Adapter fails to power DC motor

[Clive Arthur]

On 06/03/2020 16:36, jlarkin@highlandsniptechnology.com wrote:

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 Belleman

Really 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.

Homopolar motors/generators...

https://en.wikipedia.org/wiki/Homopolar_generator

--
Cheers
Clive

 

[Tauno Voipio]

On 6.3.20 17:41, Jeroen Belleman 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 Belleman

From 'electrical Machinery 101':

The equivalent circuit for a DC motor with constant
magnetization is a resistance in series with a DC
generator. The resistance is mostly from the coils
and commutator resistance.

The generator greates an opposing EMF proportional
to the RPM of the motor and the motor torque is
proportional to the feed current. The motor achieves
such rotation speed that the feed voltage minus the
back EMF creates suitable current in the circuit
resistances to create sufficient torque to overcome
mechanical friction and load torque.

A feeely spinning motor can appear as a capacitance,
looked at sufficiently low frquency.

--

-TV

 

[Rick C]

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.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209

 

[whit3rd]

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.

 

[Jeroen Belleman]

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

 

[Michael Terrell]

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?

 

[Jasen Betts]

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.

 

[Phil Hobbs]

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

 

[Michael Terrell]

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.

 

[Michael Terrell]

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.

 

[Jeroen Belleman]

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