Sunday, September 12, 2021

BASIC THEORIES OF ELECTRICITY

 THE NATURE OF ELECTRICITY

Much must of necessity remain unexplained about the basic theories of electricity, and that is why we are dealing with the subject  now. Nevertheless, we shall be able to gain some idea of the attitude of modern physicists towards electrical phenomena.

STATIC ELECTRICITY

 Although most of our time has been spent in studying electric currents, we have met the idea of a stationary charge of electricity in examining the action of capacitors. That electricity can exist at rest  is shown by the well-known experiment in which a glass rod is rubbed with a silk handkerchief and is then found to have acquired the property of momentarily attracting small pieces of paper. The word “ electricity” is derived from the Greek name for amber, which is one of the many other substances exhibiting a similar effect. The electrification of the glass rod was at one time ascribed to a “ positive charge” of electricity, a corresponding “ negative charge” being produced on the silk, but a more satisfactory explanation is now possible.

THE ELECTRON THEORY

 From elementary science we know that matter is made of solid, liquid and gas which are key composition of molecules, and that molecules are built up from atoms, of which there are various kinds each corresponding to a different element. In order to explain the nature of electricity, we must push our investigation still further and inquire into the structure of the atom.

Although the atom is far too small to be seen by the aid of the most powerful microscope, it can be studied by indirect methods, and work of this kind has led to the belief that even the atom itself is a collection of smaller particles. These are electrical in nature, and can hardly be regarded as matter at all except when they are associated to form an atom. They are the bricks of which the universe is built, and it is to them that we must look for an explanation of electrical phenomena.

An atom appears to comprise at least two kinds of particle. Thre is a positive kind called a proton, and a negative kind called an electron. We need not attach any special significance to the terms positive and negative, except as an indication that the electrical natures of the two kinds of particle are in some way equal and opposite.

In its normal condition an atom has equal numbers of protons and electrons, the number varying according to the element. Hydrogen is the most basic atom, with only one proton and one electron.

In each atom the proton are collected in a small central nucleus. The relation of the electrons to the nucleus is rather like that of the planets to the sun; this is not a complete analogy, but it will suffice for our present purpose. In the most complicated atom, that of the element uranium, there are ninety-two protons and an equal number of planetary electrons. Each of the elements has its own characteristic number.

In addition to protons, the nucleus may contain uncharged particles known as neutrons. Variatons in the number of neutrons give rise to the modified forms of the elements called isotopes, but with these we are not concerned.

The protons being collected in the nucleus, the latter may be looked upon as a positive charge of electricity. This is balanced by the negative charge of the planetary electrons, so that the atom as a whole is neither positive nor negative.

In some substances some of the planetary electrons can be easily detached from the nucleus, and these electrons can move freely in a random manner, so that there is no effective flow of electrons in any particular direction.

ELECTRON CURRENT

A conductor is a substance in which electrons can move readily from atom to atom in this manner; an insulator is one in which they can do so only with difficulty. The effect of applying a potential difference to a conductor is to direct the interchange of free electrons, causing them to be handed on from one atom to the next along the length of the conductor. It is this directed movement of electrons that we call an electric current.

A current of 1 ampere is equivalent to a movement of about six million million million electrons per second, and this is therefore the number of electrons representing one coulomb. If the whole population of the earth counted day and night at the rate of one eletron each per second, it would take about a hundred years to count them.

The electrons tend to move towards the positive pole of the source of e.m.f. the movement is therefore in the direction opposite to that in which the electric current is conventionally assumed to flow. This is unfortunate, but the current direction was fixed arbitrarily before the  true nature of the current was appreciated, and to change the accepted convention now would be very difficult. In the comparatively few cases in which it matters which way the electrons really move, we must remember to make it clear whether we are referring to the actual electron current or to the conventional current which is supposed to flow in the opposite direction.

INSULATORS

Although in insulators there are few free electrons and it is difficult to detach them from the atoms and thus to cause a current to flow, the application of an e.m.f. does cause a  state of strain in which the electrons, while remaining faithful to their parent atoms, are displaced towards the positive pole of the source of e.m.f. this is what happens in the dielectric of a capacitor. If the e.m.f. is increased until a substantial number of electrons are detached, an appreciable current passes, and we say that the insulation has broken down.

MAGNETISM

The individual atoms of a substance appear to act as tiny magnets; one explanation of this is that the movement of the electrons is equivalent to current flowing in a coil. In the normal condition of an unmagnetized piece of iron, these atomic magnets, or groups of them, are arranged in a haphazard manner, and the iron as a whole does not exhibit any magnetism. The process of magnetization consists in the alignment of the tiny magnets so that all the north poles point in one direction and all the south poles in the other. The iron as a whole then acts as a magnet. The effect which we called saturation occurs when all the atoms have been brought into alignment.

IONIZATION

If an atom loses one its planetary electrons, there are no longer the right number to balance the positive charge of the nucleus. In this condition, the atom is called a positive ion. If, on the other hand, an atom gains an electron, there is more than enough to balance the positive charge, and the atom is called a negative ion. A positive ion will attract free electrons, and a negative ion will readily discard its surplus, the tendency in both cases being to return to the neutral state.

Not only individual atoms, but whole bodies, can exhibit these effects, and the experiment of rubbing the glass rod with the silk the handkerchief can be interpreted by saying that electrons are transferred from the glass to the silk. The result is a deficit of electrons, or positive charge, on the glass and a surplus of electrons, or negative charge, on the silk.

CONDUCTION IN ELECTROLYTES

We have seen that pure water is a very poor conductor, while a solution is, in general, a comparatively good one. It is supposed that when a substance is dissolved, some of its molecules split into separate ions, one part having an excess of electrons, and the other a deficit.

The case of the copper-sulphate solution is an example. A molecule of copper sulphate consists of one atom of copper, one of sulphur and four of oxygen. When the copper sulphate is dissolved, the molecules split up into copper ions and “sulphate” (sulphur plus oxygen) ions. The former have a deficit of electrons and are therefore positive, while the latter have an excess and are therefore negative

When electrodes are immersed in the solution and an e.m.f is applied, the positive ions move towards the negative pole (cathode) and the negative ions towards the positive pole (anode). The negative ions give up their surplus electrons at the anode, and the positive ions have their deficit made good at the cathode. There is therefore a passage of current through the electrolyte, while the neutral atoms resulting from the arrival of the ions at the anode and cathode give rise to chemical effects.

CONDUCTION IN GASES

In their normal condition, gases are almost perfect insulators. They can, however, be ionized, and an appreciable current can then be made to flow.

A sufficiently high potential difference between electrodes in a glass tube containing gas at low pressure will produce ionization. One result is to cause the gas to glow, and use is made of this effect in the tubes used in advertising signs. The color of the light depends upon the kind of gas employed.

The arc lamp provides another example of the conduction of current through a gas. Very high voltages are necessary to break down the insulation between two electrodes separated by air, but if the electrodes are brought together and then separated, a comparatively small voltage will maintain an arc between them. Except in a few special applications, arc lamps have been replaced by other types, but the electric arc is widely used as a heating agent in welding processes, while its destructive effect is a factor to be guarded against in the design of switchgear.

CONDUCTION IN VACUUM

Current can also be made to pass through a vacuum, but in this case, there is no gas to be ionized and the flow is one of electrons only. In the use of a rectifier of a thermionic valve having one hot and one cold electrode. The action of this is explained by the emission or throwing off of electrons from the hot electrodes and their subsequent passage across the intervening space to the cold one. Electron emission from a heated electrode has made possible the valves used in wireless signaling, but a consideration of these is outside the scope of the present volume.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ELECTRICAL CONDUCTORS AND CABLES

 CONDUCTORS AND CABLES

 Electrical conductors and cables form the necessary connections between the machine which generates electricity and the apparatus which uses it. They comprise a very wide range of sizes and types. The necessary requirements of a cable are that it should conduct electricity efficiently, cheaply, and safely. As a result, this shouldn't be too small, as this would result in a considerable internal voltage drop. It shouldn't be too big so that it costs too much at first, and it should have all of the essential junction sections, etc. should not be too costly. Its insulation should be such as to prevent leakage of current in unwanted directions, and thus to minimize risk of fire and shock.

 A cable has three main parts – the conductor, the insulation, and the mechanical protection.

CONDUCTOR MATERIALS

Copper and aluminum are the conductor materials used in  electricity cabling.. Copper has lower resistivity and thus higher conductivity than aluminium. For the same rated current capacity, copper cables have a reduced cross-sectional area and look smaller than aluminum cables. On the other hand, aluminium has about one-third the weight of copper and so will have an advantage in some circumstances.

Copper conductors  may be annealed or hard-drawn.Annealed copper conductors are softer and more malleable than unannealed copper wires, making them ideal for laying or fixing indoor and outdoor wiring. Hard-drawn copper conductors, which have a very high tensile strength, are used as overhead wires mainly in the bare form. Copper is used in the vast bulk of cables. Copper conductors must be plated with some insulations, but not all.

Aluminium conductors  are made in all standard sizes but are used at present only in the larger sizes. IEE Regulation b1 lays down that all cable conductors of cross-sectional area 10mm2 or smaller shall be of copper. The respective resistivities of copper and aluminium are given as 17.24µΩmm 28.2µΩmm.

The measured resistance of made-up cable coductors are normally slightly higher than those calculated from the above resistivity values. For calculation purposes , values of resistivity for annealed copper and aluminiun will be taken as 17.5µΩmm and 28.5µΩmm respectively.

Stranding – Conductors are stranded to ensure flexibility and ease of handling: a lot of cables are roundly twisted together to produce a core corresponding to a single connection of the requisite size. The numbers of strands used are 1, 7, 19, 37, 61 and 127. The sizes of conductors range from 1.0mm2(1/1.13mm) to (127/2.52mm). the latter conductor, for example, consists of 127 strands of circular conductor, each strand of 2.52mm diameter, with a total cross-sectional area of 630mm2. The complete range may be found in various tables in the Regulations.

INSULATION

  The insulation's job is to keep the  electricity contained  within the conductor. To this end the insulation itself must have a very high resistance.Insulation is designed to envelop the conductor all through it length in typical operation.. For overhead wires it is normally sufficient to provide insulation (e.g., a porcelain insulator) at the point of suspension of the wire. The remainder of the cable is insulated by the air surrounding it. The types of insulating material commonly used are:

Polyvinyl chloride (pvc)

Elastomers, a generic term for vulcanized rubber(v.i.r), butyl rubber (b.r.), ethylene-propylene (e.p), silicone rubber (s.r.)

Impregnated paper.

Mineral insulation.

Other types for special purposes.

Large cables may be single-core, twin-core, triple-core, or multi-core, in which form the cores are separately insulated and laid side by side with a slight continuous ‘lay’, packed with wormings and further insulated generally to form a round shape. They can also be manufactured in twin or triple circumferential configurations. Small lighting and power wires are placed out flat to provide a tidy setup.

Voltage ratings, for cables are given as Eo/E, Eo is system voltage to earth, and E is the voltage lines, e.g. 600/1000V.

Polyvinyl chloride – This was originally introduced as a substitute for rubber. Its properties are generally similar to rubber, although it has a tendency to soften under moderate temperature and to crack at low temperatures. It is practically impervious to chemical action. P.V.C. insulated cables may be used where the combination of ambient temperature and temperature rise due to load does not exceed 65 degrees Celsius. Thes cables are manufactured in the 600/1000V range for installation purposes and in the range 1900/3300V for electricity supply. P.V.C. insulated cables are made up in many ways; lead-alloy-sheathed, rubber-sheathed, p.v.c.-sheathed, or taped, braided and compounded for use in conduit.

Vulcanized rubber(v.i.r) – This material is fast losing ground owing to the common use of pvc. It is a preparation of pure rubber with a small amount of sulphur. It is impervious to water, flexible and of high resistivity. It has a fair mechanical strength dependent upon its degree of vulcanization. It retains its properties for long periods in the absence of light and undue heat. The rubber is applied to the conductor during manufacture in two or more layres, or may be extruded. It is then vulcanized. The make-up of cables insulated with v.i.r. is similar to the make-up of p.v.c. cables.

Butyl-rubber and ethylene-propylene-rubber – These materials are suitable for use as conductor insulation where the temperature rise does not exceed 80 degrees Celsius. I.E.E. Regulation B36 deals with the us of these heat-resisting mataerials on flexible cords in pendant lighting fittings and on cables connected to bare conductors and busbars.

Silicone rubber – cables insulated with this material may be used where the conductor temperature does not exceed 145 degrees Celsius. Silicne-rubber, butyl-rubber and ethylene-rubber-insulated cables are made up according to requirements as textile-braided and compounded, glass-braided and compounded, or with various qualities of heat-resisting, oil-resisting, and flame-retardant sheathing (h.o.f.r). all the elastomer-insulated cables are made in the 600/1000V range.

Impregnated paper -  paper-insulated cables are manufactured in voltage ranges from 600/1000V to 19000/33000V mainly for underground laying. The paper insulation is laid on to the conductor in the form of paper tapes laid helically to the desired thickness. The insulation, after being vacuum-dried, is impregnated under pressure with mineral oil or other suitable compound. Owing to the affinity of paper for moisture, the insulation is completely enclosed in a continuous laid or aluminum sheath extruded over the insulation. All cable ends are sealed by special oil-filled or compound-filled sealing boxes.

Mineral insulation -  Copper or aluminium conductors with compressed powdered mineral insulation enclosed in a copper or aluminium sheath (m.i.m.s), may be used according to the termination used, up to an ambient temperature of 150 degrees Celsius and to a much higher cable temperature. Specially designed arrangements are needed at all joints and terminations. These cables are made in the 600V class (light duty0 and the 1000V class (heavy duty).

MECHANICAL PROTECTION

-  The larger cables, used for underground work and for large interior power installation, may be mechanically protected in various ways.

Unarmoured cables may be run without further protection than the lead sheathing. A further protection is one two layers of compounded jute or hessian tape yarn laid over the lead sheath. Aluminium is used as an alternative to lead for sheathing cables.

Armoured cables include single-wire armouring (a single layer of galvanized iron wire laid spirally upon a bedding of jute or hessian), double-wire armouring (two layers of armouring), and double stell tape armouring (two layers of steel tape laid spirall over the bedding with an overall finish of jute and hessian).

Wire armouring is occasionally replaced with aluminium strip armored plating.

For underground cables, wire armouring is used where the ground is liable to subsidence, to prevent the cable from breaking, whereas steel tape armouring is needed where physical damage from stones or work-men’s tolls may be expected.

FLEXIBLE CORDS AND CABLES – Flexible wires varying in size from 0.5mm2 (16/0.20) to 4mm2 (56/0.30) are called flexible cords. Flexible wires of larger sizes from 6mm2 (84/0.30) to 630mm2 (2257/0.60) are called flexible cables. Flexible cords and cables are so made of fine gauge wires as to be much more flexible than ordinary wiring cables and are used for such purposes as from ceiling rose to lampholder, or from socket-outlet to portable apparatus. They should not be utilized for permanent cabling in general. Several types are available.

Flexible cords -  The complete range can be found in BS 6500: 1969, some of these types are given here.

Vulcanized rubber-insulated

Braided, twisted twin.

Braided, circular twin and 3-core, ordinary and unkinkable.

Rubber or PCP-sheathed, single-core, twin, 3-core and 4-core.

PVC-insulated (other than heat-resisting)

Single-core, twisted twin, and parallel twin.

PVC-sheathed, single-core, twin, 3-core and 4-core.

PVC-insulated (heat resisting)

Single-core, twin, 3-core and 4-core.

Butyl-rubber or ethylene-propylene rubber-insulated

Single-core, twin, 3-core and 4-core, sheathed with either PVC compound (heat-resisting) or H.O.F.R. compound.

Silicone rubber-insulated

Single-core, twisted twin, circular twin and 3-core, all glass fiber braided.

Varnished glass fiber braided

Single-core, twisted twin, circular twin, and 3-core.

These flexible cords are manufactured in 300/300V and 300/500V ranges according to type.

flexible cables – These are made in the 600/1000V range, and include single-core, and circular twin and multi-core cables. The conductors are of tinned annealed copper, and the insulation consists of vulcanized  rubber, butyi rubber, or EP rubber. The sheath is of heavy duty O and FR sheathing, or in some cases the cables may be armoured.

Length of cables – Smaller cables, such as those used for house lights and power, are available in lengths of 100 meters on drums or in coils, while flexible cords are available in 50 meter lengths.. the larger and more expensive lead-sheathed paper-insulated cables are supplied in lengths to the customer’s requirements. 

Saturday, September 11, 2021

ELECTRICAL COMMUNICATION SYSTEMS

 ELECTRICAL COMMUNICATION

The use of electricity for purposes of communication is so widespread that site would be incomplete without some reference to the principles involved in simple telephone and telegraph circuits.

TRANSMISSION OF SPEECH

Sound consists of vibrations, usually in the air, which may be compared to ripples on the surface of a pond. Instead of being confined to a surface, however, the air vibrations spread out in all directions from their source. Moreover, while the horizontal movement of a ripple across a pond is produced by up-and-down movements of the particles of water, the vibration of the particles of air takes place in the same direction as that in which the sound is transmitted.

The frequency of the vibrations settles the pitch of the sound, a low frequency producing a low note and a high frequency a high note. The frequency of the notes on a piano ranges from thirty to three thousand cycles per second (30 to 3000 Hz). The ear can detect frequencies of from about 20 to 20,000 Hz.

The sound-waves have a waveform which can be represented by a sine curve (SEE ARTICLE) in simple cases, but which is very complex in the case of speech. The problem of telephony is to reproduce the sound waves at a distant point. This is done by the use of a transmitter or microphone which converts the sound-waves into corresponding variations in the strength of an electric current and a receiver that responds to the varying current and generates new sound-waves similar to those which affected the transmitter.

ELEMENTARY TELEPHONE

The transmitter commonly employed is a special type of variable resistance controlled by the sound-waves. An elementary form is shown below.

 


A flexible conducting diaphragm is separated from a rigid conducting disc by a number of small granules of carbon g. External connections are made to the diaphragm and the rigid disc r. Current passing through the transmitter has to traverse a large number of more or less imperfect contacts between the carbon granules, and the resistance of this oath is very sensitive to variations in pressure on the diaphragm d.

The diaphragm vibrates in response to any sound-waves directed towards it, thus producing varying pressure on the granules. The resistance of the transmitter is therefore continually altering in accordance with the frequency and waveform of the sound waves.

The receiver consists of a magnet m, a coil, and an iron diaphragm. The coil is in series with the transmitter and a cell. As the current which the cell is able to send round the circuit is varied by the transmitter, the coil at the receiver produces corresponding variations in the flux of the magnet. The result is to vibrate the iron diaphragm, thus generating new sound waves similar to those directed towards the transmitter.

It is of interest to note that the receiver itself will together be used as a telephone. When soundwaves are directed onto the diaphragm, its movement results in a continual redistribution of the flux produced by the magnet. The changing flux cuts the coil and induces currents in the winding corresponding to the speech waves. These currents affect the second receiver in the usual manner. Provided that the instruments are fitted with permanent magnets to produce the initial flux, no battery is required.

PRACTICAL TRANSMITTER AND RECEIVERS

Many modern commercial transmitters of the variable-resistance type employ two diaphragms, an outer one for receiving the sound waves and a small inner one forming part of the granule chamber. The two are joined at their centers. The carbon granules are situated between two carbon discs, one attached to the back of the inner diaphragm and the other to the back of the granule chamber. All the contact points influenced by the sound waves are thus between one carbon surface and another.

For special purposes, transmitters based upon other principles are employed. By way of example, we may mention moving-coil microphones, in which a coil coupled to the diaphragm moves in a magnetic field, and thus has speech currents induced in it, and electrostatic microphones in which the diaphragm forms one plate of a capacitor the capacitance of which varies in accordance with the speech waves.

Commercial receivers do not differ in principle from the simple form is shown, but the magnetic circuit is improved by the use of a magnet of horseshoe or some equivalent form. Both the pole-pieces can thus be brought close to the diaphragm. A separate coil is usually fitted to each pole-piece, the two coils being connected in series.

TELEPHONE CIRCUITS

A complete circuit between two telephones must make provision for a transmitter and receiver at each end. One possible arrangement is to connect all four instruments in series and to insert a battery at any convenient point in the circuit. If, however, the line is a long one, the resistance of the transmitters will then form only a small part of the whole, and the variations in current caused by the speech waves will be slight.

This difficulty can be overcome by connecting each transmitter in a local circuit in series with its own source of current and the primary winding of an induction coil. The latter is merely a small straight-core transformer of the kind shown in the previous article. The secondary winding of the induction coil is included in series with the line as shown in the diagram above.

The variations in the transmitter resistance produce large current variations in the local circuit, and these, in turn, produce large voltage variations across the secondary winding of the induction coil. The result is a much more effective transmission circuit than would be possible if the transmitters were connected directly in series with the line.

TELEPHONE EXCHANGES

 In a public telephone system, each telephone is normally provided with its own pair of wires to the telephone exchange, at which arrangements are made to connect any pair of wires to any other pair. In modern exchange systems, local batteries at the telephones are not used, all the transmitters being supplied with current from a common battery at the exchange. Special circuit arrangements are then necessary to retain the advantages of the local transmitter circuit to ensure that the varying speech currents between one pair of telephones connected to the common battery do not reach other telephones drawing their current from the same source. For particulars of these circuits, the reader is referred to specialized books on telephony.

Calling is nearly always affected by means of alternating current connected to the line at the exchange. In order that it may respond properly to this current, the bell at the telephone instrument is fitted with a permanent magnet, which causes the armature to move in one direction in response to one-half cycle of current and in the opposite direction in response to the next half-cycle. Bells of this kind are said to be polarized.

TELEGRAPHY

One of the earliest applications of electricity was to the telegraph, and this is still an important means of communication. In its simplest form, an electric telegraph consists simply of a source of current, a key for opening and closing the circuit at one end of a line, and a device responsive to the current at the other. Signaling can be carried on over such a circuit by means of the morse or any other code.

The signal receiving device may be a modified galvanometer, a sounder consisting of an electromagnet the armature of which is designed to give a distinctive click at each operation, or a relay arranged to close a local circuit for some other indicating or recording instrument when the line current flows.

SINGLE AND DOUBLE-CURRENT WORKING

If the circuit is simply opened and closed at the sending end, it is said to be arranged for single-current working. This is satisfactory for short lines on which the capacitance is small, but as the capacitance increases, rapid signaling is prevented by the time taken to charge the line when the key contacts are closed and to discharge it when they are opened.

An improvement can be effected by filling up the gaps between one impulse and the next by current flowing in the opposite direction. This is known as double-current working and necessitates the use of a receiving device the response of which is dependent upon the direction in which the current flows. The current constituting the signals proper is called the marking current, and that flowing in the opposite direction is the spacing current. Contacts on the transmitting key are arranged to send a spacing current when the key is at rest and a marking current when it is depressed.

SIMPLEX AND DUPLEX SYSTEMS

A simplex telegraph circuit is one in which a message can be sent in one direction at a time. It is possible to send non-interfering messages in both directions at the same time by means of what is called a duplex circuit. The diagram below shows the principle of one form.

The signals are received by relays, each of which has two windings, as represented conventionally at the diagram r r’.

In addition to its relay, each station has a key K K’, a cell C C’, and a line balancing resistance b b’. The object of the resistance will appear very shortly.

With the keys in the position shown, no current flows. When the key k is pressed,  the cell C ends a current through the upper windings of both relays, the circuit being completed through the back contact of the key  K’ and the earth return. No current flows in the lower windings of relay r’, as this is short-circuited by the key, but the relay is operated by the current in its upper winding.

This current also flows through the upper winding of the relay r, but in this case there is another circuit through the lower winding and the line balancing resistance b. The latter is chosen so as to make the current equal to that in the line circuit, and as the two windings oppose each other, the relay does not operate.

Suppose that while key R  is still depressed, key R’ is depressed also. Cells c and c’ are now in opposition, and there is, therefore, no current in the line. Current flows, however, in the lower windings of both relays, so that relay r operates while relay r’ remains in its operated position.

If key k is now released, current from cell c’ flows over the line. Relay r is therefore held in its operated position by current in its upper winding, while relay r’, which now has current flowing in both windings, releases.

The result is that each relay responds to the movements of the key at the other end of the line and ignores the movements of its own. When either relay operates, the armature closes a circuit for any desired form of indicating device.

Direct-current working has now been largely abandoned in telegraph systems, but the duplex circuit is still worthy of notice as an example of the unexpected results obtainable by the application of ingenuity to basic principles.

VOICE-FREQUENCY TELEGRAPH SYSTEMS

The development of the telephone has greatly influenced the technique of telegraphy, and many modern telegraph systems make use of voice-frequency currents, i.e., alternating currents the frequency of which is within the range to which the ear responds. We mentioned in previous articles that combinations of inductance and capacitance possessed a resonant frequency at which a maximum current would flow. By an extension of this principle, it is possible to design circuits that will accept currents of one frequency while rejecting those of another. Several messages can therefore be sent over the same line by using a different frequency for each, the various frequencies being sorted out at the receiving end and passed to separate receiving apparatus.

 

TYPES OF ELECTRICAL MEASURING INSTRUMENTS

 ELECTRICAL MEASURING INSTRUMENTS

We have referred many times to current and voltage, but have not yet seen how these quantities can be measure. It is now time to repair this omission.

                An instrument for measuring current (amperes) is called an ammeter, and one for measuring voltage a voltmeter. Do not confuse the terms voltmeter and voltmeter; the latter is the name of a device used in the study of electrolysis. A volt-ammeter , n the other hand, is a combine voltmeter and ammeter.

Connetions of Voltmeters and Ammeters

                Most voltmeters really measure current and rely upon ohm’s law for their ability to indicate voltage.

                              AMMETER AND VOLTMETER

Will make this clear. A cell is sending a current through a resistance R, and it is required to measure the current flowing and the voltage across the resistance. We therefore connect an ammeter A  in series with the resistance and a voltmeter v across it.

                The ammeter is of very low resistance , and the volt-meter is of very high resistance. Apart from this, their construction, which will be described later, maybe the same.Each meter includes a pointer that deflects in response to the current.In the case of the ammeter this current is that flowing in the  main circuit, and the meter can therefore be marked in amperes.

                As the resistance of the voltmeter is very high, the current passing through it is vey small. What little there is depends upon the resistance of the meter and the voltage across it. As the resistance of the meter is a fixed quantity, the current, in accordance with ohm’s law, is proportional to the voltage, which is also the voltage across the resistance R. the meter scale can therefore be marked in volts instead of amperes.

                It is important to note that accurate measurements are dependent upon the high resistance of the voltammeter. This is particularly true in the case of a voltmeter connected    across a portion only of a circuit; if the resistance of the meter is low enough to allow it to draw an appreciable current, and the mere connection of the meter lowers the voltage it is required to measure.

Moving iron instruments

                Most ammeters and voltmeter are dependent for their operation upon the magnetic effect of the current. In one type the magnetic field produced by a coil carrying the current to be measured is made to move a pivoted piece of iron having a pointer attached. Meters of this kind are known as moving iron instruments.


                the figure above illustrates two forms. In that on the left, the coil carrying the current is shown in section at c and the hairspring of a watch tends to keep the iron and is attached pointer in their normal position

                When current flows in the coils, a magnetic field is set up. The iron tries to move into the densest part of this field, and owing to its shape it rotates about the pivot in doing so, thus moving the pointer over the scale. As the iron moves, he opposition of the control spring increases , until finally the iron and the pointer come to rest . the final position of the pointer depends upon the strength of the field, and therefore upon the strength of the current.

                In the form illustrated on the right, he coil carrying the current is again shown at c, this time in end view, the moving iron consists of a short rod a, only one end of which is seen, attached to the spindle carrying the pointer. As before, a light spring tends to keep the moving parts in their normal position.

                A second iron b, parallel to the first, is fixed inside the coil. When current flows in the winding, similar poles are induced in the adjacent ends o the rods, say the rods consequently repel each other, and the one until the increasing opposition of the control spring prevents further movement. The pointer moves with it, and indicates the strength of the current.

                Moving iron meters are not vry sensitive, and are subject to errors caused by hysteresis. They are, however, simple and robust, and are largely used when the same manner for current flowing in either direction, they are suitable for use on either direct or alternating current. On alternating current, the pointer has no time to follow the individual half cycles of current, and it remains steady in a position corresponding to the root mean square value.

                The scales of moving iron instrument are irregular; ie., the divisions are of different sizes at different points on the scale. This is not always a disadvantage, as it is sometimes possible to arrange for the divisions to be largest on the  part of the scale at which the meter is most often used.

Damping

                If proper precautions were not taken, the rapid movement of the pointer of an ammeter or voltmeter when current was switched on would cause it ot overshoot the mark, and it would then oscillate for a considerable time before setting down in the correct position. As readings would be difficult to take under these conditions, most meters are fitted with some means for amping out the vibrations.

                 One method is to fit on to the moving system a piston working in a curved box after the manner shown in fig, 107, the piston does not actually touch the box, but the clearance is small enough to restrict the passage of air from one side to the other. The result is to prevent oscillation of the pointer without affecting its final position.

Damping Arrangement

                In a method of damping used on some kinds of meter` ,a light metal disc fitted to the spindle is arranged to move between the poles of a small permanent magnet. As it moves, it cuts the field of the magnet, and currents are induced in it. These currents produce a field which reacts with that of the magnet and , in accordance with lenz’s  law , tends to retard the movement. The result is effectively to prevent oscillation, again without interfering with the final position of the pointer.

                Meters which are efficiently damped, so that the pointer comes quickly to rest in the correct position, are said to be dead beat.

MOVING COIL INSTRUMENTS

In these meters a coil carrying the current to be measured (or part of it )moves in a magnetic field produced by a fixed permanent magnet. A small direct current motor with an armature that never moves more than a quarter of a rotation can be equated to just a moving coil meter.

                Consider again the loop of the wire, and suppose that it is held in the position shown by means of a light spring. If we pass current round the loop, it will try torotate, and the distance it will move against the opposition of the spring will depend upon the strength of the current. The moving coil meter works on this principle.

               The figure below shows the arrangement adopted. The coil c, one end of which is shown, consists of a number of turns of fine wire wound on a light metal frame. The field is produced by a permanent magnet of horseshoe form fitted with pole pieces N,S,. a stationary iron cylinder I improves the magnetic circuit and the uniformity


Of the field without adding to the weight of the moving system. The coil is free to rotate between the iron cylinder and the pole pieces without touching either of them.

                Current is led to the coil through two flat, spiral springs (not shown in the drawing), one at each end of the frame on which the coil is wound. These springs serve also to limit the movement of the coil when current is flowing. As the frame on which the coil is wound is made of metals, c

currents are induced in it as it turns in the magnetic field. It therefore acts in the same manner as the disc mentioned in the last section and damping is effected without further provision being made.

                Moving coil instruments are accurate and sensitive, and they have an even scale. They are, however, more expensive than those of the moving iron type. As the direction in which the coiol tends to move depends upon the direction of the current, they are not, in their normal form, suitable for use of alternating supplies.

SHUNTS AND SERIES RESISTANCES

                It is often convenient to be able to use the same meter for different ranges of current or voltage. In any case, it is not always possible to wind the meter itself to a resistance suitable for the conditions in which it is to be used. This is especially true of moving coil meters, in which the available winding space is very limited.

                For these reasons, ammeters are often used with shunts, and voltmeters with series resistances, either fixed permanently inside the case or connected externally as required. The effect of a shunt on an ammeter (fig 109)is to bypass some of the current so that more current is needed in the main circuit to produce the same deflection. The effect of a series resistance on a voltmeter is to reduce the current which a given voltage will cause to flow so that a higher voltage is necessary to produce the same deflection.

                By way of example, suppose that we have an ammeter that reads up to I ampere and we wish to provide a shunt which will make it suitable for use up to 10 amperes. Clearly , nineteenths of the current will have to flow through the shunt, and only one tenth through the meter. We therefore make the shunt exactly one ninth the resistance of the meter and, when using it, multiply all the meter readings by ten.;

                Or again, suppose that we have a voltmeter that read up to 10 volts and we wish to provide a series resistance which will enable it to be used up to 100 volts. Clearly, we need a potential difference across the series resistance of 90 volts when that across the meter is 10 volts . we therefore made the series resistance exactly nine times the resistance of the meter, and again multiply  all the meter readings by ten.

                As we can always increase the range of ammeter or voltmeter but cannot decrease it, it is an advantage to start off with a sensitive low range meter, we can, in fact, use such a meter as either a voltmeter or ammeter by fitting suitable resistances and shunts, and this is the principle of the universal test sets’ commonly employed.

Shunts and series resistance should always be made of one of the alloys having a negligible temperature coefficient.

RECTIFIER AND THERMOCOUPLE INSTRUMENT

                Moving coil meters are sometimes fitted with small metal rectifiers to enable them to be used on alternating current. The deflection of a moving coil meter used in this way does not depend upon the r.m.s value of the current of voltage, but upon the average value. The difference between  the two is explained on page 123, this does not prevent the scale from being marked in the equivalent r.m.s  values, but the readings then need correction if the waveform is very different from that of a sine curve.

                For use on high frequency alternating currentsl thermocouple instruments are made. These are moving coil meter arranged to measure the current produced by thermocouple heated by the current to be measured .

HOT WIRE AMMETERS

                These are ammeters which measure the strength of a current by its heating effect. Fig 111 shows the principle of one type. A wire a is stretched between two fixed pints and a wire b between it and a third fixed point. A silk thread c is attached to the second to the second wire and passed around a pulley d carrying the pointer. a spring tends to pull the thread in the direction of the arrow.


                The current to be measured is passed through wire a, which expands as its temperature rises. As it expands, its sag increases ,thus  allowing the silk thread to increase the sag in wire b. the movement of the silk thread turns the pulley and causes the pointer to move over the scale.

                Although hotwire ammeters are now seldom used, they are of special interest because , being directly dependent upon the heating  effect of the current, they ready r.m.s values on a.c. of any frequency or waveform.

ELECTROSTATIC VOLTMETERS

                When a capacitor is charged, the electric field produces an attractive force between the plates. This effect is used in the electrostatic voltmeter, the principle of which is shown in fig 112. A moving vane m can move between a pair of fixed vanes f, only one of which can be seen, without touching either of them. The movement is limited by the usual control spring.

                The moving vane is connected to one side of the circuit the voltage across which is to be measured, and the fixed vanes to the side. The combination then acts as a capacitor, and the attractive force between the vanes causes the moving vane to move between the other two to an extent determined by the voltage.

                Electrostatic meters are suitable for use of a.c. or d.c. supplies but not for very small voltages, on a.c. supplies they read r.m.s values they take only a very small current on a.c. and none on d.c. except when first switched on.

WATTMETERS

  The power in a d.c. circuit can be measured by means of a voltmeter and ammeter, but in an ac circuit it is necessary to take the power factor into account as well. It is possible, however, to measure power directly by means of a wattmeter. This has a moving coil similar to that of an ordinary moving coil instrument, but instead of a permanent magnet, another coil  is used to produce the magnetic field. One coil is connected in the circuit like a voltmeter, and the other like an ammeter, and the deflection of the moving coil is dependent upon the product of the current and voltage, I.e upon the power.  On an a.c. circuit it is dependent also upon the phase relationship between current and voltage, so that the power factor is taken automatically into account.

METERS FOR MEASURING RESISTANCE

                For the rough measurement of resistance without the use of a Wheatstone bridge, a combination called and ohm-meter is sometimes employed . it consists of a moving coil meter and a source of e.m.f such a small cell. Provided that the e.m.f. remains constant, the current flowing depends upon the external resistance to which the combination is applied and the scale of the meter can therefore be marked in ohms. Only comparatively high resistances can be measured, and owing to the difficulty of compensating for changes in the e.m.f., great accuracy is not to be expected.

                For the measurement of high resistances such as insulation resistance, the instrument most commonly employed comprises a small hand turned d.c. generator for sending a current through the resistance to be measured, together with a special moving coil meter. The latter has two coils on the same spindle, a current coil in series with the resistance to be measured and arrangement is such that the two coils tend to move the pointer in opposite directions and the voltage coil is thus able to correct the resistance readings given by the current coil in order to compensate for variations in the voltage produced by the generator.

GALVANOMETERS

                A simple type of galvanometer was described in chapter I, and we have referred several times to the use of such instruments for experimental purpose, galvanometers are employed both for the detection of current and for comparing one current with another, but as their scales are not marked in any particular units, they do not indicate the value of a current directly.

                The example described in chapter I is only moderately sensitive, and the compass needle is liable to be affected by stray magnetic fields. For laboratory use, therefore, more elaborate types are necessary . most to these operate on the principle of the moving coil instruments we have already described, but the suspension of the coil itself may be more delicate.

                The most sensitive galvanometers are those of the reflecting type. In these the moving part carries a small mirror instead of a pointer. A narrow beam of light is projected on to the mirror, from which it is reflected in the form of a spot of light on to an evenly divided scale. The beam of light between the mirror and the scale acts as a ling weightless pointer, so that a very small angular movement of the mirror causes a large movement of the spot of light along the scale.

                If a galvanometer is not heavily damped, and current flows through the coil for a time so short that the current impulse is over before the movement has progressed very far, the first swing of the pointer( or spot of light ) is proportional not to the current, but to the quantity of electricity current x time which flows during the impulse. An instrument designed for use in this manner is called a ballistic galvanometer. It can be used, for example, for comparing the quantities of electricity stored in two capacitors. If both the capacitors are in chapter xi, are proportional to the capacitances, it follows that if the value of either capacitor is known, that of the other can be calculated from the galvanometer deflections

SUPPLY METERS

                In order that consumers may be called upon to pay for the energy which they take from public supply mains, we need a meter which will take into account both power watt and time hours. Since, however, the voltage is usually constant, the power on d.c. supplies is proportional to the current and time, one such meter is the electrolytic type mentioned.

                In most supply meters, however, a miniature electric motor is operated by the current , its speed being proportional to the power taken at any time. By means of gearing the revolutions made by the motor are counted and displayed on a  series of dials marked in the equivalent number of kilowatt-hour. In a.c. meters, the power factor is taken automatically into account.

Friday, September 10, 2021

WORKING OF A.C GENERATOR AND MOTOR

 A.C GENERATORS AND MOTORS

The outline of a d.c machine shown earlier in the previous article will serve also as a small alternating-current generator or alternator. Alternators, of course, have no commutator, and the direct current for energizing the field magnet must be obtained from a separate source. A small d.c generator termed an exciter is sometimes mounted on the same shaft for this purpose.

As in the d.c generator, the field-magnet poles, if there are more than two, are alternately n south, and since a complete cycle of current is generated every time a conductor passes north and south pole in succession, the frequency of the current is given by the equation;

FREQUENCY = Revolution per second ×Number of pairs of poles.

The armature carries several groups of conductors, the groups being spaced around the periphery so as to occupy similar positions in relation to the field-magnet poles. The current is collected from the armature by means of slip-rings.

Since all that is necessary for the generation of current is relative motion between conductors and field, a generator could be made to work by holding the armature and rotating the field magnet around it. This would be very inconvenient, but a similar effect can be obtained by placing the field-magnet system on the central rotating part, and the conductors in which the current is generated on the surrounding stationary part. As the field-magnet windings then rotate, they must be connected to the d.c source by a pair of slip-rings.

Part of an alternator in which the conductors move and the field magnets are stationary is shown in the diagram below, while the corresponding arrangement, in which the field-magnets move and the conductors are stationary. In both cases the slots in which the conductors are housed are indicated, but the conductors themselves are omitted for the sake of clearness.

In order to avoid confusion, the terms stator and rotor are used instead of armature and field-magnet. The stator is the stationary part and the rotor is the rotating part, no matter which of them carries the armature conductors and with the field-magnet system.

The advantage of the arrangement shown below is that the slip-rings and moving windings have to deal only with the comparatively low voltage and small current necessary to produce the field.

As the conductors in which the alternating current is generated are stationary, their insulation is simplified, while the fact that the connection can be made to them without slip-rings and brushes removes another difficulty in the generation of high voltages. Alternators of this type are therefore normal and can be made for much larger outputs than are practicable in d.c generators.

Note that the right-hand rule for finding out in which direction induced current flows assumes that the conductor is moving across the field, and not vice versa. When the conductor is stationary and the field is moving, the rule must be applied as though the direction of motion were reversed.

POLYPHASE CURRENTS

From the generating point of view, it is better to produce a steady current than one that is continually varying. In the d.c generator we were able to do this by spacing a number of conductors around the armature, so that while some were generating their maximum e.m.f., others were generating their minimum. Something of the same kind can be done in the case of an alternator, by causing it to generate at the same time two or more currents differing in phase. The different currents must be taken from the machine over different circuits, and we are thus led to a polyphase system comprising several more or less independent supplies of the same frequency, as distinct from a single-phase system having only one supply.

Polyphase systems also enable the winding space on the generator to be utilized more efficiently. Moreover, they simplify the design of alternating-current motors and lead to economies in the amount of copper required for transmission lines.

 

Suppose that we provide an alternator with two sets of conductors, the grouping in relation to the spacing of the rotating field-magnets being such that one set is producing its maximum. We then have two independent sources of current, the phase relationship of the voltages being represented by curves a and b. This is a two-phase system, and the difference in phase is one-quarter of a cycle or ninety electrical degrees.

In a similar manner, we can provide a generator with three independent groups of conductors, thus obtaining three separate sources of current, the phase relationship of the three voltages being as a, b and c  in the diagram below. This is a three-phase system and the difference in phase is one-third of a cycle or 120 electrical degrees.

Instead of fitting a three-phase generator with six output terminals, we can connect one end of each winding to a common point, thus using only four.  

The arrangement will then be shown on the diagram below in which the coils represent the three generator windings, and the center point the fourth terminal. Conductors a,b and c are connected to the terminals at the free ends of the windings. The common return path to the center point. Conductors a, b and c are known as the three lines, and the common center point (which is usually earthed) as the neutral point.

Suppose that three exactly similar loads are connected, one between each of lines a, b, and c. The three currents will then be equal, and their phase relationship will be the same as that of the voltages. They can therefore be represented by the curves from which it will be seen that when anyone current is at a maximum, the other two are halfway towards a maximum in the opposite direction and that when anyone is zero, the other two are equal and opposite. Similar conditions apply at all points on the curves, so that the sum of the three currents at any moment, taking their directions into account, is zero.


It follows that so long as loads remain the same, there is no current flowing in either direction in the conductor, which under these conditions could be omitted. Each of the three lines is then acting in turn as a return path for the other two. Even if the loads are not the same, the conductor has to carry only the difference between the current flowing outwards and that flowing inwards over the three lines at any instant. It can therefore be smaller in size than the others.

Instead of connecting each load between one of the lines a, b, c, and the conductor, we can connect one load between a and b, one between b and c,  and one between c, and a. the voltage applied to each load is then derived from two of the generator windings, but as the two voltages do not reach their maximum at the same time, the joint value is not twice that of one winding, but some smaller figure. The actual value is √3 or 1.732, times the voltage of one winding. The current in each line is obviously equal to the current in one winding.

Generator windings arranged as shown below are said to be star connected.

 

The voltage between any two of the lines a, b, and c  is called the line voltage, and that between any one of them and the neutral point the phase voltage.

Line voltage = phase voltage × 1.732 = 400.

Phase voltage = 400/1.732 = 230.

An alternating arrangement of the generator windings is shown  below, and in this case they are said to be delta connected or mesh connected.

The term “delta” is taken from the Greek capital letter Δ. There is no tendency for current to circulate around the closed path because the sum of the voltages at any instant is zero. The line voltage is that produced by one winding and is therefore equal to the phase voltage. The current in each line, however, is √3 or 1.732, times the current in one winding.

Note that the two methods of connection (star and delta) apply not only to the generator windings but also to the loads. Thus, in the first case we considered, if the loads are connected between each of lines Ia, b, c, and the conductor, they are star-connected, but if between a and b, b and c, and c and a, they are delta connected. This can readily be seen by drawing an example.

A.C MOTORS

A.C generators, like d.cgenerators can be made to run as motors, but only when the frequency of the supply is in step with the frequency at which the armature conductors pass the frequency at which the armature conductors pass the pairs of poles. The  motor must therefore be rotated by some other means until it is running fast enough to continue in synchronism with the supply frequency.

Machines designed to operate in this manner are called synchronous motors. Motors that do not operate in synchronism with the supply frequency are the most important class of asynchronous motors that depend upon a special property of polyphase currents which we shall now examine.

ROTATING FIELDS

Consider the two pairs of coils shown below. If coils are energized, there will be a magnetic field in line with their axis; let us call this direction north and south. If coils b are energized, there will be a magnetic field in line with their axis; let us call this direction east and west.

Suppose now that the coils are connected to a two-phase supply. We can use a phase diagram to represents the two currents, each curve corresponding to the similarly lettered coils. Starting on the left-hand side, current a is at a maximum in one direction; let us assume that this produces a flux in coils a towards the north. At this moment, current b is at zero, so there is no flux in coils b. the flux at the center may therefore be represented by the arrow in the sketch below:

Halfway between these positions, curve a is still some distance from zero, and curve b is some distance from its maximum. This is the point at which the curves cross, and the two currents are therefore equal. The two small arrows in sketch 2 represent these conditions, and their combined effect is to produce a field towards the north-west as shown by the heavy arrow.

This combination of two fields should be noted. It follows from the fact that the lines of force can not have more than one direction at the same place and time. An analogy may be helpful. Suppose that we set up two electric fans at right angles so that one produces a wind towards the north and the other a wind towards the west. If only the first fan is blowing, a particle caught in the wind will move north. If only the second fan is blowing, it will move west. If both fans are blowing, its tendency will be to move north-west.

As the field from coils a dies away and that from coils b  grows, the combined field, starting from sketch 1, passes through all the intermediate positions to sketch 2, and then through all the intermediate positions to sketch 3. We have, therefore, a rotating field produced by fixed coils. Moreover, it can be shown mathematically that the strength of the field does not vary, being always equal to that produced by one of the coils when the current in the other is at zero.

It is important to note that the rotating field is not a matter of approximation, or of a sudden jump from north to west, or even from north to north-west. The rotation of the field is quite smooth and regular, owing to the gradual dying away of flux in one direction and its equally gradual building up at right angles.

Continuing the sequence, the current in coil b dies away again after reaching its maximum, and the field towards the west gradually weakens. At the same time, coil a is energized by a growing current in the direction opposite to that which produced a field towards the north. This produces a field towards the south, which, in conjunction with the weakening field towards the west, produces a field passing through the south-west as shown in the sketch above. when the current in coils a has reached a maximum in this direction and that in coils b is at zero, the field is towards the south. We have now traced the changes for a complete half-cycle, it will be found that the rotation continues through south-east, east, and north-east until the field is again towards the north. The number of revolutions per second made by the field is therefore equal to the frequency of the current.

We have examined the production of a rotating field by two-phase current because this is the easiest case to follow without a detailed mathematical statement. A similar effect can, however, be produced by three-phase currents. In this case, three sets of coils are needed, and the field rotates through 120 degrees between the maximum in one set of coils and the maximum in the next. As this time represents one-third of a cycle, the field again rotates at the supply frequency.

INDUCTION MOTORS

The fact that a rotating field can be produced by stationary coils has led to the development of the induction motor. In this machine, coils carried by the stator produce the rotating field and the rotor is provided with heavy copper conductors accommodated in the usual slots.  In many cases, these conductors have no external connections but are simply joined together at each end of the rotor by heavy copper rings. The term squirrel-cage is often applied to rotors of this type.

As the magnetic field rotates, it cuts the rotor conductors and induces currents in the circuits completed by the copper end rings. These currents produce a field that reacts with the rotating field. in accordance with Lenz’s law, the effect is to produce motion that will tend to prevent the change of flux linkage, i.e., to make the conductors follow the field.

The rotor therefore turns. It does not, however, actually reach the speed of the rotating field; if it did there would no longer be any induced currents, and there would be no power even to overcome the friction of the motor bearings. The difference between the speed of the rotor and that of the field is known as the slip, and at full load may amount in practice to, say, 4% of the speed.

The squirrel-cage rotor without slip-rings is very simple and robust but is not suitable for starting under heavy loads. Some induction motors are therefore fitted with wound rotors. The winding is in three sections and is brought out to three slip-rings. The object of the slip-rings is to enable starting resistances to be gradually cut out as the motor speeds up until conditions are then similar to those of the squirrel-cage machine.

MISCELLANEOUS A.C MACHINES

In addition to the machines we have described, the following types may be mentioned.

SINGLE-PHASE INDUCTION MOTORS – Although a single-phase supply cannot produce a rotating magnetic field in the way described for polyphase currents, it is nevertheless possible to design a single-phase motor operating on the induction principle. Machines of this kind are not very efficient, but in small sizes they have come into increasing use in recent years. They are not naturally self-starting but can be made so at the cost of some complication.

MAGNETO GENERATORS – These are miniature generators in which the field is produced by permanent magnets. The armature (rotor) is usually of the simple two-slot form also known as H-armature shown below.

One cycle of alternating current is generated during each revolution. Magneto generators have been used for the generation of ringing current in some telephone systems, and for ignition current in internal combustion engines. In the latter case, provision is made for interrupting the armature current periodically. The result is to induce a very high voltage in a second winding, also carried by the armature. The high-voltage current so made available is led to the sparking-plugs, where it produces the spark which ignites the explosive mixture in the cylinders.

COMMUTATOR MOTORS – Since in a d.c motor, reversal of both field and armature connections at the same time does not alter the direction of rotation, it is possible to run such motors on alternating current. Owing to the greater tendency to eddy-current losses, however, all the iron, including the field magnet, should be laminated. Small machines of this kind suitable for either d.c or a.c supplies are made and are called universal motors.

MINIATURE SYNCHRONOUS MOTORS – For driving electric clocks, miniature synchronous motors are used. They must be operated from mains on which the frequency is “controlled” i.e., kept at or near its stated value over long periods. The motor then runs at a known speed, so that by means of reduction gearing it can be made to operate the hands of a clock. Some but not all of these motors are self-starting. Miniature synchronous motors of slightly larger sizes are commonly used for rotating gramophone turntables.

 

 

 

 

 

 

BASIC THEORIES OF ELECTRICITY

  T HE NATURE OF ELECTRICITY Much must of necessity remain unexplained about the basic theories of electricity, and that is why we are dea...