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.
Adapter fails to power DC motor
Started by ●March 3, 2020
Reply by ●March 6, 20202020-03-06
Reply by ●March 6, 20202020-03-06
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. Jeroen Belleman
Reply by ●March 6, 20202020-03-06
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.
Reply by ●March 6, 20202020-03-06
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? Jeroen Belleman
Reply by ●March 6, 20202020-03-06
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.
Reply by ●March 6, 20202020-03-06
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
Reply by ●March 6, 20202020-03-06
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. 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
Reply by ●March 6, 20202020-03-06
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 BellemanThat makes sense. So the 'capacitance' is the energy you have to put into spinning up the rotor? George H.
Reply by ●March 6, 20202020-03-06
George Herold wrote:> On Friday, March 6, 2020 at 8:04:09 AM UTC-5, Jeroen Belleman wrote:[Snip!]>> >> 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 > > That makes sense. So the 'capacitance' is the energy you have to > put into spinning up the rotor? > > George H.Yep! From the motor terminals, it looks like a large leaky capacitor. Jeroen Belleman
Reply by ●March 6, 20202020-03-06
On Fri, 06 Mar 2020 16:41:41 +0100, Jeroen Belleman <jeroen@nospam.please> wrote:>George Herold wrote: >> On Friday, March 6, 2020 at 8:04:09 AM UTC-5, Jeroen Belleman wrote: >[Snip!] >>> >>> 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 >> >> That makes sense. So the 'capacitance' is the energy you have to >> put into spinning up the rotor? >> >> George H. > >Yep! From the motor terminals, it looks like a large leaky >capacitor. > >Jeroen BellemanReally big energy storage applications use what are essentially DC motor-generators instead of capacitors for storage. High-field magnets, electromagnetic aircraft launchers, things like that. -- John Larkin Highland Technology, Inc The cork popped merrily, and Lord Peter rose to his feet. "Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
