A coil of wire, usually cylindrical, partially surrounding a movable iron core. When a current flows in the coil, a magnetic field is produced which makes the core move. A solenoid converts electrical energy into mechanical energy, as in operating a switch or circuit breaker.
A solenoid is 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 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.
Electromechanical solenoids
Electromechanical solenoids consist of an electromagnetically inductive coil, wound around a movable steel or iron slug (termed the armature).
The magnetic field inside a solenoid is given by:
B = μ0NI = μ0I(n / l)where henries per metre, N is the number of turns per metre, and I is the current in amperes.
Electromechanical solenoids are commonly seen in electronic paintball markers, and dot matrix printers.
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. Since we are dealing with a long solenoid, all of the components of the magnetic field not pointing upwards cancel out by symmetry. 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. 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. 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. In order for the total number of field lines to be conserved, the field outside must go to zero as the solenoid gets longer. The horizontal components vanish, and the field outside is zero, so Ampère's Law gives us:
B * L = μ0 * N * L * IFrom which we get:
B = μ0 * N * IRotary Voice Coil
This is a rotational version of a solenoid.
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 at around 100 psi (7 bar, 0.7 MPa, 0.7 MN/m²).
Pneumatic solenoids may have one, two, or three output ports, and the requisite number of vents.
The pneumatic solenoid is akin to a transistor, allowing a relatively small signal to control a large device.
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).
The basics of solenoid valves
Solenoid valves are the most frequently used control elements in fluidics. Solenoids offer fast and safe switching, high reliability, long service life, good medium compatibility of the materials used, low control power and compact design.
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