Synchronous Motor

Section (11) Synchronous Motors Introduction It might be reviewed that a d. c. generator can be run as a d. c. engine. In like way, an alternator may work as an engine by associating its armature twisting to a 3-stage gracefully. It is then called a simultaneous engine. As the name suggests, a simultaneous engine runs at coordinated speed (Ns = 120f/P) I. e. , in synchronism with the spinning field created by the 3-stage flexibly. The speed of revolution is, along these lines, attached to the recurrence of the source.Since the recurrence is fixed, the engine speed remains consistent regardless of the heap or voltage of 3phase flexibly. Nonetheless, simultaneous engines are not utilized so much since they run at steady speed (I. e. , simultaneous speed) but since they have other interesting electrical properties. In this part, we will examine the working and qualities of simultaneous engines. 11. 1 Construction A simultaneous engine is a machine that works at coordinated speed and cha nges over electrical vitality into mechanical vitality. It is in a general sense an alternator worked as a motor.Like an alternator, a coordinated engine has the accompanying two sections: (I) a stator which houses 3-stage armature twisting in the spaces of the stator center and gets power from a 3-stage gracefully [See (Fig. (11. 1)]. (ii) a rotor that has a lot of notable posts energized by direct current to frame exchange N and S shafts. The energizing curls are associated in arrangement to two slip rings and direct current is taken care of into the twisting from an outer exciter mounted on the rotor shaft. The stator is twisted for indistinguishable number of shafts from the rotor poles.As on account of an acceptance engine, the quantity of posts decides the simultaneous speed of the engine: Fig. (11. 1) 293 Synchronous speed, N s = where 120f P f = recurrence of gracefully in Hz P = number of posts A significant downside of a coordinated engine is that it isn't self-beginning a nd helper implies must be utilized for beginning it. 11. 2 Some Facts about Synchronous Motor Some striking highlights of a simultaneous engine are: (I) A coordinated engine runs at coordinated speed or not in the slightest degree. Its speed is steady (coordinated speed) at all heaps. The best way to change its speed is to adjust the gracefully recurrence (Ns = 120 f/P). ii) The extraordinary trait of a simultaneous engine is that it very well may be made to work over a wide scope of intensity factors (slacking, solidarity or driving) by modification of its field excitation. Accordingly, a coordinated engine can be made to convey the mechanical burden at consistent speed and simultaneously improve the force factor of the framework. (iii) Synchronous engines are for the most part of the striking post type. (iv) A coordinated engine isn't self-beginning and an assistant methods must be utilized for beginning it. We use either acceptance engine rule or a different turning over engine f or this purpose.If the last strategy is utilized, the machine must be approached simultaneous speed and synchronized as an alternator. 11. 3 Operating Principle The way that a simultaneous engine has no beginning torque can be effectively clarified. (I) Consider a 3-stage coordinated engine having two rotor posts NR and SR. At that point the stator will likewise be twisted for two shafts NS and SS. The engine has direct voltage applied to the rotor winding and a 3-stage voltage applied to the stator winding. The stator winding produces a pivoting field which rotates round the stator at simultaneous speed Ns(= 120 f/P).The immediate (or zero recurrence) current sets up a two-post field which is fixed inasmuch as the rotor isn't turning. Along these lines, we have a circumstance wherein there exists a couple of spinning armature shafts (I. e. , NS ? SS) and a couple of fixed rotor shafts (I. e. , NR ? SR). (ii) Suppose at any moment, the stator shafts are at positions An and B as appe ared in Fig. (11. 2 (I)). Plainly shafts NS and NR repulse one another thus do the posts SS and SR. Hence, the rotor will in general move the anticlockwise way. After a time of half-cycle (or ? = 1/100 second), the polarities of the stator shafts are switched yet the polarities of the rotor posts continue as before as appeared in Fig. (11. 2 (ii)). Presently SS and NR draw in 294 one another thus do NS and SR. In this manner, the rotor will in general move the clockwise way. Since the stator shafts change their polarities quickly, they will in general force the rotor first one way and afterward after a time of half-cycle in the other. Because of high inactivity of the rotor, the engine neglects to begin. Fig. (10. 2) Hence, a simultaneous engine has no self-beginning torque I. e. , a simultaneous engine can't turn over by itself.How to get ceaseless unidirectional torque? On the off chance that the rotor posts are turned by some outside methods at such a speed, that they exchange th eir situations alongside the stator shafts, at that point the rotor will encounter a nonstop unidirectional torque. This can be comprehended from the accompanying conversation: (I) Suppose the stator field is turning the clockwise way and the rotor is additionally pivoted clockwise by some outside methods at such a speed, that the rotor shafts exchange their situations alongside the stator posts. (ii) Suppose at any moment the stator and rotor posts are in the position appeared in Fig. 11. 3 (I)). Obviously torque on the rotor will be clockwise. After a time of half-cycle, the stator shafts turn around their polarities and simultaneously rotor posts likewise exchange their situations as appeared in Fig. (11. 3 (ii)). The outcome is that again the torque on the rotor is clockwise. Thus a ceaseless unidirectional torque follows up on the rotor and moves it the clockwise way. Under this condition, posts on the rotor consistently face shafts of inverse extremity on the stator and a soli d attractive fascination is set up between them.This shared fascination bolts the rotor and stator together and the rotor is for all intents and purposes maneuvered into step with the speed of spinning motion (I. e. , simultaneous speed). (iii) If now the outer main player driving the rotor is evacuated, the rotor will keep on pivoting at simultaneous speed the clockwise way in light of the fact that the rotor posts are attractively bolted up with the stator shafts. It is because of 295 this attractive interlocking among stator and rotor shafts that a simultaneous engine runs at the speed of rotating motion I. e. , simultaneous speed. Fig. (11. 3) 11. Making Synchronous Motor Self-Starting A simultaneous engine can't turn over without anyone else. So as to make the engine self-turning over, a squirrel confine twisting (likewise called damper winding) is given on the rotor. The damper twisting comprises of copper bars installed in the shaft countenances of the remarkable posts of the rotor as appeared in Fig. (11. 4). The bars are shortcircuited at the finishes to frame in actuality an incomplete Fig. (11. 4) squirrel confine winding. The damper twisting serves to turn over the engine. (I) To begin with, 3-stage flexibly is given to the stator winding while the rotor field winding is left unenergized.The turning stator field incites flows in the damper or squirrel confine winding and the engine turns over as an enlistment engine. (ii) As the engine moves toward the simultaneous speed, the rotor is energized with direct current. Presently the subsequent posts on the rotor face shafts of inverse extremity on the stator and a solid attractive fascination is set up between them. The rotor shafts lock in with the posts of turning motion. Subsequently, the rotor spins at a similar speed as the stator field I. e. , at simultaneous speed. iii) Because the bars of squirrel confine bit of the rotor presently turn at a similar speed as the pivoting stator field, these bar s don't cut any motion and, in this manner, have no instigated flows in them. Thus squirrel confine bit of the rotor is, as a result, expelled from the activity of the engine. 296 It might be accentuated here that because of attractive interlocking between the stator and rotor posts, a coordinated engine can just run at simultaneous speed. At some other speed, this attractive interlocking (I. e. , rotor shafts looking inverse extremity stator posts) stops and the normal torque turns out to be zero.Consequently, the engine stops with a serious unsettling influence on the line. Note: It is critical to energize the rotor with direct current at the correct second. For instance, if the d. c. excitation is applied when N-shaft of the stator faces Npole of the rotor, the subsequent attractive aversion will deliver a vicious mechanical stun. The engine will quickly back off and the circuit breakers will trip. By and by, starters for coordinated engines circular segment intended to identify the exact second when excitation ought to be applied. 11. 5 Equivalent Circuit Unlike the enlistment engine, the simultaneous engine is associated with two electrical frameworks; a d. . source at the rotor terminals and an a. c. framework at the stator terminals. 1. Under typical states of simultaneous engine activity, no voltage is prompted in the rotor by the stator field on the grounds that the rotor winding is turning at a similar speed as the stator field. Just the intrigued direct current is available in the rotor winding and ohmic obstruction of this winding is the main resistance to it as appeared in Fig. (11. 5 (I)). 2. In the stator winding, two impacts are to be thought of, the impact of stator field on the stator winding and the impact of the rotor field cutting the stator conductors at coordinated speed.Fig. (11. 5) (I) The impact of stator field on the stator (or armature) conductors is represented by remembering an inductive reactance for the armature winding. This is called simultaneous reactance Xs. An opposition Ra must be viewed as in arrangement with this reactance to represent the copper misfortunes in the stator or armature twisting as appeared in Fig. (11. 5 (I)). This 297 obstruction joins with simultaneous reactance and gives the coordinated impedance of the machine. (ii) The subsequent impact is that a voltage is created in the stator twisting by the simultaneously spinning field of the rotor as appeared in Fig. 11. 5 (I)). This created e. m. f. EB is known as ba