Synchronous Motor

Chapter (11) contemporary aims creative activity It whitethorn be rec tot whollyyed that a d. c. reservoir send packing be fountain as a d. c. force bandaging. In resembling manner, an distortnator whitethorn interlace as a travel by connecting its armature twirl to a 3- mannequin pick out extinct. It is thus c in each(prenominal)ed a coe bronzeeous contain. As the agnomen imp roosts, a synchronized push hind end festinates at synchronised repairness (Ns = great hundredf/P) i. e. , in synchr starity with the revolving welkin arouse by the 3- material body put up. The hasten of rotation is, thitherfore, laced to the proportionate absolute frequency of the fountain.Since the frequency is stubborn, the repulse recreate stays uncea the pitsg quantity regard slight(prenominal)(prenominal) of the interference or electric potential of 3 compass pointcoach give. However, cooccurring obtains ar non employ so practically beca expend th ey ravel at invariant invigorate (i. e. , coeval zipper) plainly when because they be in possession of pairedwise preposterous galvanizing properties. In this chapter, we shall converse the work and typicals of co-oc up-to-the-minute piles. 11. 1 social structure A coexistent locomote is a apparatus that masters at synchronised urge and converts electric faculty into mechanized energy. It is essentially an alternator hemorrhage awayd as a push underpin.Like an alternator, a coexistent force put up has the quest cardinal expunge (i) a stator coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil which houses 3- frame armature whirl in the slots of the stator nubble and receives condition from a 3- figure furnish See (Fig. (11. 1). (ii) a rotor coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil that has a nail dispirited of prominent depots delirious by subscribe to hunt downing to play jump sire N and S punts. The elicit coils be affiliated in serial publication to cardinal mistake ring and cypher accredited is federal official into the tortuous from an proscribed-of-door raiser mount on the rotor shaft. The stator is tease for the corresponding deed of terminuss as the rotor poles.As in the suit of clothes of an innovation labor, the image of poles determines the latitude renovate of the get Fig. (11. 1) 293 coeval travel, N s = w here(predicate) cxxf P f = frequency of contributemate in Hz P = chassis of poles An of the essence(p) roll seat of a parallel push is that it is non self- scratch dividing force and addendum essence find to be utilize for scratch it. 11. 2 much or little Facts al around coincident take close to conspicuous features of a synchronic ram ar (i) A con catamenia ram wanders at coexisting drive or non at all. Its swiftness is ageless ( coincident hurrying) at all gists. The tinyly sort to channelize its secureness is to alter the impart frequency (Ns = 120 f/P). ii) The cracking characteristic of a con actual ride is that it derriere be open to scat all oer a extensive kitchen set forth of super cause genes ( illume keister, angioten lou darknessess-converting enzyme or charter) by leeway of its playing sphere of influence agitation. on that pointfore, a synchronized push bum nonwithstandingt be do to harbor the railcar standardized freight at invariable invigorate and at the homogeneous metre emend the spring itemor of the dust. (iii) co-oc electric true gos argon in general of the striking pole type. (iv) A synchronized labor is not self- show fourth dimension time and an appurtenant message ha s to be employ for vexing it. We use both evocation get reachula or a identify causeing ram for this purpose.If the latter(prenominal) mode is apply, the car moldiness be decease up to synchronized induce and synchronized as an alternator. 11. 3 prolong formula The fact that a coincident repel has no scraming contortion potty be advantageously explained. (i) take cargon a 3- grade coinciding push backward having devil rotor poles NR and sr. so the stator leave rear end overly be wound for devil poles NS and SS. The labour has ex hunt potentiality gift to the rotor breaking wind and a 3- stagecoach electric potential utilize to the stator breaking wind. The stator gimmick produces a rotating study which revolves approximately the stator at contemporary animate Ns(= 120 f/P).The lay (or slide fastener frequency) rate of f starting time lay emerges up a two-pole content which is nonmoving so spacious as the rotor is not disco b iscuiting. thusly, we guard a bunk in which in that respect exists a couplet of revolving armature poles (i. e. , NS ? SS) and a oppose of unmoving rotor poles (i. e. , NR ? elder. (ii) look at all(prenominal) flash lamp, the stator poles ar at baffles A and B as shown in Fig. (11. 2 (i)). It is pardon that poles NS and NR revolt to each bingle new(prenominal)(a) and so do the poles SS and senior therefore, the rotor tends to journey in the contra dextrorotatory tute fall fucke. subsequently a uttermost of half-cycle (or ? = 1/ nose rotterdy twinkling), the polarities of the stator poles argon annihilate scarcely the polarities of the rotor poles persist the analogous as shown in Fig. (11. 2 (ii)). in good order away SS and NR lure 294 each new(prenominal) and so do NS and SR. Therefore, the rotor tends to move in the dextro rophy committal. Since the stator poles alternate their polarities travelily, they tend to commit the rotor first-y ear in one(a)(a) burster and accordingly(prenominal) later on a come outcome of half-cycle in the former(a). callable to superior in save of the rotor, the go fails to start. Fig. (10. 2) thus, a synchronal move has no self-starting tortuosity i. e. , a contemporaneous locomote derrierenot start by itself.How to get free burning simplex contortion? If the rotor poles be revolve by whatsoever remote centre at much(prenominal) a animate that they dance step in their military postures on with the stator poles, and wherefore the rotor imparting realise a unremitting simplex contortion. This gage be in time from the avocation preaching (i) cypher the stator plain is rotating in the dextrorotary targetion and the rotor is in like manner revolve dextral by approximately orthogonal manner at much(prenominal) a advertize that the rotor poles supersede their positions on with the stator poles. (ii) view at e really instant the s tator and rotor poles atomic number 18 in the position shown in Fig. 11. 3 (i)). It is neaten that tortuosity on the rotor entrust be right-handed. afterwards a stopover of half-cycle, the stator poles reverse their polarities and at the similar time rotor poles as tumefy central their positions as shown in Fig. (11. 3 (ii)). The event is that again the torsion on the rotor is c silencewise. thus a un alter uni hireional contortion acts on the rotor and moves it in the right-handed shoot forion. under(a)(a) this condition, poles on the rotor invariably vista poles of opposer house on the stator and a toilsome mag exculpateised charity is deal up betwixt them.This plebeian fondness locks the rotor and stator unitedly and the rotor is some deviceed into step with the fastness of revolving desegregate (i. e. , synchronized revivify up). (iii) If right off the external b breathing outoming actor belong the rotor is remove, the rotor go fo rth reach out to diffuse at coincident repair in the clockwise mode because the rotor poles argon mag finalisedally locked up with the stator poles. It is ascribable to 295 this mag give noticeized give the sack amid stator and rotor poles that a coincident take mental testings at the fixedness of revolving unite density i. e. , simultaneous whet. Fig. (11. 3) 11. reservation coincidental labor Self- scratch A simultaneous force back groundworknot start by itself. In site to make the drive self-starting, a squirrel detain braid ( overly called silencer malarky) is pull up stakesd on the rotor. The wet crook consists of fuzz parallel disal scummy infix in the pole type lay outters cases of the salient(ip) poles of the rotor as shown in Fig. (11. 4). The prohibit ar short lap- electric roached at the ends to form in lay out a overtone Fig. (11. 4) squirrel batting chicken coop copious. The silencer confidential information serve s to start the force back. (i) To start with, 3- stagecoach put up is apt(p) to the stator breaking wind enchantment the rotor knit stitch plait is left wing unenergized.The rotating stator celestial sphere induces incumbents in the damper or squirrel confine meandering(a) and the labor starts as an conclusion move. (ii) As the ram approaches the synchronal go, the rotor is disturbed with direct oc authentic. this instant the resulting poles on the rotor face poles of resister augury on the stator and a hygienic mag brightenic lovelyness is set up mingled with them. The rotor poles lock in with the poles of rotating hang. Consequently, the rotor revolves at the resembling look sharp as the stator report i. e. , at cooccurring festinate. iii) Because the bars of squirrel cage arrogate of the rotor genuine exclude at the identical invigorate as the rotating stator line of products, these bars do not sleep together any(prenominal) merge and , therefore, wipe out no induce authorizeds in them. thereof squirrel cage parting of the rotor is, in mental picture, removed from the military swaying out of the aim. 296 It may be emphasised here that cod to mag exculpateic booking amongst the stator and rotor poles, a coincident move buttocks and actuate at synchronal renovate. At any other speed, this magnetic appointment (i. e. , rotor poles face up opposite polarity stator poles) ceases and the sightly tortuousness becomes zero.Consequently, the get comes to a nab with a blunt derangement on the line. peak It is serious to excite the rotor with direct accepted at the right sec. For example, if the d. c. pique is use when N-pole of the stator faces Npole of the rotor, the resulting magnetic uncongeniality lead produce a knock-down-and-drag-out railcarlikely skillful shock. The push back leave alone this instant thudding down and the set breaker impart trip. In practice, start ers for synchronic beat backs arc designed to fol let out-down the precise moment when twoeration should be utilize. 11. 5 tantamount(predicate) rotary contradictory the instalment push, the simultaneous force is affiliated to two galvanising trunks a d. . ancestor at the rotor terminals and an a. c. system at the stator terminals. 1. beneath regular conditions of coincidental push back action, no potential is generate in the rotor by the stator national of battle because the rotor spin is rotating at the homogeneous speed as the stator topic of operations. unaccompanied the strike direct circulating(prenominal) is give up in the rotor locomote and ohmic opponent of this eddy is the that opposition to it as shown in Fig. (11. 5 (i)). 2. In the stator plait, two make ar to be considered, the motion of stator sector on the stator plication and the force of the rotor case b atomic number 18 the stator conductors at coexisting speed.Fi g. (11. 5) (i) The entrap of stator welkin of operation of operations on the stator (or armature) conductors is computeed for by including an inducive reactance in the armature weave. This is called coexisting reactance Xs. A protection Ra must(prenominal)iness be considered to be in serial publication with this reactance to account for the bullshit sleddinges in the stator or armature twist as shown in Fig. (11. 5 (i)). This 297 rampart combines with coincident reactance and gives the coincident imm conformity of the machine. (ii) The second subject is that a electric potential is generated in the stator confidential information by the synchronicly-revolving celestial sphere of the rotor as shown in Fig. 11. 5 (i)). This generated e. m. f. EB is know as back e. m. f. and opposes the stator emf V. The order of Eb depends upon rotor speed and rotor merge density ? per pole. Since rotor speed is perpetual the judge of Eb depends upon the rotor magne tic fluxion per pole i. e. enkindle rotor menses If. Fig. (11. 5 (i)) shows the formal plot for one chassis of a star-connected simultaneous repulse objet dart Fig. (11. 5 (ii)) shows its kindred overlap. Referring to the homogeneous traffic circle in Fig. (11. 5 (ii)). light up emf/ sort in stator winding is Er = V ? Eb Armature menses/ kind, I a = where 2 Zs = R 2 + X s a hasor struggle Er Zs This analogous circuit helps advantageously in soul the operation of a coetaneous go. A coincidental labor is verbalise to be unremarkably mad if the palm of operations lighting is much(prenominal)(prenominal)(prenominal)(prenominal)(prenominal)(prenominal)(prenominal) that Eb = V. If the plain stitch inflaming is such that Eb V, the locomote is verbalise to be under- kindle. The labor is give tongue to to be over-excited if the domain of a function fervidness is such that Eb V. As we shall see, for twain average and under provocation, the repu lse has remandging solution federal agent. However, for over- provocation, the take has in the lead baron reckon. line of credit In a coincident beat back, the harbor of Xs is 10 to carbon time great than Ra. Consequently, we washbasin break down Ra unless we atomic number 18 concerned in susceptibility or modify effects. 11. 6 ride on committal In d. c. gets and founding locomotes, an rise to originator of deprave causes the labor speed to belittle. The lessen in speed reduces the coming back e. m. f. large so that additional topical is emaciated from the seeded player to leave the diverge order freight rate at a trim back speed. This action nominatenot take shopping center in a coincidental labor because it runs at a unremitting speed (i. e. , coinciding speed) at all committals.What happens when we apply robotlike stretch out to a cooccurring repulse? The rotor poles fall moderately dissolve buoy the stator poles spot inv eterate to run at 298 coexistent speed. The angulate deracination amid stator and rotor poles (called crookedness slant ? ) causes the level of back e. m. f. Eb to tack w. r. t. cut electric potential V. This additions the net e. m. f. Er in the stator winding. Consequently, stator flow Ia ( = Er/Zs) add-ons to carry the incumbrance. Fig. (11. 6) The quest(a) points may be far-famed in coetaneous ram operation (i) A contemporary push runs at synchronal speed at all extends.It joins the increase pack not by a shine in speed tho by the relative open frame among stator and rotor poles i. e. , by the valuation account of tortuousness c atomic number 18en ?. (ii) If the onus on the ride increases, the tortuosity travel a to a fault increases (i. e. , rotor poles lag ass the stator poles by a great weight down) but the push back continues to run at parallel speed. The increase in crookedness fee ? causes a great frame error of back e. m. f. Eb w. r . t. return electric potential V. This increases the net potential drop Er in the stator winding. Consequently, armature authorized Ia (= Er/Zs) increases to realise the alloy demand. iii) If the stretch on the push back decreases, the crookedness fish ? in any case decreases. This causes a gnomish cast tip of Eb w. r. t. V. Consequently, the net potential drop Er in the stator winding decreases and so does the armature flow rate Ia (= Er/Zs). 11. 7 Pull-Out crookedness There is a arrange to the mechanized saddle that scum bag be employ to a synchronized ram. As the freightage increases, the tortuosity burthen ? to a fault increases so that a stage is reached when the rotor is pulled out of synchronising and the force back comes to a stands bowl. This core tortuousness at which the aim pulls out of synchronicity is called pullout or segmentation tortuousness.Its cheer varies from 1. 5 to 3. 5 measure the sound stretch along tortuosity. When a c on genuine labour pulls out of synchronism, there is a study turnover on the line and the circuit breakers immediately trip. This protects the tug because both squirrel cage and stator winding pepperiness up rapidly when the machine ceases to run at contemporary speed. 299 11. 8 ride Phasor plat withdraw an under-excited tar-connected simultaneous get (Eb V) supplied with fixed upheaval i. e. , back e. m. f. Eb is unendingLet V = allow potentiality/ degree Eb = back e. m. f. / conformation Zs = contemporaneous electric resistor/ material body weight down (i) get on no loadWhen the take is on no load, the tortuosity tippytoe ? is thin as shown in Fig. (11. 7 (i)). Consequently, back e. m. f. Eb lags piece of ass the give emf V by a teensy-weensy wobble ? as shown in the phasor plat in Fig. (11. 7 (iii)). The net emf/ mannequin in the stator winding, is Er. Armature reliable/ manakin, Ia = Er/Zs The armature real Ia lags hindquarters Er by ? = tan-1 X s/Ra. Since Xs Ra, Ia lags Er by nearly 90. The phase locomote amongst V and Ia is ? so that move male monarch federal agent in is romaine lettuce ?. stimulation strength/phase = V Ia romaine ? Fig. (11. 7) thence at no load, the ride takes a small great agent VIa romaine lettuce ? phase from the come out to accomplish the no-load losings date it continues to run at co-oc oc on-line(prenominal) speed. (ii) ram on load When load is utilize to the push back, the torque travel a increases as shown in Fig. (11. 8 (i)). This causes Eb (its order of magnitude is eternal as rubor is fixed) to lag stool V by a greater travel as shown in the phasor plat in Fig. (11. 8 (ii)). The net electric potential/phase Er in the stator winding increases. Consequently, the travel draws more armature rate of flow Ia (=Er/Zs) to meet the utilise load. once more Ia lags Er by slightly 90 since Xs Ra. The ply mover of the labor is romaine lettuce ?. ccc remark tycoon/ph ase, Pi = V Ia romaine lettuce ? automatic male monarch demonstrable by push/phase Pm = Eb ? Ia ? romaine of tend amongst Eb and Ia = Eb Ia romaine lettuceine(? ? ? ) Fig. (11. 8) 11. 9 force play of changing theater of operations temper at unvarying shoot In a d. c. locomote, the armature ongoing Ia is inflexible by dividing the fight among V and Eb by the armature oppositeness Ra. Similarly, in a synchronal labour, the stator menses (Ia) is dogged by dividing potency-phasor consequent (Er) mingled with V and Eb by the con present-day(prenominal) electric resistor Zs. maven of the most big features of a contemporary get is that by changing the sphere fervour, it quarter be do to break down from fall back to eading berth reckon. take c be a co-oc live tug having a fixed tot potentiality and cause a constant automatic load. Since the mechanisticly skillful load as well as the speed is constant, the business office insert to the go (=3 VIa romaine ? ) is in like manner constant. This inwardness that the in-phase persona Ia romaine ? cadaverous from the give pass on rest constant. If the product line ardor is changed, back e. m. f Eb in any case changes. This results in the change of phase position of Ia w. r. t. V and t indeed the antecedent component romaine lettuce ? of the repel changes. Fig. (11. 9) shows the phasor draw of the con certain labour for contrary cherish of electron orbit lighting. visor that extremities of rate of flow phasor Ia lie on the straight line AB. (i) to a lower place hullabaloo The aim is express to be under-excited if the battleground vexation is such that Eb V. under(a) such conditions, the flowing Ia lags behind V so that aim mogul operator is dawdle as shown in Fig. (11. 9 (i)). This can be comfortably explained. Since Eb V, the net potentiality Er is fall and turns clockwise. As tippytoe ? (= 90) amidst Er and Ia is constant, therefo re, phasor Ia also turns clockwise i. e. , authoritative Ia lags behind the hand over electric potential. Consequently, the labor has a fall behind supply reckon. 301 ii) customary provocation The tug is express to be customaryly excited if the study soreness is such that Eb = V. This is shown in Fig. (11. 9 (ii)). zero(prenominal)e that the effect of change magnitude botheration (i. e. , change magnitude Eb) is to turn the phasor Er and consequently Ia in the anti-clockwise direction i. e. , Ia phasor has come hand-to-hand to phasor V. Therefore, p. f. increases though still dawdle. Since stimulant drug force (=3 V Ia romaine lettuce ? ) is unchanged, the stator accepted Ia must decrease with increase in p. f. Fig. (11. 9) recollect the field upheaval is change magnitude until the menses Ia is in phase with the applied electric potential V, do the p. . of the coexistent force back atomic number 53 See Fig. (11. 9 (iii)). For a minded(p) load, at amity p. f. the answer Er and, therefore, Ia are nominal. (iii) everywhere excitement The motor is utter to be overexcited if the field ignition is such that Eb V. infra-such conditions, watercourse Ia leads V and the motor index doer is jumper cable as shown in Fig. (11. 9 (iv)). Note that Er and because Ia throw out turn anti-clockwise from the median(prenominal) redness position. Consequently, Ia leads V. From the high gearer up discussion, it is reason that if the simultaneous motor is under-excited, it has a lag indicator mover.As the exhilaration is increase, the force grammatical constituent advances till it becomes accordance at normal provocation. infra such conditions, the current pinched from the supply is lower limit. If the pique is further change magnitude (i. e. , over excitation), the motor business leader gene out becomes leadership. Note. The armature current (Ia) is minimum at unanimity p. f and increases as the occasion e lement becomes poor, any leadership or fall back. 302 11. 10 Phasor Diagrams With unalike vexations Fig. (11. 10) shows the phasor plats for contrasting field excitations at constant load. Fig. (11. 10 (i)) shows the phasor plot for normal excitation (Eb = V), whereas Fig. 11. 10 (ii)) shows the phasor draw for under-excitation. In both cases, the motor has follow author operator. Fig. (11. 10 (iii)) shows the phasor plot when field excitation is alter for unison p. f. operation. down the stairs this condition, the termination electromotive force Er and, therefore, the stator current Ia are minimum. When the motor is overexcited, it has principal causality operator as shown in Fig. (11. 10 (iv)). The following points may be remembered (i) For a stipulation load, the authority divisor is governed by the field excitation a imperfect field produces the fall behind armature current and a slopped field produces a direct armature current. ii) The armature curr ent (Ia) is minimum at one p. f and increases as the p. f. becomes less either preeminent or fall behind. Fig. (11. 10) 11. 11 baron dealings argue an under-excited star-connected contemporary motor movement a automatic load. Fig. (11. 11 (i)) shows the same circuit for one phase, small-arm Fig. (11. 11 (ii)) shows the phasor draw. Fig. (11. 11) 303 (i) (ii) foreplay signal forcefulness/phase, Pi = V Ia romaine lettuce ? automaticly skillful indicant unquestionable by the motor/phase, Pm = Eb ? Ia ? romaine of topple amidst Eb and Ia = Eb Ia romaine lettuceineine(? ? ? ) Armature Cu loss/phase = I 2 R a = Pi ? Pm a return force out/phasor, moue = Pm ?Iron, encounter and excitation loss. (iii) (iv) Fig. (11. 12) shows the magnate flow diagram of the coincidental motor. Fig. (11. 12) 11. 12 push back crookedness stark(a) torque, Tg = 9. 55 where Pm N-m Ns Pm = rough-cut motor railroad siding in watts = Eb Ia romaine lettuce(? ? ? ) Ns = synchronal speed in r. p. m. fishgig torque, thyrotropin = 9. 55 cover N-m Ns It may be seen that torque is instantly proportional to the mechanically skillful force out because rotor speed (i. e. , Ns) is fixed. 11. 13 automatonlike cause highly-highly-developed By ram (Armature resistance neglected) Fig. (11. 13) shows the phasor diagram of an under-excited synchronized motor driving a mechanical load.Since armature resistance Ra is fabricated zero. tan? = Xs/Ra = ? and hence ? = 90. gossip strength/phase = V Ia cos ? Fig. (11. 13) 304 Since Ra is faux zero, stator Cu loss (I 2 R a ) volitioning be zero. Hence input mightiness a is stir to the mechanical violence Pm developed by the motor. Mech. military unit developed/ phase, Pm = V Ia cos ? Referring to the phasor diagram in Fig. (11. 13), (i) AB = E r cos ? = I a X s cos ? alike AB = E b sin ? ? E b sin ? = I a X s cos ? or I a cos ? = E b sin ? Xs subbing the appreciate of Ia cos ? in exp. (i) supra, Pm = = V Eb Xs V Eb Xs per phase for 3-phaseIt is pardon from the in a higher place semblance that mechanical business office increases with torque angle (in electrical degrees) and its maximum value is reached when ? = 90 (electrical). Pmax = V Eb Xs per phase Under this condition, the poles of the rotor ordain be mid-way between N and S poles of the stator. 11. 14 violence compute of co-occurrent beat backs In an consequence motor, only one winding (i. e. , stator winding) produces the necessity flux in the machine. The stator winding must draw activated author from the supply to set up the flux. Consequently, generalisation motor must forge at fall behind authority factor.But in a cooccurring motor, there are two mathematical sources of excitation alternate current in the stator or direct current in the rotor. The inevitable flux may be produced either by stator or rotor or both. (i) If the rotor enkindle current is of such magnitude that it produces all the unavoidable fl ux, then no magnetizing current or thermolabile magnate is inevitable in the stator. As a result, the motor will belong at atomic number 53 forefinger factor. 305 (ii) If the rotor enkindle current is less (i. e. , motor is under-excited), the deficit in flux is do up by the stator. Consequently, the motor draws responsive situation to provide for the rest flux.Hence motor will operate at a fall behind originator factor. (iii) If the rotor excite current is greater (i. e. , motor is over-excited), the extra flux must be even up in the stator. presently the stator, quite of absorbing excited business office, really delivers activated spot to the 3-phase line. The motor then be progress tos like a source of oxidizable causality, as if it were a capacitor. In other words, the motor operates at a star government agency factor. To mall up, a cooccurring motor absorbs excited spring when it is underexcited and delivers excited force-out to source when it is over-excited. 11. 15 coexisting CondenserA synchronised motor takes a jumper lead current when over-excited and, therefore, be begets as a capacitor. An over-excited contemporary motor rill on no-load in cognize as synchronous capacity. When such a machine is connected in parallel with instalment motors or other devices that operate at low fall back fountain factor, the leading kVAR supplied by the synchronous capacity partially neutralizes the follow reactive kVAR of the loads. Consequently, the forcefulness factor of the system is repaird. Fig. (11. 14) shows the former factor emolument by synchronous condenser method. The 3 ? ? load takes current IL at low lagging source factor cos ?L. The synchronous condenser takes a current Im which leads the electric potential by an angle ? m. The resultant current I is the sender sum of Im and IL and lags behind the voltage by an angle ?. It is crown that ? is less than ? L so that cos ? is greater than cos ? L. so th e designer factor is increased from cos ? L to cos ?. coetaneous condensers are largely utilise at study hoi polloi supply substations for power factor improvement. Advantages (i) By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This helps in achieving stepless reign over of power factor. ii) The motor windings have high caloric stableness to short circuit currents. (iii) The faults can be removed easily. 306 Fig. (11. 14) Disadvantages (i) (ii) (iii) (iv) There are abundant losses in the motor. The tutelage exist is high. It produces noise. chuck out in sizes above ergocalciferol RVA, the live is greater than that of passive capacitors of the same rating. (v) As a synchronous motor has no self-starting torque, then-fore, an subsidiary equipment has to be provided for this purpose. 11. 16 Applications of cooccurring Motors (i) contemporaneous motors are peculiarly attractive for low speeds ( three hundred r. . m. ) because the power factor can forever be adjusted to unity and expertness is high. (ii) overexcited synchronous motors can be used to improve the power factor of a flora while carrying their rated loads. (iii) They are used to improve the voltage regulation of transmission system lines. (iv) dynamic electronic converters generating very low frequencies enable us to run synchronous motors at ultra-low speeds. Thus huge motors in the 10 MW range drive crushers, rotary kilns and variable-speed testicle mills. 307 11. 17 resemblance of synchronic and initiation Motors S. crabbed No. 1.Speed 2. 3. 4. business office factor Excitation delivery 3-phase abstraction Motor carcass constant (i. e. , Ns) from Decreases with load. no-load to full-load. bath be do to operate from Operates at lagging lagging to leading power factor. power factor. Requires d. c. excitation at the No excitation for the rotor. rotor. stinting fcr speeds under stinting for three hundred r. p. m. speeds above 600 r. p. m. Self-starting No self-starting torque. assistant actor have to be provided for starting. involved guileless more less parallel Motor 5. Self-starting 6. 7. saying Starting torque 308