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Electromagnetic brake
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Electromagnetic brakes operate electrically, but transmit torque mechanically. This is why they
used to be referred to as electro-mechanical brakes. Over the years, EM brakes became known as
electromagnetic, referring to their actuation method. Since the brakes started becoming popular
over sixty years ago, the variety of applications and brake designs has increased dramatically, but
the basic operation remains the same.
Single face electromagnetic brakes make up approximately 80% of all of the power applied
brake applications. This article mainly concentrates on these brakes. Alternative designs are
shown at the end of this article.
Contents
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1 Construction
2 Basic Operation
3 Voltage/Current - And the Magnetic Field
4 Engagement Time
5 Burnishing - What Is It and Why Is It Important?
6 Torque
7 Over Excitation
8 Wear
9 Backlash
10 Environment / Contamination
11 Other Types of Electromagnetic Brakes
o 11.1 Electromagnetic Power Off Brake
o 11.2 Electromagnetic Particle Brake
o 11.3 Electromagnetic Hysteresis Power Brake
o 11.4 Multiple Disk Brakes
12 References
[edit] Construction
A-1 Horseshoe magnet red silver iron
A horseshoe magnet (A-1) has a north and south pole. If a piece of carbon steel contacts both
poles, a magnetic circuit is created. In an electromagnetic brake, the north and south pole is
created by a coil shell and a wound coil. In a brake, the armature is being pulled against the
brake field. (A-3) The frictional contact, which is being controlled by the strength of the
magnetic field, is what causes the rotational motion to stop. All of the torque comes from the
magnetic attraction and coefficient of friction between the steel of the armature and the steel of
the brake field. For many industrial brakes, friction material is used between the poles. The
material is mainly used to help decrease the wear rate. But different types of material can also be
used to change the coefficient of friction (torque) for special applications. For example, if the
brake was required to have an extended time to stop or slip time, a low coefficient material can
be used. Conversely, if the brake was required to have a slightly higher torque (mostly for low
RPM applications), a high coefficient friction material could be used.[1]
In a brake, the electromagnetic lines of flux have to attract and pull the armature in contact with
it to complete brake engagement. Most industrial applications use what is called a single-flux
two-pole brake. The coil shell is made with carbon steel that has a combination of good strength
and good magnetic properties. Copper (sometimes aluminum) magnet wire, is used to create the
coil, which is held in shell either by a bobbin or by some type of epoxy/adhesive.[2]
To help increase life in applications, friction material is used between the poles. This friction
material is flush with the steel on the coil shell, since if the friction material was not flush, good
magnetic traction could not occur between the faces. Some people look at electromagnetic brakes
and mistakenly assume that, since the friction material is flush with the steel, that the brake has
already worn down, but this is not the case.[3]
[edit] Basic Operation
There are three parts to an electrmagnetic brake: field, armature, and hub (which is the input on a
brake) (B-2). Usually the magnetic field is bolted to the machine frame (or uses a torque arm that
can handle the torque of the brake). So when the armature is attracted to the field the stopping
torque is transferred into the field housing and into the machine frame decelerating the load. This
can happen very fast (.1-3sec).
Disengagement is very simple. Once the field starts to degrade flux falls rapidly and the armature
separates. A spring(s) hold the armature away from its corresponding contact surface at a
predetermined air gap.[4]
A-3 Electromagentic brake
[edit] Voltage/Current - And the Magnetic Field
V-1 Right hand thumb rule
If a piece of copper wire was wound, around the nail and then connected to a battery, it would
create an electro magnet. The magnetic field that is generated in the wire, from the current, is
known as the “right hand thumb rule”. (V-1) The strength of the magnetic field can be changed
by changing both wire size and the amount of wire (turns). EM clutches are similar; they use a
copper wire coil (sometimes aluminum) to create a magnetic field.
The fields of EM brakes can be made to operate at almost any DC voltage and the torque
produced by the brake will be the same as long as the correct operating voltage and current is
used with the correct brake. If a 90 volt brake had 48 volts applied to it, this would get about half
of the correct torque output of that brake. This is because voltage/current is almost linear to
torque in DC electromagnetic brakes.
A constant current power supply is ideal for accurate and maximum torque from a brake. If a non
regulated power supply is used the magnetic flux will degrade as the resistance of the coil goes
up. Basically, the hotter the coil gets the lower the torque will be produced by about an average
of 8% for every 20°C. If the temperature is fairly constant, and there is a question of enough
service factor in the design for minor temperature fluctuation, by slightly over sizing the brake
can compensate for degradation. This will allow the use of a rectified power supply, which is far
less expensive than a constant current supply.
Based on V = I × R, as resistance increases available current falls. An increase in resistance,
often results from rising temperature as the coil heats up, according to: Rf = Ri × [1 + αCu × (Tf
- Ti)] Where Rf = final resistance, Ri = initial resistance, αCu = copper wire’s temperature
coefficient of resistance, 0.0039 °C-1, Tf = final temperature, and Ti = initial temperature.
[edit] Engagement Time
There are actually two engagement times to consider in an electromagnetic brake. The first one is
the time it takes for a coil to develop a magnetic field, strong enough to pull in an armature.
Within this, there are two factors to consider. The first one is the amount of ampere turns in a
coil, which will determine the strength of a magnetic field. The second one is air gap, which is
the space between the armature and the coil shell. Magnetic lines of flux diminish quickly in the
air. The further away the attractive piece is from the coil, the longer it will take for that piece to
actually develop enough magnetic force to be attracted and pull in to overcome the air gap. For
very high cycle applications, floating armatures can be used that rest lightly against the coil shell.
In this case, the air gap is zero; but, more importantly the response time is very consistent since
there is no air gap to overcome. Air gap is an important consideration especially with a fixed
armature design because as the unit wears over many cycles of engagement the armature and the
coil shell will create a larger air gap which will change the engagement time of the brakes. In
high cycle applications, where registration is important, even the difference of 10 to 15
milliseconds can make a difference, in registration of a machine. Even in a normal cycle
application, this is important because a new machine that has accurate timing can eventually see
a “drift” in its accuracy as the machine gets older.
The second factor in figuring out response time of a brake is actually much more important than
the magnet wire or the air gap. It involves calculating the amount of inertia that the brake needs
to decelerate. This is referred to as “time to stop”. In reality, this is what the end-user is most
concerned with. Once it is known how much inertia is present for the brake to stop then the
torque can be calculated and the appropriate size of brake can be chosen.
Most CAD systems can automatically calculate component inertia, but the key to sizing a brake
is calculating how much inertia is reflected back to the brake. To do this, engineers use the
formula: T = (WK2 × ΔN) / (308 × t) Where T = required torque in lb-ft, WK2 = total inertia in
lb-ft2, ΔN = change in the rotational speed in rpm, and t = time during which the acceleration or
deceleration must take place.
Inertia Calculator There are also online sites that can help confirm how much torque is required
to decelerate a given amount of inertia over a specific time. Remember to make sure that the
torque chosen, for the brake, should be after the brake has been burnished.
[edit] Burnishing - What Is It and Why Is It Important?
Burnishing is the wearing or mating of opposing surfaces. When the armature and brake faces
are produced, the faces are machined as flat as possible. (Some manufacturers also lightly grind
the faces to get them smoother.) But even with that the machining process leaves peaks and
valleys on the surface of the steel. When a new “out of the box” brake is initially engaged most
peaks on both mating surfaces touch which means that the potential contact area can be
significantly reduced. In some cases, an out of box brake may have only 50% of its torque rating.
Burnishing is the process of cycling the brake to wear down those initial peaks, so that there is
more surface contact between the mating faces
Even though burnishing is required to get full torque out of the brake it may not be required in all
applications. Simply put, if the application torque is lower than the initial out of box torque of
the brake, burnishing would not be required; however, if the torque required is higher, then
burnishing needs to be done. In general this tends to be required more on higher torque brakes
than on smaller lower torque brakes.
The process involves cycling the brake a number of times at a lower inertia, lower speed or a
combination of both. Burnishing can require from 20 to over 100 cycles depending upon the size
of a brake and the amount of initial torque required. For bearing mounted brakes where the rotor
and armature is connected and held in place via a bearing, burnishing does not have to take place
on the machine. It can be done individually on a bench or as a group at a burnishing station. Two
piece brakes that have separate armatures should try to have the burnishing done on the machine
verses a bench. The reason for this is if burnishing on a two piece brake is done on a bench and
there is a shift in the mounting tolerance when that brake is mounted to the machine the
alignment could be shifted so the burnishing lines on the armature, rotor or brake face may be off
slightly preventing that brake from achieving full torque. Again, the difference is only slight so
this would only be required in a very torque sensitive application.
[edit] Torque
Burnishing can affect initial torque of a brake but there are also factors that affect the torque
performance of a brake in an application. The main one is voltage/current. In the voltage/current
section we showed why a constant current supply is important to get full torque out of the brake.
When considering torque, the question of using dynamic or static torque for the application is
key? For example, if running a machine at relatively low rpm (5 – 50 depending upon size) there
is minimal concern with dynamic torque since the static torque rating of the brake will come
closest to where it is running. However, when running a machine at 3,000rpm and applying the
brake at its catalog torque, at that rpm, is misleading. Almost all manufacturers put the static
rated torque for their brakes in their catalog. So, when trying to determine a specific response
rate for a particular brake, the dynamic torque rating is needed. In many cases this can be
significantly lower. It can be less than half of the static torque rating. Most manufacturers
publish torque curves showing the relationship between dynamic and static torque for a given
series of brake.
T1
[edit] Over Excitation
Electromagnetic-Power-Off-Brake
Over-excitation is used to achieve a faster response time. It is when a coil momentarily receives
a higher voltage then its nominal rating. To be effective, the over-excitation voltage must be
significantly, but not to the point of diminishing returns, higher than the normal coil voltage.
Three times the voltage typically gives around 1/3 faster response. Fifteen times the normal coil
voltage will produce a 3 times faster response time.
With over-excitation, the in-rush voltage is momentary. Although it would depend upon the size
of the coil, the actual time is usually only a few milliseconds. The theory is, for the coil to
generate as much of a magnetic field as quickly as possible to attract the armature and start the
process of deceleration. Once the over-excitation is no longer required, the power supply to the
brake would return to its normal operating voltage. This process can be repeated a number of
times as long as the high voltage does not stay in the coil long enough to cause the coil wire to
overheat.
[edit] Wear
It is very rare that a coil would just stop working in an electromagnetic brake. Typically if a coil
fails it is usually due to heat which has caused the insulation of the coil wire to break down. That
heat can be caused by high ambient temperature, high cycle rates, slipping or applying too high
of a voltage. Most brakes are flanged mounted and have bearings but some brakes are bearing
mounted and like the coils, unless bearings are stressed beyond their physical limitations or
become contaminated, they tend to have a long life and they are usually the second item to wear
out.
The main wear in electromagnetic brakes occurs on the faces of the mating surfaces. Every time
a brake is engaged during rotation a certain amount of energy is transferred as heat. The transfer,
which occurs during rotation, wears both the armature and the opposing contact surface. Based
upon the size of the brake, the speed and the inertia, wear rates will differ. With a fixed armature
design a brake will eventually simply cease to engage. This is because the air gap will eventually
become too large for the magnetic field to overcome. Zero gap or auto wear armatures can wear
to the point of less than one half of its original thickness, which will eventually cause missed
engagements.
[edit] Backlash
Some applications require very tight precision between all components. In these applications
even a degree of movement between the input and the output when a brake is engaged can be a
problem. This is true in many robotic applications. Sometimes the design engineers will order
brakes with zero backlash but then key them to the shafts so although the brake will have zero
backlash there is still minimal movement occurring between the hub or rotor in the shaft.
Most applications, however, do not need true zero backlash and can use a spline type connection.
Some of these connections between the armature and the hub are standard splines, others are hex
or square hub designs. The spline will have the best initial backlash tolerance. Typically less than
2 degrees but the spline and the other connection types can wear over time and the tolerances
will increase.
[edit] Environment / Contamination
As brakes wear they create wear particles. In some applications such as clean rooms or food
handling this dust could be a contamination problem so in these applications the brake should be
enclosed to prevent the particles from contaminating other surfaces around it. But a more likely
scenario is that the brake has a better chance of getting contaminated from its environment.
Obviously oil or grease should be kept away from the contact surface because they would
significantly reduce the coefficient of friction which could drastically decrease the torque
potentially causing failure. Oil midst or lubricated particles can also cause surface contamination.
Sometimes paper dust or other contamination can fall in between the contact surfaces. This can
also result in a lost of torque. If a known source of contamination is going to be present many
clutch manufactures offer contamination shields that prevent material from falling in between the
contact surfaces.
In brakes that have not been used in a while rust can develop on the surfaces. But in general this
is normally not a major concern since the rust is worn off within a few cycles and there is no
lasting impact on the torque.
[edit] Other Types of Electromagnetic Brakes
[edit] Electromagnetic Power Off Brake
Electormagnetic Power Off Brake Spring Set
Introduction - Power off brakes stop or hold a load when electrical power is either accidentally
lost or intentionally disconnected. In the past, some companies have referred to these as "fail
safe" brakes. These brakes are typically used on or near an electric motor. Typical applications
include robotics, holding brakes for Z axis ball screws and servo motor brakes. Brakes are
available in multiple voltages and can have either standard backlash or zero backlash hubs.
Multiple disks can also be used to increase brake torque, without increasing brake diameter.
There are 2 main types of holding brakes. The first is spring applied brakes. The second is
permanent magnet brakes.
How It Works
Spring Type - When no electricity is applied to the brake, a spring pushes against a pressure
plate, squeezing the friction disk between the inner pressure plate and the outer cover plate. This
frictional clamping force is transferred to the hub, which is mounted to a shaft.
Permanent Magnet Type – A permanent magnet holding brake looks very similar to a standard
power applied electromagnetic brake. Instead of squeezing a friction disk, via springs, it uses
permanent magnets to attract a single face armature. When the brake is engaged, the permanent
magnets create magnetic lines of flux, which can turn attract the armature to the brake housing.
To disengage the brake, power is applied to the coil which sets up an alternate magnetic field that
cancels out the magnetic flux of the permanent magnets.
Both power off brakes are considered to be engaged when no power is applied to them. They are
typically required to hold or to stop alone in the event of a loss of power or when power is not
available in a machine circuit. Permanent magnet brakes have a very high torque for their size,
but also require a constant current control to offset the permanent magnetic field. Spring applied
brakes do not require a constant current control, they can use a simple rectifier, but are larger in
diameter or would need stacked friction disks to increase the torque.
[edit] Electromagnetic Particle Brake
Magnetic Particle Brake
Introduction - Magnetic particle brakes are unique in their design from other electro-mechanical
brakes because of the wide operating torque range available. Like an electro-mechanical brake,
torque to voltage is almost linear; however, in a magnetic particle brake, torque can be controlled
very accurately (within the operating RPM range of the unit). This makes these units ideally
suited for tension control applications, such as wire winding, foil, film, and tape tension control.
Because of their fast response, they can also be used in high cycle applications, such as magnetic
card readers, sorting machines and labeling equipment.
How It Works - Magnetic particles (very similar to iron filings) are located in the powder cavity.
When electricity is applied to the coil, the resulting magnetic flux tries to bind the particles
together, almost like a magnetic particle slush. As the electric current is increased, the binding of
the particles becomes stronger. The brake rotor passes through these bound particles. The output
of the housing is rigidly attached to some portion of the machine. As the particles start to bind
together, a resistant force is created on the rotor, slowing, and eventually stopping the output
shaft.
When electricity is removed from the brake, the input is free to turn with the shaft. Since
magnetic particle powder is in the cavity, all magnetic particle units have some type of minimum
drag associated with them.
[edit] Electromagnetic Hysteresis Power Brake
Electomagnetic Hysteresis Power Brake
Introduction - Electrical hysteresis units have an extremely wide torque range. Since these units
can be controlled remotely, they are ideal for test stand applications where varying torque is
required. Since drag torque is minimal, these units offer the widest available torque range of any
of the hysteresis products. Most applications involving powered hysteresis units are in test stand
requirements.
How It Works - When electricity is applied to the field, it creates an internal magnetic flux. That
flux is then transferred into a hysteresis disk passing through the field. The hysteresis disk is
attached to the brake shaft. A magnetic drag on the hysteresis disk allows for a constant drag, or
eventual stoppage of the output shaft.
When electricity is removed from the brake, the hysteresis disk is free to turn, and no relative
force is transmitted between either member. Therefore, the only torque seen between the input
and the output is bearing drag.
[edit] Multiple Disk Brakes
Electromagnetic Multiple Disk Brake
Introduction - Multiple disk brakes are used to deliver extremely high torque within a small
space. These brakes can be used either wet or dry, which makes them ideal to run in multi speed
gear box applications, machine tool applications, or in off road equipment.
How It Works - Electro-mechanical disk brakes operate via electrical actuation, but transmit
torque mechanically. When electricity is applied to the coil of an electromagnet, the magnetic
flux attracts the armature to the face of the brake. As it does so, it squeezes the inner and outer
friction disks together. The hub is normally mounted on the shaft that is rotating. The brake
housing is mounted solidly to the machine frame. As the disks are squeezed, torque is
transmitted from the hub into the machine frame, stopping and holding the shaft.
When electricity is removed from the brake, the armature is free to turn with the shaft. Springs
keep the friction disk and armature away from each other. There is no contact between breaking
surfaces and minimal drag.
[edit] References
Electromagnet
Top
Home > Library > Miscellaneous > Wikipedia
An electromagnet is a type of magnet whose magnetic field is produced by the flow of electric
current. The magnetic field disappears when the current ceases.
Electromagnet attracts paper clips when current is applied creating a magnetic field, loses them
when current and magnetic field are removed
Contents [hide]
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1 Introduction
2 How the iron core works
3 History
4 Analysis of ferromagnetic electromagnets
o 4.1 Magnetic circuit - the constant B field approximation
o 4.2 Magnetic field created by a current
o 4.3 Force exerted by magnetic field
o 4.4 Closed magnetic circuit
o 4.5 Force between electromagnets
5 Side effects in large electromagnets
o 5.1 Ohmic heating
o 5.2 Inductive voltage spikes
o 5.3 Lorentz forces
o 5.4 Core losses
6 High field electromagnets
o 6.1 Superconducting electromagnets
o 6.2 Bitter electromagnets
o 6.3 Exploding electromagnets
7 Uses of electromagnets
8 Definition of terms
9 See also
10 References
11 External links
Introduction
A wire with an electric current passing through it generates a magnetic field around it; this is a
simple electromagnet. The strength of magnetic field generated is proportional to the amount of
current.
Current (I) through a wire produces a magnetic field (B). The field is oriented according to the
right-hand rule.
In order to concentrate the magnetic field generated by a wire, it is commonly wound into a coil,
where many turns of wire sit side by side. The magnetic field of all the turns of wire passes
through the center of the coil, creating a strong field there. A coil forming the shape of a straight
tube, a helix (similar to a corkscrew) is called a solenoid; a solenoid that is bent into a donut
shape so that the ends meet is called a toroid. Much stronger magnetic fields can be produced if a
"core" of ferromagnetic material, such as soft iron, is placed inside the coil. The ferromagnetic
core magnifies the magnetic field to thousands of times the strength of the field of the coil alone,
due to the high magnetic permeability μ of the ferromagnetic material. This is called a
ferromagnetic-core or iron-core electromagnet.
The direction of the magnetic field through a coil of wire can be found from a form of the righthand rule.[1][2][3][4][5][6] If the fingers of the right hand are curled around the coil in the direction of
current flow (conventional current, flow of positive charge) through the windings, 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.
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.
Magnetic field produced by a solenoid. The crosses are wires in which current is moving into the
page; the dots are wires in which current is moving up out of the page.
How the iron core works
The material of the core of the magnet (usually iron) is composed of small regions called
magnetic domains that act like tiny magnets (see ferromagnetism). Before the current in the
electromagnet is turned on, the domains in the iron core point in random directions, so their tiny
magnetic fields cancel each other out, and the iron has no large scale magnetic field. When a
current is passed through the wire wrapped around the iron, its magnetic field penetrates the iron,
and causes the domains to turn, aligning parallel to the magnetic field, so their tiny magnetic
fields add to the wire's field, creating a large magnetic field that extends into the space around
the magnet. The larger the current passed through the wire coil, the more the domains align, and
the stronger the magnetic field is. Finally all the domains are lined up, and further increases in
current only cause slight increases in the magnetic field: this phenomenon is called saturation.
When the current in the coil is turned off, most of the domains lose alignment and return to a
random state and the field disappears. However some of the alignment persists, because the
domains have difficulty turning their direction of magnetization, leaving the core a weak
permanent magnet. This phenomenon is called hysteresis and the remaining magnetic field is
called remanent magnetism. The residual magnetization of the core can be removed by
degaussing.
History
Sturgeon's electromagnet, 1823
Danish scientist Hans Christian Ørsted discovered in 1820 that electric currents create magnetic
fields. British scientist William Sturgeon invented the electromagnet in 1824.[7][8] His first
electromagnet was a horseshoe-shaped piece of iron that was wrapped with about 18 turns of
bare copper wire (insulated wire didn't exist yet). The iron was varnished to insulate it from the
windings. When a current was passed through the coil, the iron became magnetized and attracted
other pieces of iron; when the current was stopped, it lost magnetization. Sturgeon displayed its
power by showing that although it only weighed seven ounces (roughly 200 grams), it could lift
nine pounds (roughly 4 kilos) when the current of a single-cell battery was applied. However,
Sturgeon's magnets were weak because the uninsulated wire he used could only be wrapped in a
single spaced out layer around the core, limiting the number of turns. Beginning in 1827, US
scientist Joseph Henry systematically improved and popularized the electromagnet.[9] By using
wire insulated by silk thread he was able to wind multiple layers of wire on cores, creating
powerful magnets with thousands of turns of wire, including one that could support 2063 pounds.
The first major use for electromagnets was in telegraph sounders.
The magnetic domain theory of how ferromagnetic cores work was first proposed in 1906 by
French physicist Pierre-Ernest Weiss, and the detailed modern quantum mechanical theory of
ferromagnetism was worked out by Werner Heisenberg and Lev Landau.
Analysis of ferromagnetic electromagnets
For definitions of the variables below, see box at end of article.
Industrial electromagnet lifting scrap iron, 1914
The magnetic field of electromagnets in the general case is given by Ampere's Law:
which says that the integral of the magnetizing field H around any closed loop of the field is
equal to the sum of the current flowing through the loop. Another equation used, that gives the
magnetic field due to each small segment of current, is the Biot-Savart law. Computing the
magnetic field and force exerted by ferromagnetic materials is difficult for two reasons. First,
because the strength of the field varies from point to point in a complicated way, particularly
outside the core and in air gaps, where fringing fields and leakage flux must be considered.
Second, because the magnetic field B and force are nonlinear functions of the current, depending
on the nonlinear relation between B and H for the particular core material used. For precise
calculations the finite element method is used.
Magnetic circuit - the constant B field approximation
However, for a typical DC electromagnet in which the magnetic field path is confined to a loop
or circuit most of which is in core material, a simplification can be made. A common simplifying
assumption satisfied by many electromagnets, which will be used in this section, is that the
magnetic field strength B is constant around the magnetic circuit and zero outside it. Most of the
magnetic field will be concentrated in the core material. Within the core the magnetic field will
be approximately uniform across any cross section, so if in addition the core has roughly
constant area throughout its length, the field in the core will be constant. This just leaves the air
gaps, if any, between core sections. In the gaps the magnetic field lines are no longer confined by
the core, so they 'bulge' out beyond the outlines of the core before curving back to enter the next
piece of core material, reducing the field strength in the gap. The bulges are called fringing
fields. However, as long as the length of the gap is smaller than the cross section dimensions of
the core, the field in the gap will be approximately the same as in the core. In addition, if parts of
the core are too near other parts, some of the magnetic field lines will take 'short cuts' and not
pass through the entire core circuit. This also occurs in the field near the windings, if the
windings are not wrapped tightly around the core. This is called leakage flux. It also results in a
lower magnetic field in the core. Therefore the equations in this section are valid for
electromagnets for which:
1. the magnetic circuit is a single loop.
2. the core has roughly the same cross sectional area throughout its length.
3. the air gaps between sections of core material are not large compared with the cross
sectional dimensions of the core.
4. there is negligible leakage flux
The main nonlinear feature of ferromagnetic materials is that the B field saturates at a certain
value, which is around 1.6 teslas (T) for most high permeability core steels. The B field increases
quickly with increasing current up to that value, but above that value the field levels off and
increases at the much smaller paramagnetic value, regardless of how much current is sent
through the windings. So the strength of the magnetic field possible from an iron core
electromagnet is limited to 1.6-2 T.
Magnetic field created by a current
The magnetic field created by an electromagnet is proportional to both the number of turns in the
winding, N, and the current in the wire, I, hence this product, NI, in ampere-turns, is given the
name magnetomotive force. For an electromagnet with a single magnetic circuit, of which length
Lcore is in the core material and length Lgap is in air gaps, Ampere's Law reduces to:[10][11]
where
is the permeability of free space (or air).
This is a nonlinear equation, because the permeability of the core, μ, varies with the magnetic
field B. For an exact solution, the value of μ at the B value used must be obtained from the core
material hysteresis curve. If B is unknown, the equation must be solved by numerical methods.
However, if the magnetomotive force is well above saturation, so the core material is in
saturation, the magnetic field won't vary much with changes in NI anyway. For a closed
magnetic circuit (no air gap) most core materials saturate at a magnetomotive force of roughly
800 ampere-turns per meter of flux path.
For most core materials,
.[11] So in equation (1) above, the
second term dominates. Therefore, in magnetic circuits with an air gap, the behavior of the
magnet depends strongly on the length of the air gap, and the length of the flux path in the core
doesn't matter much.
Force exerted by magnetic field
When none of the magnetic field bypasses any sections of the core (no flux leakage), the force
exerted by an electromagnet on a section of core material is:
The 1.6 T limit on the field mentioned above sets a limit on the maximum force per unit core
area, or pressure, an iron-core electromagnet can exert; roughly:
Given a core geometry, the B field needed for a given force can be calculated from (2); if it
comes out to much more than 1.6 T, a larger core must be used.
Closed magnetic circuit
Cross section of lifting electromagnet like that in above photo, showing cylindrical construction.
The windings (C) are flat copper strips to withstand the Lorentz force of the magnetic field. The
core is formed by the thick iron housing (D) that wraps around the windings.
For a closed magnetic circuit (no air gap), such as would be found in an electromagnet lifting a
piece of iron, equation (1) becomes:
Substituting into (2), the force is:
It can be seen that to maximize the force, a core with a short flux path and a wide cross sectional
area is preferred. To achieve this, in applications like lifting magnets (see photo above) and
loudspeakers a flat cylindrical design is often used. The winding is wrapped around a short wide
cylindrical core that forms one pole, and a thick metal housing that wraps around the outside of
the windings forms the other part of the magnetic circuit, bringing the magnetic field to the front
to form the other pole.
Force between electromagnets
The above methods are inapplicable when most of the magnetic field path is outside the core. For
electromagnets (or permanent magnets) with well defined 'poles' where the field lines emerge
from the core, the force between two electromagnets can be found using the 'Gilbert model'
which assumes the magnetic field is produced by fictitious 'magnetic charges' on the surface of
the poles, with pole strength m and units of Ampere-turn meter. Magnetic pole strength of
electromagnets can be found from:
The force between two poles is:
This model doesn't give the correct magnetic field inside the core, and thus gives incorrect
results if the pole of one magnet gets too close to another magnet.
Side effects in large electromagnets
There are several side effects which become important in large electromagnets and must be
provided for in their design:
Ohmic heating
The only power consumed in a DC electromagnet is due to the resistance of the windings, and is
dissipated as heat. Some large electromagnets require cooling water circulating through pipes in
the windings to carry off the waste heat.
Since the magnetic field is proportional to the product NI, the number of turns in the windings N
and the current I can be chosen to minimize heat losses, as long as their product is constant.
Since the power dissipation, P = I2R, increases with the square of the current, the power lost in
the windings can be minimized by reducing I and increasing the number of turns N
proportionally. For this reason most electromagnets have windings with many turns of wire.
However, the limit to increasing N is that the larger number of windings takes up more room
between the magnet's core pieces. If the area available for the windings is filled up, more turns
require going to a smaller diameter of wire, which has higher resistance, which cancels the
advantage of using more turns. So in large magnets there is a minimum amount of heat loss that
can't be reduced. This increases with the square of the magnetic flux B2.
Inductive voltage spikes
An electromagnet is a large inductor, and resists changes in the current through its windings.
Any sudden changes in the winding current cause large voltage spikes across the windings. This
is because when the current through the magnet is increased, such as when it is turned on, energy
from the circuit must be stored in the magnetic field. When it is turned off the energy in the field
is returned to the circuit.
If an ordinary switch is used to control the winding current, this can cause sparks at the terminals
of the switch. This doesn't occur when the magnet is switched on, because the voltage is limited
to the power supply voltage. But when it is switched off, the energy in the magnetic field is
suddenly returned to the circuit, causing a large voltage spike and an arc across the switch
contacts, which can damage them. With small electromagnets a capacitor is often used across the
contacts, which reduces arcing by temporarily storing the current, allowing the current through
the electromagnet to change more slowly. Large electromagnets are usually powered by variable
current electronic power supplies, controlled by a microprocessor, which prevent voltage spikes
by accomplishing current changes in gentle ramps. It may take several minutes to energize or
deenergize a large magnet.
Lorentz forces
In powerful electromagnets, the magnetic field exerts a force on each turn of the windings, due to
the Lorentz force
acting on the moving charges within the wire. The Lorentz force is
perpendicular to both the axis of the wire and the magnetic field. It can be visualized as a
pressure between the magnetic field lines, pushing them apart. It has two effects on an
electromagnet's windings:
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The field lines within the axis of the coil exert a radial force on each turn of the windings,
tending to push them outward in all directions. This causes a tensile stress in the wire.
The leakage field lines between each turn of the coil exert a repulsive force between
adjacent turns, tending to push them apart.
The Lorentz forces increase with B2. In large electromagnets the windings must be firmly
clamped in place, to prevent motion on power-up and power-down from causing metal fatigue in
the windings. In the Bitter design, below, used in very high field research magnets, the windings
are constructed as flat disks to resist the radial forces, and clamped in an axial direction to resist
the axial ones.
Core losses
In alternating current (AC) electromagnets, used in transformers, inductors, and AC motors and
generators, the magnetic field is constantly changing. This causes energy losses in their magnetic
cores that are dissipated as heat in the core. The losses stem from two processes:
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Eddy currents: From Faraday's law of induction, the changing magnetic field induces
circulating electric currents inside nearby conductors, called eddy currents. The energy in
these currents is dissipated as heat in the electrical resistance of the conductor, so they are
a cause of energy loss. Since the magnet's iron core is conductive, and most of the
magnetic field is concentrated there, eddy currents in the core are the major problem.
Eddy currents are closed loops of current that flow in planes perpendicular to the
magnetic field. The energy dissipated is proportional to the area enclosed by the loop. To
prevent them, the cores of AC electromagnets are made of stacks of thin steel sheets, or
laminations, oriented parallel to the magnetic field, with an insulating coating on the
surface. The insulation layers prevent eddy current from flowing between the sheets. Any
remaining eddy currents must flow within the cross section of each individual lamination,
which reduces losses greatly. Another alternative is to use a ferrite core, which is a
nonconductor.
Hysteresis losses: Reversing the direction of magnetization of the magnetic domains in
the core material each cycle causes energy loss, because of the coercivity of the material.
These losses are called hysteresis. The energy lost per cycle is proportional to the area of
the hysteresis loop in the BH graph. To minimize this loss, magnetic cores used in
transformers and other AC electromagnets are made of "soft" low coercivity materials,
such as silicon steel or soft ferrite.
The energy loss per cycle of the AC current is constant for each of these processes, so the power
loss increases linearly with frequency.
High field electromagnets
Superconducting electromagnets
Main article: Superconducting magnet
When a magnetic field higher than the ferromagnetic limit of 1.6 T is needed, superconducting
electromagnets can be used. Instead of using ferromagnetic materials, these use superconducting
windings cooled with liquid helium, which conduct current without electrical resistance. These
allow enormous currents to flow, which generate intense magnetic fields. Superconducting
magnets are limited by the field strength at which the winding material ceases to be
superconducting. Current designs are limited to 10–20 T, with the current (2009) record of 33.8
T.[12] The necessary refrigeration equipment and cryostat make them much more expensive than
ordinary electromagnets. However, in high power applications this can be offset by lower
operating costs, since after startup no power is required for the windings, since no energy is lost
to ohmic heating. They are used in particle accelerators, MRI machines, and research.
Bitter electromagnets
Main article: Bitter electromagnet
Since both iron-core and superconducting electromagnets have limits to the field they can
produce, the highest manmade magnetic fields have been generated by air-core
nonsuperconducting electromagnets of a design invented by Francis Bitter in 1933, called Bitter
electromagnets.[13] These consist of a solenoid made of a stack of conducting disks, arranged so
that the current moves in a helical path through them. This design has the mechanical strength to
withstand the extreme Lorentz forces of the field, which increase with B2. The disks are pierced
with holes through which cooling water passes to carry away the heat caused by the high current.
The highest continuous field achieved with a resistive magnet is currently (2008) 35 T.[12] The
highest continuous magnetic field, 45 T,[13] was achieved with a hybrid device consisting of a
Bitter magnet inside a superconducting magnet.
Exploding electromagnets
The factor limiting the strength of electromagnets is the inability to dissipate the enormous waste
heat, so higher fields, up to 90 T,[12] have been obtained from resistive magnets by pulsing them.
The highest magnetic fields of all have been created by detonating explosives around a pulsed
electromagnet as it is turned on. The implosion compresses the magnetic field to values of
around 1000 T[13] for a few microseconds.
Uses of electromagnets
Electromagnets are widely used in many electric devices, including:
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Motors and generators
Relays, including reed relays originally used in telephone exchanges
Electric bells
Loudspeakers
Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks
Particle accelerators
Magnetic locks
Magnetic separation of materials
Industrial lifting magnets
Electromagnetic suspension used for MAGLEV trains
Definition of terms
square meter
tesla
newton
ampere per meter
ampere
meter
cross section area of core
Magnetic field
Force exerted by magnetic field
Magnetizing field
Current in the winding wire
Total length of the magnetic field path
meter
Length of the magnetic field path in the core material
meter
Length of the magnetic field path air gap
ampere meter
Pole strength of the electromagnets
newton per square ampere Permeability of the electromagnet core material
newton per square ampere Permeability of free space (or air) = 4π(10−7)
Relative permeability of the electromagnet core material
Number of turns of wire on the electromagnet
meter
Distance be