Basic Design 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 mounting means.

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 or coil of usually cooper wire. As electrical current is applied to the solenoid, the field strength is related to the current and the number of turns in the winding.

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 serves to provide 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 plunger end and the plug in the fully retracted position.

Basic Operation of Solenoids

Solenoids act as electric 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 developed by an electrical conductor, usually a winding. Solenoids can be configured to operate in either Direct Current (DC), or Alternating Current (AC). Solenoids are electromechanical actuating devices found in many types of applications.

Basic operation :

The operation of solenoids is that of an electromagnet. The magnetic field is developed around a long straight current carrying conductor (the solenoid plunger), according to:

B = 2k'I/r

(1)

Where:

B = magnetic field density
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 19 th century. Ampere's law provides a means with which to adapt Biot-Savart to a closed electrical path, the application of which provides:

B = m 0 nI

(2)

Where:

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 them 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)

Where:

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(d f /dt)

(4)

Where:

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 illustrates 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.

Solenoid specifications

Increase DC Solenoid Life Expectancy

Guardian Home Page

Guardian Electric Manufacturing • 1425 Lake Avenue • Woodstock, IL 60098
Phone: (815) 334-3600 • Fax: (815) 337-0377