<—GiEREE PHASE INDUCTION MOTORS 4 pase induction motor isthe most widely used ae motor ey cost simple and extremely rugged construction, oi iy high efficiency, reasonably good power factor, sgh ce cost and simple starting arrangement inate fom other types of electric motors in that there is rere eonnection from the rotor winding to any source of ar: “The necessary current and voltage in the rotor circuit sup hed by induction from the stator winding This is the sn tat itis called the induction motor. q4.4. Construction. The induction motor essentially consists aftwo pars () stationary part known as stafor and (ji) the fevotving par, known as the roror. 4, Stator. Stator of a 3-phase induction motor is similar in ‘construction to that of a 3-phase synchronous machine. The Stor winding, also sometimes known as primary winding, is laced in slots on the inner periphery of the core. The core and ‘windings are enclosed in cast iron frame. The stator winding is usually arranged for 3-phase power supply, the phases of which may be connected cither in delta or star as per design of the machine. It is wound for a definite number of poles as per requirement of specd—greater the number of poles lesser the speed and vice versa for a supply of given frequency. 2.Rotor. The rotors employed in 3 phase induction motors oe types namely (a) squirrel cage (b) wound rotor and ©) double cage. (a) Squirrel Cage Rotor. Almost 90 per cent of induction ‘olos are provided with squirrel cage rotor because of its very Simple, robust and almost instrucible construction. Squirrel cage ‘Tor is constructed of a laminated core with the conductors (Copper or aluminium bars) placed parallel, or approximately a othe shaft and embedded inthe core surface, At each OF the rotor, the rotor conductors are all short circuited by ‘Putnuous end rings of similar materials to that of conductors. — the rotor are usually skewed in order to obtain a While ima torque and reduce the magnetic humming noise ig and reduce the locking tendency of the rotor. ving) W284 Rotor. Such arotoris wound with an insulated of; 'g similar to that of stator ‘winding and for the same number hase me, that of the stator, The rotor winding is always three~ Denne at stBeR the stator is wound for two phases. ‘Sirel cage le Squirrel ‘Cage Rotor. Such a rotor carries two Slots conte inlings embedded in two rows of slots, ‘The outer 2d the inn ‘high resistance and low leakage reactance winding slots contain a low resistance and high leakage Induction Machines reactance winding. The outer winding bars are usually of manganese brass while the inner winding bars are of red copper. 9.1.2, Types of 3-Phase Induction Motors. The 3-phase induction motors are of three types depending upon the type of rotor they employ, namely, (i) squirrel cage rotor (ii) wound rotor or slip-ring and (iii) double squirrel cage rotor induction motors. The principle of operation is same for all types of induction motors. 9.1.3. Principle of Operation. When the stator or primary winding of a3-phase induction motor is connected to a 3-phase ac supply, a rotating magnetic field is established which rotates at synchronous speed. The direction of revolution of this field wil depend upon the phase sequence of the primary currents and, therefore, will depend upon the order of connection of the primary terminals to the supply. The direction of rotation of the field can be reversed by interchanging the connection to the supply of any two leads of a 3-phase induction motor. The number of magnetic poles of the revolving field will be the same asthe numberof poles for which each phase ofthe primary orstator winding is wound. The speed at which the field produced by the primary currents will revolve is called the synchronous speed of the motor and is given by an equation, Ns je where fis supply frequency and P is the number poles on stator. ‘The revolving magnetic field produced by the primary ‘currents sweeps across the rotor conductors and thereby induces an emf in these conductors, Since the rotor winding is either directly shorted of closed through some extemal resistance, the emf induced in the secondary by the revolving field causes a Current to flow in the rotor conductors whose direction is such sto oppose the cause which is producing it. Because the cause producing the induced currents isthe relative speed between the rotating magnetic field and the stationary rotor conductors, therefore, they circulate in such a way that a torque is produced in the rotor tending to cause it to follow the rotating magnetic, field and thus reducing the relative speed. ‘An induction motor cannot run at synchronous speed. IF it ‘were possible, by some means, forthe rotor to attain synchronous Speed, the rotor would then be standstill with respect to the Totating flux. Then no emf would be induced in the rotor, no fotorcurrent would flow, and therefore, there would be no torque developed. ‘An induction motor running on no load will have a speed very elose to synchronous speed and, therefore, emf induced in the rotor winding will be very small. As the mechanical load is applied to the motor shaft the motor slows down, the relative Ra © scanned with OKEN Scannermotion between the rotating magnetic field and the rotor increases causing increase in rotor emf, rotor current aiid so it the torque developed. Thus the motor meets the inereased load, ‘The direction of rotation of a 3-phase induction motor can be reversed by interchanging the connections to the supply of any two terminals. 8.1.4, Slip. The rotor never attains the synchronous speed because at that speed there would be no relative speed between rotor conductors and stator revolving field and induction Pheromenon is not possible. Its speed is always less than synchronous speed ‘The difference between the speed ofthe stator field, known $5 synchronous speed (N,), and the actual speed ofthe rotor (N) ‘is known as the slip and is denoted by 5 8.1.5. Speed of Rotor Current or EMF. Both he stator and ‘Tote fields rotate synchronously, which means that they are Stationary wrt. each other at all possible rotor speeds, 8.1.6: Frequency of Rotor Current or EMF. The frequency of both the rotor emf and rotor current depends upon the vate of Cutting flux by the rotor conductors i., on the relative speed ‘een the stator revolving magnetic field and rotor and is given by the equation fas ‘here sis the slip and fis the supply frequency. 9. 9.1) Rotor EMF. When the rotoris at standstill, the motoris equivalent to a 3-phase transformer with secondary short Circuited and so emf induced per phase inthe rotors given as N; Brent (92) Where E, isthe applied voltage per phase tothe stator winding and N, and N, are the number of tums per phase on stator and rotor respectively. ‘While the motor is running with slip s, the induced emf in the rotor wll bes times the induced emf in the otorat standstill, 9.1.8. Rotor Current. If R, and L, are the resistance and inductance per phase of rotor and E, is the induced emf at ‘standstill, then rotor current per phase at standstill =B._E 7 4 IRi+x} ‘While the motor is running with slips, then the rotor phase reactance and rotor phase em willbe sX, and sE respectively ‘and rotor current will be as (9.3) by ne (94) Rs Ke and power factor,cos@, _ __R; +95) Rae 9.1.9, Rotor Torque. The torque of an due to interaction of the rotor and stator strength of those fields and phase relat given as ‘induction motor being fields depends upon the ion between them and is ‘An Integrated Course In Electrical Engineering 2 T = Kemet Reig 5 Atstrts= Land, therefore, stating torque ia vena t, - KRBF . MR RE+x? 6 ‘The operating o running torgue will be maxim slip s= RX, and is given by the equation "hy Tax - KER | 2X, Oh Starting torque will be maximum when =R2..,_ xy Since the rotor resistance is not more than 1 or2 its leakage reactance, otherwise efficiency wll below heat increase the starting torque, extra resistance must be el the rotor circuit at start and cut out gradually asthe motor up. This is possible with wound rotor motors only. 8.1.10. Torque-slip and Torque-speed Curves. ony Eq, (9.6) for rotor torque itis obvious that © at synchronous speed slips is zero and so the tone, zero, (Gat speeds near synchronous speed, the slip is very sat the term sX> is very small in comparison to R, and te torque is approximately proportional to slips RESISTANCE a — Fen cen sinchonous speeo "> Fig.91 Torque-Speed Curves RESSTANGE 1) mone je Ban Law 3 0 f vwmi.agc noror | = a —— rencenr suns ——— Fig.9.2 Torque-Slip Curves, i © scanned with OKEN ScannerFperease in slip, speed decreases, torque in rr aces te maxi value when RE, The tera aso Known as Breakdowh ep «eo Wit ase in load beyond the point of breakdown or inte toe dereases causing slow down of moto tetotor will eventually stop. ont ihe ip R Becomes neal in comparison 1; to aX; and the forgue varies as ~ Le, torque-speed or revgueslip curves are rectangular hyperbola with the {ior speed beyond pull out or breakdows point. These slips are shown in Figs. 9.1 and 9.2 respectively, itis seen that although maximum torque is independent of soreistance Ro, yet the exact Toeation of Tn depends upon rooristance Ry, Greater te value of Roy greater isthe value spat which the maximum torque OCeUrS. gt. Effect of Rotor Resistance Upon Torque. Slip or aivje Speed Relationship, Fora given value of torque T, {psi proportional 1 rotor resistance Ro ation of external a8 ance in the rotor circuit does not lower the torque curve but, rely steches itso that the same torque values occur at lower ‘rods (rhigher slips) as shown in Figs. 9.1 (a) and 9.1 (6). 94.12. Effect of Change in Supply Voltage on Starting and Running Torques. Starting torque varies as the square ofsupply voltage i.e, T,, 02. It means that the starting torque is very sensitive to any change in the applied voltage. For example, a change of 10 per cent in supply voltage will cause a change of 19 % in the rotor torque. This fact is very important in connection with starting methods of squirrel cage induction rotors. Ifthe supply voltage is changed, it changes the torque under running condition also. While running the torque varies as sV". With the decrease in supply voltage, torque under running condition decreases. As the motor now develops a reduced torque, the motor slows down (i.e., slip increases) 50 a8 to cause increase in torque to meet the load torque and the motor may come to a standstill (jf the load torque exceeds the pull-out forgue of the motor corresponding to reduced supply voltage) unless the load is removed. oan Full-Load Torque and Maximum Torque. Ratio full load torque and maximum torque is given by relationship ho ge 7 99) 4 ima Same y SL er iy SmaxT At starting s,= 1 and so we have et 2a ak et (9.10) where a= ’ Se Tax a+t x, A44.Tor ‘om ‘Torque-Speed Curve and Operating Region. The Fromagbeed eurve for an induction motor is given in Fig. 9.3. Fig. 9.3 itis evident that for any load torque, there are (wo (Say B and D). But at position B, the operation le because if there is tendency of speed rise the Induction Machines rian torque also increases than the load torque causing, forthe isin set ‘Thus at position B the operation is unstable. ‘ ion D operation is stable because a tendency to rise in ‘ea will be opposed by the decrease in developed torque ilarly when there isa tendency to fall in speed, there will be nerease in developed torque to bring the motor to oper Position D. Thus the region AC of the torque-speed characteristic is unstable region of operation while the region CF is stable region of operation, Fig. 93 ‘At full-load, the motor runs at a speed of N rpm. If the ‘mechanical load applied to the motor is increased, motor speed drops till the torque developed by the motor equals the load torque. As long as the two torques are in balance, the motor will run at constant speed. However, if the load torque exceeds the breakdown torque of the motor, the motor will immediately stop. ‘The value of breakdown torque varies with the design of the motor but ranges from 200 to 300 percent of full-load torque in standard squirrel cage motors. ‘The torque-speed characteristics of an induction motor in the operating region are quite similar to those of a de shunt motor, Even at full load, the drop in speed from no-load does not exceed 5 percent, 9.1.15. Power Stages in an Induction Motor Input to motor Rot6e input ‘Stator copper and iron losses Rotor output or ‘Rotor copper losses ‘Mechanical power developed Rotor up ‘Windage and retion tosses 9.1.16, Rotor Output. When an induction motor is operating hormally, the slip isso small that the frequency of the magnetic Teversals in the rotor core is only of the onder of one or two per foo, therefore iron losses occurring in the rotor are very small fand can be neglected. Hence output of stator = Input t rotor = Ouput of rotor + copper losses in rotor, ‘When a 3-phase induction motor is operating on a certait toad, the power transferred from the stator tothe rotor (ie. rot input) can be expressed in terms of the torgue exerted on th ator by the rotating Mux and the synchronous speed at whic this flux is rotating. © scanned with OKEN Scanner‘An Integrated Course | Power transferred from stator to rotor or input power to rotor orair-gap power, Re aN Twatts (9.11) ‘where T is the torque in newton metres, exerted on the rotor by the rotating flux and N, is the synchronous speed of rotating ‘magnetic field in rpm. Total mechanical power developed by rotor or gross output of power of rotor. Poe = 22M watts here Nis the speed of rotor, at full load, in rpm. Rotor copper losses = Rotor in put power ~ rotor gross output power ‘i.e, Rotor copper losses = s x power input to rotor, Pp Power input to rotor= Tote! copper losses s ‘Total copper losses in rotor Gross rotor output, = Pech and P : Pager # Popper = 1: (1=8) 5 (9.12) 9.1.17. Equivalent Circuit of an Induction Motor. An induction motor is essentially a transformer. In the transformer the load on the secondary is electrical whereas in case of induction motor the load is mechanical which can be replaced by an equivalent electrical load of load resistance R,, given by 11) where R, is the rotor phase resistance and Kis the turn-ratio of rotor to stator. The simplified equivalent circuit of an induction motor is shown in Fig. 9.4. Fig.94 9.1.18. Comparison Between 3-Phase Induction Motor and Transformer. The important differences between a 3- hase induction motor and transformer are as given below: 1, Because of distributed windings in an induction motor, the ratio of stator and rotor currents is not equal (0 the ‘atio of turns per phase in the rotor and stator windings, 2. Inan induction motor the magnetic leakage and leakage reactances of rotor and stator are higher than those in a transformer. 3. The friction and windage losses are also present in induction motor, so the efficiency is lower than that of a transformer, 380 n Electrical Engineering No-load current in an induction motor varia * Soo 50 percent of floa car transformer it varies fom 2 105 pe cen gh primary current. This i because of the fy tual oxi transformer competsis pu fonts ce, wicear he masa induction motor has to cross he air ap berg and rotor. Thus the reluctance offered mung) an induction motor is much higher than ihe transformer and, therefore, more magnetizing (anmi)is required o develop the required muti in case of induction motor than that ina transfonyey means that the magnetising current is large ne induction motor as compared to that fora trans, Energy component of exciting or no-load cunew, also much higher in ease of an induction motor te that in a transformer because of presence of wine and friction losses in induction motor. Thus nose current ofan induction motor is much higher thn he of an equivalent transformer. 9.1.19, Maximum Power Output. The gross mecharc| power developed will be maximum when the equivalent lat Fesistance R;, is equal tothe standstill leakage impedance (2, aoe : 5 m+ 3) Maximum gross mechanical power developed is git 3v2 2(Ror + Zo) The slip corresponding to maximum gross mechanical oe ogo R,/K? (+z) % 9.1.20. Maximum Torque. The expression for maximum torque in a 3-phase induction motor is ins 9.13) (9.19 and the expression for slip corresponding to maximum torque s Rik? + Ha(x, +43) K 5s, 9.1.21, Power Factor Characteristics. The current dav by an induction motor running at no load is largely # ‘magnetising current $0 no-load current lags behind the applied voltage by a large angle and thus the power factor of a ish loaded induction motor is quite low. As the load is in {he active or power component ofthe curent increases resins ina higher power factor. However, because ofthe large vane Imagnetising current which is present regardless of loa Power factor of an induction motor at full load seldom eX° 90 per cent. © scanned with OKEN Scannera 22. Perro performance curves of a75 kW, 2200 V, ot lone “Guim! cage induction motor are shown in “ es ep] at) BY cok 2] s.100 “le i i ; 8) cot oy 'y Bole Z ae t El aot §| ; ele H ob Alo oO 2 4 60 00 100 4 95. Performance Curves ofa.75 kW, 2,200V, "Phase, 50 Hz Squirrel Cage Motor Induction Motors. eters test is performed to determine no-load ent ly 20 Toad power factor cosy, Windage and friction Gites wo-load core 10ss, no-load input, and no-load reaatance Rp and reactance X,, This testi performed with {fiierent values of applied voltage below and above rated Noltge while the motor is running light (without load). () Short-rcuit or Blocked Rotor Test, Tis testis performed th eemine the short-circuit curren. with normal applied voltage to stator; power factor on short circuit; total ‘equivalent resistance and reactance of the motor as referred ‘to stator. This test is just equivalent to short-circuit test on atransformer and in this test rotor is held firmly with rotor short-circuited at slip-rings in case of wound rotor induction ‘motor and the stator connected across supply of variable voltage. 8124, Measurement of Slip. Slip can be determined by various methods such as (i) actual measurement of rotor speed (@ measurement of rotor frequency and (ii) stroboscopic nethods. Determination of slip by measuring rotor speed actually isso accurate, s being the difference of two approximately ‘qual quantities. Rotor frequency can be easily measured by a moving coil millivoltmeter inserted in the rotor pes Circle Dagram. The operating characteristics of an lution motor can be computed by using a circle diagram ‘sly and conveniently, The locus of stator load current Iisa ice of diameter and having coordinate (ee 0) with, Rapecttog Ot 2X 9126, olor, a Sco ‘Crawling and Cogging. Itis observed that induction Parculary the squirrel-cage type, sometimes ex! {orun stably at speeds as low as one-seventh oftheir ie nous speed. This phenomenon is called the cravling. tecag, ase Winding carying sinusoidal curents produces “sof he ordern=6N+ I, where N isan integer (+ means — the rotation and ~ means against the rotation). ‘The synchronous speed of nth order of harmonic is Vath of the Synchronous speed of the fundamental. For N = 1, a 3-phase ‘Winding produces a forward rotating 7th harmonic and backward rotating Sth harmonic. Considering 7th harmonic, the interaction between the fictitious stator and rotor 7th harmonic poles will Produce a positive torque and if the torque is sufficiently Pronounced it may prevent the motor speed to exceed one- Seventh of normal speed. Thus the motor crawls at about one- seventh the normal speed. ‘When the stator slots and rotor slots are equal in number, the speeds of all the harmonics developed by the stator slots coincide with the speed of corresponding rotor harmonics. This harmonies of every order try to exert synchronous torques at their synchronous speeds and so the machine fails to start. This is known as cogging or magnetic locking. 9.1.27. High Torque Cage Motors. Many efforts have been made to build squirrel cage motors with improved starting characteristics without sacrificing significantly its excellent running characteristics. Increased starting torque with short- circuited rotors can be obtained by using two types of rotors (ther deep bar rotors or double cage rotors) which employ change of rotor circuit impedance with change in rotor frequency to have high starting torque and also high efficiency under normal running conditions. 1.Deep bar Cage Rotor Motor. In deep bar motors use is made of skin effect with rotor bars caused by the slot leakage fluxes. At normal speed the deep bar motor isaplainsquirel-cage motor with the usual resistance, but with a somewhat higher inductive reactance, owing to which the power factor, efficiency and the overload capacity of the motor are somewhat reduced. Cutaway view of deep bar rotor is shown Fig. 9,6 Cutaway View of Deep in Fig. 9.6. Bar Rotor 2, Double Cage Rotor Motor. The rotor of a double cage motor carries two squirrel cage windings embedded in two rows of slots, The outer slots contain a high resistance and low leakage reactance winding and the inner slots contain a low resistance ‘and high leakage reactance winding, COPPER BARS ‘To (STARTING) ‘ence Boro (Runa) CARE RUNNING Fig. 97 © scanned with OKEN Scanner‘An Integrated Course In Electrical Engineering Double cage rotor motor is classed as ahigh-torgue, w= Starting curent motor and costs approximately 0030 percent Figher than that of plain squire cage motor. A deep-bar rotor constructions employed for higher tang torque applications and double cage construction for sill higher Starting requirements. For large-size motors with stringent starting torque requirements, the most expensive wound rotor construction is employed, 9.4.28. Ratings of 3-Phase Induction Motors. Thre-phase induction motors ae rated in terms of power output in kW, speed, voltage, frequency, phase (single or thee, line current and temperature riseina specified ime. Ifa motor is designed to be operated on more than one voltage ort more than one speed by reconnecting the windings, a connection diagram is also often piven on the name plate. Class of motors (A,B,C, D, Bot F)is also to be mentioned onthe name pate. 9.1.28. Standard Types of 3-Phase Squirrel Cage Induction Motors. These motors are available ina range of standard ratings up to 150 KW at various standard frequencies, voltages and speeds in order to meet the usual requirements of industry. According to thei eletrical characterises, such motors are divided ino six types as given below : 1. Class A Motors. These motors have low resistance and low reactance and have low slipand high efficiency at ul load The high staring curent (5 to 8 times full-oad curent at rated voltage isthe main drawback Such motors ae used in small ratings for machine tools, centrifugal pumps, fans, blowers etc. 2. Class B Motors. These motors have high reactance, This design is common in the 5-150 kW range. Such motors have largely replaced class A motors for new installations a its running characteristics are quite similar to those of class A motor but have smaller starting curent (around $ times the fll-load current at rated voltage). 3. Class C Motors. These motors are double cage motos and provide high stating torque with low starting curren Typical applications are in driving air compressors, conveyors, reciprocating pumps, crushers, mixers, large refrigerating machines etc. 4. Class D Motors. Such motos are provided with a high resistance squirrel cage rotors and, therefor, give high stating torgue with low stating current These motors have low operating efficiency and ther use is limited to driving of intermittent loads involving high accelerating duty and to drive high-impact loads such as punch presses, shears, bulldozers, small hosts ete. 5. Class E Motors. These motorshave relatively low stating torque, normal stating current and have a relatively low slip at ated load, 6, Class F Motors. These motors havea relatively lw starting torque, low starting current and normal sip. 9.1.30, Starting Methods of Squirrel Cage Induction ‘Motors. At stat the squirrel cage induction motor is just like a transformer with short-circuited secondary and, therefore, it will draw heavy current (5107 times the ated full-Joadcurent fromthe supply lines if rated voltage were applied at start. OF course, the induced emfin the rotor circuit decreases with the increase in speed ofrtorandconequnty bth thor and sttorcuney valuesdeterminedonlyby mechanicalload,hisesenu oP the stating current oan extent that the ine votage drop ye affect the operation of other equipment connected to ie distribution work. One obvious way to reduce the startin, {sto impress te lower voltage across stator winding at sa cbjion wits hat itreduces the stating true ashes torque varies a the square of impressed voltage. The yn’ methods of stating squirrel cage induction motorae given begs 1.Direct-on-Line Starting Method or Full-Vltage Squirrel cage motors of capacity up to 1.5 kW, double rotormotors and squirel cage motors of age capacity hang, high roto resistance are started by this method. The sar torque with DOL starting is given as ne Ore 5 where sp Ipand T, are slip, current and torque respectively at load and ste shor-ciruit current. 2. Reduced Voltage or Line Resistance Starter Method. this starting method, reduced voltage across stator windings is obtained by inserting external resistances in each stator leadat start. After a definite time, when the motor picks up the spe, accelerating contacts close and short circuit the starting resistances and apply full rated voltage to the motor terminals If voltage across stator terminals is reduced to K times of the lite voltage V then at start Voltage applied to motor = KY 019 Starting current 1,, = Kl, 0. Starting torque, T,, = K? x torque obtained by direct switching (918) ‘Though this starting method is simple and cheap but loot Power is wasted inthe external series resistors. 3. Auto-transformer Starting Method. In this starting method reduced voltage is obtained by taking tappings 31 suitable points (50, 60 and 80 percent points) from a 3-phse aulo-transformer. Auto-transformer starters may be eithet ‘manually of magnetically operated. This type of starter is used both for star-connected as well as dlta-connected motors. ith transformer tapping used reduces the voltage V to KV thet ‘Starting line current will be K2I,. and starting torque will be ¥ times the torque developed by direct switching, tar-Delta Switching Method, The star-delta stare Conneets the three stator windings in star across the rated supe Voltage at stat and after the motor picks up the speed reconnes them in delta across the same supply voltage, It reduces the ‘starting line current and starting torque to one-third of thosé obtained with direct switching 9.1.31. Starting of Stip-ring Induction Motors. Thous! the methods described above can be employed for stating phase induction motors of both types namely squimel cage well as slip-ting induction motors but are usually employed f° Starting squirrel cage type motors, Slip-ving induction mer are usually started with full-line voltage across the Sf terminals and by introducing variable resistance in each PBS® of the rotor circuit. Such a starting method, called the 1” © scanned with OKEN Scannere starter, cannot be employed in case of vtion motors because external resistances cannot be irogueed in squirrel cage motor, The extemal resistan idecarentat the starting instant but alsy increases the starting rg due f0 improvement in power factor Ror small and medium size motors face-plate starters are large size ‘used but for motors liquid starters are used, 8482 Speed Control of Induction Motors, The spe of i ie anindooion motoris given as N= 20 4 5 obviously the se ofan ndction motor can be onto by saying any ee fit namely supply frequency numberat ato slips. - 1.Frequency Control. Though this ‘method provides wide sedconrol rane with grad vation in speed thengh the range but the difficulty ish 4 ew {0 Bet the variable supply frequency. Thatis why this method isnot used for, Squire cage seat ‘general purpose speed control applications. 2.By Changing of Number of Poles, ‘Thismethodis generally rotapplied to slip-ring motors as in suc *h machines thi ‘Method would involve considerable complications of design met switching, since the interconnections of both primary and secondary would have to be changed simultaneously in a sane to produce the same number of poles in both windings, ‘With wo independent sets of stator windings, each arranged for pak changing, as many as four synchronous speeds ean fe bianed in a squirrel cage motor, This method has the advantages of simplicity, good speed regulation for each setting, high ficiency, and moderate first cost and maintenance, This metho is very satisfactory for applications such as ventilating fans, Senos: machine tools, or other applications which require operation at only two or four speeds, 3. Slip Control. The vari ‘given below: (@) Line Voltage Control. This method of speed control is ‘imple, low in first cost and has low maintenance cost, but because of limi lations which it imposes on the maximum eveloped torque, itis used. ‘only with small squirrel-cage motors driving fans, 0) Rotor Resistance Control, This method of speed contol hascharacteristics. similar to those of de shunt motors controlled by ‘Means of resistances in series with the armature. Its drawbacks are icency and poor speed regulation due o increase inrotor iste. Because of convenience and simplicity, iti often ‘PPloyed when speed isto be reduced fora short period only. {Secondary Foreign Voltage Control. In this method, the Fat ofan induction motors controlled by injecting a voltage slip frequency in the secondary circuit. I the injected emfis In phase with the secondary circuit induced emf the slip will Asrease (or Speed will increase) and if itis in phase ‘opposition ns e2tondary ciceitinduced emf sip wil increase speed Tlsestase. Is costlier due to needs of auxiliary Iacines {orinectng ‘mf of slip frequency in the secondary circuit a tag moors oF very large rain, such as for motors in ling mils, lous methods of slip controls are _ e a 8 way tha the revolving field conn M Machines are in the same direction; under this Yon the resulting synchronous, Speed will be given as 129, N, te +-(9.19) . ney here aeDely frequency and Py and Py are the number of eames ys wet sPeed contol, fected canbe obtained manag stn ai of wound rr pes ee ang ind rotor type and inserting control tof the second machine, Round on the rotor ands fed through tree sliprings, while the Tromiitys wound onthe stator, separate de desi inding, known as regulating windi N + is placed in the same slots as the Primary winding and connect brushes are arranged on the Secondary winding is connect of brushes can be adjusted as spacing between them by: ted 0 commutator. Six sets of Commutator, and each phase of ted to a pair of them. The two sets S to angular position and relative ‘means of ahand wheel, The magnitude Thisis more expensive than a slip. is efficiency is higher at all speeds andis very much high at lower speeds. 9.4.34, Merits, Demerits ai Phase Induction Motors, {induction motors are () simp except synchronous peed: ind Applications of Three ‘The advantages of squirrel cage le and rugged construction (if) low initial as well as maintenance cost (ji) nearly constant speed (o) high overload capacity (v) simple starting arrangement and (vi) high power factor. Because of low resistance of rotor in comparison to that of slip-ring induction motor, it has low rotor ‘copper loss and, therefore, higher efficiency _ The drawbacks are (large starting curent (i) very sensitive to fluctuations in supply voltage (i) low power factor at light toads (iv) speed contol very dificult and () very poor stating {0 its low rotor resistance, Penasmenseniann lust vives of smal power where speed control is not required such as for printing ‘machinery, flour mills and other shatt dives of small power, The slip-ting induction motors in comparison to squirrel ceage motors have low stating current and high starting torque but suffer due to low power factor and low efficiency. These ‘motors ate used for most industrial drives of high power and requiring high starting torque. © scanned with OKEN Scanner‘An Integrated Course in Electrical Engineering 9.1.38. Comparison of Squirrel Cage and Slip-ting Induction Motors ry lip-Ring Induction Motor = So, Characeraicy Saurel Cage induction Motor SE es Mitr 5 springs bushes, shor: cuit | Construction Simple and rugged eee eulting device 2 ‘Overhang: Less. Large | 3 ‘Space factor in slots Better Rooks | 4 Cost (initial as welll as Less aoe imines) | 5S Copper losses ‘Small More | 6. Efficiency ‘High (only for machines, not designed for | Low | ‘high starting torque) | 2 Power factor Better, Poor 8 | cooking Cootngeterbecuseofitsbarcendrings | Not quite ecient davalbliyofmore space foro. Tas 9. ‘Speed regulation Better ‘Poor when operated with external resistances in the. recat 10. ‘Starting ‘Simple The motor needs slip rings, brushgear, shor. shelng devo and sartngresisorsee 11 | staningtrgue Stag torque low wth re stating | Posty oficresing starting tau by nea caret of enteral resistances inher. ce 12, | Powerfacoratsan Poor Canteimproved 13. | Speedcontol Nopossiity Possibly insertion of extemal resitorin thera 14, | Protetionsgninsterpasion | Explasonproot Notexploson poof 9.1.36. Comparison Between. Synchronous and Induction Motors. = DE See ee 1. | thas got nose sing orgie nd some external ean seed I has got sl tartng torque and no pci means vequed foritssaing. forts sang 2. | Its average speed is constant and independent of load. Its speed falls with the increase in load and is always less than | syctvonos sped. 2. | Atcanbe operated ander wide range of ower factor, othagging| Itoprats at only lguing power fecto, which becomes ve andleadog poort ight loads | 4, | Strequires dc excitation so it isa doubly excited machine, Itrequires no de excitation soit isa singly excited machine. | 5. | Nospeed control is possible. ‘Speed can be controlled but to small extent. 6. | Itis used for supplying mechanical! oad as wells for power factor! Itis used fr supplying mechanical load only, inp verment, | 1. | tetorqu is ess sense to change in supply volage. Istorueis more seaskivetochangein supply voage. | 8. | Breakdown torque is proportional to the supply voltage. ‘Breakdown torque is proportional to the square of the supply volage 9, | tismore complicated and cots more compartvely IRismoresimple nd costs less comparatively. motors. However, synchronous motors with speeds below 500 rpm an ratings exceeding about 40 KW or with medium speds from 500 to 1,000 rpm andratingsexceding abou SO0W costes than induction 9.1.97. Effects of Operating Conduction. ( Effect of Loading. With the increase in load, speed falls and consequently torque increases. A point will reach when the maximum torque will be developed. A further inerease in load causes further drop in speed, ‘consequently the driving torque decreases and the rotor stops ultimately. Ty] (ii) Effect of Unbatance Supply Voltage. Ifthe -pb3S* supply to the motor is not balanced, the rotatisg magnetic field move ata ni ‘non-uniform strength, This will cause more unbalant Currents in the stator windings and, therefore, prod 4n unequal heating, uniform rate and ins (tii) Effect of Break in One Phase. If one phase of * Polyphase induction motor breaks y be due (0249 —_ © scanned with OKEN ScannerInduction Machi 1 motor isin operaton, the motor will reas ee igle phase, provided that the load ine 57.7% ofthe normal rating, with about does ot on ature ise as when catrying rated oad as hese Moction motor. I will not, however, star, ose ough pee y sme exten 8 "as palling om the belt, it will continue to es ST eo on Yad below 57.7% of rated one. one oe phase ofthe rotor of a wound rotor A esom motor may prevent from starting butif itis ind to speed before the circuit is opened it wll boie under reduced Toad but usually with oper erable vibrations. an ects of Variations in Line Frequeney and Line 0 aire tis usual for manufacturers to guarantee vefactory operation of motors with only slight altered ‘Sacersties with voltage or frequency variations chfynting to not more than 10% above or below arma, provided, however, that when both voltage . eMpequency vary at the same time the combined nation shall not exceed 10% above or below. se variations in supply frequency and supply voltage will sae only the speed and torque of the motor but also the ge cuent,ful-load current, starting forque, maximum es given below in Table 9.1) and operating temperature wea ous pats ofthe machine and thus make their operation vasisfctoy, 44.38, Synchronous-Induction Motors. This is fiamenully a wound rotor induction motor. ‘The rotor slots av fower and larger. The airgap is that of synchronous motor sce the machine operates a8 a synchronous motor at normal lms These machines are provided with a heavy rotor winding inower to have a low slip, which facilitates in pulling it into sckonism. Also in order that the induced emt in the field at Suing may not be to high, the field turns provided are few in tuber and the excitation voltage is kept low. Although such richines have excellent characteristics but they are expensive 224 have lower efficiency than standard types. Synchronous ‘nluction motors are used where a high starting torqu« ‘quied, They can be made to operate at any desired pf by ‘aying its de excitation. 6 9. Induction Generator. Ifa polyphase induction motor Contected to constant voltage and frequency mains is ly coupled toa prime mover and is driven by it a @ speed higher than synchronous speed, the machine will operate 48 a generator and deliver electrical energy to the mains instead Of taking it from the mains. The induction generator differs from ‘synchronous generator in some respects such as (A it does not need de excitation (ii) it only generates when its stator is connected to the mains of constant frequency, its exciting ‘current being the reactive (lagging), magnetizing current drawn from the mains (if the frequency is independent of the speed of the generator and (iv) it does not require synchronization. Induction generator is simple and rugged in construction, cheaper in cost, easy in maintenance, does not hunt or dropout of synchronism and when short circuited it delivers little or no sustained power, because its excitation quickly becomes Zero. Inspite of all these advantages itis litle used as it eannot be ‘operated independently and it can deliver only leading current. Itis very useful for braking purpose in railway work. 9.1.40. Induction Voltage Regulator. Single-phase {induction voltage regulator is similar in construction to single phase induction motor except that the rotor is not allowed to rotate continuously but can be tumed through one pole pitch usually by worm and pinion gearing from a hand whee! or pilot ‘motor drive. The primary winding is usually wound on the rotor ‘and the secondary is wound on the stator. Both windings are usually two pole and spread over approximately 120°. The primary winding is connected across the circuit to be regulated and the secondary is connected in series with the circuit. To have a voltage regulation of 20% or # 10%, the induction voltage regulator of rating 10% of the kVA rating of the circuit is required. Three phase induction regulators are somewhat similar to single phase induction regulators in constructional features, there being one series (secondary) and one shunt (primary) ‘winding for each phase placed on circular cores as in the single phase induction regulators. Induction voltage regulators like transformers are available in air-cooled or non-inflammable liquid cooled designs for indoor use. Oil cooled regulators are ordinarity used outdoors. 9.1.41. Phase Advancer. Itis a particular type of ac exciter which may be connected in the rotor circuit of an induction motor to improve the power factor. The principle used is that of injection through the slip rings of the motor a current which is leading with regard to the rotor voltage. This current relieves the stator circuit of the duty of magnetising the machine and as leading current is supplied to the rotor at the low voltage corresponding to the slip of the rotor, the KVA capacity of the TABLES. Eifecsof Variationsin Line Frequeney and Line Voltage “slip : beset ‘ - Increases with ; vrokage, V Constant ecco in av Suppl Incte regency, as Constant sy wi © scanned with OKEN ScannerPhase advancer need be only $9 or less of KVA correction affected in the main supply circuit. The fall corrective kVA Could have to be provided iffa static or rotary eapacitor were ‘sed in the supply circuit, The phase advancer is thus very advantageous for use with large motors. In its simplest form it consists of a rotor built as a de machine armature and provided ‘with commutator, and of a stator assembled of sec laminations and acting only asthe magnetic cireuit for the flux ofthe phase advancer. Three brushes are arranged on the commutator, 120° apart. °.1-42, Electric Braking With Three-Phase Induction Motors. The simplestmethod of stopping ofan induction moter, oF any other type of motor, is to disconnect the motor from the ‘Supply mains. Torque is no longer developed, and the combined effect of the rotor and the extemal load brings the motor to Standstill, When rapid and mote positive action is required, mechanical or electrical braking may be employed, but the latter has many advantages, particularly where precise control ofthe Stopping moment and smoothness of operation are required. ‘The motor is sid to operate on electrical braking when the direction of developed torque is opposite to that of rotation, During electrical braking the motor operates on torque-speed istic corresponding to the method of braking employed. There are three methods of electrical braking of induction motors: plugging (or counter-current braking), dynamic (ot ‘heostatic braking) and regenerative braking. 1, Plugging (or Counter-Current Braking). Plugging can be achieved in an induction motor merely by reversing any two of the three phases which causes a reversal of the direction ofthe rotating magnetic field, At the instant of switching the motor to the plugging position the motor runs in the opposite direction to that of the field and the relative speed is approximately twice (2-5) times] of synchronous speed i, the slip is very nearly ‘equal to two, being equal to (2 - s). So voltage induced in the rotor will be twice of normally induced voltage at standstill and the winding must be provided with the additional insulation to withstand this much voltage. In practice, for reversing drives where braking and stating up of induction motor in reverse direction comprises stages of the same continuous process, plugging is advantageously employed. 2. Dynamic (or Rheostatie Braking). The rheostatic braking with a polyphase induction motor can be obtained by disconnecting the stator winding from the ac supply and exciting it from a de source to produce a stationary de field. In rheost braking, the stator winding is employed as a de field winding and the rotor winding as an armature winding. With a wound rotor machine, external resistors can be inserted into the rotor circuit to provide a load. With squirrel cage machines, however, the rotor winding itself has to form the load. ‘The source of excitation may be provided either by an independent de source or from the ac mains through a transformer-rectifier set 3, Regenerative Braking. Regenerative ‘braking isan inherent characteristic of an induetion motor, since it operates as an induction generator when it runs at speed above synchronous and it feeds power back to the supply line. {An Integrated Course in Electrical Englnoering ‘The 3-phase induction motor can be ma speed above synchronous speed by emplo following processes. () Switching overto ow fequeney suppyn controled induction motors inorder rede) of operation ofthe drive, ‘fey (i Downward motion of a loaded hoistin, such as crane hoists, excavators etc, Switching over to a large pole number of from a smaller one in multispeed squirrel cage 10 opey rte 7M ay net i 8 Mecha, aig ot Greater number of motors of comparatively small ratings yp ‘manufactured for operation on single-phase ae supply. Nog ‘hem, builtin fractional horse power sizes, are technically me, 38 smal motors, a small motor being defined as “a motor hat {na frame smaller than having a continuous rating of | Single-phase motors perform great variety of useful service, the home, the office, the factory, in business establishment the farm and many other numerous applications where elec is available, These find interesting applications in automat, control devices of various types. Since the requirements oft numerous applications differ so widely, the motor ‘manufacturing industry has developed several types of sh ‘motors, each type having operating characteristics that meet definite demands, Single-phase ac motors may be divided ini three general classes, namely, (i) induction motors (i) ‘commutator motors and (if) synchronous motors. 9.2.1. Single-Phase Induction Motors. The construcne of single-phase induction motoris more of less similar to that 4 polyphase induction motor except that the stator is provided with a single-phase winding, An induction motor with a at rotor and a single-phase stator winding is represented schematically in Fig. 9.8. Instead of being a concentrated cl, the actual stator winding is distributed in slots in order opie ‘an approximately sinusoidally distributed mmf in space. Sucha ‘motor inherently has no starting torque, but if started by sone auxiliary means, it will continue to operate. Single-phase induction motors suffer from the several drawbacks such as lw overload capacity, low efficiency, low power factor et. ‘The explanation of single-phase motor action may be give by any one of the following two theories, (i) Double revolving field theory (i) Cross field theory. Double revolving field theory is based on the idea tht pulsating field produced in single-phase motors can be resold into two components of half its amplitude and rotating it ‘opposite directions with synchronous speed. 9.2.2. Starting Methods and Types of Single-Phas? Induction Motors, As already mentioned above the sing!” hase induction motors are not: ‘self starting and need some meats {ot aating, Comercial single-phase indution motes Classified in accordance with the methods of starting empl ‘The various methods employed for starting are given ea Split-Phase Method. If two windings connected in pal aos single-phase ac supply are displaced 90 clestical {2 SPace and Somewhat less than 90°in time, whichis pos” by connecting an ‘impedance (resistor or capacitor) in © scanned with OKEN Scanner}+— statoOR ‘no snaue Fig.9.8 Elementary Single Phase Induction Motor gh oe ofthe windings, a rotating feld, very much like the Faidof a two-phase motor, i produced. This is the principle oF phase epltting and the single-phase induction motors playing tis principle for starting are known as spir-phase hotors. Various types of split-phase motors are given below. (a) Resistance-Start Motor. The motors of this type are jded with two windings, known as auxiliary (or starting) winding and main winding, with their axes displaced 90 tlectical degrees in space. They are connected as shown in Fig, 99. The auxiliary winding has a larger resistance and lesser reactance than the main winding and sometimes extra resistance js inserted in series with the auxiliary winding, as shown in Fig. 99. The auxiliary winding is made to have large resistance and low reactance by winding it from finer wire and placing it in the top of the slots. The two windings are so designed that the ‘curents in two windings I,, and I, are about 20° to 30° out of phase in time. When the motor picks up the speed, 75 or 80%, of synchronous speed the starting winding is taken out of circuit byatransfer or centrifugal switch. Itis important that this should be done because otherwise it will result in overheating and buming out of auxiliary winding owing to its low current carrying ‘capacity and inefficient and noisy performance. ‘The resistance-start split-phase motor is commonly known 48the split-phase motor even though there are other forms of ‘plit-phase motors. B wan win SNe Phase Ligiesimnre Ly AUILARY } > wows RESISTANCE, R ‘CENTRIFUGAL SWITCH, § Fig.9.9 Resistance-Start Motor ‘The starting torque is 150 to 200 per cent of full-load running torque and the starting current is 6 to 8 times full-load ‘current. The motors are made in fractional horse power sizes and are used for a large variety of applications, such as for driving ‘washing machines, fans, blowers, centrifugal pumps, refrigerators, duplicating machines, woodworking tools, grinders, oil burners and various other low starting torque applications. Because of its low starting torque, this motor is seldom used in sizes larger than 1/4 KW. Standard sizes are from 1/8 KW to 1/4 kW with a speed of 1,440 rpm. (b) Capacitor-Start Motor. This is an improved form of split-phase motor. The capacitor-start motors, like the resistance- start motors, are provided with two stator windings displaced 90 electrical degrees from each other. The main winding is connected directly across the Tine and the starting winding is connected to the line through a static capacitor of suitable value and centrifugal switch. The line current of a resistance- start motor was found to be two-third higher than the line current of a corresponding capacitor-start motor while the starting torque of the capacitor-start motor was twice that of the resistance-start motor. The capacitor-start motor, like resistance-start motor has the starting winding disconnected by means of a centrifugal switch as the motor picks up speed. This is necessary because (i) the capacitance that provides the largest starting torque is about four times too big for the best running conditions and (ii) if the capacitor is used only for starting, it can be an electrolytic type of capacitors. This type ‘of motor had become popular because of development of cheap and reliable electrolytic condensers. Its common applications are in refrigerator, compressor or other applications involving, a hard starting torque such as in small portable hoists. The usual size range is from 1/10 to 1/2 KW but larger sizes are also ilable, The direction of rotation of motor may be reversed ferchanging the connections to the supply of either the main or auxiliary winding. (c) Capacitor-Start Capacitor-Run Motor, inorder to have the advantages that result from the use of a capacitor during the normal running operation as well as starting period, the capacitor-start capacitor-run motors have been developed. Ri © scanned with OKEN ScannerSince the value ofthe capsitance which wll esl in eptianum Snming performance is not the same as the value which will Droduce the best starting torque conditions and aso theca amployed for normal running should by of continuous duty ‘ating and that employed for starting should be of short-duty ‘ating, therefore, in this type of motor two capacitors-one of Small value oil impregnated paper continuous rating capacitor Cx and another of much larger value electrolytic short-duty Capacitor C, are connected, as shown in Fig. 9.10, When the Toror attains 70% of synchronous speed the starting capacitor Gs taken out of circuit by the operation of centrifugal switch, pacitor Cy remains permanently in series with the starting of auxiliary winding. Such motors operate as two-phase motors from single-phase supply, thereby, producing constant torque and not a pulsating torque as in other single-phase motors. Beside wy loads, they are extremely Auite in operation, have better efficiency and power factor when Toaded and develop up to 25% greater overload capacities. These ‘are indeed splendid machines where the load requirements are Severe. The disadvantage of these machines is only high cost. ‘These motors are often employed requiring a quite operating motor. The motors ofthis type have become increasingly popular and are now manufactured inthe larger sized single-phase motors Mhere previously only the repulsion start motors were available. The direction of rotation of motor may be reversed by interchanging the connections to the supply of either the main or auxiliary winding. ‘An Integrated Course in Electrical Engineering shading cot a8 shown in Fig 9.11. The single pag —~ flowing through field winding produces an alterna, ey ‘Some of the lux through each pole links with the gh You shading ring, The Muxndcesa.curentin the rng, get of flow of which wll be such that it opposes the ehanen hich inks withthe shading cil, The shading ec ge the flux inthe shaded portion to lag behind the fux in me orion of ple. This provides in effect a motion ey the pole face and under the influence ofthis moving stating torgue is developed. As Soon a8 the rotor sae? ‘under the influence of starting torque, additional torque, crea, by single-phase induction motor action. Thus the mat’ accelerates toa speed slighty below synchronous and gn” single-phase induction motor. The starting torque of sa pole motors very small, and therefore tis wed ony fos fans, electic clocks, hair dryers, time phonographs,insiruneay and other similar applications. Since the shading cil se mq taken out of circuitafter the moto attains normal speed, theron, they cause some additional energy loss. The direction of roto, ‘of motor depends upon the position of the shading coil on the Pole, ie,, which half ofthe pole is encircled by the shading ci ‘Unless the machine is constructed so that shading coil can te shifted tothe other half of the pole, motors ofthis type canny have their direction of rotation reversed, Ithas got advantages of (lack of centrifugal switch and (i use of concentrated winding up sugurouse [" whois snot mase err }—__] co n = ON pemeren Same Fig.9.10 Schematic Diagram of Capacitor Start Capacitor Run Motor (@) Permanent Capacitor Motor, In applications where the motor starts under practically zero load, itis possible to eliminate the centrifugal switch and one of the capacitors thereby making the construction simple and reducing the cost. In such motors the capacitor employed is of oil type construction and is permanently connected in series with one winding. Since {n such motors same capacitance is employed for stating and running, therefore, neither optimum starting nor running performance can be obtained. The direction of rotation of motor ‘may be reversed by interchanging the connections to the supply of either the main winding or auxiliary winding. Such motors are employed in table and ceiling fans, blowers, oil bumers, Where low starting torque is required. : (©) The Shaded-Pole Motor. The rotor of this motor is of squirrel cage type and stator has salient pole, each provided with its own exciting coil. A part of each pole is wrapped by a short-circuited copper strap forming a closed loop, known as a Fig.9.11 Shaded-Pole Single-Phase Induction motor ‘Speed of single phase induction motors can be controled by varying the applied voltage to the stator winding and this varying the slip. 9.2.3. Commutator Motors. The more common commuta" ‘motors ae the series motors, universal motors, repulsion motos ‘and the repulsion-induction motors, with various modifications ‘and combinations, 1.AC Series Motors, Ditect current shunt or series motos rotate in the same direction regardless of the polarity of tke supply. Thus, it might be expected that either motor would ‘operate on alternating current. It has been found, however hit the shunt motor develops but itl torque when i is connected ‘oan ac supply. The reason of itis thatthe field winding, ovine toits high inductance, causes the field current to lag the armstu® current by such a large angle that a very low net torque rest However in series motor, the field and armature currents bets the same, the main field and armature currents are in phase © scanned with OKEN Scannerspain same torque is developed with a given "with a like amount of direct current in a cure orinaty series motor were connected tan rf ld operate, But not very satisfactorily. The Sarl. cng torque doe to reversal of armature and are (Poser cycle, (i) excessive eddy current losses ere yoke duc to alternations in the field flux, ang due to induced voltages and currents in the eae Tap ircuited by the brushes when undergoing col () anormal voltage drop and Tow power na inductance of field winding, ce eae modcations ae necessary fr satisfactory oe, These modifications are () Series field with ax genome posible inorder 10 reduce reactance. (i) Large fe ature conductors to compensate for reduced field raevelop 2 given orgue (i Compensating winding is ro nealize the armature reaction effect. () Large pov poles with lesser flux pe pole in order to lover bet ferent in the shor-cireuited element of the armature se (Very smal ar gap dve to weak eld (¥) Laminated sy of magnetic ciel 0 reduce eddy current loses (vi) al pio operation on1ow voltage and Tow frequency n order to give less in chanctristcs: an reder to have good commutation, increased effi it creased output with a given size of armature core. ‘The single-phase ae series motor has practically the same cpering characteristics as de series motors. The torque or eave effort varies nearly as the square of the current and the speed varies inversely as the current nearly. The inductively compensated ac series motor also operates satisfactorily on de gjsem and has increased output and efficiency. The speed of the motor while working on ac system can be controlled effcienly by taps on a transformer. ‘The most important application of ac series motor is in elec traction service up to 1600 kW, 200 to 600 volts using 1525 He. 2. Universal Motors. For some applications, itis desirable Aoemploy a motor that will operate on either ac or de supply. BY ‘compromise design, fractional horse power series motors may be built to operate satisfactorily on either 50 Hz ac or direct cures 115 0 20 volts. Such motos are known as universal Universal motor is a series wound motor, which operates at ‘pproximately the same speed and output on either de or ac of ‘proximately same voltage. aso be armature of universal motors is ofthe same construction ae series motor. In the small sizes the voltage induced ee sformer action in a coil during its commutation period cog Oltend to produce sufficient eurent to cause 20Y serious ‘aid ie Problem. High-resistance brushes are ‘employed ‘pli commutation, In large motors compensa winding to improve the commutation. The stator core and are laminated to reduce eddy currents produced by ‘ductance and so to improve operating «(vii Provision of commutating or interpoles iency and oye Motor is simple, less expensive and is used for lower CSE GP 106 kW) and higher spods. The characteris of ‘re very much similar to those of de series motors. — Induction Machines us Fig. 9.12 Universal Motor ‘These motors are suitable for sewing machines, table fans, ‘vacuum cleaners, portable drills, hair dryers, blowers and kitchen appliances etc. “The direction of rotation can be changed by interchanging connections tothe field with respect to the armature as in a de series motor. 3, Repulsion Motors. Repulsion motors are similar to series motors except that rotor and stator windings are inductively coupled i.,, the rotor current is obtained by transformer action from the stator. The stator usually carries a distributed winding like the main winding of an ordinary single-phase induction rotor. The rotor or armature is similar to a de motor armature, with a drum type winding connected to the commutator. However, the brushes fixed directly opposite to each other are not connected to the supply, but are connected to each other OF short-circuited. The magnetic axis of the rotor is determined by the brush position. Ifthe rotor axis were in line with stator field ‘axis, the short-circuited brushes would join the points of maximum potential difference, current would flow between the ‘brushes, but no torque would be produced as the torque angle would be zero. Ifthe rotor axis were in quadrature with stator Field axis, the torque angle would be at the optimum value of {90° but the emfs induced in the two sections of the winding ‘would be equal and the short-circuited brushes would join the fequipotental points, therefore, no current would flow resulting there by no torque. Actually, the brushes are placed in an jntermediate position, as shown in Fig. 9.13. The current induced in the rotor winding has component in time phase with the flux and produces a torque. ‘The direction of rotation depends upon the position of the brushes, If the brushes are shifted round the commutator, the direction of rotation will reverse. Speed control is affected by sarying the impressed voltage or by changing the position of brushes. ‘The characteristics of the repulsion motor are similar to those of a series motor ie. high starting torque and high speed light loads. The motor is used where sturdy motor with the forge starting torque and adjustable but constant speed is required. Most common use ofthis type of motor is inthe coil ‘Minders, in which the operator adjusts the speed by shifting the brushes; the motor is equipped with a special lever mechanism, that shifts the brushes when a foot treadle is pressed. 4 Repulsion-Start Induction Motor. As the name implies, n-start induction motor starts as a repulsion motor © scanned with OKEN ScannerPOLE Fig.9.13 Repulsion Motor ‘and runs as an induction. built like that of a commutator. To ¢ is necessary to shi motor. The rotor ofa repulsion motoris de motor i.e, the windings are connected to onvert such a motor to an induction motor, it ‘ort circuit the commutator segments, which is accomplished by a centrifugal switch that operates as the motor atans nearly synchronous speed o throw a circular conducting disc on the commutator In some motors the centifigal device also lifts the brushes from the eomimutator in order to reduce ‘ear ofthe brushes and commutator bars and to make the runing operation more quiet. ‘he motor is connected across single-phase ac supply the current induced in the armature winding is cartied by the brushes and commutator resulting in high starting torque. The Sarting torque is 2.5 104.5 times of full-load torque and starting current is approximately 3.5 times of full-load current. When the motor armature attains nearly synchronous speed, the commutator is short circuited by the centrifugal switch and the ‘armature acts as a squirrel cage armature. Thus repulsion start induction motor combines the desirable starting characteristics of the repulsion motor with operating characteristics of the induction motor. The direction of rotation can be reversed by ‘changing the position of brushes. Although itis the most costly ofthe various types of single Phase motors, it nevertheless possesses two extremely desirable characteristics (i) high starting torque for periods that are of comparatively long duration and low starting current. (i) In Connection with the later point, these motors draw starting Current that are about 60 to 70% of that taken by corresponding sizes in the capacitor-start or capacitor-start-capacitor-run motors. This type of motor is suitable for refrigerators, compressors and other drives, particularly those which have a high inenia and prolonged starting period. The usual range of sizes is from 1/3 KW to 12 KW but for special applications, ratings as high as 30 KW are available, 5. Repulsion-Induetion Motor. The repulsion-induction ‘motor has a single-phase stator winding but it has two separate ‘windings on rotor in common slots, The inner winding isa squirrel cage winding with the rotor bars permanently short circuited, Placed over the squirrel cage winding is a repulsion winding similar to a de armature winding. The repulsion winding is ‘connected toa commutator on which ride short-circuited brushes, ‘When the motor starts, the squirrel cage winding due to its high ‘eactance has no effect and the motor starts as a repulsion motor giving high starting torque. As the motor picks up speed, the ‘squirrel cage winding comes into action, The shifting from the repulsion to the induction motor characteristics is thus done without any switching arrangement, Both of the armature 390 ‘An Integrated Course In Electrical Engineering windigs are active during the entire period of Onetog >> starting torque is about 2.25 to 3.00 times of the FUL toad to Te th lowe beng forgets andthe stating un 4 times the full-load current. 834, The chief disadvantages of this type of motor careful maintenance and tendency of sparking, This type of motor is used for applications requir, 7 staring torque with an essentially constant running geet common zesare 60 44 although ages ara ‘common application is the air compressor. Mbit, sHorr-cncurey gy BRUSHES a ‘CoMmutaton WOW. sNole Prase 0.0) saurenes cat AcsuPriy ‘WINDING Fig.9.14 Repulsion-Induction Motor 8.2.4. Servomotors, The motors which respond to the enor signal abruptly and accelerate the load quickly are called te servomorors. The torque-inertia (T) ratio i an important figure Of a servomotor because the load is to be accelerated by it abruptly. The fundamental characteristics tobe sou motorare : © The motor output torque should be proportional tote voltage applied to it (é.e., control voltage developed by the amplifier in response to an error signal) (i The direction of the torque developed by the Servomotor should depend upon the instantaneous polarity of the control voltage. Servomotors are of two types namely de servomotors and fc Servomotors. AC servomotors are generally preferred to de Servomotors except foruse in very high-power svstems, Forty high power systems de motors are preferred because they ope ‘more efficiently than comparable ac servomotors, This enables them to stay cooler. An efficient motor also prevents excesie Power waste, although power waste is generally not a prime concer in servomechanism, ight in any sen. DC Servomotors, Among the various de servomotors, art the Series motors, split series motors, shunt control motor, athe Permanent magnet (fixed exeitation) shunt motor. These ui develop high output power ina given size and in the case of the field controlled shunt motor, little contrat power is requited An ac servomotor is basically a two-phase induction mot except for certain special design features. There is one import difference between a standard splitphase motor and an Servomotor—servomotor has thinner conducting bars it he Sauitrel cage rotor so thatthe motor resistance is higher. © scanned with OKEN Scanner