Introduction To Magnetic Coupling Engineering Essay

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Two conductors are denoted to as inductively coupled or magnetically coupled when they are constructed such that change in current flow through one wire induces a voltage across the ends of the other wire through electromagnetic induction. The quantity of inductive coupling between two conductors is measured by their mutual inductance.

The coupling among two wires can be increased by winding them into coils and placing them close together on a common axis, so the magnetic field of one coil passes through the other coil. The two coils may be physically enclosed in a single unit, as in the primary and secondary sides of a transformer, or may be separated. Coupling may be intended or unintended.

Unintentional coupling is called cross-talk, and is a form of electromagnetic interference. Inductive coupling favors low frequency energy sources. High frequency energy sources generally use capacitive coupling.

An inductively coupled transponder involves an electronic data carrying device, usually a single microchip, and a large coil that functions as an antenna. Inductively coupled transponders are almost always operated inactively.

Uses

Devices that use inductive coupling comprises:

Transformers

A transformer is a device that handovers electrical energy from one circuit to another through inductively

coupled conductors-the transformer's coils. A changing current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This changing magnetic field induces a fluctuating electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP) as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS less than NP.

In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.

Electric motors and generators

An electric motor uses electrical energy to produce mechanical energy, very typically through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on vehicles often perform both tasks. Many types of electric motors can be run as generators, and vice versa.

Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They may be powered by direct current (for example a battery powered portable device or motor vehicle), or by alternating current from a central electrical distribution grid. The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the millions of watts. Electric motors may be classified by the source of electric power, by their internal construction, by their application, or by the type of motion they give.

The physical principle of production of mechanical force by the interactions of an electric current and a magnetic field was known as early as 1821. Electric motors of increasing efficiency were constructed throughout the 19th century, but commercial exploitation of electric motors on a large scale required efficient electrical generators and electrical distribution networks.

Some devices, such as magnetic solenoids and loudspeakers, although they generate some mechanical power, are not generally referred to as electric motors, and are usually termed actuators and transducers, respectively.

Induction loop communication systems

Induction loop is a term used to describe an electromagnetic communication- and detection system, relying on the fact that a moving magnet will induce a electrical current in a nearby conducting wire. Induction loops are used for transmission and reception of communication signals, or for detection of metal objects in metal detectors or vehicle presence indicators. A common modern use for induction loops is to provide hearing assistance to hearing aid users.

Graphics tablet

A graphics tablet (or digitizing tablet, graphics pad, drawing tablet[1]) is a computer input device that allows one to hand-draw images and graphics, similar to the way one draws images with a pencil and paper. These tablets may also be used to capture data or handwritten signatures.

A graphics tablet (also called pen pad or digitizer) consists of a flat surface upon which the user may "draw" an image using an attached stylus, a pen-like drawing apparatus. The image generally does not appear on the tablet itself but, rather, is displayed on the computer monitor. Some tablets however, come as a functioning secondary computer screen that you can interact with directly using the stylus.

Some tablets are intended as a general replacement for a mouse as the primary pointing and navigation device for desktop computers.

Radio Frequency Identification

Radio-frequency identification (RFID) is the use of an object (typically referred to as an RFID tag) applied to or incorporated into a product, animal, or person for the purpose of identification and tracking using radio waves. Some tags can be read from several meters away and beyond the line of sight of the reader.

Radio-frequency identification comprises interrogators (also known as readers), and tags (also known as labels).

Most RFID tags contain at least two parts. One is an integrated circuit for storing and processing information, modulating and demodulating a radio-frequency (RF) signal, and other specialized functions. The second is an antenna for receiving and transmitting the signal.

Inductive Modems

Resonant energy transfer

Resonant energy transfer or resonant inductive coupling is the short-distance wireless transmission of energy between two coils that are highly resonant at the same frequency. The equipment to do this is sometimes called a resonant transformer. While many transformers employ resonance, this type has a high Q and is nearly always air-cored to avoid 'iron' losses.[citation needed] The coils may be present in a single piece of equipment or in separate pieces of equipment.

Resonant transfer works by making a coil ring with an oscillating current. This generates an oscillating magnetic field. Because the coil is highly resonant any energy placed in the coil dies away relatively slowly over very many cycles; but if a second coil is brought near to it, the coil can pick up most of the energy before it is lost, even if it is some distance away.

One of the applications of the resonant transformer is for the CCFL inverter. Another application of the resonant transformer is to couple between stages of a superheterodyne receiver, where the selectivity of the receiver is provided by tuned transformers in the intermediate-frequency amplifiers.[1] Resonant transformers such as the Tesla coil can generate very high voltages without arcing, and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator.[

Inductive charging products charge batteries using inductive coupling, such as eCoupled; Torches, Cochlear Implants and many electric toothbrushes.

Inductive charging uses the electromagnetic field to transfer energy between two objects. A charging station sends energy through inductive coupling to an electrical device, which stores the energy in the batteries. Because there is a small gap between the two coils, inductive charging is one kind of short-distance wireless energy transfer.

The other kind of charging, direct wired contact (also known as conductive charging or direct coupling) requires direct electrical contact between the batteries and the charger. Conductive charging is achieved by connecting a device to a power source with plug-in wires, such as a docking station, or by moving batteries from a device to charger.

Induction chargers typically use an induction coil to create an alternating electromagnetic field from within a charging base station, and a second induction coil in the portable device takes power from the electromagnetic field and converts it back into electrical current to charge the battery. The two induction coils in proximity combine to form an electrical transformer

Induction cookers and induction heating systems

An induction cooker uses a type of induction heating for cooking. It is chiefly distinguished from other common forms of stovetop cooking by the fact that the heat is generated directly in the cooking vessel, as opposed to being generated in the stovetop (as by electrical coils or burning gas) and then transferred to the cooking vessel.

In an induction stovetop, a coil of copper wire--an electromagnet--is placed underneath the cooking pot. An oscillating current is applied to that coil, which produces an oscillating magnetic field. That magnetic field creates heat in the cooking vessel over it, in two different ways. First, it induces a current in the electrically conductive pot, which produces Joule (I2R) heat. Second, it also creates magnetic hysteresis losses in the ferromagnetic pot. The first effect dominates: hysteresis losses typically account for less than ten percent of the total heat generated.[1]

Low frequency induction

Low frequency induction is an unwanted form of inductive coupling, which can occur when a metallic pipeline is installed parallel to a high-voltage power line. The pipeline, which is a conductor, and is insulated from the earth by its protective coating, can develop voltages which are hazardous to personnel operating valves or otherwise contacting the pipeline.

Significance and Applications.

Magnetic couplings are used to transmit rotational and/or linear motion without direct contact and

Rotary couplings

Linear couplings

Hysteresis couplings

Eddy current couplings

Rotary couplings are principally used to eliminate the use of seals in rotating and reciprocating machines, such as seal-less pumps and pistons.  Use of magnetic couplers improves the reliability and safety aspects of such machines because seals are prone to deterioration over time and cause leaks.

Rotary magnetic couplers used in these applications are designed in two configurations - co-axial and face-to-face. 

 In the co-axial configuration, the two halves of the coupler are mounted co-axially with each other and nested one within the other.  The outer member is typically connected to the motor and the inner member to the driven system, for example, the pump in a seal-less pump. A cup-shaped stationary member, mounted to the pump body, resides between the driver and follower and separates the fluids on the pump side from the environment on the motor side.  Materials for the barrier cup and exposed surfaces of the follower are chosen to survive continuous contact with the fluids being pumped.  The thickness of the barrier is designed to withstand any pressure differential without significant deformation.

Face-to-face type couplings are used where axial length is at premium and some misalignment needs to be tolerated.  The two pancake-shaped parts comprising this type of coupler have magnets mounted on the near faces.  The separation barrier in this case can be as simple as a flat wall.  One aspect of face-to-face type couplings is considerable attraction between the two members.

Linear and rotary magnetic couplings, and hybrids of the two, also find application in vacuum technology where position or motion must be transmitted across a vacuum barrier.  An added consideration in these applications is stiffness of the coupling; minimizing the lag between driver and follower. 

Linear magnetic couplings, following similar principles, allow precise control of robotics inside vacuum systems.  These couplers are used in the semiconductor industry to position objects within a clean chamber. Elimination of seals and reduction of the number of components inside the chamber improves contamination control and enhances system reliability.

In all the above cases, greater torque/force capacity is realized with stronger magnets, increased diameter and reduced radial gap.  An added consideration in these designs is the stiffness of the coupling which results in more precise control.  Devices operating at elevated temperatures (> 120 °C) typically employ Sm-Co magnets while others may use Nd-Fe-B or ceramic magnets.

Hysteresis couplings are typically used where a torque limiting is needed, such as in the bottle capping industry. 

Eddy current couplers exhibit torque that increases linearly with increasing revolutions per minute.  They are often employed in clutches and in couplers where extreme misalignment needs to be tolerated.  Hysteresis and eddy current principles may also be used in the design of linear couplers.

Magnetic Couplings are used in the industry to transmit torque through a gap. This gap is the distance between two members of the coupling. Between this gap, one may have air, vacuum, fluids, separator cups, or other similar items.

There are two basic configurations that are utilized; the Axial and Radial design.

The Axial design requires that the two magnet systems face each other, similar to two pancakes facing each other. As one member rotates, the other follows. The maximum torque will be determined by many factors, such as air gap, number of poles, materials selected, working temperature, etc.

The Radial design requires that the magnet systems are concentric to each other. As one member, typically the outer rotates and the other follows.

When selecting the barrier material between the coupling members, consider eddy current effects, since the flux lines of the magnetic coupling will cut the barrier material.  Conductive materials will start to heat as the RPM is increased.  Conductive materials used as the barrier material will lead resistance of the coupling motion, as some of the input work will turn into the eddy current losses (heat).  At higher RPM, over 600 RPM, this loss can be significant.

Typically, coupling assemblies do not exist as a standard "off the shelf" item. They have to be designed for each application, then manufactured. Depending on the complexity of your design, engineering charges may apply. If you are able to use a configuration that is close to something that we already have designed, and you are not concerned about "optimizing" the configuration, engineering/design charges will not apply.

Couplings may be hermetically sealed so that they may work in harsh environments, such as chemical applications.  Applications of magnetic couplings include nuclear environments, chemically hazardous environments, high temperature environments, oil drilling applications (downhole), vacuum applications, and vibration isolation applications.

Industries that have benefited from magnetic couplings include aerospace, medical, chemical, pharmaceutical, food, biotechnology, industrial ovens, compressors, metering,  and hydraulics, because magnetic couplings are the only device that can transmit contact free torque.

Magnetically-Coupled Circuits

A current i1 at L1 produces opencircuit voltage v2 at L2.

A current i2 at L2 produces an open circuit voltage v1 at L1.

Current entering the dotted terminal of one coil produces a voltage that is sensed positively at the dotted terminal of the second coil. Current entering the undotted terminal of one coil produces a voltage that is sensed positively at the undotted terminal of the second coil.

(a) A circuit containing mutual inductance in which the voltage

ratio V2/ V1 is desired. (b) Self and mutual inductances are

replaced by the corresponding impedances.

Zin = Zp + jw L1 + w 2M2/Jw L2 + Zs

Transformer

A given transformer which is to be replaced by an equivalent network.

The T equivalent.

M <= under root of L1L2

The coupling coefficient k is M /under root of L1L2

Inductive Coupling

Inductance

"L" depends on circuit geometry and medium property

"L" has meaning only for a closed circuit. However when talk about inductance of only a part of a circuit, we means the contribution that a segment of a circuit make ti the total inductance of the closed circuit

If sinusoidally ,

VN = j w B A cosθ

Induced noise depends on the area enclosed by the distributed circuit( VN = j w B A cosθ

Or VN = j w M I1 = M d i1 / d t

Magnetic coupling between two circuits

Magnetic coupling between two circuits

Suppression technique : separation circuit ( B↓); twisting ( B canceling)

closer to ground plane ( A↓) orientation (cosθ↓)

Comparison between inductive and capacitive coupling

① Capacitive coupling : noise picked up is decreased when impedance ↓, but inductive coupling not

â‘¡ Noise voltage is produced in series with receiver conductor in magnetic field coupling, while in electric field coupling noise voltage is produced between receiver and ground

Magnetic coupling when a shield is placed around the receiver (with ungrounded and nonmagnetic shield)

still VS = j w M1S I1

and VN = j w M12 I1

Conclusion: Even shield grounded at one end has no effects on the magnetically induced voltage in the center conductor

§ 3.5 Magnetic Coupling Between Shield and Inner Conductor

Magnetic coupling between a hollow conducting tube and conductor placed inside it

Magnetic field produced by current in a tubular conductor

No field inside cavity

Coaxial cable with shield current flowing

so Ls = M

Condition : The validity of above depends only on the fact that there is no magnetic field in the cavity of the tube due to shield current.

Noise voltage VN due to shield current

Equivalent circuit of the shield conductor

VN = j w M IS

Since

Considering M = Ls

Finally plot of noise voltage in shielded conductor due to shield current

Notes: Break frequency is defined as the shield cutoff frequency

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