Measuring science fair motor current ?

E

ehsjr

Guest
I helped a friend's 10 year old son build a science fair motor:
a coil of wire (20t around a pill bottle former) supported
above a magnet by paper clips with the tops bent into a U.
The coil has #14 copper soldered to each end, looks like this:

**
___* *___
* *
**

The construction is such that the #14 is rigidly attached
to the coil, so it acts as the axle as well as the conductor.
A D cell makes the thing spin nicely. (Usually you have
to start it spinning by hand.)

The lad asked "how long will the battery last?" The problem is
that, as it spins, it arcs & the current is interrupted often
and with no repetitive pattern. In addition, the arcing burns
the paper clip & wire which eventually stops the motor and
(I assume) lowers the current draw over time as the motor
runs.

He could just run the thing until the battery is "dead", where
dead is some chosen terminal voltage, and note the elapsed
time. That is the "science fair" answer we went with for the
"how long" question. But, from a practical viewpoint, how would
he measure the current? My knee jerk reaction is a series
resistance to develop a voltage & charge a cap with a DMM
measuring the voltage. Then, assuming worst case, he could
predict a minimum run time. Is that approach valid? (I realize
that with intermittent contacts and resistance changing, the
battery won't discharge as quickly as worst case predicts.)
Bear in mind that he is a 10 year old who needed help with
this "electrical project", so he's not a computer wiz kid
who can whip up an interface and take a whole bunch of samples
over time, record them with a PC & spit out a prediction. :)

A "side note" - the paper clips have been replaced by a couple
of small angle irons screwed to a piece of wood. Paper clips
scotch taped to the lid of a plastic container can't take the
"handling" of a 10 year old boy. :)

Ed
 
On Wed, 25 Feb 2009 05:16:21 +0000, ehsjr wrote:

I helped a friend's 10 year old son build a science fair motor: a coil
of wire (20t around a pill bottle former) supported above a magnet by
paper clips with the tops bent into a U. The coil has #14 copper
soldered to each end, looks like this:

**
___* *___
* *
**

The construction is such that the #14 is rigidly attached to the coil,
so it acts as the axle as well as the conductor. A D cell makes the
thing spin nicely. (Usually you have to start it spinning by hand.)

The lad asked "how long will the battery last?" The problem is that, as
it spins, it arcs & the current is interrupted often and with no
repetitive pattern. In addition, the arcing burns the paper clip & wire
which eventually stops the motor and (I assume) lowers the current draw
over time as the motor runs.

He could just run the thing until the battery is "dead", where dead is
some chosen terminal voltage, and note the elapsed time. That is the
"science fair" answer we went with for the "how long" question. But,
from a practical viewpoint, how would he measure the current? My knee
jerk reaction is a series resistance to develop a voltage & charge a cap
with a DMM measuring the voltage. Then, assuming worst case, he could
predict a minimum run time. Is that approach valid? (I realize that with
intermittent contacts and resistance changing, the battery won't
discharge as quickly as worst case predicts.) Bear in mind that he is a
10 year old who needed help with this "electrical project", so he's not
a computer wiz kid who can whip up an interface and take a whole bunch
of samples over time, record them with a PC & spit out a prediction. :)

A "side note" - the paper clips have been replaced by a couple of small
angle irons screwed to a piece of wood. Paper clips scotch taped to the
lid of a plastic container can't take the "handling" of a 10 year old
boy. :)

Ed
An old-style analog current meter would do nicely.

A new-style DVM would work if it averaged the current enough -- some
will, some won't, I have no clue which is which.

--
http://www.wescottdesign.com
 
On Feb 25, 6:16 pm, ehsjr <eh...@NOSPAMverizon.net> wrote:
I helped a friend's 10 year old son build a science fair motor:
a coil of wire (20t around a pill bottle former) supported
above a magnet by paper clips with the tops bent into a U.
The coil has #14 copper soldered to each end, looks like this:

           **
      ___*    *___
         *    *
           **

The construction is such that the #14 is rigidly attached
to the coil, so it acts as the axle as well as the conductor.
A D cell makes the thing spin nicely. (Usually you have
to start it spinning by hand.)

The lad asked "how long will the battery last?" The problem is
that, as it spins, it arcs & the current is interrupted often
and with no repetitive pattern. In addition, the arcing burns
the paper clip & wire which eventually stops the motor and
(I assume) lowers the current draw over time as the motor
runs.

He could just run the thing until the battery is "dead", where
dead is some chosen terminal voltage, and note the elapsed
time.  That is the "science fair" answer we went with for the
"how long" question. But, from a practical viewpoint, how would
he measure the current?  My knee jerk reaction is a series
resistance to develop a voltage & charge a cap with a DMM
measuring the voltage.  Then, assuming worst case, he could
predict a minimum run time. Is that approach valid? (I realize
that with intermittent contacts and resistance changing, the
battery won't discharge as quickly as worst case predicts.)
Bear in mind that he is a 10 year old who needed help with
this "electrical project", so he's not a computer wiz kid
who can whip up an interface and take a whole bunch of samples
over time, record them with a PC & spit out a prediction. :)

A "side note" - the paper clips have been replaced by a couple
of small angle irons screwed to a piece of wood.  Paper clips
scotch taped to the lid of a plastic container can't take the
"handling" of a 10 year old boy. :)

Ed
Add a capacitor to kill the arcing...
 
On Wed, 25 Feb 2009 05:16:21 +0000, ehsjr wrote:
I helped a friend's 10 year old son build a science fair motor: a coil
of wire (20t around a pill bottle former) supported above a magnet by
paper clips with the tops bent into a U. The coil has #14 copper
soldered to each end, looks like this:

**
___* *___
* *
**

The construction is such that the #14 is rigidly attached to the coil,
so it acts as the axle as well as the conductor. A D cell makes the
thing spin nicely. (Usually you have to start it spinning by hand.)

The lad asked "how long will the battery last?" The problem is that, as
it spins, it arcs & the current is interrupted often and with no
repetitive pattern. In addition, the arcing burns the paper clip & wire
which eventually stops the motor and (I assume) lowers the current draw
over time as the motor runs.

He could just run the thing until the battery is "dead", where dead is
some chosen terminal voltage, and note the elapsed time. That is the
"science fair" answer we went with for the "how long" question. But,
from a practical viewpoint, how would he measure the current? My knee
jerk reaction is a series resistance to develop a voltage & charge a cap
with a DMM measuring the voltage. Then, assuming worst case, he could
predict a minimum run time. Is that approach valid? (I realize that with
intermittent contacts and resistance changing, the battery won't
discharge as quickly as worst case predicts.) Bear in mind that he is a
10 year old who needed help with this "electrical project", so he's not
a computer wiz kid who can whip up an interface and take a whole bunch
of samples over time, record them with a PC & spit out a prediction. :)

A "side note" - the paper clips have been replaced by a couple of small
angle irons screwed to a piece of wood. Paper clips scotch taped to the
lid of a plastic container can't take the "handling" of a 10 year old
boy. :)

Ed

TW > An old-style analog current meter would do nicely.
TW > A new-style DVM would work if it averaged
TW > the current enough -- some will, some won't,
TW > I have no clue which is which.

Good point about current averaging.

MC > Add a capacitor to kill the arcing...

That would have taken away half of the joy
when I did some other teaching motors like that.

Got a link to a photo or drawing of this motor
design you tried to describe?

Is it like this?

http://fly.hiwaay.net/~palmer/motor1.gif

or this?

http://www.frwindey.org/motor-running.jpg


When I did hand made training motors
one problem to try to avoid was when the
spring brushes bridged the gap between
commutator segments on both sides.
Since there were only two segments this
would cause a dead short spark, seriously
impairing battery life and work output/speed.

In this animated GIF image notice the spring
brushes are dimpled to cut down on this shorting.

http://upload.wikimedia.org/wikipedia/commons/e/e0/Kommutator_animiert.gif

http://en.wikipedia.org/wiki/Commutator_(electric)

All practical commutators contain at least
three rotor segments to prevent a dead spot
in the rotation of the commutator
http://en.wikipedia.org/wiki/File:Simplest_Possible_Commutator_-_Rotor_View.JPG

To me a dead short spark is not the same as arcing.
Is it technically actually arcing and would a
capacitor actually help in that case?

http://blog.makezine.com/208844120_86e874df1c.jpg

http://www.bcscience.com/images/stlouismotor.jpg
 
Thanks to you and Tim & MC.
Replies are inline.

Greegor wrote:
On Wed, 25 Feb 2009 05:16:21 +0000, ehsjr wrote:
<snip>
TW > An old-style analog current meter would do nicely.
TW > A new-style DVM would work if it averaged
TW > the current enough -- some will, some won't,
TW > I have no clue which is which.

Good point about current averaging.

MC > Add a capacitor to kill the arcing...

That would have taken away half of the joy
when I did some other teaching motors like that.

Got a link to a photo or drawing of this motor
design you tried to describe?

Is it like this?

http://fly.hiwaay.net/~palmer/motor1.gif

or this?

http://www.frwindey.org/motor-running.jpg

Yes. The construction is a little different, but those two
pictures show the identical type of motor.

Wish I had those pictures earlier when we were making
the thing. You gave some nice references! Thanks. :)


When I did hand made training motors
one problem to try to avoid was when the
spring brushes bridged the gap between
commutator segments on both sides.
Since there were only two segments this
would cause a dead short spark, seriously
impairing battery life and work output/speed.

In this animated GIF image notice the spring
brushes are dimpled to cut down on this shorting.

http://upload.wikimedia.org/wikipedia/commons/e/e0/Kommutator_animiert.gif

http://en.wikipedia.org/wiki/Commutator_(electric)


All practical commutators contain at least
three rotor segments to prevent a dead spot
in the rotation of the commutator


http://en.wikipedia.org/wiki/File:Simplest_Possible_Commutator_-_Rotor_View.JPG

To me a dead short spark is not the same as arcing.
Is it technically actually arcing and would a
capacitor actually help in that case?
Both. The contact between the support and the copper
conductor/axle is interrupted (not by design) as the
thing spins, so you get an L di/dt arc on break. And
you get a short circuit arc on make.

Thanks,
Ed


http://blog.makezine.com/208844120_86e874df1c.jpg

http://www.bcscience.com/images/stlouismotor.jpg
 

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