Electromagnet

Related Topics:
Electromagnetism :: Electromagnetic_Field

An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current. The magnetic field disappears when the current ceases.

Introduction

The simplest type of electromagnet is a coiled piece of wire. A coil forming the shape of a straight tube (similar to a corkscrew) is called a solenoid; a solenoid that is bent so that the ends meet is a toroid. Much stronger magnetic fields can be produced if a "core" of paramagnetic or ferromagnetic material (commonly iron) is placed inside the coil. The core concentrates the magnetic field that can then be much stronger than that of the coil itself.

Magnetic fields caused by coils of wire follow a form of the right-hand rule. If the fingers of the right hand are curled in the direction of current flow through the coil, the thumb points in the direction of the field inside the coil. The side of the magnet that the field lines emerge from is defined to be the north pole.

Electromagnets and permanent magnets

The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated over a wide range by controlling the amount of electric current. However, a continuous supply of electrical energy is required to maintain the field.

As a current is passed through the electromagnet, small magnetic regions within the material, called magnetic domains, align with the applied field, causing the magnetic field strength to increase. As current is increased, all of the domains eventually become aligned, a condition called saturation. Once the core becomes saturated, a further increase in current will only cause a relatively minor increase in the magnetic field. In some materials, some of the domains may realign themselves. In this case, part of the original magnetic field will persist even after power is removed, causing the core to behave as a permanent magnet. This phenomenon called remanent magnetism, and is due to the hysteresis of the material. Applying a decreasing AC current to the coil, removing the core and hitting it, or heating it above its Curie point will reorient the domains, causing the residual field to weaken or disappear.

In applications where a variable magnetic field is not required, permanent magnets are generally superior. Additionally, permanent magnets can be manufactured to produce stronger fields than electromagnets of similar size.

Devices that use electromagnets

Electromagnets are used in many situations where a rapidly or easily variable magnetic field is desired. Many of these applications involve deflection of charged particle beams; the cathode ray tube and mass spectrometer fall into this category.

Other devices cause electromagnetic fields to interact with fields from permanent magnets or induced fields from ferromagnetic materials to produce forces. Electromagnetic actuators take advantage of the fact that, if a ferromagnetic core is displaced toward one end of a solenoid, a force will occur which tends to center the core within the solenoid. Also, a nearby plate of steel will be strongly attracted to the core within the solenoid. Typical uses include relays, electromagnetic door locks, and solenoid valves. Doorbells and similar devices are commonly made by causing the moving core to strike a bell. Electromagnets are the essential components of many circuit-breakers, they are used in cars in electromagnet brakes and clutches. In some trams, electromagnetic brakes grip directly on to the rails. Very high powered electromagnets are even used to lift heavy scraps of iron and steel, and to magnetically separate metals at junkyards and recycling centers. Magnetic levitation trains use powerful electromagnets to hover without touching the track. Some trains use attractive forces, while others use repulsive forces.

Electromagnets are used in a rotary electric motor to produce a rotating magnetic field that turns the rotor, or in a linear motor to produce a travelling magnetic field that propels the armature. Although silver is the best conductor of electricity, copper is the most often used conductor due to its low cost, and aluminum is sometimes used to save weight and cost.

Electric guitars also use electro-magnetic pickups, which sense the motions of the strings, and the energy is converted into sound.

Force on ferromagnetic materials

Computing the force on ferromagnetic materials is, in general, quite complex. This is due to fringing field lines and complex geometries. It can be simulated using finite element analysis. However, it is possible to estimate the maximum force under specific conditions. If the magnetic field is confined within a high permeability material, such as certain steel alloys, the maximum force is given by:

$F = \frac{B^2 A}{2 \mu_o}$

Where:

F is the force in newtons
B is the magnetic field in teslas
A is the area of the pole faces in square meters
μ is the permeability of free space

See energy in a magnetic field for more details on the derivation.

In the case of free space (air), $\mu_o = 4 \pi \cdot 10^{-7}\,\mbox{H}\cdot \mbox{m}^{-1}$ , the force per unit area (pressure) is:

$P \approx 398 \, \mathrm{kPa}$ or $57.7 \, \mbox{lbf}\cdot\mbox{in}^{-2}$ @ B = 1 tesla

$P \approx 1592 \, \mathrm{kPa}$ or $230.8 \, \mbox{lbf}\cdot\mbox{in}^{-2}$ @ B = 2 teslas

In a closed magnetic circuit:

$B = \frac{\mu N I}{L}$

Where:

N is the number of turns of wire around the electromagnet
I is the current in amperes
L is the length of the magnetic circuit

Substituting above,

$F = \frac{\mu N^2 I^2 A}{2 L^2}$

In order to build a strong electromagnet, a short magnetic circuit with large area is preferred. Most ferromagnetic materials saturate around 1 to 2 teslas. This occurs at a field intensity of:

$H\approx 787\ \mbox{ampere.turns/meter or}\ 20\ \mbox{ampere.turns/inch}$ .

For this reason, there is no point in building an electromagnet with a higher field intensity. Industrial lifting electromagnets are designed with both pole faces at one side (the bottom). This confines the field lines to maximize the magnetic field. It's like a cylinder within a cylinder. Many loudspeaker magnets use a similar geometry, although the field lines are radial from the inner cylinder rather than perpendicular to the face. Notice the large surface area compared with the height. With pole faces of one square foot or more, thousands of pounds can be lifted with drive currents of just a few amperes.

Introduction

Electromagnets and permanent magnets

Devices that use electromagnets

Force on ferromagnetic materials

Patents

See also

Gourt Home::Gourt :: Electromagnet

Introduction

Electromagnets and permanent magnets

Devices that use electromagnets

Force on ferromagnetic materials

Patents

See also