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On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
klauskvik@hotmail.com> wrote:
On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...
Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...
That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm
Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.
We used pulse-width-modulation with an H-bridge to deliver an
essentially DC current - up to 3A - to drive the Peltier junction.
The H-bridge switched no faster than 200kHz, and the pulse-width
pattern repeated at 17.4kHz., but it was fudged so that a 50% duty
cycle showed up as a 139Khz square wave, which made filtering out the
low frequency components a lot easier. E-mail me if you want a copy
of the paper.
We brought a couple of new electronic designers onto the ventilator
project I\'m working on. One of them was suggesting we control the
motor with a variable voltage rather than an H-bridge to switch the
current in a PWM manner. He mentioned some high power amps to drive
this, but they are class AB and so would have high losses at anything
other than the maximum output.
Is it a common practice to use a switching circuit to supply a
controlled voltage to a motor? The motor is an inherently inductive
load, so a PWM drive at a high enough frequency would approximate a
constant current to the motor. A smoothing cap at the source (we
already have 2000 uF) helps that a lot. The H-bridge would be
retained to allow the polarity to the motor to be reversed.
Is this a method used? It would lower the I2R heating in the motor
with a lower continuous current rather than a higher pulsed current
with a duty cycle.
We have scope plots showing a 500 Hz pulse rate is not fast enough to
make that work. I need to ask if we can get some better measurements
to see how long it takes for the motor current to drop significantly
when the H-bridge removes the power connection and the reverse diode
conducts.
The H-bridge has a 10 kHz max rate, but that might be good enough.
I\'ll see if I can get someone to make a measurement.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor?
The motor is an inherently inductive load, so a PWM drive at a
high enough frequency would approximate a constant current to the motor.
A smoothing cap at the source (we already have 2000 uF) helps that a lot..
We have scope plots showing a 500 Hz pulse rate is not fast enough to make that work. I need to ask if we can get some better measurements to see how long it takes for the motor current to drop significantly when the H-bridge removes the power connection and the reverse diode conducts.
so a PWM drive at a high enough frequency would approximate a constant current to the motor.
Which will get you a constant torque.
huh what method, cap where?
tirsdag den 20. oktober 2020 kl. 23.43.40 UTC+2 skrev John Larkin:
On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
klauskvik@hotmail.com> wrote:
On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...
Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...
That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm
Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.
if you you keep switching so it works like a boost converter
Ricketty the Clueless Idiot wrote:
====================================
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.
** A class D amp would be better.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor?
** Absolutely for both AC and DC ( brush) motors.
The motor is an inherently inductive load, so a PWM drive at a
high enough frequency would approximate a constant current to the motor.
** However it is still constant voltage drive.
The average value of the PWM voltage is the equivalent DC drive.
A smoothing cap at the source (we already have 2000 uF) helps that a lot.
** You cannot have a large value cap across a motor using PWM drive.
Is this a method used? It would lower the I2R heating in the motor with a lower continuous current rather than a higher pulsed current with a duty cycle.
** Oh dear.
On Tue, 20 Oct 2020 14:57:29 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:
tirsdag den 20. oktober 2020 kl. 23.43.40 UTC+2 skrev John Larkin:
On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
klauskvik@hotmail.com> wrote:
On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...
Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...
That is not correct. At switching frequency, the winding is inductive.. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm
Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.
if you you keep switching so it works like a boost converter
Right. An H-bridge is bidirectional, sort of a DC Variac.
Decelerating a big load can blow up the power supply. Even a modest
motor and load could store hundreds of joules.
onsdag den 21. oktober 2020 kl. 01.15.23 UTC+2 skrev John Larkin:
On Tue, 20 Oct 2020 14:57:29 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:
tirsdag den 20. oktober 2020 kl. 23.43.40 UTC+2 skrev John Larkin:
On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
klauskvik@hotmail.com> wrote:
On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...
Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...
That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm
Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.
if you you keep switching so it works like a boost converter
Right. An H-bridge is bidirectional, sort of a DC Variac.
Decelerating a big load can blow up the power supply. Even a modest
motor and load could store hundreds of joules.
if you look at the schematic is obvious that it is a sync buck from supply to motor and a sync boost from motor to supply
On Tue, 20 Oct 2020 16:22:33 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:
onsdag den 21. oktober 2020 kl. 01.15.23 UTC+2 skrev John Larkin:
On Tue, 20 Oct 2020 14:57:29 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:
tirsdag den 20. oktober 2020 kl. 23.43.40 UTC+2 skrev John Larkin:
On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
klauskvik@hotmail.com> wrote:
On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load....
Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...
That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm
Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.
if you you keep switching so it works like a boost converter
Right. An H-bridge is bidirectional, sort of a DC Variac.
Decelerating a big load can blow up the power supply. Even a modest
motor and load could store hundreds of joules.
if you look at the schematic is obvious that it is a sync buck from supply to motor and a sync boost from motor to supply
Mechanical engineers dream of having a continuously-variable gear set.
A lot of silly versions exist.
We can just vary the duty cycle.
On 20/10/2020 20:16, Ricketty C wrote:
The motor turns an arm with a \"hand\" that pushes on a bag that expels
air that inflates a patient\'s lungs. The mode of operation is
constant pressure. There is a feedback system to maintain the
pressure during the inspiratory portion of the cycle. None of that
is the target of this matter. I\'m just looking to reduce the losses
and noise of operating the motor.
Why is the motor power requirement here so large?
You are trying to inflate and deflate the patients lungs slowly and
steadily with gentle periodic movements not pump up a car tyre quickly.
It should be possible to use a relatively modest motor and gearing to
convert that basic rotary motion into whatever movement is required.
It would be useful to find a H-bridge motor controller that is rated
to work at PWM rates more in the 20 kHz range with 15A+ loads. The
VNH3SP30-E we are planning to use has an upper limit of 10 kHz.
That is more the sort of motor power control kit I would expect to see
on electric scale model trains on 3.5\" or 5\" gauge. They can typically
pull three or four bogeys loaded with kids round on a track. Or at least
they could until Covid restrictions came into play. eg.
https://www.4qd.co.uk/product/dno/
RFI is something else to look out for motor power control in a hospital
setting. It caught out Crossrail in the UK badly as their signalling
system was confused by dynamic interference from the rolling stock motor
controllers. They are not the only ones Hitachi\'s Azuma trains didn\'t
get on with the signalling system north of York when first delivered.
https://www.railmagazine.com/news/network/signalling-interference-halts-azuma-tests-north-of-york
On Tue, 20 Oct 2020 14:15:24 -0700 (PDT), Klaus Kragelund
klauskvik@hotmail.com> wrote:
On Tuesday, October 20, 2020 at 8:44:25 PM UTC+2, whit3rd wrote:
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator project I\'m working on. One of them was suggesting we control the motor with a variable voltage rather than an H-bridge to switch the current in a PWM manner. He mentioned some high power amps to drive this, but they are class AB and so would have high losses at anything other than the maximum output.
Is it a common practice to use a switching circuit to supply a controlled voltage to a motor? The motor is an inherently inductive load...
Motors have iron in the windings, and there\'s leakage inductance, BUT that\'s at audio and below. At
typical SMPS frequencies, that iron is resistive, and rotor inertia is capacitive...
That is not correct. At switching frequency, the winding is inductive. Rotor inerita has almost no effect on the winding. If you have burried versus surface magnets, the winding inductance varies with respect to rotor orientation, but that is another issue, relevant for the motor control algoritm
Rotor/load inertia gets interesting when you need deceleration. That
stored energy has to go somewhere. An H-bridge will happily extract
the mechanical energy from the system and stuff it back into the power
supply.
On 20/10/2020 03:32:05, Ricketty C wrote:
We brought a couple of new electronic designers onto the ventilator
project I\'m working on. One of them was suggesting we control the
motor with a variable voltage rather than an H-bridge to switch the
current in a PWM manner. He mentioned some high power amps to drive
this, but they are class AB and so would have high losses at anything
other than the maximum output.
Is it a common practice to use a switching circuit to supply a
controlled voltage to a motor? The motor is an inherently inductive
load, so a PWM drive at a high enough frequency would approximate a
constant current to the motor. A smoothing cap at the source (we
already have 2000 uF) helps that a lot. The H-bridge would be
retained to allow the polarity to the motor to be reversed.
You might want to add some filtering to remove some of the HF being
radiated.
Is this a method used? It would lower the I2R heating in the motor
with a lower continuous current rather than a higher pulsed current
with a duty cycle.
As long as ripple is low then rms/ave current will be close to unity.
That\'s as good as it gets.
We have scope plots showing a 500 Hz pulse rate is not fast enough to
make that work. I need to ask if we can get some better measurements
to see how long it takes for the motor current to drop significantly
when the H-bridge removes the power connection and the reverse diode
conducts.
You can add your own inductance for a quick easy fix.
The H-bridge has a 10 kHz max rate, but that might be good enough.
I\'ll see if I can get someone to make a measurement.
One thing to bear in mind. If you want this to work long term and use
ball bearings I have come across premature failures that were explained
as HF circulating currents in the stator, rotor and frame that
effectively spark eroded the bearings.
If the bearings are electrically isolated then obviously this isn\'t
going to be an issue.
On Tuesday, October 20, 2020 at 5:07:41 PM UTC-4, Martin Brown
wrote:
On 20/10/2020 20:16, Ricketty C wrote:
The motor turns an arm with a \"hand\" that pushes on a bag that
expels air that inflates a patient\'s lungs. The mode of
operation is constant pressure. There is a feedback system to
maintain the pressure during the inspiratory portion of the
cycle. None of that is the target of this matter. I\'m just
looking to reduce the losses and noise of operating the motor.
Why is the motor power requirement here so large?
You are trying to inflate and deflate the patients lungs slowly
and steadily with gentle periodic movements not pump up a car tyre
quickly.
It should be possible to use a relatively modest motor and gearing
to convert that basic rotary motion into whatever movement is
required.
I like the fact that you likely know nothing about the actual problem
and yet present yourself as an expert. Ok, so please tell me what
pressure constitutes \"gentle\"? How much air movement is required?
How much power is required of the motor? What is the timing
required?
On 21/10/2020 01:23, Ricketty C wrote:
On Tuesday, October 20, 2020 at 5:07:41 PM UTC-4, Martin Brown
wrote:
On 20/10/2020 20:16, Ricketty C wrote:
The motor turns an arm with a \"hand\" that pushes on a bag that
expels air that inflates a patient\'s lungs. The mode of
operation is constant pressure. There is a feedback system to
maintain the pressure during the inspiratory portion of the
cycle. None of that is the target of this matter. I\'m just
looking to reduce the losses and noise of operating the motor.
Why is the motor power requirement here so large?
You are trying to inflate and deflate the patients lungs slowly
and steadily with gentle periodic movements not pump up a car tyre
quickly.
It should be possible to use a relatively modest motor and gearing
to convert that basic rotary motion into whatever movement is
required.
I like the fact that you likely know nothing about the actual problem
and yet present yourself as an expert. Ok, so please tell me what
pressure constitutes \"gentle\"? How much air movement is required?
How much power is required of the motor? What is the timing
required?
You could do worse than look at the UK government specification:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/879382/RMVS001_v4.pdf
Or the user manual for one of the units in use:
https://www.bartshealth.nhs.uk/download.cfm?doc=docm93jijm4n13401.pdf&ver=23079
Either way the total power consumption for the whole thing is ~100W and
obviously a power cut battery backup capability is essential.
Hot swapping of batteries is an optional extra useful in places where
the mains is particularly dodgy.