed
(per mile driven, per engine revolution, etc.)
This was a good "in your head" exercise to undertake during my
morning walk (so, treat it as you would a slide-rule calculation
and hope for one or two significant digits)
Assume 6 engine displacements = 1 cu ft.˙˙ (5L plant)
One engine displacement = 2 revolutions (VE=1.0)
So, 12 revolutions per cu ft.
Assume 1200 RPM (to make the math easier while walking!)
means 100 CFM.
As a datapoint, a bathroom exhaust fan is in the 60-100 CFM ballpark.
And, an HVAC system (refrigeration) typically moves 400 CFM/ton
WHILE RUNNING (a cooler runs continuously but refrigeration is
typically thermostatically controlled and more efficient at
cooling -- CFM per comfort unit)
Of course, when you look at the diameter of a tail pipe, the
numbers make more sense!
An air conditioner runs 30-60% duty cycle (pretend day+nite)
so about 8-16 hours per day -- moving about 16,000 (to 32,000)
cu ft (@4T) in that time.˙ Or, roughly half a million (to a full
million) cubic feet per month (typical replacement interval)
A car's air filter is roughly an annual replacement interval.
How many driving minutes per year?˙ At 12K mi/yr and 40MPH,
(city driving) that would be 300 hours or 18000 minutes.
At 100 CFM that's 1,800,000 cubic feet
Still seems that we're changing HVAC filters more often!
But, they aren't operated 24/7/365 (as HVAC systems are).
And, the volume of air processed by an HVAC system is pretty
impressive (our cooler moves 6000 CFM -- continuously,
regardless of whether we are awake, asleep, etc.).
On 9/9/2026 4:38 AM, Liz Tuddenham wrote:
Although the air filter appeared to be perfectly clean, something must
have been blocking the pores sufficiently to cause enough suction under heavy acceleration to collapse the PVC hose when it was hot and pliable.
In my vehicle, the hose from the PCV (!) valve vents to the "outside"
of the air filter.
Don Y <blockedofcourse@foo.invalid> wrote:Ah! I thought you were discussing the Positive Crankcase Ventilation
On 9/9/2026 4:38 AM, Liz Tuddenham wrote:
[...]
Although the air filter appeared to be perfectly clean, something must
have been blocking the pores sufficiently to cause enough suction under
heavy acceleration to collapse the PVC hose when it was hot and pliable.
In my vehicle, the hose from the PCV (!) valve vents to the "outside"
of the air filter.
Have we got at cross-purposes here? PVC = polyvinyl chloride, the
material the hose was made from.
On 9/09/2026 7:53 pm, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 9/09/2026 3:46 am, Carlos E.R. wrote:
On 2026-09-08 16:12, Bill Sloman wrote:
On 8/09/2026 9:09 pm, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through
an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are >>>>> talking about).
Synchronous motors are driven by alternating current, as are induction >>>>> motors.
https://www.tlclam.net/synchronous-motor-vs-induction-motor/
Induction motors slip - the more heavily loaded they are the more the >>>>> rotation rate lags the rotation rate of the applied magnetic field.
Synchronous motors don't. The rotation rate always follows that of the >>>>> applied magnetic field, with a phase lag that gets larger as the load >>>>> gets heavier.
They can slip if the torque is big enough. I have seen it.
They don't slip. They skip steps and eventually stop rotating at all.
Or rotate backwards - or oscillate. The magnetic field has compliance
and the rotor has inertia, so they will form an oscillating system at
certain stepping frequencies - this can happen even it there is no load.
A 'soggy' resilient coupling between the motor and the inertia of the
load is a convenient way of damping this out.
The designer may choose to to mount the motor resiliently because of the
noise caused by the stepping; in combination with the mass of the motor,
this can also be excited into resonance by the torque reaction. Stepper
motors have a lot of hidden snags.
Most of which you can eliminate by driving them with a better
approximation to a sine wave - that is by treating them as the
synchronous motors they actually are.
If a square wave is a first approximation to a sine wave, the second >approximation to to connect a winding to V+ for 33% of the period, to
ground for the next 17%, to V- for the next 33% and to ground again for
the last 17%. Don Lancaster listed a whole lot more patterns for his
"magic sine waves" which were all pretty horrible, but didn't have much >higher harmonic content.
On 9/9/2026 7:01 AM, Liz Tuddenham wrote:
Don Y <blockedofcourse@foo.invalid> wrote:Ah!˙ I thought you were discussing the Positive Crankcase Ventilation
On 9/9/2026 4:38 AM, Liz Tuddenham wrote:
[...]
Although the air filter appeared to be perfectly clean, something must >>>> have been blocking the pores sufficiently to cause enough suction under >>>> heavy acceleration to collapse the PVC hose when it was hot and pliable. >>>In my vehicle, the hose from the PCV (!) valve vents to the "outside"
of the air filter.
Have we got at cross-purposes here?˙ PVC = polyvinyl chloride, the
material the hose was made from.
valve which recirculates "fumes" from the crankcase back into the
carburetor to be burned off.˙ It's the only hose that enters my
air cleaner (the hose being rubber).
On 9/9/26 10:53, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
[...]
If they were put into a fan they would be called synchronous motors
(which what stepper motors are called by people who know what they are
talking about).
Whilst stepper motors are technically a type of synchronous motor,
people who know what they are talking about will not call them
"synchronous motors" so as to avoid confusion with sinewave-driven
synchronous motors.
Stepper motors are not good at high speed, unless designed
for that class of service, as the rotors are usually solid,
eddy current losses, and will typically get quite hot.
The old synchronous motor driven clocks, 1950's vintage,
typically had a mechanism to ensure that they started
running in the right direction, not in reverse. Even
earlier versions had a shaft that you had to spin to start
the motor. Have a 1930's deco instance, Temco, of such
design. restored many years ago, that is still running on
the wall here, and is accurate within a minute or so,
over months, but that shows how accurate the mains
frequency is here in the uk, averaged over time.
On Wed, 9 Sep 2026 21:25:14 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
On 9/09/2026 7:53 pm, Liz Tuddenham wrote:
Bill Sloman <bill.sloman@ieee.org> wrote:
On 9/09/2026 3:46 am, Carlos E.R. wrote:
On 2026-09-08 16:12, Bill Sloman wrote:
On 8/09/2026 9:09 pm, John R Walliker wrote:
On 08/09/2026 11:49, Don Y wrote:
On 9/8/2026 12:37 AM, John R Walliker wrote:
On 08/09/2026 03:33, Don Y wrote:
If, for example, I drove two stepper motors with the same
drive (ingoring back EMF), I would expect each to move through >>>>>>>> an identical rotation as I would be directly commutating them.
I have never come across a fan that uses stepper motors.
If they were put into a fan they would be called synchronous motors >>>>>> (which what stepper motors are called by people who know what they are >>>>>> talking about).
Synchronous motors are driven by alternating current, as are induction >>>>>> motors.
https://www.tlclam.net/synchronous-motor-vs-induction-motor/
Induction motors slip - the more heavily loaded they are the more the >>>>>> rotation rate lags the rotation rate of the applied magnetic field. >>>>>>
Synchronous motors don't. The rotation rate always follows that of the >>>>>> applied magnetic field, with a phase lag that gets larger as the load >>>>>> gets heavier.
They can slip if the torque is big enough. I have seen it.
They don't slip. They skip steps and eventually stop rotating at all.
Or rotate backwards - or oscillate. The magnetic field has compliance
and the rotor has inertia, so they will form an oscillating system at
certain stepping frequencies - this can happen even it there is no load. >>> A 'soggy' resilient coupling between the motor and the inertia of the
load is a convenient way of damping this out.
The designer may choose to to mount the motor resiliently because of the >>> noise caused by the stepping; in combination with the mass of the motor, >>> this can also be excited into resonance by the torque reaction. Stepper >>> motors have a lot of hidden snags.
Most of which you can eliminate by driving them with a better
approximation to a sine wave - that is by treating them as the
synchronous motors they actually are.
If a square wave is a first approximation to a sine wave, the second
approximation to to connect a winding to V+ for 33% of the period, to
ground for the next 17%, to V- for the next 33% and to ground again for
the last 17%. Don Lancaster listed a whole lot more patterns for his
"magic sine waves" which were all pretty horrible, but didn't have much
higher harmonic content.
For smoother motion, use PWM microstepping.
I did that to tune the superconducting cavities for the CEBAF electron accelerator at Jlabs.
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
On 2026-09-08, Don Y <blockedofcourse@foo.invalid> wrote:
How "repeatable" are 4-wire fans -- in terms of getting the same
"airflow performance" out of two same make/same model devices
driven identically?
And, how consistently do they report speed vs. control?
[I.e., if I put two, side-by-side, and drive them from the same
PWM signal (and Vcc), will they both move "identical" amounts
of air AND report the same tacho outputs (ignoring phase)?]
It's a brushless DC motor with one of the position sensors fed to the
pulse output, and the PWM input switching the stator supply on and off.
Given the way that air-flow determines torque in blowers,
side by side is probably near the worst circumstance.
If you want exact same they will probably response to some sort of PLL
if you want near same some sort of charge-pump feedback
If you want closely matched fans - quadcopter drones do that using
multiple motor controllers.
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