Basic Solenoid Operations and Increasing Solenoid Life Expectancy
Basic Operation of Solenoids
Field: The outermost part of the solenoid. The body is constructed of ferrous metal, usually iron, and serves as a part of the magnetic circuit. The body also provides structural integrity as well as a means for mounting.
Plunger: The plunger is the moving element of the solenoid and responds to the magnetic field. The solenoid plunger is also known as the armature. The motion of the plunger being drawn into the magnetic field will produce a mechanical force.
Winding: The winding consists of a number of turns of wire (usually copper), creating a coil.
Plug: The plug is connected to the body at the distal end of the solenoid from the plunger. It serves to both communicate the magnetic circuit and to provide a receiver for the plunger.
Cone: The cone is the end of the plunger that is within the solenoid. In pull solenoids, the cone moves towards the plug. In push solenoids, the cone moves away from the plug. The cone is usually defined by its included angle.
Bobbin: The bobbin provides an internal structural support for the winding. Normally constructed of polymer, it aids in the assembly of the solenoid and provides a low friction guide for the plunger.
Stroke: The stroke is the space that exists between the end of the plunger and the plug in the fully retracted position.
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Solenoids are electro-mechanical actuating devices found in many types of applications. They act as electrical to mechanical energy converters, taking an electrical signal and converting it to work. The operation is based upon the reaction of a moving element, the armature or plunger, in response to a magnetic field created by an electrical conductor, usually a winding. Solenoids can be configured to operate in either direct current (DC), or alternating current (AC).
The operation of solenoids is that of an electromagnet. The magnetic field is generated around a long, straight current-carrying conductor (the solenoid plunger), according to:
B = 2k'I/r (1)
B = magnetic field strength
k' = constant relating field strength, distance and current
I = current in the conductor
r = distance from the conductor
This relationship was deduced by Biot and Savart in the early 19th century. Ampere's law provides a means with which to adapt Biot-Savart to a closed electrical path:
B = m 0 nI (2)
m 0 = solenoid-specific constant relating field strength, distance and current
n = number of turns in the solenoid coil
The magnetic field that is developed will produce a mechanical force on ferromagnetic materials, i.e. the armature (or solenoid plunger), drawing it to the densest part of the field. In the special case of an end-effecting solenoid, the force on the armature can be written as:
F = B 2A/2 m 0 (3)
A = cross-sectional area
The motion of the armature (or solenoid plunger) being drawn into the magnetic field will alter the flux within the iron coil. The flux change will induce a voltage in the solenoid coil.
AC solenoids and induction:
Flux changes within the coil can be due to armature motion or excitation changes, such as AC voltage. The resulting flux change will induce a voltage across the coil in an effect called self-inductance. This effect was first described by Michael Faraday in 1831. Faraday's law states that the voltage induced in the coil is related to the rate of change of the flux:
E = n(df /dt) (4)
E = induced voltage
f = flux (in Webers)
The direction of the induced voltage is described by Lenz's law so as to always produce an opposing current.
Ampere's and Faraday's laws were later codified in the Maxwell equations, which illustrate the relationships of charge, potential, current and magnetics.
Because of the self-inductance, AC solenoids can be operated more efficiently, as the impedance of the coil can be quite high once the armature has completed the magnetic circuit.
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Increase DC Solenoid Life Expectancy
The single most effective technique to increase the life of a DC solenoid is to prevent the metal-to-metal contact between the plug and plunger. This can be accomplished by using a non-metallic plunger stop, either on the outside of the solenoid or at the interface between plug and plunger on the inside. In some cases, longevity can be increased by a factor of 50 or more by this means. It must be understood that the gap introduced between plug and plunger must be taken into account so that the force delivered by the solenoid is appropriate.
The Long Life tubular solenoids are "anti-bottoming" units and the holding forces indicated on our specification sheets are the forces the solenoids hold against their anti-bottoming means.
Proper alignment of plunger and load is important. It is desirable that solenoids be operated in a vertical position (plunger moving vertically) whenever possible. Side loading, that is, loading which is not centered directly along the line of plunger travel, should be avoided. The results of misalignment are premature wear of plungers and plunger bores and possible impairment of seating.
Selection of a solenoid which closely matches the mechanical load requirement is most desirable. Energy in excess of that required to do the necessary work must be dissipated. If the extra force imparted to the plunger is not used either by the load or by some spring cushioning means, the energy will be dissipated at the time of impact of the plunger against the plug. This excessive impacting causes early failure of the solenoid.
Speed of Operation:
The speed of operation of a DC solenoid is dependent upon the input power (NI), the stroke and the load against which it operates and to some degree upon its orientation, whether vertical, horizontal or other. The operate times shown on our data and specification sheets are typical at the values of NI, stroke, load and orientation indicated. Operate time will increase with an increase of load and stroke and a decrease in NI input. Conversely, any decrease in load and stroke or increase in NI will shorten operate time.
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