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Transcript of Final Report - Copy
ELECTROMAGNETIC BRAKING SYSTEM
A Seminar Report Submitted By
Name : Shashank Gupta
Class : Seventh Sem
Branch : Mechanical Engineering
Section : C
Roll Number : 145
Electromagnetic Braking System
CONTENTS
1. INTRODUCTION
2. PRINCIPLE OF OPERATION
3. CONSTRUCTION
4. FEATURES
5. CHARACTERISTICS OF ELECTROMAGNETIC BRAKES
6. TYPES OF ELECTROMAGNETIC BRAKES
7. ADVANTAGES AND DISADVANTAGES
8. REFERENCES
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
1. INTRODUCTION
Electromagnetic brakes have been used as supplementary retardation
equipment in addition to the regular friction brakes on heavy vehicles.
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.
A non-contact brake design actuated when an electric current charges a coil
that acts as an electromagnet. Electromagnetic brakes are widely used in
automated machinery and provide a high cycling rate. On trams and trains,
an electromagnetic brake is a track brake where the braking element is
pressed by magnetic force to the rail, i.e. the braking is by friction, not the
magnetic effect directly. This is different from an Eddy current brake where
there is no mechanical contact between the braking element on the moving Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
vehicle and the rail.
An eddy current brake, like a conventional friction brake, is responsible
for slowing an object, such as a train or a roller coaster. Unlike friction brakes,
which apply pressure on two separate objects, eddy current brakes slow an
object by creating eddy currents through electromagnetic induction which
create resistance, and in turn either heat or electricity.
Electromagnetic brakes are similar to electrical motors; non-
ferromagnetic metal discs (rotors) are connected to a rotating coil, and
a magnetic field between the rotor and the coil creates a resistance used
to generate electricity or heat. When electromagnets are used, control of the
braking action is made possible by varying the strength of the magnetic field. A
braking force is possible when electric current is passed through the
electromagnets. The movement of the metal through the magnetic field of the
electromagnets creates eddy currents in the discs. These eddy currents
generate an opposing magnetic field, which then resists the rotation of the
discs, providing braking force. The net result is to convert the motion of the
rotors into heat in the rotors.
2. PRINCIPLE OF OPERATION
There are three parts to an electromagnetic brake: field, armature,
and hub (which is the input on a brake). 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
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
transferred into the field housing and into the machine frame decelerating the
load. This can happen very fast (.1-3sec).
When the magnet is moved along the rail, it generates in non-
stationary magnetic field in the head of the rail, which then generates electrical
tension (Faraday's induction law), and that causes eddy currents. These disturb
the magnetic field in such a way that the magnetic force F, mentioned above, is
diverted to the opposite of the direction of the movement, thus creating a
parallelogram of forces consisting of the remaining vertical force FV and the
horizontal force FH, which works against the movement of the magnet.
The braking energy of the vehicle is converted in eddy current losses which lead
to a warming of the rail. The regular magnetic brake which is in wide use in
railways, exerts its braking force by friction with the rail, which also creates heat.
The eddy current brake does not have any mechanical contact with the rail, and
thus no wear and tear of it, and creates no noise or odor. The eddy
current brake is, as should be clear from the above explanation, unusable at low
speeds, but can be used at high speeds both for emergency braking as well as
regular and regulated braking.
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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
2.1 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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
2.2 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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Over-excitation is used to achieve a faster response time. It is when a coil
momentarily receives a higher voltage than 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.
2.3 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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
2.4 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.
3.CONSTRUCTION
The construction of the electromagnetic brake motor is shown below. The
electromagnetic brake is off. When voltage is applied to the coil, the armature is
retracted to the spring. This creates an air gap between the armature and brake
lining. The motor shaft is then released from braking to run freely. When the
voltage to the coil is shut off (the power is turned off), the armature is pressed
against the brake lining by the spring force to stop the motor shaft.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
4.FEATURES
• It is suitable for holding the load. Because the electromagnetic brake is off,
when the power is turned off, it will be activated and hold the load securely.
• The brake can be used as an excellent safety brake. Among the examples are
emergency braking at the time of power failure, load holding for a long period of
time and the prevention of free-run of the machine.
• The brake will be activated instantly. The overrun is only 2 to 4 revolutions when
the motor is used alone.
• A quick-reversal run can be frequently. Up to 6 cycles of start/stop can be
performed through simple switching. (Secure 3 seconds or longer for a pause.)
• Common power for both motor and brake can be used. Because the
electromagnetic brake section contains a rectifier circuit, it can use the same .AC
power supply as the motor. The construction of the electromagnetic brake motor
is shown below. The electromagnetic brake is off.
-When voltage is applied to the coil, the armature is retracted to the spring. This
creates an air gap between the armature and brake lining. The motor shaft is
then released from braking to run freely.
-When the voltage to the coil is shut off (the power is turned off), the armature is
pressed against the brake lining by the spring force to stop the motor shaft.
5. CHARACTERISTICS OF ELECTROMAGNETIC BRAKES
It was found that electromagnetic brakes can develop a negative power which
represents nearly twice the maximum power output of a typical engine,and at
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
least three times the braking power of an exhaust brake (Reverdin1974). These
performance of electromagnetic brakes make them much more competitive
candidate for alternative retardation equipments compared with other retarders.
By using the electromagnetic brake as supplementary retardation equipment, the
friction brakes can be used less frequently, and Therefore practically never reach
high temperatures. The brake linings would last considerably longer before
requiring maintenance, and the potentially “brake fade” problem could be
avoided.
The characterstics of the electromagnetic motor include responses regarding a
start time, stop time, overrun, etc. And these are all affected by the load inertia.
The characteristics of the electromagnetic motor depend on the following three
elements.
1) Average acceleration torque of the motor
2) Average value of brake torque
3) Load torque and inertia
When these elements are identified, the start time and stop time will be
determined. It is necessary to give sufficient attention to the load inertia in
particular because it varies depending on the equipment used together with the
motor.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
6. TYPES OF ELECTROMAGNETIC BRAKE
6.1 Electromagnetic Power Off Brake.
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
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
6.2 Electromagnetic 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 Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
6.3 Electromagnetic 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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
6.4 Multiple Disk Brakes
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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
7. ADVANTAGES AND DISADVANTAGES
Electromagnetic brakes rely purely on magnetic action working through an air
gap to develop torque. They have an extremely wide torque range. Since
torque is produced without physical contact of parts Electromagnetic devices are
not subject to wear. This feature makes them distinctly superior to mechanical-
friction brakes in life expectancy, servicing requirements and consistency of
performance. Since their working members have no physical contact they do not
depend on mechanical friction. Therefore, hysteresis units are absolutely and
constantly smooth at any slip ratio. Torque is reasonably independent of slip
speed and is also directly proportional to coil current, making response time
extremely quick. Electromagnetic brakes are also the most repeatable braking
devices known. They will repeat their performance precisely, an indefinite
number of times, whenever operating factors are repeated. This makes it ideal
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
for many precision tension control and testing applications. These devices have
a number of advantages over magnetic particle brakes, in particular eliminating
the problem of confining the magnetic particles inside the gap. These
advantages include long life, environmental stability, precise repeatability and
consistency of performance and extremely low power consumption. They can
tolerate extreme temperatures and have high heat-dissipation capability. They
also have the widest speed range of all electronically torque-control devices.
Hysteresis units will outlast any other type of electromechanical unit. The
transmitted torque remains constant and smooth as the hysteresis element is
forced to rotate within the air gap and will respond to increases and decreases in
coil current with corresponding increases and decreases in torque.
7.1 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
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
8.REFERENCE
1. http://en.wikipedia.org/wiki/Electromagnetic_brake
2. Automotive chassis: brakes, suspension, and steering By Tim GilleS
3. http://www.magtorx.com/
faq.htm#6._Why_we_using_Current_Regulated_Power_Supply_for_electr
omagnetic_brakes_and_clutches_
4. http://scholar.lib.vt.edu/theses/available/etd-5440202339731121/
unrestricted/CHAP2_DOC.pdf
5. http://industrial.panasonic.com/ww/i_e/25000/fa_pro_sgeard_shing1_e/
fa_pro_sgeard_shing1_e/ctlg_geared_e_14.pdf
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
CONTENTS
9. INTRODUCTION
10.PRINCIPLE OF OPERATION
11.CONSTRUCTION
12.FEATURES
13.CHARACTERISTICS OF ELECTROMAGNETIC BRAKES
14.TYPES OF ELECTROMAGNETIC BRAKES
15.ADVANTAGES AND DISADVANTAGES
16.REFERENCES
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
1. INTRODUCTION
Electromagnetic brakes have been used as supplementary retardation
equipment in addition to the regular friction brakes on heavy vehicles.
Electromagnetic brakes operate electrically, but ransmit 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.
A non-contact brake design actuated when an electric current charges a coil
that acts as an electromagnet. Electromagnetic brakes are widely used in
automated machinery and provide a high cycling rate. On trams and trains,
an electromagnetic brake is a track brake where the braking element is
pressed by magnetic force to the rail, i.e. the braking is by friction, not the
magnetic effect directly. This is different from an Eddy current brake where
there is no mechanical contact between the braking element on the moving Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
vehicle and the rail.
An eddy current brake, like a conventional friction brake, is responsible
for slowing an object, such as a train or a roller coaster. Unlike friction brakes,
which apply pressure on two separate objects, eddy current brakes slow an
object by creating eddy currents through electromagnetic induction which
create resistance, and in turn either heat or electricity.
Electromagnetic brakes are similar to electrical motors; non-
ferromagnetic metal discs (rotors) are connected to a rotating coil, and
a magnetic field between the rotor and the coil creates a resistance used
to generate electricity or heat. When electromagnets are used, control of the
braking action is made possible by varying the strength of the magnetic field. A
braking force is possible when electric current is passed through the
electromagnets. The movement of the metal through the magnetic field of the
electromagnets creates eddy currents in the discs. These eddy currents
generate an opposing magnetic field, which then resists the rotation of the
discs, providing braking force. The net result is to convert the motion of the
rotors into heat in the rotors.
PRINCIPLE OF OPERATION
There are three parts to an electromagnetic brake: field, armature,
and hub (which is the input on a brake). 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
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
transferred into the field housing and into the machine frame decelerating the
load. This can happen very fast (.1-3sec).
When the magnet is moved along the rail, it generates in non-
stationary magnetic field in the head of the rail, which then generates electrical
tension (Faraday's induction law), and that causes eddy currents. These disturb
the magnetic field in such a way that the magnetic force F, mentioned above, is
diverted to the opposite of the direction of the movement, thus creating a
parallelogram of forces consisting of the remaining vertical force FV and the
horizontal force FH, which works against the movement of the magnet.
The braking energy of the vehicle is converted in eddy current losses which lead
to a warming of the rail.The regular magnetic brake which is in wide use in
railways, exerts its braking force by friction with the rail, which also creates heat.
The eddy current brake does not have any mechanical contact with the rail, and
thus no wear and tear of it, and creates no noise or odor. The eddy
current brake is, as should be clear from the above explanation, unusable at low
speeds, but can be used at high speeds both for emergency braking as well as
regular and regulated braking.
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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
2.1 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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
2.2 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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Over-excitation is used to achieve a faster response time. It is when a coil
momentarily receives a higher voltage than 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.
2.3 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.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
2.4 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.
3.CONSTRUCTION
The construction of the electromagnetic brake motor is shown below. The
electromagnetic brake is off. When voltage is applied to the coil, the armature is
retracted to the spring. This creates an air gap between the armature and brake
lining. The motor shaft is then released from braking to run freely. When the
voltage to the coil is shut off (the power is turned off), the armature is pressed
against the brake lining by the spring force to stop the motor shaft.
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
4.FEATURES
• It is suitable for holding the load. Because the electromagnetic brake is off,
when the power is turned off, it will be activated and hold the load securely.
• The brake can be used as an excellent safety brake. Among the examples are
emergency braking at the time of power failure, load holding for a long period of
time and the prevention of free-run of the machine.
• The brake will be activated instantly. The overrun is only 2 to 4 revolutions when
the motor is used alone.
• A quick-reversal run can be frequently. Up to 6 cycles of start/stop can be
performed through simple switching. (Secure 3 seconds or longer for a pause.)
• Common power for both motor and brake can be used. Because the
electromagnetic brake section contains a rectifier circuit, it can use the same .AC
power supply as the motor. The construction of the electromagnetic brake motor
is shown below. The electromagnetic brake is off.
-When voltage is applied to the coil, the armature is retracted to the spring. This
creates an air gap between the armature and brake lining. The motor shaft is
then released from braking to run freely.
-When the voltage to the coil is shut off (the power is turned off), the armature is
pressed against the brake lining by the spring force to stop the motor shaft.
5. CHARACTERISTICS OF ELECTROMAGNETIC BRAKES
It was found that electromagnetic brakes can develop a negative power which
represents nearly twice the maximum power output of a typical engine,and at
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Electromagnetic Braking System
least three times the braking power of an exhaust brake (Reverdin1974). These
performance of electromagnetic brakes make them much more competitive
candidate for alternative retardation equipments compared with other retarders.
By using the electromagnetic brake as supplementary retardation equipment, the
friction brakes can be used less frequently, and Therefore practically never reach
high temperatures. The brake linings would last considerably longer before
requiring maintenance, and the potentially “brake fade” problem could be
avoided.
The characterstics of the electromagnetic motor include responses regarding a
start time, stop time, overrun, etc. And these are all affected by the load inertia.
The characteristics of the electromagnetic motor depend on the following three
elements.
1) Average acceleration torque of the motor
2) Average value of brake torque
3) Load torque and inertia
When these elements are identified, the start time and stop time will be
determined. It is necessary to give sufficient attention to the load inertia in
particular because it varies depending on the equipment used together with the
motor.
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Electromagnetic Braking System
6. TYPES OF ELECTROMAGNETIC BRAKE
6.1 Electromagnetic Power Off Brake.
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
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Electromagnetic Braking System
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.
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Electromagnetic Braking System
6.2 Electromagnetic 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 Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
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.
6.3 Electromagnetic 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.
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Electromagnetic Braking System
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.
6.4 Multiple Disk Brakes
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.
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Electromagnetic Braking System
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.
7. ADVANTAGES AND DISADVANTAGES
Electromagnetic brakes rely purely on magnetic action working through an air
gap to develop torque. They have an extremely wide torque range. Since
torque is produced without physical contact of parts Electromagnetic devices are
not subject to wear. This feature makes them distinctly superior to mechanical-
friction brakes in life expectancy, servicing requirements and consistency of
performance. Since their working members have no physical contact they do not
depend on mechanical friction. Therefore, hysteresis units are absolutely and
constantly smooth at any slip ratio. Torque is reasonably independent of slip
speed and is also directly proportional to coil current, making response time
extremely quick. Electromagnetic brakes are also the most repeatable braking
devices known. They will repeat their performance precisely, an indefinite
number of times, whenever operating factors are repeated. This makes it ideal
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Electromagnetic Braking System
for many precision tension control and testing applications. These devices have
a number of advantages over magnetic particle brakes, in particular eliminating
the problem of confining the magnetic particles inside the gap. These
advantages include long life, environmental stability, precise repeatability and
consistency of performance and extremely low power consumption. They can
tolerate extreme temperatures and have high heat-dissipation capability. They
also have the widest speed range of all electronically torque-control devices.
Hysteresis units will outlast any other type of electromechanical unit. The
transmitted torque remains constant and smooth as the hysteresis element is
forced to rotate within the air gap and will respond to increases and decreases in
coil current with corresponding increases and decreases in torque.
7.1 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
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Electromagnetic Braking System
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.
8.REFERENCE
6. http://en.wikipedia.org/wiki/Electromagnetic_brake
7. Automotive chassis: brakes, suspension, and steering By Tim GilleS
8. http://www.magtorx.com/
faq.htm#6._Why_we_using_Current_Regulated_Power_Supply_for_electr
omagnetic_brakes_and_clutches_
9. http://scholar.lib.vt.edu/theses/available/etd-5440202339731121/
unrestricted/CHAP2_DOC.pdf
10.http://industrial.panasonic.com/ww/i_e/25000/fa_pro_sgeard_shing1_e/
fa_pro_sgeard_shing1_e/ctlg_geared_e_14.pdf
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Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.
Electromagnetic Braking System
Dept. of Mechanical and Manufacturing Engg.