Electrical & Electronic Principles

Electrical and Electronic PrinciplesBTEC National Diploma

OP7, P8, P9, D1Magnetism Assessment Criteria P7. describe the characteristics of a magnetic field.

P8. describe the relationship between flux density (B) & field strength (H).

P9. describe the principles & applications of electromagnetic induction.

D1. analyse the operation and the effects of varying component parameters of a power supply circuit that includes a transformer, diodes and capacitors.

2Know the principles and properties of magnetism: contentMagnetic field:Magnetic field patterns eg flux, flux density (B), magnetomotive force (mmf) and field strength (H), permeability, B/H curves and loops; Ferromagnetic materials; reluctance; magnetic screening; hysteresisElectromagnetic induction:Principles eg induced electromotive force (emf), eddy currents, self and mutual inductance;Applications (electric motor/generator eg series and shunt motor/generator; transformer eg primary and secondary current and voltage ratios); Application of Faradays and Lenzs laws

Using iron filings to show magnetic field lines

Bar magnetWire carrying a DC currentCurrent-carrying solenoid (notice magnetic field pattern similar to that for bar magnet)These images show that magnetism and electricity are linkedA solenoid is a coil in the form of a cylinder:

Using plotting compasses to showmagnetic field direction

Magnetic polesAn electric dipole is a paired arrangement of a positive (+) electric charge and a negative () one. They are equal and opposite. A magnetic dipole is a paired north (N) and south (S) pole arrangement. An atom is a tiny magnetic dipole. Whereas a single electric charge can exist on its own, a single magnetic pole on its own (a so-called magnetic monopole) has never been observed and can never be created from normal matter (though some theories in physics predict it does exist).

If a bar magnet is cut in half, it is not the case that one half has only the north pole and the other half has only the south. Instead, each piece has its own pair of north and south poles.Man-made permanent magnetsNaturally occurring ferromagnets were used in first experiments.Man-made products based on a mixture of naturally occurring magnetic elements or compounds.Magnets often manufactured by sintering (a sort of baking).Some common man-made magnets in table below:

Magnet typeCompositionNeodymiumNeodymium, iron, boronSamCoSamarium, cobalt (+ iron, copper)AlnicoAluminium, nickel, cobaltSr-ferriteStrontium oxide, iron(II) oxideFerrimagnetismAlmost every item of electronic equipment produced today contains some ferrimagnetic material: loudspeakers, motors, deflection yokes, interference suppressors, antenna rods, proximity sensors, recording heads, transformers and inductors are frequently based on ferrites.

Ferrimagnets possess permeability to rival most ferromagnets but their eddy current losses are far lower because of the material's greater electrical resistivity. Also it is practicable to fabricate different shapes by pressing or extruding - both low cost techniques.

Ferrimagnetic materials are usually oxides of iron combined with one or more of the transition metals such as manganese, nickel or zinc. Permanent ferrimagnets often include barium.

The raw material is turned into a powder which is then fired in a kiln or sintered.

Magnetic field linesAt any point where two magnetic fields are acting and a compass needle does not point in any particular direction, then there is no resultant field at the point.

Such a point is called a neutral point or a null point. (See np on bottom diagram.)

Strength of magnetic field around a bar magnet

www.coolmagnetman.com Strength of magnetic field around a bar magnet's north pole: close-up

www.coolmagnetman.com Magnetic field lines at north pole of bar magnet

www.coolmagnetman.com Two mutually attracting horseshoe magnets

Can you identify a neutral point?Magnetic flux and flux densityAround the magnet there is a magnetic field which we think of as corresponding to a flow of magnetic energy from the north pole to the south pole. We call this flow magnetic flux () and the units are Webers (Wb). The diagram shows that there is as much flux flowing from the north pole as there is flowing into the south pole.However, the amount of magnetic flux flowing through a given area will change from one point to another. At position X there is a greater number of field lines passing through the loop than there is when the same loop is at A.The amount of flux passing through a unit area (1 m2) at right angles to the field lines is called the magnetic flux density (B) at that point. B is measured in Tesla (T) where 1 T = 1 Wbm-2

Magnetic flux density formula

If we now use a coil of N turns instead of just one single loop, as shown in position Z, the effect of the flux through the N turns is N times that through the single loop.

(The quantity N is called the flux linkage for the coil at that point not required for the BTEC Diploma.)

WORKED EXAMPLE: flux and flux densityThe flux flowing through a horse-shoe magnet is 0.16 Wb.

The cross sectional area of the gap is200 mm2.

Calculate the magnetic flux density in the gap.

SOLUTION

= 0.16 WbA = 200 x 10-6 m2.So B = /A = 0.16/200 x 10-6 = 800 T

Wilhelm Eduard Weber (1804-91)

Important role in electrical science.

The unit of magnetic flux - weber (Wb) - is named after him.Nikola Tesla (18561943)

Serbian American inventor, electrical engineer, mechanical engineer, physicist, and futurist Best known for his contributions to the design of the modern AC electricity supply system

Made a lot of money from his patents and lived for most of his life in New York hotels. Spent a lot of income financing own projects -eventually declared bankrupt.

Regarded as a bit of a "mad scientist.

The unit of magnetic flux density tesla (T) named after him.Magnetic field round a current-carrying solenoidAdapted from the Penguin IB physics guide

Magnetic field round acurrent-carrying solenoid

This graphic has been created mathematically by computer

The LHC and liquid helium

Top left: Large Hadron Collider (LHC) beam pipe

Top right: Liquid helium and liquid nitrogen are both pumped in to different parts of the cyromodules

Bottom left: liquid helium in an open containerSuperconducting magnets at the LHC, CERN

The Compact Muon Solenoid (CMS - left) is one of the Large Hadron Collider's massive particle detectors. The Solenoid is a cryomagnet, i.e. an electromagnet that operates at extremely low temperatures.

Cryomagnets are also used for the Large Hadron Collider itself (right). The main magnets operate at around 8 tesla and a temperature of 271.3C (1.9 K), colder than the temperature of outer space (2.7K). At these very low temperatures, the wire is superconducting, i.e. its electrical resistance is exactly zero. This means it can conduct much larger electric currents than ordinary wire, creating intense magnetic fields. Because no energy is dissipated as heat in the windings, they can be cheaper to operate.

Cross-section of LHC beam pipes, containing a vacuum as empty as interplanetary spaceMeasuring magnetic fields: the flux density meter(this one uses a Hall probe)The Hall probe consists of a slice of semiconducting material with a small current passing through it. When it is placed in the magnetic field a p.d. that is directly proportional to the magnetic flux density is produced across the slice at right angles to the current direction.A flux density meter is sometimes called a Tesla meter. The Hall probe is only suitable for measuring steady magnetic fields.

Types of magnetism and the periodic tableIf interested, look up:Antiferromagnetism(due to neighbouring ions equal & opposite dipole moments)Ferrimagnetism(due to neighbouring ions UNequal & opposite dipole moments)Paramagnetic materials create a magnetic field in alignment with an externally applied magnetic field. They are weakly attracted to a magnet. [Due to orbital electron motion] Diamagnetic materials create a magnetic field in opposition to an externally applied magnetic field. There are weakly repelled by a magnet. [Due to unpaired electron spins] Ferromagnetic materials are strongly attracted to a magnet. Iron, nickel and cobalt are ferromagnetic. It is these your BTEC course is most interested in. [Due to magnetic domains]This periodic table shows magnetic properties of ELEMENTS, not minerals, alloys or compounds.

Paramagnetism & diamagnetism

Pyrolytic carbon, which is highly diamagnetic, levitating over permanent magnets

Diamagnetic forces acting upon the water within its body levitating a live frog. The frog is inside a special solenoid that generates an extremely powerful magnetic field (16 T).

Oxygen is paramagnetic and so is attracted to a magnet.See https://www.youtube.com/watch?v=KcGEev8qulA Nijmegen High Field Magnet Laboratory.Ferromagnetism

Unmagnetised ferromagnetic material: magnetic domains are unalignedMagnetised ferromagnetic material: magnetic domains are alignedYou may like to look up paramagnetism, diamagnetism, ferromagnetism, ferrimagnetism and antiferromagnetism.Ferromagnetism is a very strong form of magnetisation.

This is due to the existence of magnetic domains in ferromagnetic materials.Iron, nickel, cobalt (and some of the rare earth elements) exhibit a behaviour called ferromagnetism because iron (Latin: ferrum) is the most common and dramatic example.Effect of matter onapplied magnetic field

For ferromagnetic matter, this effect is more extreme.Magnetic flux density B, magnetic field strength H and permeability .When a magnetic field is applied to a material, the resulting overall magnetic flux density B within the material has two components, arising from:

The original applied fieldAn extra induced field resulting from the effect of the applied field on the atoms of the material (the material itself has become magnetised even if only minutely owing to the effect of the applied field and has produced a field of its own)

A common formula to express this situation isB = HWhere B is the overall magnetic flux density, H is the magnetic (or applied) field strength and is the permeability of the material, measured in henry per metre (Hm-1).

The permeability is a measure of the extent to which the material enhances the existing applied field. It is measured in amps per metre (Am-1)

The permeability is composed of two components: = 0 rWhere 0 is the permeability of free space (4 10-7 Hm-1) and r is the relative permeability of the substance (no units).Relative permeability (r) values for some materials Paramagnetic (r > 1)Platinum1.000265Aluminium1.000022Air1.00000043Wood1.0000004 Diamagnetic (r < 1)Bismuth0.999834Water0.999992Copper0.999994Sapphire0.9999998 Ferromagnetic(r >> 1)Metglas1,000,000Iron (annealed)to 350,000Mumetalto 100,000Permalloyto 25,000Rhometalto 5000Steelto 800Nickelto 600Cobaltto 250r for a vacuum = 1 exactly, by definitionHere, ferrite means a chemical compound of ceramic materials with iron(II) oxide as its main constituent.It was invented in Japan in 1930.(Ferrite also has other meanings.)

A stack of ferrite magnetsFor paramagnetic & diamagnetic materials,r is very close to 1. Ferrimagnetic(r >> 1)Ferrite (Ni-Zn)to 640Magnetisation in different materials

These are often called B-H curves.

Note: the B axis here is in tesla, whereas for the paramagnetic & diamagnetic graphs it is in millitesla.Magnified B-H curve for a ferromagnetic material

(These steps are called Barkhausen jumps - not required for BTEC Diploma! They occur because of the magnetic domain structure of ferromagnetic materials.)

Typical hysteresis loop(Greek hystrsis = lagging behind)Magnetic domains and hysteresis

Magnetically hard and soft materials

Magnetic memory(permanent magnet)Transformer core(temporary magnet)Incremental permeabilityThe permeability of a material, as already discussed, is given bySo at point P on the curve (see diagram), = 6.7 Hm-1The incremental permeability is given by the gradient of the curve at P:So at P, inc = 1.3 Hm-1

Quite often, books confuse readers by alluding to both B/H and B/H as the permeability, whereas they can have very different values!ShieldingElectromagnetic or magnetic shielding is the practice of isolating electrical equipment from the 'outside world.

Electromagnetic shielding is used against relatively high frequency electromagnetic fields. It is made from conductive or magnetic materials. A conductive enclosure used to block electrostatic fields is known as a Faraday cage. Such shielding is also used in cables to isolate wires from the environment.

Magnetic shielding is used against static or slowly varying magnetic fields. Shields made of high magnetic permeability metal alloys can be used, such as sheets of Permalloy (80% iron, 20% nickel) and Mu-Metal (77% nickel, 16% iron plus a little copper and chromium or molybdenum). These materials don't block the magnetic field, as is the case with electric shielding, but rather draw the field into themselves. Magnetic shields often consist of several enclosures one inside the other.

How magnets are madeThere are four main ways to magnetize a magnetisable object or substance:bringing the substance near a magnet; using electric current; stroking the substance with a magnet; and striking a blow to the substance while it is in a magnetic field.

A permanent magnet can be made by stroking a magnetic substance with either the N or the S pole of a magnet. Stroking lines up the domains in the material.A piece of iron can be magnetized by holding it parallel to a compass needle (along the lines of force in the earth's field) and hitting the piece of iron with a hammer. The blow will overcome the resistance of the domains to movement, and they will line up parallel to the earth's field.To demagnetize an object, a strong magnetic field is used. In one method, the magnetic field is made to fluctuate very rapidly. In another method, the magnetized object is placed so that a line drawn between its poles would be at right angles to the field. The object is then tapped or hit until its domains are no longer lined up magnetically.

Strengths of some magnetic fieldsA neodymium magnet (developed in 1982) is the most widely used type of rare-earth magnetmade from an alloy of neodymium, iron and boronthe strongest type of permanent magnet commercially availableused in applications that require strong permanent magnets, such as motors in cordless tools, hard disk drives and magnetic fasteners.SourceMagnetic flux density (tesla)Magnetically shielded room10-14Interstellar space 10-10Earth's magnetic field (UK)510-5Small bar magnet0.01Sunspot0.2Neodymium magnet1Big electromagnet; big transformer; speaker coil1-2.4Superconducting magnet1-40Regular neutron star107Magnetar108-1011

Neodymium magnets can easily lift thousands of times their own weight such as these steel spheresThere are 17 rare earth metals in the periodic table. They are actually not rare in themselves, but are scattered far and wide rather than being concentrated in easily found minerals. It is the minerals that are rare.Magnetomotive force & reluctanceMagnetomotive force (mmf) is what causes there to be a magnetic flux in a magnetic circuit. The mmf is defined as = NIwhere N is the number of turns of wire in the coil and I is the current in the coil. The unit for mmf is ampere-turns (At).

Example: calculate the mmf for a coil with 2000 turns and a 5 mA current.Answer: = N I = 2000 5 10-3 = 10 At

For a magnetic circuit we have = SSee table below for comparison of magnetic scenario with electrical scenario.Magnetic circuitElectrical circuit = Swhere is the mmf is the magnetic fluxS is the reluctance of the material through which the flux passes. = IRwhere is the emfI is the currentR is the total circuit resistanceElectromagnetism

= Bl vF = BilElectromagnetic inductionworked exampleWorked example. A plane of wingspan 30 m flies through a vertical field of strength 5 x 10-4 T. Calculate the emf induced across its wing tips if its velocity is 150 ms-1.

= Bl v = 5x10-4 x 30 x 150 = 2.25VElectromagnetic Induction

A galvanometer is a type of very sensitive ammeter used to detect tiny currents.

(They were the original ammeters)

Principles linkingmagnetism and electricity:Every electric current has a magnetic field surrounding it.

Alternating currents have fluctuating magnetic fields.

A fluctuating magnetic fields produces an emf which causes a current to flow in conductors lying within the fields. This is known as electromagnetic induction.

Electromagnetic induction applicationsElectromagnetic induction is the principle that makes possible devices such as:

electrical generators, transformers and certain kinds of motor

rechargeable electric toothbrushes and wireless communication devices

rice cookers.Ways that EMFsare generatedInductors(self induction)Transformers(mutual induction)Electricity generatorsPhotoelectric /thermoelectric / junction / etcdevicesIn accordance with Faradays Lawe.g. = Bl v Faradays law of electromagnetic inductionThe emf induced is equal to the rate of change of magnetic flux linkage or the rate of flux cutting.

= Bl vfor the motional emf induced in a straight conductor of length l , both positioned and moving (at a velocity v) at right angles to a uniform magnetic field of density B. See diagram.The general equation above simplifies toLENZS LAW: An induced electric current flows in a direction such that the current opposes the change that induced it. Hence the sign in the Faraday equation.Eddy currents

A kayaker can use river eddies. On the downstream side of every rock that breaks the surface of a river, you will find an eddy large enough for the front of your kayak to sit in while you have a rest and admire the view.

Eddyhopping is where a white water kayaker sprints upstream from one eddy to another.

This 93 mile wide deep underwater eddy was spotted off the coast of South Africa by satellite.Electrical eddy currents

Mutual and self induction

Unit of inductance:the henry (H)Typical values: H mHFaradays LawMutual induction(switch being closed in the primary circuit)Does the galvanometers pointer remain deflected to the right? Which way will it go if S is now opened?

Mutual induction(AC in the primary circuit)

How Induction Cooktops Work

http://home.howstuffworks.com/induction-cooktops3.htm

Diagram of simple inductor

Examples of Inductors

More on inductorsAn inductor is somewhat like a capacitor. They both store electromagnetic energy. They both oppose changes in a circuit.

A capacitor likes to maintain a constant voltage. It stores this energy in an electric field. Its reactance decreases with frequency.An inductor likes to maintain a constant current. It stores this energy in a magnetic field. Its reactance increases with frequency. [NOTE: reactance means a capacitors or inductors resistance to AC.] Because of this constant current feature, when current through an inductor is increased or decreased, the inductor "resists" this change by producing a voltage between its leads in opposing polarity to the change. Inductors when combined with capacitors become useful when you want to make filters that let only chosen frequencies through (e.g. In radio tuner circuits and speaker crossovers.) The capacitor blocks off low frequencies, the inductor blocks off high frequencies.

Inductor circuit symbols

The transformerA transformer steps up or steps down an AC voltage.

Core laminations

A symbol for a transformer

US (and original UK) symbol for a resistor

Transformer lossesIn addition to the above, there is a very small amount of mechanical loss due to vibrations, which result in an audible transformer humCore losses are sometimes called no-load lossesWinding losses are sometimes called load lossesStray losses are relatively smallFlux leakage (stray losses) in a transformer

LEAKAGELEAKAGELEAKAGELEAKAGE

A simple AC electric generatorAC generator (continued)1234

EMF induced in a coil rotating in a magnetic fieldThe motor effect

F = Bilwhere F is the force on a conductor of length l carrying a current i and perpendicular to a magnetic field of flux density B.Worked example. Calculate the force on a power cable of length 100 m carrying a current of 200 A at place where the Earth's magnetic field is 10-5 T and is perpendicular to the cable.

The cable will experience a force given by F = Bil = 10-5x200x100 = 0.2 NThe catapult effect,

Used in motor effect situations.

Using Flemings LHRThe homopolar motorhttps://www.youtube.com/watch?v=xbCN3EnYfWU

With the motor effect or generator effect, we have three vectors:The magnetic fieldThe electric currentThe motion of (i.e. thrust on) the object.

In diagrams, two of these are likely to lie within the plane of the page. The third is likely to go into or come out of the page.

If it goes into the page, the direction is denoted by a cross inside a small circle. If it comes out of the page, its direction is denoted by a dot inside a small circle. (These represent an arrow going into or coming out of the page.)

In the diagram to the left, the magnetic field B and the current I lie within the plane of the paper. The direction of motion of the wire is out of the page on the left hand side, and into the page on the right.Homopolar machines(they hardly ever used due to inefficiency)The first superconducting electric motor, made in 1966 by NEI for the MOD - a homopolar machine containing no iron and rated at 50 horsepower (hp).1hp = 746 watts. The term horsepower was adopted in the late 18th century by James Watt to compare the output of steam engines with that of draft horses. Brake horsepower (bhp) is the measure of an engine's horsepower before the loss in power caused by the gearbox, alternator, differential, water pump, and other auxiliary components such as power steering pump & muffled exhaust.

The powerful NPT301 turbojet was designed for use primarily in Remotely Piloted Vehicle (RPV) applications. The nose bullet housed a homopolar alternator. RPVs are more often called UAVs (Unmanned Aerial Vehicles) or drones these days. NPT went bust in 1990 due to competition from overseas companies.YOU WONT BE TESTED ON THIS

Basic electric motorCatapult effecton a coil in a magnetic field

Commercial motorsA commercial motor is different in several ways from our simple model.It uses:

Field windingsA commercial motor is different in several ways from our simple model. It uses:carbon brushes for good electrical contact with the commutator and also so that when the brushes wear away, they can easily be replaced. Carbon brushes do not wear away as quickly as metal brushes.a multi-section commutator - two sections for each of several rotating coils wound in different planes. Although only one of these coils carries a current at any one time, having a lot of them makes the rotation far smoother.field coils rather than a permanent magnet. These coils become magnetised when a current is passed through them. Field coils give a stronger, more easily shaped magnetic field than permanent magnets.AppendixMagnetism FormulaeAC MotorAlternative names for B and HHistory of magnet strengthsBBC Learning Zone 1BBC Learning Zone 2Building a tunnelSome magnetism formulae

Example of AC motor developed locally

According to the Green Motorsport website

This water-cooled 48 volt high frequency AC motor is capable of pulling 650 amps peak.It delivers its power in a very different way from the conventional DC motor. Its high performance capability is obtained by means of a water-cooling system and highly efficient windings. The water cooling jacket is totally seamless.

The GMS M1 motor is brushless and totally sealed from the elements, making it durable and robust. This makes it suitable for almost any application, fromelectric cars to water craft. The technology will be proven in motorsport, the most demanding environment known. Woking(opposite McClarens)Environmentally conscious motorsportAlternative names for B and HAlternative names for BAlternative names for HMagnetic flux densityMagnetic inductionMagnetic fieldMagnetic field intensityMagnetic field strengthMagnetic fieldMagnetizing fieldHow the strength of magnets has increased over the years

BBC Learning Zone (1)TRP reference codeClip numberBBC titleBrief overview of the topicLZ16616How wind energy produces electricityEngineers at a wind farm in Wales explain: choice of site, transportation of turbines to the site, the farms construction, production of electricity for the national grid, and positive and negative aspects of wind energy.LZ26617A solar power plant in Spain is producing enough power for thousands of homesEngineers explain how hundreds of mirrors are used to reflect sunlight to a receiver on a central tower. There, water is heated to create steam, which drives a turbine and generates electricity. A second system using parabolic reflectors is shown, together with new ways to store heat to increase the useful output from the power plant.LZ36618How electricity can be produced by nuclear fusion, and arguments for and against its useEngineers at JET in Oxfordshire explain their research into fusing hydrogen isotopes to create energy to produce electricity. The aim is to allow them to get closer to being able to design and build a commercial fusion power plant. Positive and negative aspects of harnessing fusion energy are considered.LZ46619How does an electric shaver work, and how is it made?Engineers at Braun explain how an electric razor works, and the innovations incorporated into the latest shaver designs. The different stages of manufacture from design to mass production are shown and discussed.www.bbc.co.uk/learningzone/clips/ BBC Learning Zone (2)LZ56620How does a loudspeaker work, and how is it made?Engineers explain how a loudspeaker is made from a number of components assembled into an enclosure, and the technical basis on which it operates. Its operation is demonstrated and a post-production testing procedure described.LZ66621How does a hover lawnmower work?A design engineer at Flymo explains and demonstrates the principle of operation and the safety tests that a mower must pass. Cut-away sections through the mower allow the internal components to be seen.LZ76622The worlds longest, deepest tunnelSwiss engineers describe the design and construction of the Gotthard Base tunnel. They explain using an electronic system to correctly align the tunnel and recycling excavated rubble into concrete for its lining.LZ86623The Synchrotron: the worlds biggest microscopeEngineers describe the design and construction of a device that can accelerate electrons to almost the speed of light in order to produce x-rays that can see deep inside metals and other substances.LZ96624The use of the Synchrotron, the worlds biggest microscopeAn engineer from Rolls-Royce explains how the materials which go into the manufacture of aero engines can be made stronger and lighter if more is known about their internal structures. To do this the engineers use x-rays from the Synchrotron to look deep into metals. Components are subjected to forces, and the stresses and deformations within them investigated.www.bbc.co.uk/learningzone/clips/ Building a tunnel for high-speed trains er the link www.bbc.co.uk/learningzone/clips/6622.html has got very little to do with this unit except for the electronic system used to align the tunnel, but its quite interesting and its a BTEC-recommended video clip, so I suppose I might as well show it Swiss engineers explain the need for the Gotthard Base Tunnel to reduce the amount of traffic on the roads. The long, flat rail tunnel through the Alps will allow both passenger trains and shuttles carrying lorries to cross the Alps using far less energy. The tunnel is being built in sections and electronic systems are used to ensure the sections meet up to within 25 cm. The engineers explain how they have developed a way to use the rubble from the excavations in the concrete used to build the tunnel.

Measuring magnetic fields: the search coilThe search coil method can be used to measure both constant and varying fields. Typical characteristics: 1000 turns; cm diameter.

Measuring varying magnetic fields. An e.m.f is induced in it which is directly proportional to the flux density. This e.m.f. is conveniently displayed as a vertical line on an oscilloscope whose time-base is switched off.Measuring steady magnetic fields. The search, connected to a ballistic galvanometer, is placed in the field and held still, then removed quickly. The maximum galvanometer deflection is proportional to the field strength.

End