# Solenoid operation

Discussion in 'Electronic Basics' started by R.Spinks, Jun 30, 2005.

1. ### R.SpinksGuest

I'm trying to understand basic solenoid operation a little better. I was
under the impression that if I applied DC to a coil (inductor) and the
inductor had a metal object (like a nail or something) say in it and
slightly out to one side the current through the inductor would create a
right hand rule flux in the inductor and the nail would move in the flux
direction. Then if I switched the polarity the nail would move the opposite
direction as the current direction would be opposite. I tried building a
little coil out of magnet wire and put a nail in it - then applied 14V - but
it didn't move (the nail did get a bit magnetic as I touched it w/another
nail). Is my understanding of the theory right or is there still something
missing (like does the nail have to be a permanent magnet or something
coil always pulls to the center... but ... the right hand rule should apply
so .. I think it should have a direction not just to center of coil.

2. ### John FieldsGuest

---
The "handedness" only refers to the polarity of the field, (That is,
which end is north or south) not its amplitude. If you put a ferrous
material in the field it will be attracted by the north pole as well
as the south pole and will come to rest where the forces on it are
equal.

3. ### ehsjrGuest

Generally, solenoids have a spring that pulls or pushes
the plunger away from the center of the coil:COIL PLUNGER SPRING
)))))))))=========ssssssss

In the diagram above, the tension spring exerts a force
toward the right. When energized, the coil exerts a
larger force, toward the left:
<<<<----- -->
COIL SPRING
)))))))))===== s s ssssss

Note that the plunger has begun to move inside the coil,
and the spring has stretched a bit. The arrow showing the
spring's rightward force is now stronger, but the
attraction of the coil on the plunger is strong enough
to overcome it.

<<<--- --->>>
COIL SPRING
=)))))))))= s s s s s s s s

Finally, the force exerted by the coil to move the plunger
toward the left equals the force exerted on the plunger
by the spring to move the plunger to the right, so the
plunger stops moving. When you de-energize the coil, the
spring pulls the plunger back to the right.

If you had no spring and started with this:

COIL PLUNGER
)))))))))=========

And then energized the coil, you would end up with this:

COIL
=)))))))))=

The plunger would move in to the coil and stop when the
magnetic center of the coil and the magnetic center of
the plunger lined up. Unless there is a force to pull
the plunger out of the coil - another coil, air pressure,
a spring, whatever - the plunger will stay there when the
coil is de-energized.

Ed

4. ### Bob EldredGuest

Your basic assumptions are wrong. The handed rules do not apply. A ferrous
object, iron,or steel will always move toward the highest flux concentration
which is in the center of the coil, length wise. The handedness determines
magnetic polarity, north or south but not attraction. Just as both ends of a
permanent magnet, north or south will attract an iron object, so does a
solenoid attract regardless of the polarity of the current. The nail should
be sucked right into the coil if there is sufficient flux density to move
it. Based on what you said, there is very little flux density in the coil
and the iron barely moves but does show a little magnetic effect. It means
you do not have sufficient magnetizing force or amp-turns per length to
create much flux. To get a healthy pull on an object like a nail you may
need about 500amp-turns or more in an inch or so of coil length. The higher
the number, the more the force. Also, the greater the crosssectional area of
the iron, the greater the force. A nail is a bit small, try a 1/4inch bolt.
In fact, the iron sould fill the area in the center of the coil as much as
possible but still move.

For example, a 14volt coil drawing about 300mA, the resistance would have
to be 14/.3 = 47ohms. To get 500Amp-turns, the winding would have to be
500/.3 = 1666 turns of wire. It would take about 450 feet of 30AWG wire to
get 47 ohms. And, for 1670 turns, the average length of one turn would be
3.2 inches, about 1 inch dia. A smaller diameter would give more turns and
higher force for the same length of wire and iron size. I suspect you used
far less than 1600 turns of unknown wire size and that's why the performance
is less than you expected. Try getting at least 500Amp-turns and see what
happens.
Bob

5. ### R.SpinksGuest

Ok -- that makes sense, I think. Thanks. If the ferrite was instead a
permanent magnet (ie. has it's own north and south pole) and I beefed up my
example to have sufficient pull (like 500 amp-turns) as you have
indicated -- would it then work that if I switched the direction of current
in the coil that the magnet would move in opposite direction (as I would
have changed the polarity of the coil magnet -- or would it still just pull
to center)?

6. ### Bob EldredGuest

It might work if done right, however the forces on a permanent magnet are
at right angles to the forces of attraction for a piece of unmagnetized
iron. Most likely, a permanent magnet in a solenoid would filp around until
in was "most happy" then it would stick to the side walls of the coil.
Reversing polarity might flip it some other way. It's hard to analyze
because the field lines and their interactions are very complex. It most
certainly would not work like you want where it pulls in one way and pushes
out on reversed polarity.

The voice coil on a loud speaker moves in the way you have described. Here
the current, the magnetic flux and the resultant force are all at right
angles to each other and a hand rule applies. When the current reverses, the
force also reverses. It is possible but difficult to arrange a solenoid and
permanent magnet to simulate this action but the magnet would have to be
arranged so that its flux lines cross the coil at nearly right angles as it
moves. The armature of a permanent magnet motor does this as it rotates. To
generate linear motion, a voice coil is the best way to go, like a speaker.
Bob  