What is Solenoid?

A solenoid is a 3-dimensional coil.

In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electrical current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. The term solenoid refers specifically to a magnet designed to produce a uniform magnetic field in a volume of space (where some experiment might be carried out).

In engineering, the term solenoid may also refer to a variety of transducer devices that convert energy into linear motion. The term is also often used to refer to a solenoid valve, which is an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid, or a linear solenoid, which is an electromechanical solenoid.

Derivation of magnetic field around a long solenoid

This is a derivation of the magnetic field around a solenoid, that is long enough so that fringe effects can be ignored.

In the diagram to the right, we immediately know that the field points in the positive z direction inside the solenoid, and in the negative z direction outside the solenoid. We see this by applying the right-hand rule for the field around a wire. If we wrap our right hand around a wire with the thumb pointing in the direction of the current, the fingers show how the field behaves. Since we are dealing with a long solenoid, all of the components of the magnetic field not pointing upwards cancel out by symmetry. Outside, a similar cancellation occurs, and the field is only pointing downwards.

Now consider loop "c". By Ampère's law, we know that the path integral of B around this loop is zero, since no current passes through it. We have shown above that the field is pointing upwards inside the solenoid, so the horizontal portions of loop "c" don't contribute anything to the integral. Thus the integral up side 1 is equal to the integral down side 2. Since we can arbitrarily change the dimensions of the loop and get the same result, the only physical explanation is that the integrands are actually equal, that is, the magnetic field inside the solenoid is constant. A similar argument can be applied to loop "a" to conclude that the field outside the solenoid is constant.

An intuitive argument can be used to show that the field outside the solenoid is actually zero. Magnetic field lines only exist as loops, they cannot diverge from or converge to a point like electric field lines can. The magnetic field lines go up the inside of the solenoid, so they must go down the outside so that they can form a loop. However, the volume outside the solenoid is much greater than the volume inside, so the density of magnetic field lines outside is greatly reduced. Recall also that the field outside is constant. In order for the total number of field lines to be conserved, the field outside must go to zero as the solenoid gets longer.

Now we can consider loop "b". Take the path integral of B around the loop, with the height of the loop set to L. The horizontal components vanish, and the field outside is zero, so Ampère's Law gives us:

:

From which we get:

:

Rotary Voice Coil

This is a rotational version of a solenoid. Typically the fixed magnet is on the outside, and the coil part moves in an arc controlled by the current flow through the coils. Rotary voice coils are widely employed in devices such as disk drives.

Electromechanical solenoids

Electromechanical solenoids consist of an electromagnetically inductive coil, wound around a movable steel or iron slug (termed the armature). The coil is shaped such that the armature can be moved in and out of the center, altering the coil's inductance and thereby becoming an electromagnet. The armature is used to provide a mechanical force to some mechanism (such as controlling a pneumatic valve). Although typically weak over anything but very short distances, solenoids may be controlled directly by a controller circuit, and thus have very low reaction times.

The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil. The force applied to the armature will always move the armature in a direction that increases the coil's inductance.

The magnetic field inside a solenoid is given by:

:

where henries per metre, B is the magnetic field magnitude in teslas, N is the number of turns per metre, I is the current in amperes, n is the number of turns and L is the length of the solenoid in metres. See also: Electromagnet.

Electromechanical solenoids are commonly seen in electronic paintball markers, dot matrix printers and fuel injectors.

Pneumatic solenoid valves

A pneumatic solenoid valve is a switch for routing air to any pneumatic device, usually an actuator of some kind. A solenoid consists of a balanced or easily moveable core, which channels the gas to the appropriate port, coupled to a small linear solenoid. The valve allows a small current applied to the solenoid to switch a large amount of high pressure gas, typically up to 100 psi (7 bar, 0.7 MPa, 0.7 MN/m²). Some solenoids are capable of operating at far greater pressures.

Pneumatic solenoids may have one, two, or three output ports, and the requisite number of vents. The valves are commonly used to control a piston or other linear actuator.

The pneumatic solenoid is akin to a transistor, allowing a relatively small signal to control a large device. It is also the interface between electronic controllers and pneumatic systems.

Hydraulic solenoid valves

Hydraulic solenoid valves are in general similar to pneumatic solenoid valves except that they control the flow of hydraulic fluid (oil), often at around 3000 psi (210 bar, 21 MPa, 21 MN/m²). Hydraulic machinery uses solenoids to control the flow of oil to rams or actuators to (for instance) bend sheets of titanium in aerospace manufacturing. Solenoid-controlled valves are often used in irrigation systems, where a relatively weak solenoid opens and closes a small pilot valve, which in turn activates the main valve by applying fluid pressure to a piston or diaphragm that is mechanically coupled to the main valve.

Transmission solenoids control fluid flow through an automatic transmission and are typically installed in the transmission valve body.

Solenoids in fiction

The use of the word solenoid (particularly in science fiction) could be grouped in with other terms such as conduit, socket, firewall, capacitor, wormhole and laser to lend some kind of scientific/engineering credibility from a layman's perspective.

• In the anime and manga series Neon Genesis Evangelion, one plot device was a "Super Solenoid Engine" (or S²), a limitless power source.
• In Steven Spielberg's 2005 War of the Worlds movie, the alien tripods disable all flow of electric current in a wide area, thus rendering vehicles useless; the problem is fixed by replacing the solenoid.
• In Gene Roddenberry's Andromeda TV-Series, an Anti-Proton Solenoid Valve is used to control the flow of the anti-protons which are used to power the ship and propel it through space. In the Episode "The Vault of The Heavens" (3.18), Engineer Harper orders Trance to shut it, filling the Anti Proton tanks to capacity, then expelling it quickly, thereby accelerating the ship much more than normal.
• In GUNNM, Sechs and Alita use coilguns, which makes use of solenoids to accelerate projectiles to supersonic speed. The result is a gun that can still cause major damage even if it misses. Alita's fires a rocket slug which has incredibly high power; Sechs' modified gun can fire multiple rounds in quick succession.

External links

• Solenoid Basics for Robotics
• Discussion of Solenoids at Hyperphysics
• Basics of Rotary Voice Coils
• Solenoid General Application Guidelines