Overview of Direct CurrentMachines Direct-current (DC) machines are divided into dcgenerators and dc motors. DC generators are not as common as they used tobe, because direct current, when required, is mainlyproduced by electronic rectifiers. While dc motors are widely usedin many applications.
DC Generator A dc generator is a machinethat converts mechanicalenergy into electrical energy(dc voltage and current) byusing the principle ofmagnetic induction. In this example, the ends ofthe wire loop have beenconnected to two slip ringsmounted on the shaft, whilebrushes are used to carrythe current from the loop tothe outside of the circuit. Principle of magnetic induction in DC machine
DC Motor DC motors are everywhere!In a house, almost everymechanical movement thatyou see around you iscaused by an DC (directcurrent) motor. An dc motor is a machinethat converts electricalenergy into mechanicalenergy by supplying a dcpower (voltage and current).
Construction Major parts are rotor (armature) and stator(field).
ARMATURE More loops of wire = higher rectified voltage In practical, loops are generally placed in slots of an iron core The iron acts as a magnetic conductor by providing a low-reluctance path formagnetic lines of flux to increase the inductance of the loops and provide ahigher induced voltage. The commutator is connected to the slotted iron core.The entire assembly of iron core, commutator, and windings is called thearmature. The windings of armatures are connected in different waysdepending on the requirements of the machine.Loops of wire are wound around slot in a metal core DC machine armature
ARMATURE WINDINGS Lap Wound Armatures– are used in machines designed for lowvoltage and high current– armatures are constructed with large wirebecause of high current– Eg: - are used is in the starter motor ofalmost all automobiles– The windings of a lap wound armature areconnected in parallel. This permits thecurrent capacity of each winding to beadded and provides a higher operatingcurrent– No of current path, C=2p ; p=no of polesLap wound armatures
ARMATURE WINDINGS (Cont) Wave Wound Armatures– are used in machines designed for highvoltage and low current– their windings connected in series– When the windings are connected inseries, the voltage of each winding adds,but the current capacity remains thesame– are used is in the small generator inhand-cranked megohmmeters– No of current path, C=2Wave wound armatures
FIELD WINDINGS Most DC machines use woundelectromagnets to provide the magnetic field. Two types of field windings are used :– series field– shunt field
FIELD WINDINGS (Cont) Shunt field windings– is constructed with relatively many turns of smallwire, thus, it has a much higher resistance thanthe series field.– is intended to be connected in parallel with, orshunt, the armature.– high resistance is used to limit current flowthrough the field.
FIELD WINDINGS (Cont) When a DC machineuses both series andshunt fields, each polepiece will contain bothwindings. The windings are woundon the pole pieces in sucha manner that whencurrent flows through thewinding it will producealternate magneticpolarities.Both series and shunt field windings are contained in each pole pieceS – series fieldF – shunt field
Principle operation of Generator Whenever a conductor is moved within amagnetic field in such a way that the conductorcuts across magnetic lines of flux, voltage isgenerated in the conductor. The AMOUNT of voltage generated depends on:i. the strength of the magnetic field,ii. the angle at which the conductor cuts the magneticfield,iii. the speed at which the conductor is moved, andiv. the length of the conductor within the magnetic field
Principle of operation (Cont) The POLARITY of the voltagedepends on the direction of themagnetic lines of flux and thedirection of movement of theconductor. To determine the direction ofcurrent in a given situation, theLEFT-HAND RULE FORGENERATORS is used.•thumbthumb in the direction the conductor is being movedconductor is being moved•forefingerforefinger in the direction of magnetic flux (from north to south)magnetic flux (from north to south)•middle fingermiddle finger will then point in the direction of current flowthe direction of current flow in anexternal circuit to which the voltage is appliedvoltage is appliedLeft Hand Rules
THE ELEMENTARY GENERATOR The simplest elementary generatorthat can be built is an ac generator.Basic generating principles aremost easily explainedeasily explained through theuse of the elementary ac generator.For this reason, the ac generatorwill be discussed first. The dcgenerator will be discussed later. An elementary generator consistsconsistsof a wire loop mounted on the shaft,of a wire loop mounted on the shaft,so that it can be rotated in aso that it can be rotated in astationary magnetic fieldstationary magnetic field. This willproduce an induced emf in the loopproduce an induced emf in the loop.Sliding contacts (brushes) connect(brushes) connectthe loop to an external circuit loadthe loop to an external circuit loadin order to pick up or use thein order to pick up or use theinduced emf.induced emf.Elementary Generator
The pole pieces (marked N and S) provide the magnetic fieldprovide the magnetic field.The pole pieces are shaped and positioned as shown toconcentrate the magnetic field as close as possible to the wireloop. The loop of wire that rotates through the field is called theThe loop of wire that rotates through the field is called theARMATUREARMATURE. The ends of the armature loop are connected toThe ends of the armature loop are connected torings called SLIP RINGSrings called SLIP RINGS. They rotate with the armature. The brushes, usually made of carbon, with wires attached tothem, ride against the rings. The generated voltage appearsThe generated voltage appearsacross these brushes.across these brushes. ((These brushes transfer power from the battery toThese brushes transfer power from the battery tothe commutator as the motor spins – discussed later in dc elementarythe commutator as the motor spins – discussed later in dc elementarygeneratorgenerator).).THE ELEMENTARY GENERATOR(Cont)
THE ELEMENTARY GENERATOR (A) An end view of the shaft andwire loop is shown. At thisparticular instant, the loop ofwire (the black and whiteconductors of the loop) isparallel to the magnetic linesof flux, and no cutting actionis taking place. Since thelines of flux are not being cutby the loop, no emf isinduced in the conductors,and the meter at this positionindicates zero. This position is called theNEUTRAL PLANE.00Position (Neutral Plane)
THE ELEMENTARY GENERATOR (B) TheThe shaft has been turned 90shaft has been turned 9000clockwise,clockwise, the conductors cutthe conductors cutthrough more and more lines of flux,through more and more lines of flux,andand voltage is induced in thevoltage is induced in theconductor.conductor. at a continuallyat a continually increasing angleincreasing angle ,,the inducedthe induced emf in the conductorsemf in the conductorsbuilds up from zero to a maximumbuilds up from zero to a maximumvalue or peak value.value or peak value. Observe that from 0Observe that from 000to 90to 9000, the, theblack conductor cuts DOWNblack conductor cuts DOWNthrough the field. At the same timethrough the field. At the same timethe white conductor cuts UP throughthe white conductor cuts UP throughthe field. The induced emfs in thethe field. The induced emfs in theconductors are series-adding. Thisconductors are series-adding. Thismeans the resultant voltage acrossmeans the resultant voltage acrossthe brushes (the terminal voltage) isthe brushes (the terminal voltage) isthe sum of the two inducedthe sum of the two inducedvoltages. The meter at position Bvoltages. The meter at position Breads maximum value.reads maximum value.900Position
THE ELEMENTARY GENERATOR (c) After another 900of rotation,the loop has completed 1800of rotation and is againparallel to the lines of flux.As the loop was turned, thevoltage decreased until itagain reached zero. Note that : From 00to 1800the conductors of thearmature loop have beenmoving in the same directionthrough the magnetic field.Therefore, the polarity of theinduced voltage hasremained the same1800Position
THE ELEMENTARY GENERATOR (D) As the loop continues to turn, theconductors again cut the lines ofmagnetic flux. This time, however, the conductorthat previously cut through theflux lines of the south magneticfield is cutting the lines of thenorth magnetic field, and vice-versa. Since the conductors are cuttingthe flux lines of opposite magneticpolarity, the polarity of theinduced voltage reverses. After270 of rotation, the loop hasrotated to the position shown, andthe maximum terminal voltage willbe the same as it was from A to Cexcept that the polarity isreversed.2700Position
THE ELEMENTARY GENERATOR (A) After another 900of rotation,the loop has completed onerotation of 3600and returnedto its starting position. The voltage decreased fromits negative peak back tozero. Notice that the voltageproduced in the armature isan alternating polarity. Thevoltage produced in allrotating armatures isalternating voltage.3600Position
Elementary Generator (Conclusion) Observes– The meterdirection- Theconductorsof thearmatureloop– Direction ofthe currentflowOutput voltageof an elementary generatorduring one revolution
THE ELEMENTARY DC GENERATOR Since DC generators must produceDC current instead of AC current, adevice must be used to change the ACvoltage produced in the armaturewindings into DC voltage. This job isperformed by the commutatorcommutator.. The commutator is constructed from acopper ring split into segments withinsulating material between thesegments (See next page). Brushesriding against the commutatorsegments carry the power to theoutside circuit. The commutator in a dc generatorreplaces the slip rings of the acgenerator. This is the main differencein their construction. The commutatormechanically reverses the armatureloop connections to the external circuit.
THE ELEMENTARY DC GENERATOR(Armature) The armature has an axle, and thecommutator is attached to the axle.In the diagram to the right, you cansee three different views of thesame armature: front, side and end-front, side and end-on.on. In the end-on view, the winding iseliminated to make the commutatormore obvious. You can see that thecommutator is simply a pair ofsimply a pair ofplates attached to the axleplates attached to the axle. TheseTheseplates provide the two connectionsplates provide the two connectionsfor the coil of the electromagnet.for the coil of the electromagnet.Armature with commutator view
THE ELEMENTARY DC GENERATOR(Commutator & Brushes worktogether) The diagram at the right shows how thehow thecommutator and brushes work togethercommutator and brushes work togetherto let current flow to the electromagnetto let current flow to the electromagnet,and also to flip the direction that theelectrons are flowing at just the rightmoment. The contacts of theThe contacts of thecommutator are attached to the axle ofcommutator are attached to the axle ofthe electromagnet, so they spin with thethe electromagnet, so they spin with themagnet. The brushes are just two piecesmagnet. The brushes are just two piecesof springy metal or carbon that makeof springy metal or carbon that makecontact with the contacts of thecontact with the contacts of thecommutator.commutator. Through this process the commutatorchanges the generated ac voltage to apulsating dc voltage which also knownas commutation process.Brushes and commutator
THE ELEMENTARY DC GENERATOR The loop is parallel tothe magnetic lines offlux, and no voltage isinduced in the loop Note that the brushesmake contact with bothof the commutatorsegments at this time.The position is calledneutral plane.00Position (DC Neutral Plane)
THE ELEMENTARY DC GENERATOR As the loop rotates, theconductors begin to cut throughthe magnetic lines of flux. The conductor cutting throughthe south magnetic field isconnected to the positive brush,and the conductor cuttingthrough the north magnetic fieldis connected to the negativebrush. Since the loop is cutting lines offlux, a voltage is induced intothe loop. After 900of rotation,the voltage reaches its mostpositive point.900Position (DC)
THE ELEMENTARY DC GENERATOR As the loop continues torotate, the voltage decreasesto zero. After 1800of rotation, theconductors are again parallelto the lines of flux, and novoltage is induced in theloop. Note that the brushes againmake contact with bothsegments of the commutatorat the time when there is noinduced voltage in theconductors1800Position (DC)
THE ELEMENTARY DC GENERATOR During the next 900of rotation, theconductors again cut through themagnetic lines of flux. This time, however, the conductor thatpreviously cut through the southmagnetic field is now cutting the flux linesof the north field, and vice-versa. . Since these conductors are cutting thelines of flux of opposite magneticpolarities, the polarity of induced voltageis different for each of the conductors.The commutator, however, maintains thecorrect polarity to each brush. The conductor cutting through the northmagnetic field will always be connectedto the negative brush, and the conductorcutting through the south field will alwaysbe connected to the positive brush. Since the polarity at the brushes hasSince the polarity at the brushes hasremained constant, the voltage willremained constant, the voltage willincrease to its peak value in the sameincrease to its peak value in the samedirection.direction.2700Position (DC)
THE ELEMENTARY DC GENERATOR As the loop continues to rotate,the induced voltage againdecreases to zero when theconductors become parallel to themagnetic lines of flux. Notice that during this 3600rotation of the loop the polarity ofvoltage remained the same forboth halves of the waveform. Thisis called rectified DC voltage. The voltage is pulsating. It doesturn on and off, but it neverreverses polarity. Since thepolarity for each brush remainsconstant, the output voltage isDC.00Position (DC Neutral Plane)
THE ELEMENTARY DC GENERATOR Observes– The meterdirection– Theconductorsof thearmatureloop– Direction ofthe currentflowEffects of commutationhttp://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm
Effects of additional turns To increase the amount ofoutput voltage, it is commonpractice to increase thenumber of turns of wire foreach loop. If a loop contains 20 turns ofwire, the induced voltage willbe 20 times greater than thatfor a single-loop conductor. The reason for this is thateach loop is connected inseries with the other loops.Since the loops form a seriespath, the voltage induced inthe loops will add. In this example, if each loophas an induced voltage of 2V,the total voltage for thiswinding would be 40V(2V x 20 loops = 40 V).Effects of additional turns
Effects of additional coils When more than one loop is used, theaverage output voltage is higher and there isless pulsation of the rectified voltage. Since there are four segments in thecommutator, a new segment passes eachbrush every 900instead of every 1800. Since there are now four commutatorsegments in the commutator and only twobrushes, the voltage cannot fall any lowerthan at point A. Therefore, the ripple is limited to the rise andfall between points A and B on the graph. Byadding more armature coils, the ripple effectcan be further reduced. Decreasing ripplein this way increases the effective voltageof the output.Effects of additional coils
The Practical DC Generator The actual construction and operation of apractical dc generator differs somewhatfrom our elementary generators Nearly all practical generators useelectromagnetic poles instead of thepermanent magnets used in ourelementary generator The main advantages of usingelectromagnetic poles are:(1) increased field strength and(2) possible to control the strength of thefields. By varying the input voltage, thefield strength is varied. By varying the fieldstrength, the output voltage of thegenerator can be controlled.Four-pole generator (without armature)
Notes !!! If you ever have the chance to take apart a small electric motor, you willfind that it contains the same pieces described above: two small permanentmagnets, a commutator, two brushes, and an electromagnet made bywinding wire around a piece of metal. Almost always, however, the rotor willhave three poles rather than the two poles as shown in this article. Thereare two good reasons for a motor to have three poles:– It causes the motorIt causes the motor to have better dynamicsto have better dynamics.. In a two-pole motor, if theIn a two-pole motor, if theelectromagnet is at the balance point, perfectly horizontal between the two poleselectromagnet is at the balance point, perfectly horizontal between the two polesof the field magnet when the motor starts, you can imagine theof the field magnet when the motor starts, you can imagine the armature gettingarmature getting"stuck" there. That never happens in a three-pole motor."stuck" there. That never happens in a three-pole motor.– Each time the commutator hits the point where it flips the field in a two-poleEach time the commutator hits the point where it flips the field in a two-polemotor,motor, the commutator shorts out the batterythe commutator shorts out the battery ((directly connects the positive anddirectly connects the positive andnegative terminals) for a moment.negative terminals) for a moment. This shorting wastes energy and drains theThis shorting wastes energy and drains thebattery needlesslybattery needlessly.. A three-pole motor solves this problem as well.A three-pole motor solves this problem as well. It is possible to have any number of poles, depending on the size of themotor and the specific application it is being used in.