الأربعاء، 25 يناير 2017
الاثنين، 10 ديسمبر 2012
Electromagnetic field
This Article is written to bring together two topics: circuit theory and field theory.
Electromagnetic field theory is an important part of basic physics. In school it
is usually taught as a separate course. Because physics is a very mathematical
subject, the connection to everyday problems is not emphasized. Circuit theory,
by its very nature, is very practical. It provides a methodology that connects
with the many problems that students will encounter in practice. It is natural
for most technical people to reinforce circuit concepts and push basic physics
into the background.
Circuit theory is not a match for describing the nature of a facility, the
interconnection of many pieces of hardware, or the power grid that interfaces
each piece of hardware. In circuit theory the emphasis is on components, not
on such items as facilities or power distribution. A building or a power grid
is not a topic for discussion in a physics course, as these areas are far too
complex to consider. Basic physics can handle only very simple geometries,
not buildings. Given an interference problem, an engineer defaults to circuit
theory and circuit diagrams, as this is where he or she is usually most successful.
The circuits that might be considered for a facility usually do not
communicate well and bring little understanding to the problem. The fact that
there are no actual circuit components to consider is just one of the problems.
Circuit theory is a very powerful tool. If the right circuits are considered,
the answers can be meaningful. In this book we place the concepts of fields
into every aspect of circuit behavior. Every component functions because of
internal or external fields. A facility has its own fields, and these fields enter
into every circuit. When all the fields are considered, many problem areas
become clearer. A solution may require changing the geometry of a system
to limit the influence of the extraneous fields. Circuit theory is still used, but
the influence of the environment becomes a part of the design. In effect, field
theory brings geometry into circuit design. Experienced designers understand
how important geometry can be to circuit performance.
Fields are fundamental even in static circuits, and this is where the first
chapter starts. All circuits function through the motion of field energy, and this
idea must be considered at all circuit speeds. This includes batteries, utility
power, audio, radio frequencies, and microwaves. Fields are needed to operate
every circuit component, and conductors are needed to bring fields to each
1
2 THE ELECTRIC FIELD
component. This means that the flow of field energy, to every component
describes performance. The environment also includes field energy and this
energy cannot be ignored. Understanding this fact makes it possible to design
practical products.
Today’s circuits operate at very high speeds. The demand to process vast
amounts of data in very short periods is ever present. To understand high-speed
problems, it is necessary to start slowly. The fields involved in all electrical
phenomena are the same. In the first chapter we treat static charge and the
concept of voltage. These very elementary ideas lay the foundation for understanding
circuit behavior at all speeds. In later chapters, when the fields are
changing more rapidly, the problems of radiation are discussed. All circuits,
including the lowly flashlight, are explained using the same physics. This is
where the book starts: fields, batteries, and resistors.
Electromagnetic field theory is an important part of basic physics. In school it
is usually taught as a separate course. Because physics is a very mathematical
subject, the connection to everyday problems is not emphasized. Circuit theory,
by its very nature, is very practical. It provides a methodology that connects
with the many problems that students will encounter in practice. It is natural
for most technical people to reinforce circuit concepts and push basic physics
into the background.
Circuit theory is not a match for describing the nature of a facility, the
interconnection of many pieces of hardware, or the power grid that interfaces
each piece of hardware. In circuit theory the emphasis is on components, not
on such items as facilities or power distribution. A building or a power grid
is not a topic for discussion in a physics course, as these areas are far too
complex to consider. Basic physics can handle only very simple geometries,
not buildings. Given an interference problem, an engineer defaults to circuit
theory and circuit diagrams, as this is where he or she is usually most successful.
The circuits that might be considered for a facility usually do not
communicate well and bring little understanding to the problem. The fact that
there are no actual circuit components to consider is just one of the problems.
Circuit theory is a very powerful tool. If the right circuits are considered,
the answers can be meaningful. In this book we place the concepts of fields
into every aspect of circuit behavior. Every component functions because of
internal or external fields. A facility has its own fields, and these fields enter
into every circuit. When all the fields are considered, many problem areas
become clearer. A solution may require changing the geometry of a system
to limit the influence of the extraneous fields. Circuit theory is still used, but
the influence of the environment becomes a part of the design. In effect, field
theory brings geometry into circuit design. Experienced designers understand
how important geometry can be to circuit performance.
Fields are fundamental even in static circuits, and this is where the first
chapter starts. All circuits function through the motion of field energy, and this
idea must be considered at all circuit speeds. This includes batteries, utility
power, audio, radio frequencies, and microwaves. Fields are needed to operate
every circuit component, and conductors are needed to bring fields to each
1
2 THE ELECTRIC FIELD
component. This means that the flow of field energy, to every component
describes performance. The environment also includes field energy and this
energy cannot be ignored. Understanding this fact makes it possible to design
practical products.
Today’s circuits operate at very high speeds. The demand to process vast
amounts of data in very short periods is ever present. To understand high-speed
problems, it is necessary to start slowly. The fields involved in all electrical
phenomena are the same. In the first chapter we treat static charge and the
concept of voltage. These very elementary ideas lay the foundation for understanding
circuit behavior at all speeds. In later chapters, when the fields are
changing more rapidly, the problems of radiation are discussed. All circuits,
including the lowly flashlight, are explained using the same physics. This is
where the book starts: fields, batteries, and resistors.
Controlling an a.c. induction motor
Controlling an a.c. induction motor by the technique of sinewave-weighted pulse-width modulation (PWM) switching gives the benefits of smooth torque at low speeds, andalso complete speedcontrol from zero upto the nominal rated speed of the motor, with only small additional motor losses.
Traditional power switches such as thyristors need switching frequencies in the audible range, typically between 400 and 1500Hz. In industrial environments, the small amount of acoustic noise produced by the motor with this type of control can be regarded as insignificant. By contrast, however, the same amount of noise in a domestic or office application, such as speed control of a ventilation fan, might prove to be unacceptable.
Now, however, with the advent of power MOSFETs, three-phase PWM inverters operating at ultrasonic frequencies can be designed. A three-phase motor usually makes even less noise when being driven from such a system than when being run directly from the mains because the PWM synthesis generates a purer sinewave than is normally obtainable from the mains.
The carrier frequency is generally about 20kHz and so it is far removed from the modulation frequency, which is typically less than 50Hz, making it economic to use a low-pass filter between the inverter and the motor. By removing the carrier frequency and its sidebands and
harmonics, the waveform delivered via the motor leads can be made almost perfectly sinusoidal. RFI radiated by the motor leads, or conducted by the winding-to-frame capacitance of the motor, is therefore almost entirely eliminated. Furthermore, because of the high carrier frequency, it is possible to drive motors which are designed for frequencies higher than the mains, such as 400Hz aircraft motors.
This section describes a three-phase a.c. motor control system which is powered from the single-phase a.c. mains. It is capable of controlling a motor with up to 1kW of shaft output power. Before details are given, the general principles of PWM motor control are outlined.
Traditional power switches such as thyristors need switching frequencies in the audible range, typically between 400 and 1500Hz. In industrial environments, the small amount of acoustic noise produced by the motor with this type of control can be regarded as insignificant. By contrast, however, the same amount of noise in a domestic or office application, such as speed control of a ventilation fan, might prove to be unacceptable.
Now, however, with the advent of power MOSFETs, three-phase PWM inverters operating at ultrasonic frequencies can be designed. A three-phase motor usually makes even less noise when being driven from such a system than when being run directly from the mains because the PWM synthesis generates a purer sinewave than is normally obtainable from the mains.
The carrier frequency is generally about 20kHz and so it is far removed from the modulation frequency, which is typically less than 50Hz, making it economic to use a low-pass filter between the inverter and the motor. By removing the carrier frequency and its sidebands and
harmonics, the waveform delivered via the motor leads can be made almost perfectly sinusoidal. RFI radiated by the motor leads, or conducted by the winding-to-frame capacitance of the motor, is therefore almost entirely eliminated. Furthermore, because of the high carrier frequency, it is possible to drive motors which are designed for frequencies higher than the mains, such as 400Hz aircraft motors.
This section describes a three-phase a.c. motor control system which is powered from the single-phase a.c. mains. It is capable of controlling a motor with up to 1kW of shaft output power. Before details are given, the general principles of PWM motor control are outlined.
Defeating Electromagnetic Door Locks
Electromagnetic locks on the doors to keep them locked while the clerk is occupied cleaning the store at night so robbers can't ambush him while he's out from behind the bulletproof glass. These locks have around a 1,500 pound hold strength so it's impossible to force them open manually.
But you can open them from inside easily without the clerk having to buzz you out because there's a circuit that detects when a person touches the exit bar that de-energizes the electromagnet. This is a simple touch sensitive circuit that detects a change in capacitance when a conductive body (like yours) drains it.
It won't work if you touch it with gloves since they'd insulate you from the circuit. But touch it with a bare metal object...
Take a flat piece of steel about 2 feet long and 3/8" wide and bend it with a curve on the end in a half-circle 4" in diameter.
Now, by simply slipping the curved part through the gap in the doors, I can touch the bar on the inside with this conductive metal and fool the lock into thinking I'm inside and unlocking. Takes all of 3 seconds to insert this jimmy and unlock the doors.
This shows what an electromagnetic lock looks like. Basically a flat steel plate attached to the door with the electromagnet attached to the frame. These have a pull force of several thousand pounds, thus impossible to manually pull open. Here's the jimmy being inserted into the gap between doors. Here we see how the jimmy curves around to touch the door handle, thus fooling the lock into releasing.
The end must touch the bare metal of the bar in order to conduct. You have to be holding the jimmy with a bare hand too, because it won't work if you're wearing gloves.
But you can open them from inside easily without the clerk having to buzz you out because there's a circuit that detects when a person touches the exit bar that de-energizes the electromagnet. This is a simple touch sensitive circuit that detects a change in capacitance when a conductive body (like yours) drains it.
It won't work if you touch it with gloves since they'd insulate you from the circuit. But touch it with a bare metal object...
Take a flat piece of steel about 2 feet long and 3/8" wide and bend it with a curve on the end in a half-circle 4" in diameter.
Now, by simply slipping the curved part through the gap in the doors, I can touch the bar on the inside with this conductive metal and fool the lock into thinking I'm inside and unlocking. Takes all of 3 seconds to insert this jimmy and unlock the doors.
This shows what an electromagnetic lock looks like. Basically a flat steel plate attached to the door with the electromagnet attached to the frame. These have a pull force of several thousand pounds, thus impossible to manually pull open. Here's the jimmy being inserted into the gap between doors. Here we see how the jimmy curves around to touch the door handle, thus fooling the lock into releasing.
The end must touch the bare metal of the bar in order to conduct. You have to be holding the jimmy with a bare hand too, because it won't work if you're wearing gloves.
Current Actuated Relay
Fuses
The most commonly used protective device in a distribution circuit is the fuse. Fuse characteristics vary
considerably from one manufacturer to another and the specifics must be obtained from their appropriate
literature. Figure 3.3 shows the time-current characteristics which consist of the minimum melt
and total clearing curves.
Minimum melt is the time between initiation of a current large enough to cause the current
responsive element to melt and the instant when arcing occurs. Total Clearing Time (TCT) is the total
time elapsing from the beginning of an overcurrent to the final circuit interruption; i.e., TCT¼
minimum melt plus arcing time.
In addition to the different melting curves, fuses have different load-carrying capabilities. Manufacturer’s
application tables show three load-current values: continuous, hot-load pickup, and cold-load
pickup. Continuous load is the maximum current that is expected for three hours or more for which the
fuse will not be damaged. Hot-load is the amount that can be carried continuously, interrupted, and
immediately reenergized without melting. Cold-load follows a 30-min outage and is the high current
that is the result in the loss of diversity when service is restored. Since the fuse will also cool down during
this period, the cold-load pickup and the hot-load pickup may approach similar values.
The most commonly used protective device in a distribution circuit is the fuse. Fuse characteristics vary
considerably from one manufacturer to another and the specifics must be obtained from their appropriate
literature. Figure 3.3 shows the time-current characteristics which consist of the minimum melt
and total clearing curves.
Minimum melt is the time between initiation of a current large enough to cause the current
responsive element to melt and the instant when arcing occurs. Total Clearing Time (TCT) is the total
time elapsing from the beginning of an overcurrent to the final circuit interruption; i.e., TCT¼
minimum melt plus arcing time.
In addition to the different melting curves, fuses have different load-carrying capabilities. Manufacturer’s
application tables show three load-current values: continuous, hot-load pickup, and cold-load
pickup. Continuous load is the maximum current that is expected for three hours or more for which the
fuse will not be damaged. Hot-load is the amount that can be carried continuously, interrupted, and
immediately reenergized without melting. Cold-load follows a 30-min outage and is the high current
that is the result in the loss of diversity when service is restored. Since the fuse will also cool down during
this period, the cold-load pickup and the hot-load pickup may approach similar values.
The Nature of Relaying
1: Reliability
Reliability, in system protection parlance, has special definitions which differ from the usual planning or
operating usage. A relay can misoperate in two ways: it can fail to operate when it is required to do so, or
it can operate when it is not required or desirable for it to do so. To cover both situations, there are two
components in defining reliability:
Dependability—which refers to the certainty that a relay will respond correctly for all faults for which
it is designed and applied to operate; and
Security—which is the measure that a relay will not operate incorrectly for any fault.
Most relays and relay schemes are designed to be dependable since the system itself is robust enough
to withstand an incorrect tripout (loss of security), whereas a failure to trip (loss of dependability) may
be catastrophic in terms of system performance.
Reliability, in system protection parlance, has special definitions which differ from the usual planning or
operating usage. A relay can misoperate in two ways: it can fail to operate when it is required to do so, or
it can operate when it is not required or desirable for it to do so. To cover both situations, there are two
components in defining reliability:
Dependability—which refers to the certainty that a relay will respond correctly for all faults for which
it is designed and applied to operate; and
Security—which is the measure that a relay will not operate incorrectly for any fault.
Most relays and relay schemes are designed to be dependable since the system itself is robust enough
to withstand an incorrect tripout (loss of security), whereas a failure to trip (loss of dependability) may
be catastrophic in terms of system performance.
Types of Transformer Faults
Types of Transformer Faults
Any number of conditions have been the reason for an electrical transformer failure. Statistics show that
winding failures most frequently cause transformer faults (ANSI=IEEE, 1985). Insulation deterioration,
often the result of moisture, overheating, vibration, voltage surges, and mechanical stress created during
transformer through faults, is the major reason for winding failure.
Voltage regulating load tap changers, when supplied, rank as the second most likely cause of a transformer
fault. Tap changer failures can be caused by a malfunction of the mechanical switching mechanism,
high resistance load contacts, insulation tracking, overheating, or contamination of the insulating oil.
Transformer bushings are the third most likely cause of failure. General aging, contamination,
cracking, internal moisture, and loss of oil can all cause a bushing to fail. Two other possible reasons
are vandalism and animals that externally flash over the bushing.
Transformer core problems have been attributed to core insulation failure, an open ground strap, or
shorted laminations.
Other miscellaneous failures have been caused by current transformers, oil leakage due to inadequate
tank welds, oil contamination from metal particles, overloads, and overvoltage.
Any number of conditions have been the reason for an electrical transformer failure. Statistics show that
winding failures most frequently cause transformer faults (ANSI=IEEE, 1985). Insulation deterioration,
often the result of moisture, overheating, vibration, voltage surges, and mechanical stress created during
transformer through faults, is the major reason for winding failure.
Voltage regulating load tap changers, when supplied, rank as the second most likely cause of a transformer
fault. Tap changer failures can be caused by a malfunction of the mechanical switching mechanism,
high resistance load contacts, insulation tracking, overheating, or contamination of the insulating oil.
Transformer bushings are the third most likely cause of failure. General aging, contamination,
cracking, internal moisture, and loss of oil can all cause a bushing to fail. Two other possible reasons
are vandalism and animals that externally flash over the bushing.
Transformer core problems have been attributed to core insulation failure, an open ground strap, or
shorted laminations.
Other miscellaneous failures have been caused by current transformers, oil leakage due to inadequate
tank welds, oil contamination from metal particles, overloads, and overvoltage.
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