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Signal Processing & Control System / What is an MRI and its preperation
« on: October 03, 2018, 02:39:48 PM »
Magnetic resonance imaging (MRI) is a test that uses powerful magnets, radio waves, and a computer to make detailed pictures inside your body.

Your doctor can use this test to diagnose you or to see how well you've responded to treatment. Unlike X-rays and CT scans, an MRI doesn't use radiation.

Why Would You Get an MRI?
An MRI helps a doctor diagnose a disease or injury, and it can monitor how well you’re doing with a treatment. MRIs can be done on different parts of your body.

An MRI of the brain and spinal cord looks for:

Blood vessel damage
Brain injury
Multiple sclerosis
Spinal cord injuries
An MRI of the heart and blood vessels looks for:

Blocked blood vessels
Damage caused by a heart attack
Heart disease
Problems with the structure of the heart
An MRI of the bones and joints looks for:

Bone infections
Damage to joints
Disc problems in the spine
MRI can also be done to check the health of these organs:

Breasts (women)
Ovaries (women)
Prostate (men)
A special kind of MRI called a functional MRI (fMRI) maps brain activity.

This test looks at blood flow in your brain to see which areas become active when you do certain tasks. An fMRI can detect brain problems, such as the effects of a stroke, or for brain mapping if you need brain surgery for epilepsy or tumors. Your doctor can use this test to plan your treatment.

How Do I Prepare for an MRI?
Before your MRI, let your doctor know if you:

Have any health problems, such as kidney or liver disease
Recently had surgery
Have any allergies to food or medicine, or if you have asthma
Are pregnant, or might be pregnant
No metal is allowed in the MRI room, because the magnetic field in the machine can attract metal. Tell your doctor whether you have any metal-based devices that might cause problems during the test. These can include:

Artificial heart valves
Body piercings
Cochlear implants
Drug pumps
Fillings and other dental work
Implanted nerve stimulator
Insulin pump
Metal fragments, such as a bullet or shrapnel
Metal joints or limbs
Pacemaker or implantable cardioverter-defibrillator (ICD)
Pins or screws

Power System and Renewable Energy / Principle of Alternators
« on: September 23, 2018, 05:03:16 PM »

Early 20th-century alternator made by Ganz Works in 1909 in Budapest, Hungary, in the power generating hall of the biggest hydroelectric station of the Russian Empire (photograph by Prokudin-Gorsky, 1911)[1]
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current.[2] For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.[3] Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines. An alternator that uses a permanent magnet for its magnetic field is called a magneto. Alternators in power stations driven by steam turbines are called turbo-alternators. Large 50 or 60 Hz three phase alternators in power plants generate most of the world's electric power, which is distributed by electric power grids.[4]

1   History
2   Principle of operation
3   Synchronous speeds
4   Classifications
4.1   By excitation
4.1.1   Direct connected DC generator
4.1.2   Transformation and rectification
4.1.3   Brushless alternators
4.2   By number of phases
4.3   By rotating part
4.4   Cooling methods
5   Specific applications
5.1   Electric generators
5.2   Automotive alternators
5.3   Diesel electric locomotive alternators
5.4   Marine alternators
5.5   Radio alternators
6   See also
7   References
8   External links

In what is considered the first industrial use of alternating current in 1891, workmen pose with a Westinghouse alternator at the Ames Hydroelectric Generating Plant. This machine was used as a generator producing 3000 volt, 133 hertz, single-phase AC, and an identical machine 3 miles away was used as an AC motor.[5][6][7]
Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of electric current in the 1830s. Rotating generators naturally produced alternating current but, since there was little use for it, it was normally converted into direct current via the addition of a commutator in the generator.[8] The early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii. Faraday developed the "rotating rectangle", whose operation was heteropolar – each active conductor passed successively through regions where the magnetic field was in opposite directions.[9] Lord Kelvin and Sebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 Hz.

The late 1870s saw the introduction of first large scale electrical systems with central generation stations to power Arc lamps, used to light whole streets, factory yards, or the interior of large warehouses. Some, such as Yablochkov arc lamps introduced in 1878, ran better on alternating current, and the development of these early AC generating systems was accompanied by the first use of the word "alternator".[10][8] Supplying the proper amount of voltage from generating stations in these early systems was left up to the engineer's skill in "riding the load".[11] In 1883 the Ganz Works invented the constant voltage generator[12] that could produce a stated output voltage, regardless of the value of the actual load.[13] The introduction of transformers in the mid-1880s led to the widespread use of alternating current and the use of alternators needed to produce it.[14] After 1891, polyphase alternators were introduced to supply currents of multiple differing phases.[15] Later alternators were designed for various alternating current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.[16] Specialized radio frequency alternators like the Alexanderson alternator were developed as longwave radio transmitters around World War 1 and used in a few high power wireless telegraphy stations before vacuum tube transmitters replaced them.

Principle of operation

Diagram of a simple alternator with a rotating magnetic core (rotor) and stationary wire (stator) also showing the current induced in the stator by the rotating magnetic field of the rotor.
A conductor moving relative to a magnetic field develops an electromotive force (EMF) in it (Faraday's Law). This emf reverses its polarity when it moves under magnetic poles of opposite polarity. Typically, a rotating magnet, called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an induced EMF (electromotive force), as the mechanical input causes the rotor to turn.

The rotating magnetic field induces an AC voltage in the stator windings. Since the currents in the stator windings vary in step with the position of the rotor, an alternator is a synchronous generator.[3]

The rotor's magnetic field may be produced by permanent magnets, or by a field coil electromagnet. Automotive alternators use a rotor winding which allows control of the alternator's generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are restricted in size, due to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger than those used in automotive applications.

An automatic voltage control device controls the field current to keep output voltage constant. If the output voltage from the stationary armature coils drops due to an increase in demand, more current is fed into the rotating field coils through the voltage regulator (VR). This increases the magnetic field around the field coils which induces a greater voltage in the armature coils. Thus, the output voltage is brought back up to its original value.

Alternators used in central power stations also control the field current to regulate reactive power and to help stabilize the power system against the effects of momentary faults. Often there are three sets of stator windings, physically offset so that the rotating magnetic field produces a three phase current, displaced by one-third of a period with respect to each other.

Synchronous speeds
One cycle of alternating current is produced each time a pair of field poles passes over a point on the stationary winding. The relation between speed and frequency is {\displaystyle N=120f/P} N=120f/P, where {\displaystyle f} f is the frequency in Hz (cycles per second). {\displaystyle P} P is the number of poles (2,4,6...) and {\displaystyle N} N is the rotational speed in revolutions per minute (RPM). Very old descriptions of alternating current systems sometimes give the frequency in terms of alternations per minute, counting each half-cycle as one alternation; so 12,000 alternations per minute corresponds to 100 Hz.

The output frequency of an alternator depends on the number of poles and the rotational speed. The speed corresponding to a particular frequency is called the synchronous speed for that frequency. This table[17] gives some examples:

Poles   RPM for 50 Hz   RPM for 60 Hz   RPM for 400 Hz
2   3,000   3,600   24,000
4   1,500   1,800   12,000
6   1,000   1,200   8,000
8   750   900   6,000
10   600   720   4,800
12   500   600   4,000
14   428.6   514.3   3,429
16   375   450   3,000
18   333.3   400   2,667
20   300   360   2,400
40   150   180   1,200
Alternators may be classified by method of excitation, number of phases, the type of rotation, cooling method, and their application.[18]

By excitation
There are two main ways to produce the magnetic field used in the alternators, by using permanent magnets which create their own persistent magnetic field or by using field coils. The alternators that use permanent magnets are specifically called magnetos.

In other alternators, wound field coils form an electromagnet to produce the rotating magnetic field.

A device that uses permanent magnets to produce alternating current is called a permanent magnet alternator (PMA). A permanent magnet generator (PMG) may produce either alternating current, or direct current if it has a commutator.

Direct connected DC generator
This method of excitation consists of a smaller direct-current (DC) generator fixed on the same shaft with the alternator. The DC generator generates a small amount of electricity just enough to excite the field coils of the connected alternator to generate electricity. A variation of this system is a type of alternator which uses direct current from the battery for initial excitation upon start-up, after which the alternator becomes self-excited.[18]

Transformation and rectification
This method depends on residual magnetism retained in the iron core to generate weak magnetic field which would allow a weak voltage to be generated. This voltage is used to excite the field coils for the alternator to generate stronger voltage as part of its build up process. After the initial AC voltage buildup, the field is supplied with rectified voltage from the alternator.[18]

Brushless alternators
A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on the rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field as described above and a stationary armature (power generation windings).

Varying the amount of current through the stationary exciter field coils varies the 3-phase output from the exciter. This output is rectified by a rotating rectifier assembly, mounted on the rotor, and the resultant DC supplies the rotating field of the main alternator and hence alternator output. The result of all this is that a small DC exciter current indirectly controls the output of the main alternator.

Small-scale examples are ubiquitous in engine-driven motive-power applications. For example, early Honda four-cylinder motorcycles (CB750F, CB350F, CB500F, CB550F) used a brushless Hitachi 200W alternator. This had a fixed "rotor" winding on the outer cover; the outer end of the iron core was a disc that closed the outer rotor pole. The rotor comprised two intermeshed six-pole "claws" welded to and spaced apart by a non-magnetic ring. This was bolted directly to the end of the five-bearing crank via the hub of one pole. The other pole had an open end to receive the stator winding. The outer cover also held the three-phase stator windings. The magnetic circuit had two auxiliary air gaps between the rotor and its stationary core. The regulator was a conventional automotive type with vibrating points. As it had no slip rings, it was very compact and rugged, but due to the auxiliary air gaps, it had poor efficiency.

By number of phases
Main articles: Single-phase generator and Polyphase coil
Another way to classify alternators is by the number of phases of their output voltage. The output can be single phase, or polyphase. Three-phase alternators are the most common, but polyphase alternators can be two phase, six phase, or more.[18]

By rotating part
The revolving part of alternators can be the armature or the magnetic field. The revolving armature type has the armature wound on the rotor, where the winding moves through a stationary magnetic field. The revolving armature type is not often used.[18] The revolving field type has magnetic field on the rotor to rotate through a stationary armature winding. The advantage is that then the rotor circuit carries much less power than the armature circuit, making the slip ring connections smaller and less costly; only two contacts are needed for the direct-current rotor, whereas often a rotor winding has three phases and multiple sections which would each require a slip-ring connection. The stationary armature can be wound for any convenient medium voltage level, up to tens of thousands of volts; manufacture of slip ring connections for more than a few thousand volts is costly and inconvenient.

Cooling methods
Many alternators are cooled by ambient air, forced through the enclosure by an attached fan on the same shaft that drives the alternator. In vehicles such as transit buses, a heavy demand on the electrical system may require a large alternator to be oil-cooled. [19] In marine applications water-cooling is also used. Expensive automobiles may use water-cooled alternators to meet high electrical system demands.

Specific applications
Electric generators
Further information: Electric generator
Most power generation stations use synchronous machines as their generators. Connection of these generators to the utility grid requires synchronization conditions to be met.[20]

Automotive alternators
Further information: Alternator (automotive)

Alternator mounted on an automobile engine with a serpentine belt pulley (belt not present.)
Alternators are used in modern automobiles to charge the battery and to power the electrical system when its engine is running.

Until the 1960s, automobiles used DC dynamo generators with commutators. With the availability of affordable silicon diode rectifiers, alternators were used instead.

Diesel electric locomotive alternators
In later diesel electric locomotives and diesel electric multiple units, the prime mover turns an alternator which provides electricity for the traction motors (AC or DC).

The traction alternator usually incorporates integral silicon diode rectifiers to provide the traction motors with up to 1200 volts DC (DC traction, which is used directly) or the common inverter bus (AC traction, which is first inverted from dc to three-phase ac).

The first diesel electric locomotives, and many of those still in service, use DC generators as, before silicon power electronics, it was easier to control the speed of DC traction motors. Most of these had two generators: one to generate the excitation current for a larger main generator.

Optionally, the generator also supplies head end power (HEP) or power for electric train heating. The HEP option requires a constant engine speed, typically 900 RPM for a 480 V 60 Hz HEP application, even when the locomotive is not moving.

Marine alternators
Marine alternators used in yachts are similar to automotive alternators, with appropriate adaptations to the salt-water environment. Marine alternators are designed to be explosion proof so that brush sparking will not ignite explosive gas mixtures in an engine room environment. They may be 12 or 24 volt depending on the type of system installed. Larger marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht. On single alternator circuits, the power may be split between the engine starting battery and the domestic or house battery (or batteries) by use of a split-charge diode (battery isolator) or a voltage-sensitive relay.

Radio alternators
High frequency alternators of the variable-reluctance type were applied commercially to radio transmission in the low-frequency radio bands. These were used for transmission of Morse code and, experimentally, for transmission of voice and music. In the Alexanderson alternator, both the field winding and armature winding are stationary, and current is induced in the armature by virtue of the changing magnetic reluctance of the rotor (which has no windings or current carrying parts). Such machines were made to produce radio frequency current for radio transmissions, although the efficiency was low.

See also
Bottle dynamo
Electric generator
Folsom Powerhouse State Historic Park
Hub dynamo
Induction generator, using regular induction (asynchronous) motor
Jedlik's dynamo
Linear alternator
Polyphase coil
Revolving armature alternator
Single-phase generator
Flux switching alternator
 "Abraham Ganz at the Hindukush". Poemas del río Wang. Studiolum.
 Aylmer-Small, Sidney (1908). "Lesson 28: Alternators". Electrical railroading; or, Electricity as applied to railroad transportation. Chicago: Frederick J. Drake & Co. pp. 456–463.
 Gordon R. Selmon, Magnetoelectric Devices, John Wiley and Sons, 1966 no ISBN pp. 391-393
 "List of Plug/Sockets and Voltage of Different Countries". World Standards. World Standards.
 D. M. Mattox, The Foundations of Vacuum Coating Technology, page 39
 CHARLES C. BRITTON, An Early Electric Power Facility in Colorado, Colorado Magazine v49n3 Summer 1972, page 185
 "Milestones:Ames Hydroelectric Generating Plant, 1891". IEEE Global History Network. IEEE. Retrieved 29 July 2011.
 Christopher Cooper, The Truth about Tesla: The Myth of the Lone Genius in the History of Innovation, Quarto Publishing Group USA – 2015, page 93
 Thompson, Sylvanus P., Dynamo-Electric Machinery. p. 7.
 Jill Jonnes, Empires of Light: Edison, Tesla, Westinghouse, And The Race To Electrify The World, Random House – 2004, page 47

Electronic Devices & Circuit / Basic Circuits with Operational Amplifiers
« on: September 11, 2018, 06:26:26 PM »
The attached document can be used to understand the basic of the use of OP Amps

Laboratory / Lab Sheet for EEE 458
« on: September 11, 2018, 06:24:28 PM »
Find the attached Lab Sheet for EEE 458: Power System Protection Laboratory

Extra-curricular Activities / How to make a Video resume?
« on: September 11, 2018, 06:20:20 PM »
How to make a video resume
How do you avoid internet infamy and create something potential employers (and your grandma) would love?
1.   Draft a script. Get the key points you’d like to discuss down on paper. Don’t use vague phrases like “team player” and “detail orientated.” Show the viewer you’ve got the skills by using concrete examples of what you’ve done.
2.   Create with your audience in mind. Who’s going to see this video? What problems do they have? How can you help solve them? Show the viewer why you’re valuable and why they should have you on their team.
3.   Keep it short. People are busy. Get to the point quickly while keeping it engaging. Stick to 60 to 90 seconds.
4.   Play to your strengths. Are you an in front of the camera or a behind the scenes person? Choose a video format that works best for you and that helps you put your best foot forward.
5.   Use quality equipment. If you decide to stand in front of the camera, use a good mic and camera, light the scene properly and film somewhere quiet.
6.   Slow down. Most of us have a tendency to speed up when we’re nervous or uncomfortable. Slow down. Remember, you can take as many takes as it takes (now say that three time fast.)
7.   Include relevant links. The video resume grabbed their attention. It’s time to bring it home. Include links to your resume, your portfolio and relevant social media channels.
8.   Be yourself. Working for a company is a two-way street. You need to fit well together. Breathe, relax and be yourself. Easy, right?

1.   Film in your pajamas. This is your first impression, make it a good one. Dress and present yourself like you would in an interview.
2.   Sound like a robot. Don’t read off the script or list off all your achievements in a monotone voice. Keep the viewer engaged or you’ll lose them and the opportunity.
3.   Parrot your resume. This is your chance to show why you are a good fit and highlight the skills that can get lost on paper.
4.   Make something you don’t want others to see. This is the internet. Don’t put something out there you don’t want your mom or your friends (or your enemies) to see.
5.   Exaggerate your achievements. Be honest about your skills, your experience and what you bring to the table.

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