Switcher Driving Motor for Speed Control

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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 va
riable voltage rather than an H-bridge to switch the current in a PWM manne
r.  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 v
oltage to a motor?  The motor is an inherently inductive load, so a PWM dri
ve at a high enough frequency would approximate a constant current to the m
otor.  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 t
o 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 cy
cle.  

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-bridg
e 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 se
e if I can get someone to make a measurement.  

--  

  Rick C.

  - Get 1,000 miles of free Supercharging
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Re: Switcher Driving Motor for Speed Control
On Tuesday, October 20, 2020 at 1:32:11 PM UTC+11, Ricketty C wrote:
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ct I'm working on. One of them was suggesting we control the motor with a v
ariable voltage rather than an H-bridge to switch the current in a PWM mann
er. 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.  
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 voltage to a motor? The motor is an inherently inductive load, so a PWM dr
ive 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.  
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 lower continuous current rather than a higher pulsed current with a duty c
ycle.  
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e that work. I need to ask if we can get some better measurements to see ho
w long it takes for the motor current to drop significantly when the H-brid
ge removes the power connection and the reverse diode conducts.  
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ee if I can get someone to make a measurement.  

Sloman A.W., Buggs P., Molloy J., and Stewart D. ?A microcontroller
-based driver to stabilise the temperature of an optical stage to 1mK in th
e range 4C to 38C, using a Peltier heat pump and a thermistor sensor?
? Measurement Science and Technology, 7 1653-64 (1996)  

We used pulse-width-modulation with an H-bridge to deliver an essentially D
C 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., b
ut 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-mai
l me if you want a copy of the paper.

You can certainly use an H-bridge to produce a waveform which works as a si
ne wave of lower peak  voltage amplitude that the supply rail. I've done it
 in other applications.  You do have to worry about the high frequency comp
onents coming out of the H-brige, but you can filter them close to the H-br
idge and stop them radiating form the motor leads and warming up motor.

--  
Bill Sloman, Sydney

Re: Switcher Driving Motor for Speed Control
On 20/10/2020 03:57:31, Bill Sloman wrote:

<snip>

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If there was no L or LC filter then the rms/ave current would be far  
higher than with a well filtered supply. The peltier would be far more  
efficient when PWMing.

--  
Mike Perkins
Video Solutions Ltd
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Re: Switcher Driving Motor for Speed Control
On 10/19/2020 10:32 PM, Ricketty C wrote:
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This project again? What motor? Need three engineers to PWM drive a  
motor? Don't understand. Here's Gordon Ramsay on electronic design:  
<https://youtu.be/6Gp5AYEKETs?t82%4

Re: Switcher Driving Motor for Speed Control
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no, but if you're interested in controlloing the speed it is.

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but the innertia of the mechanincal load looks inherently capacitive.

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Which will get you a constant torque.

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huh what method, cap where?

--  
  Jasen.

Re: Switcher Driving Motor for Speed Control
On 20/10/2020 07:14, Jasen Betts wrote:
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Though closed loop and monitoring rpm against the target would be  
better. Optical sensor and a mirror flat on the shaft.

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Which might be good enough if the mechanical load is fairly constant.
Or monitor the voltage at the motor and control the PWM that way.

Linear power amplifiers would tend to generate a lot of waste heat.

--  
Regards,
Martin Brown

Re: Switcher Driving Motor for Speed Control
On Tuesday, October 20, 2020 at 2:30:56 AM UTC-4, Jasen Betts wrote:
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"At the source"... the power source to the circuit.  The H-bridge data sheet shows a large capacitor at this point, I believe to supply the peak current and also to mitigate the dI/dt voltage spike when the motor is stopped or reversed.  

--  

  Rick C.

  + Get 1,000 miles of free Supercharging
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Re: Switcher Driving Motor for Speed Control
Jasen Betts wrote:

==================
        
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** No it wont, the fool is using ambiguous terms.  

 He means smooth current not "constant current".  


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** The fool's posts are riddled with ambiguities.  

  100% has to be a dumb as soup code scribbler.  


....   Phil  

Re: Switcher Driving Motor for Speed Control
On Monday, October 19, 2020 at 10:32:11 PM UTC-4, Ricketty C wrote:
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ct 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 man
ner.  He mentioned some high power amps to drive this, but they are class A
B and so would have high losses at anything other than the maximum output.  
  
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 voltage to a motor?  The motor is an inherently inductive load, so a PWM d
rive 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.  

I suspect that you do not need a very constant current.   Consider a AC/DC  
motor run on AC.  Not constant current ,but the rpm is essentially  constan
t.  And does some variation in the motor speed make  any difference for a v
entilator?

Just suggesting you check the requirements.

                                      Dan
  
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a lower continuous current rather than a higher pulsed current with a duty  
cycle.  
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e that work.  I need to ask if we can get some better measurements to see h
ow long it takes for the motor current to drop significantly when the H-bri
dge removes the power connection and the reverse diode conducts.  
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see if I can get someone to make a measurement.  
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Re: Switcher Driving Motor for Speed Control
:
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ject 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 ma
nner. He mentioned some high power amps to drive this, but they are class A
B and so would have high losses at anything other than the maximum output.
  
Quoted text here. Click to load it
ed 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 th
e 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 t
o be reversed.
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motor run on AC. Not constant current ,but the rpm is essentially constant.
 And does some variation in the motor speed make any difference for a venti
lator?  

All electric motors are essentially synchronous motors and depend on gettin
g  sine waves of current through the windings. DC motors just use brushes a
nd commutators to do the switching.

The frequency and phase of the sine waves have to line up with the rotation
 of the motor - in a DC motor the commutator does that for you. Getting syn
chronous motors spun up and spinning  at the right speed can be tricky.

When you go away from sinewaves, you get more heating when generating the s
ame torque. If you put lots of high-frequency current into the motor windin
gs it doesn't do a thing to generate extra torque, but will heat up the pol
e pieces, which won't help their magnetic properties.

The question didn't suggest that Rick C. hard thought all that hard about w
hat sort of current he need to feed into to the motor. He didn't actually i
dentify the sort of motor he was using, which wasn't a good start.

--  
Bill Sloman, Sydney


Re: Switcher Driving Motor for Speed Control
On Tuesday, October 20, 2020 at 4:15:08 PM UTC+2, Bill Sloman wrote:
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e:  
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roject 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
.  
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lled voltage to a motor? The motor is an inherently inductive load, so a PW
M 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 th
at a lot. The H-bridge would be retained to allow the polarity to the motor
 to be reversed.  
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C motor run on AC. Not constant current ,but the rpm is essentially constan
t. And does some variation in the motor speed make any difference for a ven
tilator?
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ing sine waves of current through the windings. DC motors just use brushes  
and commutators to do the switching.  
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on of the motor - in a DC motor the commutator does that for you. Getting s
ynchronous motors spun up and spinning at the right speed can be tricky.  
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 same torque. If you put lots of high-frequency current into the motor wind
ings it doesn't do a thing to generate extra torque, but will heat up the p
ole pieces, which won't help their magnetic properties.  
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 what sort of current he need to feed into to the motor. He didn't actually
 identify the sort of motor he was using, which wasn't a good start.  
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He is asking the right question. The inductance of a motor winding is very  
high, and a 10kHz to 20kHz PWM is typical to control the motor winding curr
ent. it will not generate a lot of losses since the inductance reduces the  
ripple current to very low amplitude

Cheers

Klaus

Re: Switcher Driving Motor for Speed Control
On Tuesday, October 20, 2020 at 9:54:06 AM UTC-4, snipped-for-privacy@krl.org wrote:
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ject 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 m
anner.  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
.  
Quoted text here. Click to load it
ed 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 t
he motor.  A smoothing cap at the source (we already have 2000 uF) helps th
at a  lot.  The H-bridge would be retained to allow the polarity to the mot
or to be reversed.  
Quoted text here. Click to load it
C motor run on AC.  Not constant current ,but the rpm is essentially  const
ant.  And does some variation in the motor speed make  any difference for a
 ventilator?
Quoted text here. Click to load it

The operation of the motor is controlled to produce the desired effect.  I  
am talking about upping the PWM rate to approximate a constant current driv
e rather than the present pulsed drive we have now.  The switching losses i
n the H-bridge will be higher, but the losses in the motor where we are hav
ing some trouble are lower.  Drive with 3 amps continuously or drive with 6

es in the motor, about 2x I believe.  

--  

  Rick C.

  -- Get 1,000 miles of free Supercharging
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Re: Switcher Driving Motor for Speed Control
On 20/10/2020 15:23, Ricketty C wrote:
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This might help a bit, this guy works for Portescap and they make nice  
motors and generally seem to know what they are doing. (I'm assuming it  
is a brushed motor you want to control.)

https://www.machinedesign.com/materials/article/21125511/controlling-brushed-dc-motors-using-pwm

 From what I know of your application you might need some kind of  
feedback control of motor speed.

MK


Re: Switcher Driving Motor for Speed Control
On Tuesday, October 20, 2020 at 12:08:26 PM UTC-4, Michael Kellett wrote:
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roject I'm working on.  One of them was suggesting we control the motor wit
h 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 cla
ss AB and so would have high losses at anything other than the maximum outp
ut.
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lled voltage to a motor?  The motor is an inherently inductive load, so a P
WM 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 m
otor to be reversed.
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C/DC motor run on AC.  Not constant current ,but the rpm is essentially  co
nstant.  And does some variation in the motor speed make  any difference fo
r a ventilator?
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  I am talking about upping the PWM rate to approximate a constant current  
drive rather than the present pulsed drive we have now.  The switching loss
es in the H-bridge will be higher, but the losses in the motor where we are
 having some trouble are lower.  Drive with 3 amps continuously or drive wi

losses in the motor, about 2x I believe.
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hed-dc-motors-using-pwm
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Thank you.  Yes, we have a PID control loop.  There are a number of issues  
in the thing being controlled starting with the motor itself that we don't  
have characterized.  The guy doing most of the motor work is a mechanical e
ngineer who seems to be happy with a hunt and peck approach to finding a wo
rkable solution.  He has created a rather large number of hands to press on
 the bag, each time changing it without properly understanding how it might
 interact with the bag it is pressing on... resulting in yet another attemp
t needed.  He's also the project leader so not much I can tell him.  

We are making progress though and may have something ready for a manufactur
er to look at by the end of the year.  Not sure about the software.  That's
 not really being worked on at the moment while the hardware is being bange
d on.  Not sure why we can't come to terms on an interface spec and work on
 both in parallel.  Too many members seem to have a hobbyist approach using
 the REPL method I guess, not that REPL is bad, just not the only approach.
  

I've been trying to explain the nature of PWM control to these guys, but it
 isn't sinking in.  Your link should help a lot.  But my biggest problem is
 getting people to stop experimenting and start doing thoughtful, rational  
design.  

Even when Larkin is doing his experiments, it's not just to find out if som
ething will work or not.  He has a plan to take measurements that will give
 him information.  The garage mentality is the pits.  

--  

  Rick C.

  -+ Get 1,000 miles of free Supercharging
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Re: Switcher Driving Motor for Speed Control
tirsdag den 20. oktober 2020 kl. 16.23.45 UTC+2 skrev Ricketty C:
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roject I'm working on.  One of them was suggesting we control the motor wit
h 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 cla
ss AB and so would have high losses at anything other than the maximum outp
ut.  
Quoted text here. Click to load it
lled voltage to a motor?  The motor is an inherently inductive load, so a P
WM 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 m
otor to be reversed.  
Quoted text here. Click to load it
/DC motor run on AC.  Not constant current ,but the rpm is essentially  con
stant.  And does some variation in the motor speed make  any difference for
 a ventilator?
Quoted text here. Click to load it
I am talking about upping the PWM rate to approximate a constant current dr
ive rather than the present pulsed drive we have now.  The switching losses
 in the H-bridge will be higher, but the losses in the motor where we are h
aving some trouble are lower.  Drive with 3 amps continuously or drive with

sses in the motor, about 2x I believe.  
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http://jfsimon.net/public/Power_Semiconductor_Applications_Philips.pdf

page 285-

Re: Switcher Driving Motor for Speed Control
On Monday, October 19, 2020 at 7:32:11 PM UTC-7, Ricketty C wrote:
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ct I'm working on. One of them was suggesting we control the motor with a v
ariable voltage rather than an H-bridge to switch the current in a PWM mann
er. 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.  
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 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 capa
citive...

Yes, switching is a way to supply a motor, but it's better to consider usin
g a HF-capable choke in series,
rather than modeling the motor as an inductor, if you want to keep  your de
sign on-track.   Why
does one want variable motor control, can't you just do ON/OFF?

Re: Switcher Driving Motor for Speed Control
On Tuesday, October 20, 2020 at 2:44:25 PM UTC-4, whit3rd wrote:
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ject 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 ma
nner. He mentioned some high power amps to drive this, but they are class A
B and so would have high losses at anything other than the maximum output.
  
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ed voltage to a motor? The motor is an inherently inductive load...
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t's at audio and below.   At
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pacitive...
ing a HF-capable choke in series,
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design on-track.   Why
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The motor turns an arm with a "hand" that pushes on a bag that expels air t
hat inflates a patient's lungs.  The mode of operation is constant pressure
.  There is a feedback system to maintain the pressure during the inspirato
ry portion of the cycle.  None of that is the target of this matter.  I'm j
ust looking to reduce the losses and noise of operating the motor.  

It would be useful to find a H-bridge motor controller that is rated to wor
k 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.  

--  

  Rick C.

  +- Get 1,000 miles of free Supercharging
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Re: Switcher Driving Motor for Speed Control
On 20/10/2020 20:16, Ricketty C wrote:

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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.

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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

--  
Regards,
Martin Brown

Re: Switcher Driving Motor for Speed Control
On Tuesday, October 20, 2020 at 5:07:41 PM UTC-4, Martin Brown wrote:
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I like the fact that you likely know nothing about the actual problem and y
et present yourself as an expert.  Ok, so please tell me what pressure cons
titutes "gentle"?  How much air movement is required?  How much power is re
quired of the motor?  What is the timing required?  

Your remind me of me.  lol  


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zuma-tests-north-of-york

Even more reason to mitigate the PWM impulses.  We can apply any voltage we
 wish, but the motor determines the current.  With a faster impulse rate we
 can at least allow the current to flow more smoothly.  The RFI isn't at hu
gely high frequencies at least.  The current PWM rate is 500 Hz but the ind
uctance of the motor prevents fast edge rates.  It looks like at 10 kHz the
 current fluctuations will be ~1 amp AC triangle wave. I'd like to reach 20
 kHz so we can't hear the noise.  Right now it is very grating, not that I'
ll hear much above 12 kHz.  It starts falling off in one ear below 4 kHz, b
ut oddly that's the one I hear better with.  

--  

  Rick C.

  ++ Get 1,000 miles of free Supercharging
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Re: Switcher Driving Motor for Speed Control
On 21/10/2020 01:23, Ricketty C wrote:
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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&ver23%079

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.

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--  
Regards,
Martin Brown

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