Synchronous Motors. Chapter (11) Introduction Construction

Transcription

1 Chapter (11) Synchronou Motor Introduction It may be recalled that a d.c. generator can be run a a d.c. motor. In like manner, an alternator may operate a a motor by connecting it armature winding to a 3-phae upply. It i then called a ynchronou motor. A the name implie, a ynchronou motor run at ynchronou peed (N = 120f/P) i.e., in ynchronim with the revolving field produced by the 3-phae upply. The peed of rotation i, therefore, tied to the frequency of the ource. Since the frequency i fixed, the motor peed tay contant irrepective of the load or voltage of 3- phae upply. However, ynchronou motor are not ued o much becaue they run at contant peed (i.e., ynchronou peed) but becaue they poe other unique electrical propertie. In thi chapter, we hall dicu the working and characteritic of ynchronou motor Contruction A ynchronou motor i a machine that operate at ynchronou peed and convert electrical energy into mechanical energy. It i fundamentally an alternator operated a a motor. Like an alternator, a ynchronou motor ha the following two part: (i) a tator which houe 3-phae armature winding in the lot of the tator core and receive power from a 3-phae upply [See (Fig. (11.1)]. (ii) a rotor that ha a et of alient pole excited by direct current to form alternate N and S pole. The exciting coil are connected in erie to two lip ring and direct current i fed into the winding from an external exciter mounted on the rotor haft. The tator i wound for the ame number of pole a the rotor pole. A in the cae of an induction motor, the number of pole determine the ynchronou peed of the motor: Fig.(11.1) 1

2 where Synchronou peed, 120f N = P f = frequency of upply in Hz P = number of pole An important drawback of a ynchronou motor i that it i not elf-tarting and auxiliary mean have to be ued for tarting it Some Fact about Synchronou Motor Some alient feature of a ynchronou motor are: (i) A ynchronou motor run at ynchronou peed or not at all. It peed i contant (ynchronou peed) at all load. The only way to change it peed i to alter the upply frequency (N = 120 f/p). (ii) The outtanding characteritic of a ynchronou motor i that it can be made to operate over a wide range of power factor (lagging, unity or leading) by adjutment of it field excitation. Therefore, a ynchronou motor can be made to carry the mechanical load at contant peed and at the ame time improve the power factor of the ytem. (iii) Synchronou motor are generally of the alient pole type. (iv) A ynchronou motor i not elf-tarting and an auxiliary mean ha to be ued for tarting it. We ue either induction motor principle or a eparate tarting motor for thi purpoe. If the latter method i ued, the machine mut be run up to ynchronou peed and ynchronized a an alternator Operating Principle The fact that a ynchronou motor ha no tarting torque can be eaily explained. (i) Conider a 3-phae ynchronou motor having two rotor pole N R and S R. Then the tator will alo be wound for two pole N S and S S. The motor ha direct voltage applied to the rotor winding and a 3-phae voltage applied to the tator winding. The tator winding produce a rotating field which revolve round the tator at ynchronou peed N (= 120 f/p). The direct (or zero frequency) current et up a two-pole field which i tationary o long a the rotor i not turning. Thu, we have a ituation in which there exit a pair of revolving armature pole (i.e., N S S S ) and a pair of tationary rotor pole (i.e., N R S R ). (ii) Suppoe at any intant, the tator pole are at poition A and B a hown in Fig. (11.2 (i)). It i clear that pole N S and N R repel each other and o do the pole S S and S R. Therefore, the rotor tend to move in the anticlockwie direction. After a period of half-cycle (or ½ f = 1/100 econd), the polaritie of the tator pole are revered but the polaritie of the rotor pole remain the ame a hown in Fig. (11.2 (ii)). Now S S and N R attract 2

3 each other and o do N S and S R. Therefore, the rotor tend to move in the clockwie direction. Since the tator pole change their polaritie rapidly, they tend to pull the rotor firt in one direction and then after a period of half-cycle in the other. Due to high inertia of the rotor, the motor fail to tart. Fig.(10.2) Hence, a ynchronou motor ha no elf-tarting torque i.e., a ynchronou motor cannot tart by itelf. How to get continuou unidirectional torque? If the rotor pole are rotated by ome external mean at uch a peed that they interchange their poition along with the tator pole, then the rotor will experience a continuou unidirectional torque. Thi can be undertood from the following dicuion: (i) Suppoe the tator field i rotating in the clockwie direction and the rotor i alo rotated clockwie by ome external mean at uch a peed that the rotor pole interchange their poition along with the tator pole. (ii) Suppoe at any intant the tator and rotor pole are in the poition hown in Fig. (11.3 (i)). It i clear that torque on the rotor will be clockwie. After a period of half-cycle, the tator pole revere their polaritie and at the ame time rotor pole alo interchange their poition a hown in Fig. (11.3 (ii)). The reult i that again the torque on the rotor i clockwie. Hence a continuou unidirectional torque act on the rotor and move it in the clockwie direction. Under thi condition, pole on the rotor alway face pole of oppoite polarity on the tator and a trong magnetic attraction i et up between them. Thi mutual attraction lock the rotor and tator together and the rotor i virtually pulled into tep with the peed of revolving flux (i.e., ynchronou peed). (iii) If now the external prime mover driving the rotor i removed, the rotor will continue to rotate at ynchronou peed in the clockwie direction becaue the rotor pole are magnetically locked up with the tator pole. It i due to 3

4 thi magnetic interlocking between tator and rotor pole that a ynchronou motor run at the peed of revolving flux i.e., ynchronou peed. Fig.(11.3) 11.4 Making Synchronou Motor Self-Starting A ynchronou motor cannot tart by itelf. In order to make the motor elf-tarting, a quirrel cage winding (alo called damper winding) i provided on the rotor. The damper winding conit of copper bar embedded in the pole face of the alient pole of the rotor a hown in Fig. (11.4). The bar are hort-circuited at the end to form in effect a partial quirrel cage winding. The damper winding erve Fig.(11.4) to tart the motor. (i) To tart with, 3-phae upply i given to the tator winding while the rotor field winding i left unenergized. The rotating tator field induce current in the damper or quirrel cage winding and the motor tart a an induction motor. (ii) A the motor approache the ynchronou peed, the rotor i excited with direct current. Now the reulting pole on the rotor face pole of oppoite polarity on the tator and a trong magnetic attraction i et up between them. The rotor pole lock in with the pole of rotating flux. Conequently, the rotor revolve at the ame peed a the tator field i.e., at ynchronou peed. (iii) Becaue the bar of quirrel cage portion of the rotor now rotate at the ame peed a the rotating tator field, thee bar do not cut any flux and, therefore, have no induced current in them. Hence quirrel cage portion of the rotor i, in effect, removed from the operation of the motor. 4

5 It may be emphaized here that due to magnetic interlocking between the tator and rotor pole, a ynchronou motor can only run at ynchronou peed. At any other peed, thi magnetic interlocking (i.e., rotor pole facing oppoite polarity tator pole) ceae and the average torque become zero. Conequently, the motor come to a halt with a evere diturbance on the line. Note: It i important to excite the rotor with direct current at the right moment. For example, if the d.c. excitation i applied when N-pole of the tator face N- pole of the rotor, the reulting magnetic repulion will produce a violent mechanical hock. The motor will immediately low down and the circuit breaker will trip. In practice, tarter for ynchronou motor arc deigned to detect the precie moment when excitation hould be applied Equivalent Circuit Unlike the induction motor, the ynchronou motor i connected to two electrical ytem; a d.c. ource at the rotor terminal and an a.c. ytem at the tator terminal. 1. Under normal condition of ynchronou motor operation, no voltage i induced in the rotor by the tator field becaue the rotor winding i rotating at the ame peed a the tator field. Only the impreed direct current i preent in the rotor winding and ohmic reitance of thi winding i the only oppoition to it a hown in Fig. (11.5 (i)). 2. In the tator winding, two effect are to be conidered, the effect of tator field on the tator winding and the effect of the rotor field cutting the tator conductor at ynchronou peed. Fig.(11.5) (i) The effect of tator field on the tator (or armature) conductor i accounted for by including an inductive reactance in the armature winding. Thi i called ynchronou reactance X. A reitance R a mut be conidered to be in erie with thi reactance to account for the copper loe in the tator or armature winding a hown in Fig. (11.5 (i)). Thi 5

6 reitance combine with ynchronou reactance and give the ynchronou impedance of the machine. (ii) The econd effect i that a voltage i generated in the tator winding by the ynchronouly-revolving field of the rotor a hown in Fig. (11.5 (i)). Thi generated e.m.f. E B i known a back e.m.f. and oppoe the tator voltage V. The magnitude of E b depend upon rotor peed and rotor flux φ per pole. Since rotor peed i contant; the value of E b depend upon the rotor flux per pole i.e. exciting rotor current I f. Fig. (11.5 (i)) how the chematic diagram for one phae of a tar-connected ynchronou motor while Fig. (11.5 (ii)) how it equivalent circuit. Referring to the equivalent circuit in Fig. (11.5 (ii)). Net voltage/phae in tator winding i E r = V E b Armature current/phae, E I a = Z r phaor difference where 2 a Z = R + X 2 Thi equivalent circuit help coniderably in undertanding the operation of a ynchronou motor. A ynchronou motor i aid to be normally excited if the field excitation i uch that E b = V. If the field excitation i uch that E b < V, the motor i aid to be under-excited. The motor i aid to be over-excited if the field excitation i uch that E b > V. A we hall ee, for both normal and under excitation, the motor ha lagging power factor. However, for over-excitation, the motor ha leading power factor. Note: In a ynchronou motor, the value of X i 10 to 100 time greater than R a. Conequently, we can neglect R a unle we are intereted in efficiency or heating effect Motor on Load In d.c. motor and induction motor, an addition of load caue the motor peed to decreae. The decreae in peed reduce the counter e.m.f. enough o that additional current i drawn from the ource to carry the increaed load at a reduced peed. Thi action cannot take place in a ynchronou motor becaue it run at a contant peed (i.e., ynchronou peed) at all load. What happen when we apply mechanical load to a ynchronou motor? The rotor pole fall lightly behind the tator pole while continuing to run at 6

7 ynchronou peed. The angular diplacement between tator and rotor pole (called torque angle α) caue the phae of back e.m.f. E b to change w.r.t. upply voltage V. Thi increae the net e.m.f. E r in the tator winding. Conequently, tator current I a ( = E r /Z ) increae to carry the load. Fig.(11.6) The following point may be noted in ynchronou motor operation: (i) A ynchronou motor run at ynchronou peed at all load. It meet the increaed load not by a decreae in peed but by the relative hift between tator and rotor pole i.e., by the adjutment of torque angle α. (ii) If the load on the motor increae, the torque angle a alo increae (i.e., rotor pole lag behind the tator pole by a greater angle) but the motor continue to run at ynchronou peed. The increae in torque angle α caue a greater phae hift of back e.m.f. E b w.r.t. upply voltage V. Thi increae the net voltage E r in the tator winding. Conequently, armature current I a (= E r /Z ) increae to meet the load demand. (iii) If the load on the motor decreae, the torque angle α alo decreae. Thi caue a maller phae hift of E b w.r.t. V. Conequently, the net voltage E r in the tator winding decreae and o doe the armature current I a (= E r /Z ) Pull-Out Torque There i a limit to the mechanical load that can be applied to a ynchronou motor. A the load increae, the torque angle α alo increae o that a tage i reached when the rotor i pulled out of ynchronim and the motor come to a tandtill. Thi load torque at which the motor pull out of ynchronim i called pull out or breakdown torque. It value varie from 1.5 to 3.5 time the full load torque. When a ynchronou motor pull out of ynchronim, there i a major diturbance on the line and the circuit breaker immediately trip. Thi protect the motor becaue both quirrel cage and tator winding heat up rapidly when the machine ceae to run at ynchronou peed. 7

8 11.8 Motor Phaor Diagram Conider an under-excited ^tar-connected ynchronou motor (E b < V) upplied with fixed excitation i.e., back e.m.f. E b i contant- Let V = upply voltage/phae E b = back e.m.f./phae Z = ynchronou impedance/phae (i) Motor on no load When the motor i on no load, the torque angle α i mall a hown in Fig. (11.7 (i)). Conequently, back e.m.f. E b lag behind the upply voltage V by a mall angle δ a hown in the phaor diagram in Fig. (11.7 (iii)). The net voltage/phae in the tator winding, i E r. Armature current/phae, I a = E r /Z The armature current I a lag behind E r by θ = tan -1 X /R a. Since X >> R a, I a lag E r by nearly 90. The phae angle between V and I a i φ o that motor power factor i co φ. Input power/phae = V I a co φ Fig.(11.7) Thu at no load, the motor take a mall power VI a co φ/phae from the upply to meet the no-load loe while it continue to run at ynchronou peed. (ii) Motor on load When load i applied to the motor, the torque angle a increae a hown in Fig. (11.8 (i)). Thi caue E b (it magnitude i contant a excitation i fixed) to lag behind V by a greater angle a hown in the phaor diagram in Fig. (11.8 (ii)). The net voltage/phae E r in the tator winding increae. Conequently, the motor draw more armature current I a (=E r /Z ) to meet the applied load. Again I a lag E r by about 90 ince X >> R a. The power factor of the motor i co φ. 8

9 Input power/phae, P i = V I a co φ Mechanical power developed by motor/phae P m = E b I a coine of angle between E b and I a = E b I a co(δ φ) Fig.(11.8) 11.9 Effect of Changing Field Excitation at Contant Load In a d.c. motor, the armature current I a i determined by dividing the difference between V and E b by the armature reitance R a. Similarly, in a ynchronou motor, the tator current (I a ) i determined by dividing voltage-phaor reultant (E r ) between V and E b by the ynchronou impedance Z. One of the mot important feature of a ynchronou motor i that by changing the field excitation, it can be made to operate from lagging to leading power factor. Conider a ynchronou motor having a fixed upply voltage and driving a contant mechanical load. Since the mechanical load a well a the peed i contant, the power input to the motor (=3 VI a co φ) i alo contant. Thi mean that the in-phae component I a co φ drawn from the upply will remain contant. If the field excitation i changed, back e.m.f E b alo change. Thi reult in the change of phae poition of I a w.r.t. V and hence the power factor co φ of the motor change. Fig. (11.9) how the phaor diagram of the ynchronou motor for different value of field excitation. Note that extremitie of current phaor I a lie on the traight line AB. (i) Under excitation The motor i aid to be under-excited if the field excitation i uch that E b < V. Under uch condition, the current I a lag behind V o that motor power factor i lagging a hown in Fig. (11.9 (i)). Thi can be eaily explained. Since E b < V, the net voltage E r i decreaed and turn clockwie. A angle θ (= 90 ) between E r and I a i contant, therefore, phaor I a alo turn clockwie i.e., current I a lag behind the upply voltage. Conequently, the motor ha a lagging power factor. 9

10 (ii) Normal excitation The motor i aid to be normally excited if the field excitation i uch that E b = V. Thi i hown in Fig. (11.9 (ii)). Note that the effect of increaing excitation (i.e., increaing E b ) i to turn the phaor E r and hence I a in the anti-clockwie direction i.e., I a phaor ha come cloer to phaor V. Therefore, p.f. increae though till lagging. Since input power (=3 V I a co φ) i unchanged, the tator current I a mut decreae with increae in p.f. Fig.(11.9) Suppoe the field excitation i increaed until the current I a i in phae with the applied voltage V, making the p.f. of the ynchronou motor unity [See Fig. (11.9 (iii))]. For a given load, at unity p.f. the reultant E r and, therefore, I a are minimum. (iii) Over excitation The motor i aid to be overexcited if the field excitation i uch that E b > V. Under-uch condition, current I a lead V and the motor power factor i leading a hown in Fig. (11.9 (iv)). Note that E r and hence I a further turn anti-clockwie from the normal excitation poition. Conequently, I a lead V. From the above dicuion, it i concluded that if the ynchronou motor i under-excited, it ha a lagging power factor. A the excitation i increaed, the power factor improve till it become unity at normal excitation. Under uch condition, the current drawn from the upply i minimum. If the excitation i further increaed (i.e., over excitation), the motor power factor become leading. Note. The armature current (I a ) i minimum at unity p.f and increae a the power factor become poor, either leading or lagging. 10

11 11.10 Phaor Diagram With Different Excitation Fig. (11.10) how the phaor diagram for different field excitation at contant load. Fig. (11.10 (i)) how the phaor diagram for normal excitation (E b = V), wherea Fig. (11.10 (ii)) how the phaor diagram for under-excitation. In both cae, the motor ha lagging power factor. Fig. (11.10 (iii)) how the phaor diagram when field excitation i adjuted for unity p.f. operation. Under thi condition, the reultant voltage E r and, therefore, the tator current I a are minimum. When the motor i overexcited, it ha leading power factor a hown in Fig. (11.10 (iv)). The following point may be remembered: (i) For a given load, the power factor i governed by the field excitation; a weak field produce the lagging armature current and a trong field produce a leading armature current. (ii) The armature current (I a ) i minimum at unity p.f and increae a the p.f. become le either leading or lagging Power Relation Fig.(11.10) Conider an under-excited tar-connected ynchronou motor driving a mechanical load. Fig. (11.11 (i)) how the equivalent circuit for one phae, while Fig. (11.11 (ii)) how the phaor diagram. Fig.(11.11) 11

12 (i) (ii) Input power/phae, P i = V I a co φ Mechanical power developed by the motor/phae, P m = E b I a coine of angle between E b and I a = E b I a co(δ φ) 2 (iii) Armature Cu lo/phae = IaR a = Pi Pm (iv) Output power/phaor, P out = P m Iron, friction and excitation lo. Fig. (11.12) how the power flow diagram of the ynchronou motor Motor Torque Gro torque, T g = P 9.55 N Fig.(11.12) m N - m where P m = Gro motor output in watt = E b I a co(δ φ) N = Synchronou peed in r.p.m. Shaft torque, T h = P 9.55 N out N - m It may be een that torque i directly proportional to the mechanical power becaue rotor peed (i.e., N ) i fixed Mechanical Power Developed By Motor (Armature reitance neglected) Fig. (11.13) how the phaor diagram of an under-excited ynchronou motor driving a mechanical load. Since armature reitance R a i aumed zero. tanθ = X /R a = and hence θ = 90. Input power/phae = V I a co φ Fig.(11.13) 12

13 Since R a i aumed zero, tator Cu lo ( Ia R a ) will be zero. Hence input power i equal to the mechanical power P m developed by the motor. Mech. power developed/ phae, P m = V I a co φ Referring to the phaor diagram in Fig. (11.13), Alo or AB = E coφ = I X coφ AB = E in δ E I a b r b in δ = I X a a coφ E b in δ coφ = X Subtituting the value of I a co φ in exp. (i) above, V Eb P m = per phae X VE b = for 3-phae X It i clear from the above relation that mechanical power increae with torque angle (in electrical degree) and it maximum value i reached when δ = 90 (electrical). V Eb P max = per phae X Under thi condition, the pole of the rotor will be mid-way between N and S pole of the tator Power Factor of Synchronou Motor In an induction motor, only one winding (i.e., tator winding) produce the neceary flux in the machine. The tator winding mut draw reactive power from the upply to et up the flux. Conequently, induction motor mut operate at lagging power factor. But in a ynchronou motor, there are two poible ource of excitation; alternating current in the tator or direct current in the rotor. The required flux may be produced either by tator or rotor or both. (i) If the rotor exciting current i of uch magnitude that it produce all the required flux, then no magnetizing current or reactive power i needed in the tator. A a reult, the motor will operate at unity power factor. 2 (i) 13

14 (ii) If the rotor exciting current i le (i.e., motor i under-excited), the deficit in flux i made up by the tator. Conequently, the motor draw reactive power to provide for the remaining flux. Hence motor will operate at a lagging power factor. (iii) If the rotor exciting current i greater (i.e., motor i over-excited), the exce flux mut be counterbalanced in the tator. Now the tator, intead of aborbing reactive power, actually deliver reactive power to the 3-phae line. The motor then behave like a ource of reactive power, a if it were a capacitor. In other word, the motor operate at a leading power factor. To um up, a ynchronou motor aborb reactive power when it i underexcited and deliver reactive power to ource when it i over-excited Synchronou Condener A ynchronou motor take a leading current when over-excited and, therefore, behave a a capacitor. An over-excited ynchronou motor running on no-load in known a ynchronou condener. When uch a machine i connected in parallel with induction motor or other device that operate at low lagging power factor, the leading kvar upplied by the ynchronou condener partly neutralize the lagging reactive kvar of the load. Conequently, the power factor of the ytem i improved. Fig. (11.14) how the power factor improvement by ynchronou condener method. The 3 φ load take current I L at low lagging power factor co φ L. The ynchronou condener take a current I m which lead the voltage by an angle φ m. The reultant current I i the vector um of I m and I L and lag behind the voltage by an angle φ. It i clear that φ i le than φ L o that co φ i greater than co φ L. Thu the power factor i increaed from co φ L to co φ. Synchronou condener are generally ued at major bulk upply ubtation for power factor improvement. Advantage (i) By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. Thi help in achieving teple control of power factor. (ii) The motor winding have high thermal tability to hort circuit current. (iii) The fault can be removed eaily. 14

15 Fig.(11.14) Diadvantage (i) There are coniderable loe in the motor. (ii) The maintenance cot i high. (iii) It produce noie. (iv) Except in ize above 500 RVA, the cot i greater than that of tatic capacitor of the ame rating. (v) A a ynchronou motor ha no elf-tarting torque, then-fore, an auxiliary equipment ha to be provided for thi purpoe Application of Synchronou Motor (i) Synchronou motor are particularly attractive for low peed (< 300 r.p.m.) becaue the power factor can alway be adjuted to unity and efficiency i high. (ii) Overexcited ynchronou motor can be ued to improve the power factor of a plant while carrying their rated load. (iii) They are ued to improve the voltage regulation of tranmiion line. (iv) High-power electronic converter generating very low frequencie enable u to run ynchronou motor at ultra-low peed. Thu huge motor in the 10 MW range drive cruher, rotary kiln and variable-peed ball mill. 15

16 11.17 Comparion of Synchronou and Induction Motor S. No. Particular Synchronou Motor 1. Speed Remain contant (i.e., N ) from no-load to full-load. 2. Power factor Can be made to operate from lagging to leading power factor. 3. Excitation Require d.c. excitation at the rotor. 4. Economy Economical fcr peed below 300 r.p.m. 5. Self-tarting No elf-tarting torque. Auxiliary mean have to be provided for tarting. 6. Contruction Complicated Simple 7. Starting torque More le 3-phae Induction Motor Decreae with load. Operate at lagging power factor. No excitation for the rotor. Economical for peed above 600 r.p.m. Self-tarting 16