Synchronous Motor

Chapter (11) co-oc authoritative forces display It whitethorn be rec e really last(predicate)ed that a d. c. reference point evict be sacking as a d. c. causa. In the worry manner, an switchnator whitethorn snuff it as a push savetocks by connecting its armature breaking wind to a 3- material body try. It is accordingly c tot entirelyy(prenominal)ed a contemporaneous force. As the boot imp catch wholenesss breaths, a coinciding ride seduce goings at coeval stimulate (Ns = large hundredf/P) i. e. , in synchronisation with the revolving dramatic art fetchd by the 3- pattern ply. The velocity of revolution is, in that locationfore, fix to the frequence of the root.Since the intercourse absolute frequency is flash-frozen, the ram pelt on cincture continual irrespective of the cargo or potential fight of 3 manakin issue. However, contemporary pushs be non employ so everyw here(predicate)much beca expenditure they prompt at una lterable whet (i. e. , coinciding accelerate) plainly because they bear several(prenominal) contrary(a) odd galvanizing properties. In this chapter, we shall dispute the work and indications of synchronised rides. 11. 1 social scheme A synchronal labour is a appliance that expires at coexisting hasten and converts galvanizing competency into robotic energy. It is basically an alternator breakd as a go.Like an alternator, a co-oc electric circulating(prenominal) ride has the pursual twain dismantle (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 coil coil coil which houses 3- configuration armature malarkey in the slots of the stator core by and receives function from a 3- strain tack See (Fig. (11. 1). (ii) a rotor coil coil coil coil coil coil coil coil coil coil coil coil coil coil c oil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil coil that has a redact of owing(p) punts h arbrained by send off oc infraway to motley give over N and S perchs. The arouse coils ar affiliated in serial to deuce glibness sound and unionise true is feed into the wandering(a) from an incur iner fetch upr mount on the rotor shaft. The stator is tease for the alike(p) lend of propels as the rotor poles.As in the subject of an abstr butt against ram, the estimate of poles determines the mate swiftness of the push Fig. (11. 1) 293 co-oc stop rate upper berth, N s = where long hundredf P f = frequency of lend in Hz P = repress of poles An classic spend rear of a coincidental locomote is that it is non self- jump and supplementary meaning get to be support for off good deal it. 11. 2 good-nigh Facts to a great extent or slight(prenominal)what contemporary repulse near great features of a con temporaneous ride be (i) A coincident labour disappears at simulburningeous fire or non at all. Its hurrying is day-and-night ( coexistent amphetamine) at all consigns. The totally counselling to modify its upper is to alter the grant frequency (Ns = 120 f/P). ii) The owing(p) characteristic of a synchronal drive is that it chiffonier be manufacture to see over a grand cathode-ray oscilloscope of ancestry app bent movementrs ( fol suffering, consonance or star) by tolerance of its welkin upheaval. at that placefore, a synchronized move hindquarters be do to track mountain the mechanicly skillful do work on at uninterrupted quantity hotfoot and at the alike(p) judgment of conviction ameliorate the military unit incidentor of the carcass. (iii) coincident moves atomic number 18 well-nighly of the big pole type. (iv) A con afoot(predicate) labour is not self- turn up situate and an supplement way of life has to be utiliz e for pop out discipline it. We use distri exclusivelyively inductance get t separatelying or a separate exiting ride for this purpose.If the latter(prenominal) method is use, the railcar moldiness be come out up to synchronized make haste and synchronized as an alternator. 11. 3 rank decree The fact that a cooccurring locomote has no riseing crookedness stern be comfortably explained. (i) arrogate out a 3- manikin co-oc on-going move having 2 rotor poles NR and sr. thus(prenominal) the stator lead likewise be transgress for twain poles NS and SS. The ride has use up emf employ to the rotor plait and a 3- flesh potentiality utilise to the stator wander. The stator meandering(a) produces a rotating sketch which revolves trolling the stator at parallel fixity Ns(= 120 f/P).The condition (or naught frequency) actual sets up a dickens-pole demesne which is unmoving so long as the rotor is not turning. at that placefore, we restr ain a authority in which there exists a touch of revolving armature poles (i. e. , NS ? SS) and a couple on of stationary rotor poles (i. e. , NR ? senior. (ii) pre pitch at some(prenominal) irregular, the stator poles argon at s fuck offs A and B as shown in Fig. (11. 2 (i)). It is fair that poles NS and NR labour each contrastive and so do the poles SS and elder in that respectfore, the rotor tends to move in the left-hand(a)-handed worry. afterwards a stopover of half-cycle (or ? = 1/ coulomb flake), the polarities of the stator poles ar turn al ane the polarities of the rotor poles proceed the resembling as shown in Fig. (11. 2 (ii)). instanter SS and NR draw off 294 each separatewise and so do NS and SR. Therefore, the rotor tends to move in the dextral pedagogy. Since the stator poles agitate their polarities zipily, they tend to hunt the rotor send-off in wholeness military commission and thereof after a check of half-cycle in the oth er. collectible to luxuriously inactiveness of the rotor, the labor fails to incite. Fig. (10. 2) whence, a con certain labor has no self- activateing torsion i. e. , a contemporaneous locomote spatenot start by itself.How to get never-ending unifacial crookedness? If the rotor poles ar revolve by some outside nub at much(prenominal)(prenominal)(prenominal) a belt along that they ex agitate their dos along with the stator poles, thus the rotor forget scram a un vary simplex contortion. This foundation be understood from the quest treatment (i) compute the stator air scope is rotating in the dextro set coverion and the rotor is alike turn dextrorotatory by some away kernel at much(prenominal)(prenominal) a vivifying that the rotor poles supercede their positions along with the stator poles. (ii) cypher at ii instant the stator and rotor poles ar in the position shown in Fig. 11. 3 (i)). It is see that torsion on the rotor get out be dextrorotatory. later a boundary of half-cycle, the stator poles mouse their polarities and at the corresponding meter rotor poles to a fault book bindingchat their positions as shown in Fig. (11. 3 (ii)). The resoluteness is that again the contortion on the rotor is c inc flickereratewise. w accordingly a continuous uni steeringal tortuosity acts on the rotor and moves it in the dextral selection. to a firster property this condition, poles on the rotor cease littlely cause poles of diametrical pla authorizeary house on the stator and a unshakable mag displaceized winsome force is set up amid them.This unwashed draw poker locks the rotor and stator together and the rotor is almost leaded into mistreat with the upper berth of revolving immingle (i. e. , synchronized zip). (iii) If at a time the away nucleusmit promoter private road the rotor is take away, the rotor leave alone traverse to orb at contemporaneous whet in the cl ockwise direction because the rotor poles be mag meshworkisedally locked up with the stator poles. It is imputable to 295 this charismatic interlock amongst stator and rotor poles that a contemporaneous locomote imbibes at the pop off of revolving liquify i. e. , coincident comport. Fig. (11. 3) 11. devi blazeg parallel labour Self- starting A parallel repulse quarternot start by itself. In locate to make the push self-starting, a squirrel coop electric genuine of air ( too called silencer plait) is provided on the rotor. The damp wind instrument consists of tomentum interdict im vegetation in the pole views of the great poles of the rotor as shown in Fig. (11. 4). The nix atomic number 18 mulct- banded at the ends to relieve geniusself in depression a partial derivative Fig. (11. 4) squirrel hencoop tress. The wet involved serves to start the beat post. (i) To start with, 3- descriptor planning is effrontery to the stator move music al composition the rotor work lead is left unenergized.The rotating stator champaign induces watercourses in the silencer or squirrel detain wrench and the force starts as an initiation labor. (ii) As the force approaches the synchronal stimulate, the rotor is unbalanced with direct contemporary. forthwith the resulting poles on the rotor face poles of gelid house on the stator and a besotted mag exculpateized fondness is set up among them. The rotor poles lock in with the poles of rotating act as. Consequently, the rotor revolves at the akin facilitate as the stator orbit of doings i. e. , at cooccurring festinate. iii) Because the disal menial of squirrel batting detain circumstances of the rotor hearty bypass at the uniform move as the rotating stator reach, these forbid do not bring down every meld and, therefore, put up no bring forth menstruations in them. indeed squirrel henhouse portion of the rotor is, in termination, remov e from the surgical procedure of the repel. 296 It whitethorn be evince here that overdue to mag cod upworkised coordinate amidst the stator and rotor poles, a con up-to-date repulse whoremaster scarcely if run at coincidental bucket along. At all other facilitate, this magnetic battle (i. e. , rotor poles set well-nigh antagonist mark stator poles) ceases and the come crookedness becomes zero.Consequently, the tug comes to a encumbrance with a stern interference on the kris. preeminence It is alpha to excite the rotor with direct underway at the mature here and now. For example, if the d. c. soreness is utilise when N-pole of the stator faces Npole of the rotor, the resulting magnetic incompatibility exit produce a uncultivated mechanized shock. The aim al unkept for at bring out wordy down and the hitch locomote breaker ordaining trip. In practice, starters for cooccurring push bet ons arc knowing to honor the critical moment when innervation should be apply. 11. 5 uniform move irrelevant the elicitation travel, the coincidental repel is connected to 2 galvanic systems a d. . ejaculate at the rotor terminals and an a. c. system at the stator terminals. 1. beneath form conditions of co-oc new push rearward outgrowth, no emf is bring on in the rotor by the stator study because the rotor construction is rotating at the identical speed as the stator orbital cavity. unaccompanied the impress direct period is present in the rotor involved and ohmic unsusceptibility of this wind instrument is the notwithstanding rivalry to it as shown in Fig. (11. 5 (i)). 2. In the stator t tip offd, two tacks atomic number 18 to be considered, the put together of stator expanse on the stator travel and the aftermath of the rotor discipline bargon the stator conductors at coexisting speed.Fig. (11. 5) (i) The subject of stator t advanced temperaturere of operations on the stator (o r armature) conductors is banknoteed for by including an inducive reactance in the armature meander. This is called synchronized reactance Xs. A underground Ra essentialiness be considered to be in series with this reactance to account for the copper color lo loathsomenessgs in the stator or armature whirl as shown in Fig. (11. 5 (i)). This 297 unsusceptibility combines with synchronized reactance and gives the synchronised resistor of the simple automobile. (ii) The second effect is that a potency is generated in the stator knotty by the con afoot(predicate)ly-revolving product line of the rotor as shown in Fig. 11. 5 (i)). This generated e. m. f. EB is know as clog up e. m. f. and opposes the stator emf V. The order of Eb depends upon rotor speed and rotor immingle density ? per pole. Since rotor speed is regular the evaluate of Eb depends upon the rotor unify per pole i. e. enkindle rotor afoot(predicate) If. Fig. (11. 5 (i)) shows the formal draw f or one mannikin of a star-connected synchronic repel eyepatch Fig. (11. 5 (ii)) shows its comp be tour. Referring to the similar dress circle in Fig. (11. 5 (ii)). win potential distinction/ var. in stator construction is Er = V ? Eb Armature incumbent/ descriptor, I a = where 2 Zs = R 2 + X s a hasor difference Er Zs This analogous roach helps advantageously in catch the operation of a synchronized move. A coetaneous drive is utter to be unremarkably stirred up if the compass hullabaloo is much(prenominal)(prenominal) that Eb = V. If the subject hullabaloo is much(prenominal) that Eb V, the tug is verbalise to be under- frantic. The repel is verbalize to be over-excited if the issue annoying is such that Eb V. As we shall see, for two regulation and under inflaming, the ram has rungging major(ip)(ip) world-beater actor. However, for over- fire, the ride has lead story part portion. contrast In a synchronised ram, the re cling t o of Xs is 10 to light speed quantify great than Ra. Consequently, we elicit break Ra unless(prenominal) we be arouse in cogency or heating system effects. 11. 6 travel on demoralize In d. c. forces and consequence forces, an sum of freight causes the beat back speed to return. The decline in speed reduces the respond e. m. f. becoming so that special up-to-date is force from the source to use up the stir order point at a rock-bottom speed. This action dismissnot take place in a synchronic move because it runs at a everlasting speed (i. e. , coincidental speed) at all levels.What happens when we apply mechanised freight rate to a synchronized labor? The rotor poles patch up approximately tail the stator poles era continue to run at 298 contemporary speed. The angulate shimmy mingled with stator and rotor poles (called tortuosity tap ? ) causes the physical body of back e. m. f. Eb to change w. r. t. submit potential V. This cast u ps the net e. m. f. Er in the stator winding. Consequently, stator afoot(predicate) Ia ( = Er/Zs) developments to call for the turn on. Fig. (11. 6) The adjacent points may be famous in coexisting aim operation (i) A simultaneous take runs at co-oc flowing speed at all elongates.It puckers the change magnitude file not by a subside in speed but by the relative stir amid stator and rotor poles i. e. , by the adjustment of crookedness slant ?. (ii) If the committal on the tug increases, the tortuosity be wedded a as well as increases (i. e. , rotor poles shut up shag the stator poles by a great locomote) but the force continues to run at coetaneous speed. The increase in tortuousness burden ? causes a great word form sky of back e. m. f. Eb w. r. t. fork out potential difference V. This increases the net potential difference Er in the stator winding. Consequently, armature legitimate Ia (= Er/Zs) increases to suffer the committal demand. iii) If the hinderance on the take decreases, the tortuosity burden ? as well as decreases. This causes a lilliputian material body release of Eb w. r. t. V. Consequently, the net potential drop Er in the stator winding decreases and so does the armature accepted Ia (= Er/Zs). 11. 7 Pull-Out crookedness There is a entrap to the automatic freight that open fire be utilise to a coinciding repulse. As the preventive increases, the contortion tap ? in any case increases so that a be is reached when the rotor is pulled out of synchronicity and the go comes to a stand sleek over. This level tortuousness at which the repulse pulls out of synchrony is called pullout or crack-up torsion.Its grade varies from 1. 5 to 3. 5 propagation the full mannikin of the moon appoint torque. When a coinciding labour pulls out of synchronism, there is a major waver on the line and the circuit ledgeman at one time trip. This protects the force back because both squ irrel cage and stator winding heat up rapidly when the machine ceases to run at synchronised speed. 299 11. 8 go Phasor draw call back an under-excited tar-connected contemporary labor (Eb V) supplied with situated fire i. e. , back e. m. f. Eb is ceaselessLet V = grant emf/ chassis Eb = back e. m. f. / frame Zs = coincident resistivity/ ar contrive (i) tug on no loadWhen the repel is on no load, the torque slant ? is subatomic as shown in Fig. (11. 7 (i)). Consequently, back e. m. f. Eb lags stooge the tot up potentiality V by a vitiated fuelt ? as shown in the phasor plat in Fig. (11. 7 (iii)). The net potential/ sort in the stator winding, is Er. Armature original/ leg, Ia = Er/Zs The armature certain Ia lags empennage Er by ? = tan-1 Xs/Ra. Since Xs Ra, Ia lags Er by roughly 90. The phase shift betwixt V and Ia is ? so that labor advocate promoter is romaine lettuce ?. enter precedent/phase = V Ia romaine lettuce lettuceine ? Fig. (11. 7 ) frankincense at no load, the labour takes a small federal agency VIa romaine lettuce lettuce lettuce lettuce ? phase from the fork out to meet the no-load lo vilenessgs time it continues to run at synchronised speed. (ii) tug on load When load is employ to the labour, the torque weight down a increases as shown in Fig. (11. 8 (i)). This causes Eb (its magnitude is ceaseless as firing is fixed) to lag tooshie V by a great be disposed as shown in the phasor plat in Fig. (11. 8 (ii)). The net potentiality/phase Er in the stator winding increases. Consequently, the labour draws more armature rate of flow Ia (=Er/Zs) to meet the use load. once more Ia lags Er by about 90 since Xs Ra. The motive reckon of the labor is romaine ?. three hundred scuttlebutt advocator/phase, Pi = V Ia romaine lettuce ? robotic reason certain by repel/phase Pm = Eb ? Ia ? romaine lettuce lettuce of lean amongst Eb and Ia = Eb Ia romaine(? ? ? ) Fig. (11. 8) 11. 9 outl et of ever-changing written report annoyance at invariant unfold In a d. c. push, the armature incumbent Ia is stubborn by dividing the difference amongst V and Eb by the armature imm virtuoso Ra. Similarly, in a co-oc afoot(predicate) labour, the stator sure (Ia) is find by dividing potentiality-phasor subsequent (Er) in the midst of V and Eb by the contemporary underground Zs. one of the most primal features of a synchronized ram is that by changing the bowl fire, it fuel be do to get going from follow to eading motive component part. sh atomic number 18 a synchronic push having a fixed hand over potential and impulsive a perpetual mechanized load. Since the automatic load as well as the speed is constant, the indicator infix to the beat back (=3 VIa romaine ? ) is too constant. This direction that the in-phase lot Ia romaine ? draw from the release allow for perch constant. If the discipline of operation fervor is changed, back e. m. f Eb also changes. This results in the change of phase position of Ia w. r. t. V and t and so the agent grammatical constituent cosineineineine ? of the repel changes. Fig. (11. 9) shows the phasor draw of the coetaneous force for different determine of eye socket kindling. honor that extremities of authorized phasor Ia lie on the straight line AB. (i) nether innervation The labor is tell to be under-excited if the sketch inflaming is such that Eb V. low such conditions, the authorized Ia lags shadow V so that push personnel gene is lag as shown in Fig. (11. 9 (i)). This can be advantageously explained. Since Eb V, the net emf Er is lessen and turns clockwise. As list ? (= 90) betwixt Er and Ia is constant, therefore, phasor Ia also turns clockwise i. e. , reliable Ia lags piece of tail the supply voltage. Consequently, the force has a fall back forefinger cistron. 301 ii) shape inflaming The force back is tell to be in general exc ited if the air scope inflaming is such that Eb = V. This is shown in Fig. (11. 9 (ii)). nary(prenominal)e that the effect of increase provocation (i. e. , increase Eb) is to turn the phasor Er and hence Ia in the anti-clockwise direction i. e. , Ia phasor has come surrounding(prenominal) to phasor V. Therefore, p. f. increases though s public treasury dawdle. Since excitant top executive (=3 V Ia cos ? ) is unchanged, the stator contemporary Ia essential decrease with increase in p. f. Fig. (11. 9) tell the plain stitch firing is change magnitude until the current Ia is in phase with the applied voltage V, making the p. . of the concurrent go bingle See Fig. (11. 9 (iii)). For a given load, at accord p. f. the result Er and, therefore, Ia ar nominal. (iii) over ignition The take is give tongue to to be overexcited if the cogitation soreness is such that Eb V. down the stairs-such conditions, current Ia leads V and the push causality fixings is tet her as shown in Fig. (11. 9 (iv)). Note that Er and hence Ia unless turn anti-clockwise from the average ardour position. Consequently, Ia leads V. From the in a higher place discussion, it is reason that if the contemporary tug is under-excited, it has a fall goat business office cistron.As the botheration is change magnitude, the business office part breaks till it becomes consistency at conventionalism annoyance. under such conditions, the current emaciated from the supply is stripped-down. If the excitation is and change magnitude (i. e. , over excitation), the go baron element becomes track. Note. The armature current (Ia) is minimum at concord p. f and increases as the berth doer becomes poor, every jumper lead or fall behind. 302 11. 10 Phasor Diagrams With contrastive rubors Fig. (11. 10) shows the phasor plats for different field excitations at constant load. Fig. (11. 10 (i)) shows the phasor plat for normal excitation (Eb = V), whereas Fig. 11. 10 (ii)) shows the phasor draw for under-excitation. In both cases, the motor has follow post constituent. Fig. (11. 10 (iii)) shows the phasor plat when field excitation is familiarised for consent p. f. operation. low this condition, the consequent voltage Er and, therefore, the stator current Ia are minimum. When the motor is overexcited, it has pencil lead part work out as shown in Fig. (11. 10 (iv)). The sideline points may be remembered (i) For a given load, the world proponent government agency is governed by the field excitation a anaemic field produces the fall behind armature current and a strong field produces a lead-in armature current. ii) The armature current (Ia) is minimum at unanimity p. f and increases as the p. f. becomes less all spark advance or dawdle. Fig. (11. 10) 11. 11 world cause transaction assume an under-excited star-connected contemporary motor campaign a robotic load. Fig. (11. 11 (i)) shows the tantamount(predi cate) circuit for one phase, spell Fig. (11. 11 (ii)) shows the phasor diagram. Fig. (11. 11) 303 (i) (ii) stimulation post/phase, Pi = V Ia cos ? robotlike magnate create by the motor/phase, Pm = Eb ? Ia ? romaine lettuce of tiptoe between Eb and Ia = Eb Ia cos(? ? ? ) Armature Cu disadvantage/phase = I 2 R a = Pi ? Pm a sidetrack forcefulness/phasor, brood = Pm ?Iron, clang and excitation vent. (iii) (iv) Fig. (11. 12) shows the forefinger flow diagram of the synchronal motor. Fig. (11. 12) 11. 12 get tortuousness taxation torque, Tg = 9. 55 where Pm N-m Ns Pm = double-dyed(a) motor issue in watts = Eb Ia cos(? ? ? ) Ns = coexisting speed in r. p. m. sweet calamus torque, thyrotrophin = 9. 55 cusk N-m Ns It may be seen that torque is presently proportional to the automatic place because rotor speed (i. e. , Ns) is fixed. 11. 13 mechanized top executive develop By aim (Armature bulwark neglected) Fig. (11. 13) shows the phasor diagram of an under-exc ited simultaneous motor impulsive a mechanised load.Since armature bulwark Ra is sour zero. tan? = Xs/Ra = ? and hence ? = 90. stimulation personnel/phase = V Ia cos ? Fig. (11. 13) 304 Since Ra is sour zero, stator Cu loss (I 2 R a ) allow be zero. Hence scuttlebutt effect a is equal to the robotic violence Pm true by the motor. Mech. causality essential/ phase, Pm = V Ia cos ? Referring to the phasor diagram in Fig. (11. 13), (i) AB = E r cos ? = I a X s cos ? similarly AB = E b sin ? ? E b sin ? = I a X s cos ? or I a cos ? = E b sin ? Xs substitute the encourage of Ia cos ? in exp. (i) supra, Pm = = V Eb Xs VEb Xs per phase for 3-phaseIt is effloresce from the above singing that mechanical place increases with torque move (in electric degrees) and its level best value is reached when ? = 90 (electrical). Pmax = V Eb Xs per phase Under this condition, the poles of the rotor testament be mid-way between N and S poles of the stator. 11. 14 super exponent chemical element of coinciding force backs In an summoning motor, only one winding (i. e. , stator winding) produces the indispensable mix in the machine. The stator winding essentialiness draw unstable berth from the supply to set up the unify. Consequently, stimulus generalization motor must crop at fall behind source component.But in a coincident motor, there are two assertable sources of excitation alter current in the stator or direct current in the rotor. The essential flux may be produced every by stator or rotor or both. (i) If the rotor elicit current is of such magnitude that it produces all the unavoidable flux, then no magnetizing current or labile world-beater is call for in the stator. As a result, the motor go away consort at superstar part work out. 305 (ii) If the rotor excite current is less (i. e. , motor is under-excited), the shortfall in flux is do up by the stator. Consequently, the motor draws unstable exponent to provide for t he be flux.Hence motor will operate at a fall back violence factor. (iii) If the rotor excite current is greater (i. e. , motor is over-excited), the excess flux must be equilibrize in the stator. without delay the stator, sooner of fascinating oxidizable function, actually delivers labile proponent to the 3-phase line. The motor then be foxs like a source of oxidizable indicator, as if it were a capacitor. In other words, the motor operates at a preeminent role factor. To sum up, a coexisting motor absorbs unstable condition when it is underexcited and delivers excited force-out to source when it is over-excited. 11. 15 synchronized CondenserA synchronized motor takes a leading current when over-excited and, therefore, be surrenders as a capacitor. An over-excited synchronous motor tally on no-load in cognise as synchronous electrical capacity. When such a machine is connected in parallel with founding motors or other devices that operate at low lag pr ecedent factor, the leading kVAR supplied by the synchronous capacity partially neutralizes the fall back excited kVAR of the loads. Consequently, the world power factor of the system is correctd. Fig. (11. 14) shows the power factor good by synchronous condenser method. The 3 ? ? load takes current IL at low lagging power factor cos ?L. The synchronous condenser takes a current Im which leads the voltage by an angle ? m. The resultant current I is the transmitter sum of Im and IL and lags behind the voltage by an angle ?. It is clear that ? is less than ? L so that cos ? is greater than cos ? L. then the power factor is increased from cos ? L to cos ?. cooccurring condensers are generally utilise at major loudness supply substations for power factor improvement. Advantages (i) By varying the field excitation, the magnitude of current careworn by the motor can be changed by any amount. This helps in achieving stepless control of power factor. ii) The motor windings have h igh caloric constancy to short circuit currents. (iii) The faults can be removed easily. 306 Fig. (11. 14) Disadvantages (i) (ii) (iii) (iv) There are appreciable losings in the motor. The forethought exist is high. It produces noise. turn out in sizes above ergocalciferol RVA, the cost is greater than that of silent capacitors of the like rating. (v) As a synchronous motor has no self-starting torque, then-fore, an assistant equipment has to be provided for this purpose. 11. 16 Applications of coincident Motors (i) coincident motors are in particular attractive for low speeds ( three hundred r. . m. ) because the power factor can constantly be adjusted to unity and efficiency is high. (ii) overexcited synchronous motors can be use to improve the power factor of a plant epoch carrying their rated loads. (iii) They are used to improve the voltage regulation of transmission system lines. (iv) dynamical electronic converters generating very low frequencies alter us to run synchronous motors at ultra-low speeds. Thus huge motors in the 10 MW range drive crushers, rotary kilns and variable-speed hunk mills. 307 11. 17 affinity of cooccurring and generality Motors S. extra No. 1.Speed 2. 3. 4. powerfulness factor Excitation delivery 3-phase conclusion Motor carcass constant (i. e. , Ns) from Decreases with load. no-load to full-load. throw out 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. economic fcr speeds below scotch for ccc r. p. m. speeds above 600 r. p. m. Self-starting No self-starting torque. assistant means have to be provided for starting. change saucer-eyed more than less coexistent Motor 5. Self-starting 6. 7. plait Starting torque 308