On 2020-03-14 19:52, tabbypurr@gmail.com wrote:
> On Friday, 6 March 2020 10:13:11 UTC, Michael Terrell  wrote:
>> On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
>>> Michael Terrell wrote:
>>>> On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
>>>>
>>>>> Long ago I learnt that mains power was far more reliable than any
>>>>> battery. The ability to manually open & close will also be far more
>>>>> rleiable than any battery.
>>>>>
>>>>> I've not measured anything but suspect the motor is likely drawing way
>>>>> above 4A for tiny fractions of time. A 2.4v screwdriver motor that
>>>>> self limited to 4A on 5v wouldn't be a lot of use.
>>>>
>>>> Yes. That motor is an inductive load that takes more than the measured
>>>> stall current to start.
>>>
>>> I see DC motors as a big lossy *capacitive* load. Think about
>>> it: Initially, it draws a large starting current, which then
>>> progressively drops to more modest values when the motor
>>> comes up to speed and develops a back-EMF. The EMF persists
>>> when you disconnect, dropping exponentially as the loss
>>> consumes the stored energy until the motor is stopped.
>>
>>
>>     I've never seen a wire wound, Permanent Magnet capacitor.
>
> Motors have long been run purely for their capacitive loading, though I don't recall them being pm.
>
>
> NT
>

Free running synchronous AC motors can be made to appear capacitive
or inductive, depending on the field excitation. That feature was
used to provide power factor correction for certain industrial 
installations. These days they are largely obsolete.

Jeroen Belleman

On Friday, 6 March 2020 10:13:11 UTC, Michael Terrell  wrote:
> On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
> > Michael Terrell wrote:
> > > On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
> > > 
> > >> Long ago I learnt that mains power was far more reliable than any
> > >> battery. The ability to manually open & close will also be far more
> > >> rleiable than any battery.
> > >> 
> > >> I've not measured anything but suspect the motor is likely drawing way
> > >> above 4A for tiny fractions of time. A 2.4v screwdriver motor that
> > >> self limited to 4A on 5v wouldn't be a lot of use.
> > > 
> > > Yes. That motor is an inductive load that takes more than the measured
> > > stall current to start.
> > 
> > I see DC motors as a big lossy *capacitive* load. Think about
> > it: Initially, it draws a large starting current, which then
> > progressively drops to more modest values when the motor
> > comes up to speed and develops a back-EMF. The EMF persists
> > when you disconnect, dropping exponentially as the loss
> > consumes the stored energy until the motor is stopped.
> 
> 
>    I've never seen a wire wound, Permanent Magnet capacitor.

Motors have long been run purely for their capacitive loading, though I don't recall them being pm.


NT

On Saturday, March 7, 2020 at 7:00:36 PM UTC-5, Phil Hobbs wrote:
> On 2020-03-06 21:59, Michael Terrell wrote:
> > On Friday, March 6, 2020 at 9:08:24 AM UTC-5, Phil Hobbs wrote:
> >> On 2020-03-06 05:13, Michael Terrell wrote:
> >>> On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
> >>>> Michael Terrell wrote:
> >>>>> On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
> >>>>>
> >>>>>> Long ago I learnt that mains power was far more reliable than any
> >>>>>> battery. The ability to manually open & close will also be far more
> >>>>>> rleiable than any battery.
> >>>>>>
> >>>>>> I've not measured anything but suspect the motor is likely drawing way
> >>>>>> above 4A for tiny fractions of time. A 2.4v screwdriver motor that
> >>>>>> self limited to 4A on 5v wouldn't be a lot of use.
> >>>>>
> >>>>> Yes. That motor is an inductive load that takes more than the measured
> >>>>> stall current to start.
> >>>>
> >>>> I see DC motors as a big lossy *capacitive* load. Think about
> >>>> it: Initially, it draws a large starting current, which then
> >>>> progressively drops to more modest values when the motor
> >>>> comes up to speed and develops a back-EMF. The EMF persists
> >>>> when you disconnect, dropping exponentially as the loss
> >>>> consumes the stored energy until the motor is stopped.
> >>>
> >>>
> >>>      I've never seen a wire wound, Permanent Magnet capacitor.
> >>>
> >>
> >> PM motors do look like very large capacitors at low frequency.
> > 
> > 
> > How low of a frequency?
> > 
> 
> Low enough that the winding inductance isn't important. ;)
> 
> Seriously, we're talking ~100 Hz and below.



100Hz makes more sense than 1Hz.

On Saturday, March 7, 2020 at 12:32:43 AM UTC-5, Jasen Betts wrote:
> On 2020-03-07, Michael Terrell wrote:
> > On Friday, March 6, 2020 at 9:08:24 AM UTC-5, Phil Hobbs wrote:
> >> On 2020-03-06 05:13, Michael Terrell wrote:
> >> > On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
> >> >> Michael Terrell wrote:
> >> >>> On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
> >> >>>
> >> >>>> Long ago I learnt that mains power was far more reliable than any
> >> >>>> battery. The ability to manually open & close will also be far more
> >> >>>> rleiable than any battery.
> >> >>>>
> >> >>>> I've not measured anything but suspect the motor is likely drawing way
> >> >>>> above 4A for tiny fractions of time. A 2.4v screwdriver motor that
> >> >>>> self limited to 4A on 5v wouldn't be a lot of use.
> >> >>>
> >> >>> Yes. That motor is an inductive load that takes more than the measured
> >> >>> stall current to start.
> >> >>
> >> >> I see DC motors as a big lossy *capacitive* load. Think about
> >> >> it: Initially, it draws a large starting current, which then
> >> >> progressively drops to more modest values when the motor
> >> >> comes up to speed and develops a back-EMF. The EMF persists
> >> >> when you disconnect, dropping exponentially as the loss
> >> >> consumes the stored energy until the motor is stopped.
> >> > 
> >> > 
> >> >     I've never seen a wire wound, Permanent Magnet capacitor.
> >> > 
> >> 
> >> PM motors do look like very large capacitors at low frequency.
> >
> > How low of a frequency?
> 
> low enough that the motor starts to move and back-emf of the moving
> coils exceeds the inductance of the electromagnet.
> 
> 1Hz should be low enough for most easily portable motors. the input
> signal needs suficient amplitude to overcome static friction and
> magnetic cogging.


1Hz = 1 second. Most DC motors are already running by that time, and chopping the DC into pseudo AC pulses.

On 2020-03-06 21:59, Michael Terrell wrote:
> On Friday, March 6, 2020 at 9:08:24 AM UTC-5, Phil Hobbs wrote:
>> On 2020-03-06 05:13, Michael Terrell wrote:
>>> On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
>>>> Michael Terrell wrote:
>>>>> On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
>>>>>
>>>>>> Long ago I learnt that mains power was far more reliable than any
>>>>>> battery. The ability to manually open & close will also be far more
>>>>>> rleiable than any battery.
>>>>>>
>>>>>> I've not measured anything but suspect the motor is likely drawing way
>>>>>> above 4A for tiny fractions of time. A 2.4v screwdriver motor that
>>>>>> self limited to 4A on 5v wouldn't be a lot of use.
>>>>>
>>>>> Yes. That motor is an inductive load that takes more than the measured
>>>>> stall current to start.
>>>>
>>>> I see DC motors as a big lossy *capacitive* load. Think about
>>>> it: Initially, it draws a large starting current, which then
>>>> progressively drops to more modest values when the motor
>>>> comes up to speed and develops a back-EMF. The EMF persists
>>>> when you disconnect, dropping exponentially as the loss
>>>> consumes the stored energy until the motor is stopped.
>>>
>>>
>>>      I've never seen a wire wound, Permanent Magnet capacitor.
>>>
>>
>> PM motors do look like very large capacitors at low frequency.
> 
> 
> How low of a frequency?
> 

Low enough that the winding inductance isn't important. ;)

Seriously, we're talking ~100 Hz and below.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 2020-03-07, Michael Terrell <terrell.michael.a@gmail.com> wrote:
> On Friday, March 6, 2020 at 9:08:24 AM UTC-5, Phil Hobbs wrote:
>> On 2020-03-06 05:13, Michael Terrell wrote:
>> > On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
>> >> Michael Terrell wrote:
>> >>> On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
>> >>>
>> >>>> Long ago I learnt that mains power was far more reliable than any
>> >>>> battery. The ability to manually open & close will also be far more
>> >>>> rleiable than any battery.
>> >>>>
>> >>>> I've not measured anything but suspect the motor is likely drawing way
>> >>>> above 4A for tiny fractions of time. A 2.4v screwdriver motor that
>> >>>> self limited to 4A on 5v wouldn't be a lot of use.
>> >>>
>> >>> Yes. That motor is an inductive load that takes more than the measured
>> >>> stall current to start.
>> >>
>> >> I see DC motors as a big lossy *capacitive* load. Think about
>> >> it: Initially, it draws a large starting current, which then
>> >> progressively drops to more modest values when the motor
>> >> comes up to speed and develops a back-EMF. The EMF persists
>> >> when you disconnect, dropping exponentially as the loss
>> >> consumes the stored energy until the motor is stopped.
>> > 
>> > 
>> >     I've never seen a wire wound, Permanent Magnet capacitor.
>> > 
>> 
>> PM motors do look like very large capacitors at low frequency.
>
> How low of a frequency?

low enough that the motor starts to move and back-emf of the moving
coils exceeds the inductance of the electromagnet.

1Hz should be low enough for most easily portable motors. the input
signal needs suficient amplitude to overcome static friction and
magnetic cogging.

-- 
  Jasen.

On Friday, March 6, 2020 at 9:08:24 AM UTC-5, Phil Hobbs wrote:
> On 2020-03-06 05:13, Michael Terrell wrote:
> > On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
> >> Michael Terrell wrote:
> >>> On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
> >>>
> >>>> Long ago I learnt that mains power was far more reliable than any
> >>>> battery. The ability to manually open & close will also be far more
> >>>> rleiable than any battery.
> >>>>
> >>>> I've not measured anything but suspect the motor is likely drawing way
> >>>> above 4A for tiny fractions of time. A 2.4v screwdriver motor that
> >>>> self limited to 4A on 5v wouldn't be a lot of use.
> >>>
> >>> Yes. That motor is an inductive load that takes more than the measured
> >>> stall current to start.
> >>
> >> I see DC motors as a big lossy *capacitive* load. Think about
> >> it: Initially, it draws a large starting current, which then
> >> progressively drops to more modest values when the motor
> >> comes up to speed and develops a back-EMF. The EMF persists
> >> when you disconnect, dropping exponentially as the loss
> >> consumes the stored energy until the motor is stopped.
> > 
> > 
> >     I've never seen a wire wound, Permanent Magnet capacitor.
> > 
> 
> PM motors do look like very large capacitors at low frequency.


How low of a frequency?

On 2020-03-06 21:50, whit3rd wrote:
> On Friday, March 6, 2020 at 5:04:09 AM UTC-8, Jeroen Belleman wrote:
>> Michael Terrell wrote:
>
>> So again, even though a DC motor has coils, its dominant
>> impedance is a parallel RC. The often heard claim that
>> it's inductive is false.
>
> Well, not false, just misleading.   We use motors IN ROTATION, not braked
> to a total stall, so the current requirement is high when starting, and drops
> afterward, which is similar to capacitor loading (if there's a time-scale of
> milliseconds to seconds under consideration).   At microsecond-to-millisecond
> time scales, a nonrotating motor is inductive.
>

Agreed. I said so already.

Jeroen Belleman

On Friday, March 6, 2020 at 8:04:09 AM UTC-5, Jeroen Belleman wrote:
> Michael Terrell wrote:
> > On Friday, March 6, 2020 at 5:36:36 AM UTC-5, Jeroen Belleman wrote:
> >> Michael Terrell wrote:
> >>> On Friday, March 6, 2020 at 2:36:11 AM UTC-5, Jeroen Belleman wrote:
> >>> 
> >>>> Michael Terrell wrote:
> >>>>> On Thursday, March 5, 2020 at 9:06:20 PM UTC-5, NT wrote:
> >>>>> 
> >>>>>> Long ago I learnt that mains power was far more reliable than
> >>>>>> any battery. The ability to manually open & close will also be
> >>>>>> far more rleiable than any battery.
> >>>>>> 
> >>>>>> I've not measured anything but suspect the motor is likely
> >>>>>> drawing way above 4A for tiny fractions of time. A 2.4v
> >>>>>> screwdriver motor that self limited to 4A on 5v wouldn't be a
> >>>>>> lot of use.
> >>>>> Yes. That motor is an inductive load that takes more than the
> >>>>> measured stall current to start.
> >>>> I see DC motors as a big lossy *capacitive* load. Think about it:
> >>>> Initially, it draws a large starting current, which then 
> >>>> progressively drops to more modest values when the motor comes up
> >>>> to speed and develops a back-EMF. The EMF persists when you
> >>>> disconnect, dropping exponentially as the loss consumes the stored
> >>>> energy until the motor is stopped.
> >>> 
> >>> I've never seen a wire wound, Permanent Magnet capacitor.
> >> I suppose you can enlighten us with a reasoned discussion on the
> >> apparent impedance of permanent magnet DC motors?
> > 
> > 
> > 
> > i shouldn't have to. You have to overcome the inductance to build the
> > required magnetic field. Once the motor starts to turn, the commutator
> > converts the DC input into a crude, chopped AC field that is required
> > for the motor to continue to turn.
> > 
> > Now try to explain your capacitor theory.
> 
> I already did, see above. You should read it. Now, I'll admit
> that a DC motor isn't a pure capacitance. The equivalent
> circuit might be a parallel RC in series with an inductance.
> The capacitor models the back EMF and the energy stored in the
> rotating mass. The resistor models the work that needs to
> expended to keep the thing rotating. And yes, the inductance
> models the rotor coil with its iron yoke. My point is that
> this latter component isn't dominant, except maybe on single-
> digit milli-second time scales or shorter.
> 
> Finer detail would add still more components. My model still
> ignores rotor winding and commutator resistance, variable
> reluctance effects, iron losses and magnetic saturation, to
> name a few.
> 
> It's the parallel RC that largely dominates the impedance,
> except at 'high' frequency. The meaning of 'high' depends on
> the inertia of the motor and its load.
> 
> So again, even though a DC motor has coils, its dominant
> impedance is a parallel RC. The often heard claim that
> it's inductive is false.
> 
> Jeroen Belleman

I get what you are saying.  A motor is an electromechnical device, not entirely unlike a piezoelectric crystal.  The mechanical effects become reflected in the electrical effects and vice versa.  In fact, that is what makes it work.  The only question is which effect is dominant for any given aspect you are measuring.  Sort of like the blind men describing the elephant.  

-- 

  Rick C.

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