Introduction to Generators and Motors PDF

INTRODUCTION TO GENERATORS AND MOTORS 5-1 Generators and Motors BasIc of us and work at the end of an electric wire. You will live Most trom mber "How Electricity Is Produced and Used" in Volume I of...

INTRODUCTION TO GENERATORS AND MOTORS 5-1 Generators and Motors BasIc of us and work at the end of an electric wire. You will live Most trom mber "How Electricity Is Produced and Used" in Volume I of 1ictriatythat different forms of energy can be converted to elec. clectrical energy) and that, likewise, electricity (electrical energy) converted to different forms of energy. The electric generator is the be converts mechanical energy to electrical energy-the electric that motor. which is essentially a generator used differently, converts the elec- energy back to mechanical energy. As you may remember, the gen- used| to provide almost all of the electrical energy that we use to- ratoris ofour major problems erae is finding energy sources to run these gen crators. Because of this, we may find it necessary increasingly to use new alternative sources of energy in the future. But for now, we depend and the electric generator for the electrical energy we use. Of almostentirely on other than providing ourse, we use electrical ener8y tor many purposes mechanical energy via motors. GENERATOR SMOTOR MECHANICAL-’ ELECTRICAL ENERGY ELECTRICAL’ MECHANICAL ENERGY land other uses) 5-2 INTRODUCTION TOGENERATORS AND MOTO8S Basic Generators and Motors (continued) You already know the principles necessary to generators and motors work. In this volume, you will find understand how generators and motors work, how they are controlled, and out how troubleshoot them. how to Although there are many variations in generators and motors willfind them all basically very similar. All electric generators and use the interaction between moving conductors and magnetic fieldsmotors vice versa). It is the difference in the way these conductorsS and (or magnetic fields are arranged and in their outputs (electrical or mechanical) hat results in the differences in the devices that we will learn about here t. would be wise at this point to review the information on magnetism magnetic effects in Volumes 1, 3, and 4 before proceeding with and your study of generators and motors. Although you willlearn about de generators and motors and ac gen. erators and motors, it is important to realize now that the operation of any one of these depends simply on interacting magnetic fields involving current-carrying conductors. DRIVING WIND TO DRIVEN WIND Drives to Create Generator Motor Utilizes Electrical Energy...to Windmill Energy. Drive Fan, GENERATORS MOTORS A Generator converts AMotor converts MECHANICAL ENERGY ELECTRICAL ENERGY to to ELECTRICAL ENERGY MECHANICAL ENERGY by producing currents in by current-carrying Conductors rotated Conductors rotated THROUGH BY a magnetic field. a magnetic field. Waterwheel... Drives toCreate Motor Utilizes Generator. Electrical Energy... to Energy. Drive Water Pump. FLOWING WATER TO FLOWING WATER  INTRODUCTION TO GENERATORS 5-3 Generator Energy Sources As you learned in Volume 1, voltage, or an emf, is induced in acon Anctor moving through amagnetic field. All of the power starions supply almost all of the electric power consumed in the world today make use his simple principle to convert some source of energy into electrical energy. Most power stations use the heat energy produced by burning fossil knols such as coal, oil, or natural gas to produce steam. The steam is then, sed to drive a turbine coupled to agenerator. Several changes in the form Aonerov thus take place: the chemical energy of the fuel is first con werted into heat energY; the heat energy is converted intomechanical energy of motion in the turbine; and finally, the mechanical energy of motion is converted into electrical energy in the generator. Whatever the original source of energycoal, oil., gas, plutonium, uranium, a head of water, the Sun, the wind-the final step is always the conversion of the mechanica! energy of rotation into electical energy in a generator. This is also true of all the small transportable generating sets that you Fiod in ships and motor vehicles or in sources of emergency power. N 5-4 INTRODUCTION TO GENERATORS Revlew of Electrlclty trom Magnetiem You will ecall that electricity can be generated by moving a wie. through a magnetic field. As long as there is relative motion between the conductor and the magnetic field, electricity is generated. The generated voltage is called an induced vollage,or inducad emf, and the method of gen erating i by cutring a magnetic field with aconductor is called induction. You know that the amount of voltage induced in the wire cuttino through he magnetic field depends on a pumber of factorsFirst, if th Speed of ihe relative cutting action bertween the conductor ad the mag netic field increases, the induced emf or voltage increases. Second, if the strength of the magnetic field is increased, the induced voltage or emí also increases. Third, if the number of turns cutting through the magneric field is increased, the induced emf (voltage) is increased again. The polarity of this induced voltage or emf will be auch that the resulting current flow will build up a field to react with the field of the magnet and to oppose the movement of the coil. This phenomenon illue. trates Lenz's law, which states that in electromagnetic induction, the direction of the induced emf, and bence current flow, is such that the magnetic field senp op. poses the motion which produces it. FACTORS WHICH DETERMINE INDUCED EMF STRENGTH...the SPEED of conductor through magnetic field 2 the STRENGTH of magnetic field the NUMBER of turng  INTRODUCTION 1O GENERATORS novlewot Electrlolty trreom Magnetlem (oontnued) ned volnge ) in eh onduton is propsional mthe field imes the speed of the onducor through he lux Xopeed Van albe koow bat the polariy of the induced voage, and hence the yenernedurrent (ow, isdetermined by the divevon of the moton between the mapoetie field and the cutiny condueor DIRECTION OF RELATIVE MOTION DETERMINES DIRECTION OF CURRENT FLOW So to um up what you already know about elecricity from magnetism: 1. Movng a conductor through a magnetic field generates an emf or volt age that produces a current flow. 2. The faster the conductor cuts through the field, the more turns there are, and the stronger the magnetic field--the greater the induced emf or volt age, and the greater the current flow. 3. Reverng the direction of movement of the conductor reverses the poly of the induced enf (voltage), and therefore the direction of ureD flow is reversed. 4. Ir doesn't mater whether theconductor or the magnetic field changes or mOves, the relt is the same. 5-6 INTRODUCTION TO GENERATORS The Left-Hend Rule You have seen how an emf or voltage is generated in the cojl of elementary generator. There is a simple method for remembering the he direction of the emf induced in a conductor moving through a magnetic field; it is called the left-hand rule for generators. This rule states that if vo hold the thumb and the first and middle fingers of the left hand at riohr angles to one another, with the first finger pointing in the flux direction and the thumb pointing in the direction of motion of the conductor h middle inger will point in the direction of the induced emf. Direction of induced emf means the direction inwhich current will flow as a result of this induced emf. You will remember from Volume 1 that there are two conventions (views) as to the direction of current flow: the convention based on elec. tron theory that states that current flows from the aegative terminal of a source of electricity, and the older convention that supposes that curreat flows from the positive terminal of a source of electricity. The first of these conventions is used throughout these volumes, and the left-hand rule for generators indicates the direction of electron current flow in accordance with that convention. The same rule explained above, but applied to the right hand instead of to the left hand, will indicate the direction of current flow in accor. dance with the older convention. oTON THF GFNERATOR HAND RULF ONECTION OF ENF DIREG cONOUGToR pIRCCTION MOTOW FLUX EN DIREGTON CoNOUCTOR ONOUGTOR MOTNoN DON We will also use the convention that a conductor shown as means that the current is flowing away from the observer, whereas one shown a5 O means that the current is flowing toward the observer.  HE ELEMENTARY GENEnATOR ofan Elementary Qenerator The Parte elementary generator conits of aloop of wire so placed that it Iin a uniform magneie field to cause an indued emf in he 4pairof liding contacts are employed to connect the loop to an e in order to use the induced emt Ihe pale piees are the noth and souh poles of the magnet that magnetic field. The loop of wire hat roates through the field the armature.. The ends of the armature loop are conneed to alled dip rngu,that rotate with the armature. Urushey ride up againn ihe lip ogs to connect the armature to the exMernal circuit, You will Ialltharthis is the same elementary generator used to generate an ac dexcribed at the beginning of Volume 9. THE ELEMENTARY GENERATOR Pole Pleces Armature Loop Brush sllp Ring Lond External Circuit Zero Center Ammeter In the description of the generator action given in the following pages, you should visualize the loop rotating through the magnetic ield. (However, please remember hat you could just as easily rotate the magnet asembly.) As the sides of the loop cut through the magnetic field, an emf is induced in them which causes a current to flow through the loop, slip rings, brushes, ammeter, and load resistor--all connected in series. The magnitude of the induced emf generated in the loop, and therefore of the current that flows, depends on the instantaneous position of the loop in the magnetic field. 5-8 THE ELEMENTARY GENERATOR Elementary Generator Operatlon As you remember from Volume 3, the elementary generator Works like this: Assume that the armature loop is rotating in a clockwise directionA,andth that its initial position is at A(0°) (see diagram below). In position loop is perpendicular to the magnetic field, and the black and white con- ductors of the loop are moving parallel to the magnetic field. If a conduc. tornot iscut moving parallel to a magnetic field, its relative motion is zero, it does through any lines of force, and no emf is generated in the conduc. tor. This applies to the conductors of the loop at the instant through position A. Thus, no emf is induced in the loop, annd no they go current flows through the circuit. The ammeter reads zero. As the loop rotates from position A to position B, the conductors are cutting through the lines of force at a faster and faster rate, until at o00 (position B) they are cutting through lines of force at the maximum rate ln other words, between 0° and 90°, the induced emf in the conductors builde up from zero to a maximum value. Observe that from 0° to 90° the black conductor cuts down through the field while at the same time the white conductor cuts up through the field. As you can show by Lenz's law, the in. duced emfs in both conductors are in series; and the resultant yoltage across the brushes (the terminal voltage) is the sum of the two induced emfs,or double that ofone conductor,since the induced voltages are equal to each other. The current through the circuit will vary just as the induced emf varies, being zero at 0° and rising to a maximum at 90°. If the ammeter could follow the variations in current, it would show an increasing deflec. tion to the right between positions A and B, indicating that the current through the load was flowing in the direction shown. The direction of current flow and the polarity of the induced emf de. pend on the direction of the magnetic field and the direction of rotation of the armature loop. The waveform shows how the terminal voltage of the elementary generator varies from position A to position B. HOW THE ELEMENTARY GENERA TOR WORKS t A Generator Teroinal Position A Position B Voltage 90°  HE ELEMENTARY GENERATOR 6.0 Wementany deneralor Operatton (eonttnued) maing rom poiion B(90°) the lines position h ntoe, which ate uting C laite tate at jition B, heginthrough of force at a to co hrough ines more and wil pition hey are moving i relative parallelto the magnetic will thereloreveloity berween the ield and the conduetor. ile et Ilow will decrene as the loop moves from alo vary As the voltage 90° to varies. Posltlon B S 90° 90 120 150 Posltlon C 180° 5-10 THE ELEMENTARY GENERATOR Elementary Generator Operatlon (continued) From 0° to 180° he conductors of the loop have been same direction through the magnetic field, and the polarity movi of the ng in the emf has therefore remained the same. As the loop starts rotating induced 180° back to position A, however, the direction of the cutting actionbeyond of the conductors through the magnetic field reverses. Now the black cuts up through the field, and the white conductor cuts down conductor through field. As a result, the polarity of induced emf and the current flow, the will reverse. The voltage output waveform the complete revolution of for loop is shown below. the Position D 2700 Generator 180° 270° 360° Terminal 909 Voltage Position A 360° (or o As you know, the emf generated will cause ac current to flow in an ex ternal circuit connected to its output terminals. If the armature were rotated 60 times (cycles) per second (hertz or Hz), then the frequency of the ac output would be 60 Hz. Ac generators are often called alternators. When the loop is rotated at constant speed, it cuts the lines of force at the highest rate when the loop moves from the horizontal position and at the lowest rate when the loop is vertical. This causes the voltage output to be sinusoidal because the rate at which the loop cuts lines of force is sinusoidal, and as you know it is the rate of cutting lines of force that deter mines output voltage.  THE ELEMENTARY GENERATOR 6-11 The Commutator youhave seen, the elementary generator is an ac generator. If an ac As is complete. You willscudy practlcal desired, then the generator Lencrators later in this volume. the ac vol1age induced in the loop the elementary generator, the 0° and I80° polarity every time the loop goes through points. At these points, the conductors of he loop reverse their direction field. As you already know, the polarity of the in- through he magnetic the direction in which a conductor moves through a ducedenf depends on magneicfield: and ifthe direction reverses,the polarity of the inducedemf reverses. Since the loop continues rotating through the field, the emf in- duced in the conductors of the loop will always be alternating. Therefore. that de can be obtained from the generator is to convert the ac the only way output to dc. connected across the generator Oneway to do this is to have a switch that it will reverse the connections to the load every output in such a way the generator. The polarity of the induced emf changes inside time theillustrated switch, in the diagram below, must be operated manually every If this is done, the voltage applied ime the polarity of the voltage changes. same polarity, and the current flow to the load will always have the direction, although it will rise and fall through the resistor willnot reverse in value as the loop rotates. N Load Changing AC to DC using a Pulsating DC AC REVERSING SWITCH Load Current and Voltage Generalor Output 5-12 THE ELEMENTARY GENERATOR The Commutator (continued) To convert the generated ac voltage into a pulsating switch must be operated twice for every cycle. If the generatorvoltage the alternating at 60 Hz, the switch must be operated 120 times per output is convert the ac to dc. Obviously, it would be to operatesecond impossible a to switch manually at such a high speed. The problemhas been solved simply by mounting the switch on d. rotating shaft. This is done by changing theslip rings, so that hey giveibe same result as the mechanical switch. Essentially, one of the eliminated, and the other is split along its axis. The ends of slip ringsareis the coil then connected, one to each of the segments of the slip ring. The segmen are insulated so that there is no electrical contact between them, the shaf or any other part of the armature. The entire split ring is known as the co mutator, and its action inconverting the ac into dc is known as commutation The brushes are positioned opposite each other, and the commutator segments are mounted so that they are short-circuited by the brushes as the loop passes through the zero voltage points; thus, no current flow in the short circuit. Notice also that, as the loop rotates, each conductor will be connected by means of the commutator, first to the positive brush and then to the negative brush. When the armature loop is rotated, the commutator automatically switches each end of the loop from one brush to the other, every time the loop completes a half revolution. The action is exactly the same as that of the manual reversing switch. N S changing ACto DC using a COMMUTATOR COMMUTATOR As you will learn later when you study de generators, a called the rectifier, which you learned about when special device you studied ac meters, can also be used to convert ac to pulsatingdc.  THE ELEMENTAAY GENERATOA 5-13 AC DC by Use of the Commutator converting to analyze the action of the commutator in tonverting Suppocvou no In dc. Aposition (0°). the loop is perpendicular to tcd into ac cencta ficld, no emf is generated in the conductors of the loop, and the maenctC flow. Notice that the brushes are in contact with both current the effectively short-circuiting the loop. This the commutator, seCmcntso f any problem, since there is no current flow. does not create momentthe loop moves slightly beyond Aposition, shortcrcuit however. the The The black exists. brush is in contact with che black circuit no longer short the white brush is in contact with the white segment. segment. and rotates clockwise from A position to Bposition, the in- zero, until at Bposition (909) he in- Asthe loop emfstarts building up from current varies with the induced emf. duced maximum. Since the duced emf is at will also be at a maximum 90°. As the loop continues current rotating the flow clockwise. from Bposition to C, the induced emf decreases, until zero once again. position (180°), it is below shows how the terminal voltage of the at C The waveformfrompictured 0° to 180, generator varies TION-CONVERTING AC TO DC cOMMUTA Load B (90°) Load position A(0°) position Generated Termina! Voltage C(180°) Position Load 5-14 THE ELEMENTARY GENERATOR Converting AC to DC by Use of the Commutator Nouce that in Cposition the black brush is slipping ment onto the white segment while at the same time the white (continued) offthe blackis seg- ping off the white segment onto the black segment. In brush is always in contact with the conductor of the brush this way, slip- the black ward, and the white brush is always in contact with theloop moving down- upward. Since the upward-moving conductor has a current the brush, the white brush is the negative terminal and conductot the flow moving towardis the positive terminal of the dc black brush generator. You can easily verify this by th left-hand rule. As the loop continued rotating from C position (270°) and back to Aposition (360°position or 0°), (180°) through D connected to the white wire, which is moving down, andthe black brush the white brush ie connected to the black wire, which is moving up. As a result, the same polarity voltage waveform is generated across the brushes from 180o 360° as was generated from 0° to 180°. Notice that the current flows in same direction through the ammeter after the commutation reverses in direction every half cycle in the loop itself. even though it The voltage output then has the same polarity atall times; but it varies in value, rising from zero to maximum, falling to zero, then rising to maxi. mum and falling to zero again for each complete revolution of the at. mature loop. As you will recall, such a waveform is called pulsating do. COMMUTATIONCUNVERTING AC TO DC Load W Load W Load Cq80) D(270) A(360) position position position (or 0°) As you can' see, the only major difference between elementary ac and de generators is in the way the output is manipulated.  THE ELEMENTARY GENERATOA 5-15 the DC Output rOVing tOu learned about generators, the only de voltaze yru were Betoreh4as thesmooth and unvaryíng volta ze produced, for eam v. Noyoufind that the de output of an elementary de yer uneven-apulsating de voltage varying periodically fron mum. Although this pulsating voltage is dc, it is not onvant to operate many dc appliances and equipment. Therefore, the dc generator must be modified so that ít will ptduce a dlenentary OOher output. This is accomplished by adding more coik of wire to the Lmature. The illustration shows a generator with atwo-coil armature, the two B) positioned at right angles to each other. Notice that the (A andis broken up into four segments, with opposite sezments con- commutator coils the ends of a coil. In the position shown, the brushes connect to pectedto which a maximum voltage is generated, since ít is white coil (A) in to the field. As the armature rotates clockwise, the atright angles he moviDgfrom coil Astarts dropping off. After an eighth of a revolution output brushes slide over to the black commutator segments in whse (459)the induced emf is increasing. The output voltage starts to pick up the coil(B) peak at 90°, and starts dropping off as coilB cuts through gain, reaches a the fewer lines of force. At 135°, commutation takes place again and coil A hrzshes are once more i0 contact with shown Io the illustration below, the voltages from both coils are Notice that the output never drops superimposed on the single coil voltage.commutation will also notice that occurs at the points below point Y. You is a momentary short shere the voltages in the coils are equal. Since there that the points are of developed on thecommutator at this instant, the fact two commu erual potential means that no current will flow across the tator segments. The rise and fall in voltage is therefore now limited to the distance between Y and the maximum, rather than to that between zero output voltage of a dc and the maximum. This reduced variation in the that the output of the generator is known as generator ripple. It is apparent the output of the one rwO-coilarmature is much closer to steady dc than coil armature. MANY CDiSREIDUCE GESERA TOR RIPPLE -ToCoil Armature Coil A +Coil B 313 3 j80 225 270 Commutation Points (every 90  5-16 THE ELEMENTARY GENERATOR Improving the DC Output (continued) Although the output of the two-coil generator is stant de than the output of the one-coil generator, there ismuch closer ple in the output to make it useful for some electrical still too much rig generators, however, can be very useful in applications important (or can be filtered), for example, in battery equip wherement. tipple Siismplnote chargers, with alarge number of coils, and the commutator is armature isweldmade ing equipment, etc. To make the output reallysmooth, the alarge number of segments. The coils are so arranged similarlyaround divided into mature that at every instant there aresome turns cutting through the ar- the mag- netic field at right angles. As a result, the generator output contains little ripple and is for all practical purposes a constant, or pure, de very MANY-TURN COILS INCREASE VOLTAGE OUTPUur FLEMENTA RY GENERATOR 0UTPU VOLTAGE TwoCoil Armature Practical Generators PRACTICAL GENERATOR 0UTPUT Contain many-turn coils The voltage induced in a one-turn coil or loop is not very large. In order to generate a large voltage output, each coil on the armature of a practical generator consists of many turns of wire connected in series. AS a result, the output voltage is much greater than that generated in acoil hay ing only one turn.  THE ELEMENTARY GENERATOR 5-17 the Generator Output onsidered the way an elementary generator works, you col WAs rotated in a uniform magnetic field. To obtain he A that m trcldinpractical generators, concave pole pieces are used, and otls are carried by an iron rotor sO as to confine the mag- devired areas. It should be apparent that the generator greatly stormer in principle and that what you learned about the tcan arlier will help you understand generators and motors. Pole Piece A Iron Rotor S even more uniform by using more The flux can be made greater and more may be pair of poles. Two pairs of poles are common, but han one USEd in large machines. N Pole Pieces Iron s Rotor N arrangedin sev- that the generator can be moving should also be apparent principle of conductors eral long as the will study tMomehroughconfi gurations, just so in this volume we Later magnetic fields is preserved. of these different coonfigurations.  6-18 HE LMENIARY GENERATOR Generator Review of the Elementery emf is the vesu ing througha magneie ing the ines of tone, 2. PACTORS APPRCTIN, INDUCPD EM. Sped of condueuor netic field. hrgh m b. The srength of the magoeie fiel4 the TREGTIH ¬. The number of turns). inductrs tm d, The direction of relaúve main the NUMBEH determines polarity, 3, ELEMENTARY GENERATOR Aloop of wire rotating through , magnetic field forms an elementary generator and is connected t9 an ternal circuit through slip ríngs. The induced emf causes current flow in the external circuit. 4, ELEMENTARY GENERATOR OUTPUT-The emf and curren flow of an elementary generator LoNE AEVOLUTIOh - reverse in polarity every time the armature loop rotates 180°, The out put of such a generator is ac. 5. COMMUTATOR-An automatic reversing switch on the generator shaft, whích switches coil connec. tions to the brushes every half revolu. tion of an elementary generator. ls mo purpose is to provide a de output. The process is called commutation. cOMMUTATOR 6. PRACTICAL DC GENERA TOR-To smooth out the de taken from agenerator, many coils are used Practical Generators in the armature, and more segments are used to form the commutator. A practical de generator bas a valtage output that is near maximum at all times and has only a light ripple.  THE ELEMENTARY GENERATOR 5-19 Teet Review Questions sell pactical uses for generators. c Ltors control generator output? doubledthe speed of a generator, what would happen to the roltage doubledthe number of turns on the coil of an elementary gen- vou If uhar would happen to the output? erator, wouldthe output voltage change if the Hon strength of the magnetic s heldwereincreased? Decreased? Drawanelementary generator and show, using the left-hand rule, how 6.acommutator works. Lenz's law. 1 State back into Volume 3on Faraday's law and restate it here 1. Drawan elementary generator. Label and define the functions of all of 9, the parts. the essential difference between an ac and adc elementary gen- 10. What is erator? Learning Objectives--Next Section Overview-Now that you know something about the principle of Drac. the elementary generator, you are ready to learn about the section, tical dc generator in the next THE DC GENERATOR  DC- Geneaato otating-armatwe ac- Gene YOtoy field aohhng field 5-20 atorTHE DCGENERATOR Omahue Generator Construction staiay So far you have learned the fundamentals of generator action and. theory of operation of elementary ac and dc generators. Now you are ready to learn about actual generators and how they are constructed. In fact generators have become relatively unimportant because of the ease iel which ac can be converted to dc. However, an understanding of dc genera. tors provides abasis for understanding all the generators and motors that you will study later in this volume. There are various components essential to the operation of a complete generator. Once you learn to recognize these components and become familiar with their unction, you will find it much easier to do fault. finding and maintenance work on generators. All generators--whether ac or dc-consist of a rotating part and a stationary part. In most dc generators the coil that the outputis taken from is mounted on the rotating part, which is referred to as the armature. The coils that generate the magnetic field are mounted on the stationary bart which is referred toas the field. In most ac generators the opposite is true that is, the field is mounted on the rotating part--the rotor ; and the arma ture is wound on the stationary part--the stator. In modern generators there are exceptions to the above cases wherea permanent magnet or asim ple iron core is the rotor, and both the field and the output coils are mounted on the stator. You will learn about these later in this volume A TYPICAL DC GENERATOR Armature Field Winding In all cases, there is relative motion such that a coil cuts through mag netic lines of force. As a result, an emf is induced in the coil, causing a current to flow through the external load. Since the generator supplies electrical power to a load, mechanical power must be put into the generator to cause the rotor to turn and togen erate electricity. The generator converts mechanical power into electrical power. Consequently, all generators must have machines associated with them that will supply the mechanical power necessary to turn the rotors. These machines may be steam, gasoline,or diesel engines; electric motors; or steam turbines actuated by the heat given off in the combustion of coal or oil, or nuclear fission; or turbines driven by water power.  THE DC GENERATOR 5-21 DC Generator Construction The relationship of the various components making up a de genera- illustrated below. In the generator shown the field coils are the Is tor one end housing (not illustrated) is bolred to che generator stator, and The armature is inserted between the field poles and the housing frame. with the brush assemblies mounted last. end. End Brush Bell Spring Brush Brush Holder Commutator Armature Brush Assembly Frame Fleld Pole (lam inated) Fleld Coll Fleld Pole Screw 5-22 THE DC GENERATOR DC Generator Constructlon (continued) Generator design varies depending on the size, type, and turer; but the general arrangement of parts is as illustrated the manufac. above. The major parts of the dc generator are described on following pages. pare each part and its function Com. with the elementary generator described earlier. MAIN FRAME.: The main frame is sometimes called the yoke. Ittis i the foundation of the machine and supports the other components. It also serves to complete the magnetic field between the pole pieces POLE PIECES: The pole pieces, like transformer cores, are made of many thin laminations of iron, joined together and bolted usually to inside of the frame. These pole pieces provide asupport for the field coils and are designed to produce a concentrated field. By laminating the poles, eddy currents are reduced. CONSTRUCTION OF DC GENERATOR Field Winding Main Framel Pole Pieces End Housing Brush Holders FIELD WINDINGS: The field windings mounted on the pole pieces form electromagnets,which provide the magnetic field necessary for gen erator action. The windings and pole pieces together are often called the field. The windings are coils of insulated wire wound to fit closely around pole pieces. Current flowing through these coils generates the magnetic field. In some small generators, permanent magnets replace the field coils. Agenerator may have only two poles,or it may have a number of pairs of poles. Regardless of the number of poles, alternate poles willalways be of opposite polarity. Field windings can be connected either in series or in parallel (or shunt, as the parallel connection is often called) with the ar mature. Shunt field windings consist of many turns of fine wire, whlie series fieldwindings are composed of fewer turns of fairly heavier ire  THE DC GENERATOR 5-23 Generator Constructlon (continued) DC END HOUSINGS: These are attached to the ends of the main frame conrainthe bearings for the armature. The rear housing usually sup- only he bearing, whereas the front housing also supports the brush LNd ports ISSemblies, BRUSH HOLDER: This component supports the, brushes and their coanecting wires. The brush holders are secured to the front end housing On.some generators, the brush holders can be rotated around withclamps.adjustment. he shaft for APMATURE ASSEMBLY: In practically all dc generators, the arma rotates between the poles of the field. The armature assembly is made (ure upof a shaft, an armature core, armature windings, and a commutator. and is slotted The armature core is laminated to reduce eddy current losses windings are usually n receive the armature windings. The armature core. woundin forms and then placed in the slots of the from one The commutator is made up of copper segments insulated These anocher and from the shaft by mica or heat-resistant plastic. out slipping segments are secured by retainer rings to prevent them from the ends of the seg under the force of rotation. Small slots are provided in shaft supports the ients to whichthe armature windings are soldered. The end bearings. entire armature assembly and rotates in the pieces to pre There is a small air gap between the armature and pole This pieces during rotation. vent rubbing berween the armature and pole field strength at a maximum. gap is kept to a minimum in order to keep the BRUSHES: The brushes ride on the commutator and carry the gen of a high grade of car erated voltage to the load. They are usually made place are held in by brush holders. bon, or a carbon-copper mixture, and their holders so that they are able to The brushes can slide up and down in commutator. A flexible braided tollow irregularities in the surface of the the external circuit. conductor, called a pigtail, connects each brush to Armature AssemblySlots Shaft. Winding Commutator Laminated Core Brush Assembly Pigtail Spring Lead Commutator Brush Brush Holder 5-24 THE DC GENERATOR Types of Armatures Armatures used in dc generators are divided into two general These are the armature and the drum-type armature. In the types. type armature, ing-type the insulated armature coils are wrapped around an iron ring- ring, with taps taken off at regular intervals to form connections to the commutator segments. The ring-type armature was used in early designs tor rotating electrical machinery but is not used today. The drum-type armature is the modern standard armature construe. tion. The insulated coils are inserted into slots in the cylindrical armature core. The ends of the coils are then connected to each other. Types of Armatures Leg of Coil Armature Armature Commutator Ring Type Segments Drum Type Commutator As a rule, most dc armatures use form-made coils. These coils are wound by machine with the proper number of turns and in the proDer shape. The entire coil is then wrapped in tape, or otherwise insulated, and inserted into the armature slots as one unit. The coils are so inserted that the two legs of each coil are under unlike poles. In a two-pole machine, the legs of each coil are situated on opposite sides of the core and therefore automatically come under opposite poles. In a four-pole machine, the legs of the coils are placed in slots about one quarter the distance around the armature, thus again keeping the legs of the coils under unlike poles. In electrical machinery, the number of poles specified is actually the number of field poles. Coil Placement Armature Armau: Ci Coils S S Four-pole Two-pole N  THE DC GENERATOR 5-26 Armature Winding of 0d1ngson a drum-type armature may be one of two Tpes kinds, a lap Ihe nding the two ends of each coil are connected to adjacent cgments, and the winding forms the pattern drawing is made as if the armature were laid out illustrated flat, so the ex. lhe hclon. hand side actually connects to the extreme left-hand side. The rcnnc Kh i sfor a four-pole generator. Wustraton 6 5' 9 5 07 e' 2 9 2 1o'4 6 2' B A D The direction in which the emf in each conductor is induced is indi ated in the diagram. The way in which the coils in the lap winding are connected can be seen more easily in the simplified diagram below. B D C Youcan see that there are two points where the emfs in adjacent con ductors meet: Aand C; and two points where the emfs in adjacent conduc tors are diverging: B and D. If brushes are placed at these points, current willflow from the armature winding at Aand Cand into the winding at B and D. Brushes having the same polarity can be connected together, and the armature is thus effectively divided into four parallel paths.