Module 9. DC Machines. Version 2 EE IIT, Kharagpur

Transcription

1 Module 9 DC Mchines

2 Lesson 39 D.C Motors

3 Contents 39 D.C Shunt Motor (Lesson-39) Gols of the lesson Introduction Importnt Ides Strting of D.C shunt motor Problems of strting with full voltge A simple strter point strter Speed control of shunt motor Speed control by vrying rmture resistnce Speed control by vrying field current Speed control by rmture voltge vrition Wrd Leonrd method: combintion of V nd I f control Series motor Chrcteristics of series motor Speed control of series motor Speed control below bse speed Speed control bove bse speed Brking of d.c shunt motor: bsic ide Rheosttic brking Plugging or dynmic brking Regenertive brking Tick the correct nswer Solve the following. 24

4 39.1 Gols of the lesson In this lesson spects of strting nd speed control of d.c motors re discussed nd explined. At the end principles of electric brking of d.c. shunt motor is discussed. After going through the lesson, the reder is expected to hve cler ides of the following. 1. The problems of strting d.c motors with full rted voltge. 2. Use nd selection of vrible resistnce s simple strter in the rmture circuit of d.c motor. 3. Superiority of commercil strter (3-point strter) over resistnce strter. Vrious protective fetures incorported in commercil strter. 4. Vrious strtegies (nmely-rmture resistnce control, rmture voltge control nd field current control) dopted for controlling speed of d.c motors. 5. Importnce of chrcteristics such s (i) speed vs. rmture current nd (ii) speed vs. torque which re relevnt for cler understnding of speed control technique. 6. Principle of electric brking qulittive explntion Introduction Although in this section we shll minly discuss shunt motor, however, brief descriptions of (i) D.C shunt, (ii) seprtely excited nd (iii) series motor widely used re given t the beginning. The rmture nd field coils re connected in prllel in d.c shunt motor s shown in figure 39.1 nd the prllel combintion is supplied with voltge V. I L, I nd I f re respectively the current drwn from supply, the rmture current nd the field current respectively. The following equtions cn be written by pplying KCL, nd KVL in the field circuit nd KVL in the rmture circuit. I = I +I pplying KCL L f V I f = from KVLin field circuit R f V-Eb I = from KVL in the rmture circuit r V-kφn = r

5 39.3 Importnt Ides We hve lernt in the previous lecture (37), tht for motor opertion: 1. Electromgnetic torque T=k e φ I developed by the motor cts long the direction of rottion. 2. The lod torque T L cts in the opposite direction of rottion or in opposition to T e. 3. If T=T e L, motor opertes with constnt speed.. 4. If t ny time T>T e L, the motor will ccelerte. 5. If t ny time T<T e L, the motor will decelerte. Although our min focus of study will be the opertion of motor under stedy stte condition, knowledge of how motor moves from one stedy stte operting point to nother stedy operting point is importnt to note. To begin with let us study, how motor from rest condition settles to the finl operting point. Let us ssume the motor is bsolutely under no-lod condition which essentilly mens T L = 0 nd there is friction present. Thus when supply is switched on, both I =V r nd φ will be estblished developing T e. As T L = 0, motor should pick up speed due to ccelertion. As motor speed increses, rmture current decreses since bck emf E b rises. The vlue of T e lso progressively decreses. But so long T e is present, ccelertion will continue, incresing speed nd bck emf. A time will come when supply voltge nd E b will be sme mking rmture current I zero. Now T e becomes zero nd ccelertion stops nd motor n=v kφ nd drwing no rmture continues to run stedily t constnt speed given by ( ) current. Note tht input power to the rmture is zero nd mechnicl output power is zero s well. Let us bring little relity to the previous discussion. Let us not neglect frictionl torque during ccelertion period from rest. Let us lso ssume frictionl torque to be constnt nd equl to T fric. How the finl operting point will be decided in this cse? When supply will be switched on T e will be developed nd mchine will ccelerte if T e > T fric. With time T e will decrese s I decreses. Eventully, time will come when T e becomes equl to T L nd motor will continue to run t constnt stedy no lod speed n 0. The motor in the finl stedy stte

6 however will continue to drw definite mount of rmture current which will produce T e just enough to blnce T fric. Suppose, the motor is running stedily t no lod speed n 0, drwing no lod rmture I 0 nd producing torque T e0 (= T fric ). Now imgine, constnt lod torque is suddenly imposed on the shft of the motor t t = 0. Since speed cn not chnge instntneously, t t = 0 +, I (t = 0 + ) = I 0 nd T e (t = 0 + ) = T e0. Thus, t t = 0 +, opposing torque is (T L + T fric ) < T e0. Therefore, the motor should strt decelerting drwing more rmture current nd developing more T e. Finl stedy operting point will be reched when, T e = T fric + T L nd motor will run t new speed lower thn no lod speed n 0 but drwing I greter thn the no lod current I 0. In this section, we hve lernt the mechnism of how D.C motor gets loded. To find out stedy stte operting point, one should only del with stedy stte equtions involving torque nd current. For shunt motor, operting point my chnge due to (i) chnge in field current or φ, (ii) chnge in lod torque or (iii) chnge in both. Let us ssume the initil operting point to be: Armture current = I 1 Field current = I f 1 Flux per pole = φ 1 Speed in rps = n 1 Lod torque = T L1 T e1 = kφ 1I 1 = T L1 (39.1) E b1 = kφ 2n2 = V-I 1r (39.2) Now suppose, we hve chnged field current nd lod torque to new vlues I f2 nd T L2 respectively. Our problem is to find out the new stedy stte rmture current nd speed. Let, New rmture current = I 2 New field current = I f 2 New flux per pole = φ 2 Speed in rps = n 2 New lod torque = T L2 T e2 = kφ 2I 2 = T L2 (39.3) E b2 = kφ 2n2 = V-I2r (39.4) Now from equtions 39.1 nd 39.3 we get: Te2 TL 2 kφ = = T T kφ I 2I 2 e1 L1 1 1 (39.5) From eqution 39.5, one cn clculte the new rmture current I 2, the other things being known. Similrly using equtions 39.2 nd 39.4 we get:

7 Eb 1 kφ 2n2 V-I 1r = = E kφ n V -I r b (39.6) Now we cn clculte new stedy stte speed n 2 from eqution Strting of D.C shunt motor Problems of strting with full voltge We know rmture current in d.c motor is given by I V-Eb V-kφn = = r r At the instnt of strting, rotor speed n = 0, hence strting rmture current is I =. Since, rmture resistnce is quite smll, strting current my be quite high (mny times lrger thn the rted current). A lrge mchine, chrcterized by lrge rotor inerti (J), will pick up speed rther slowly. Thus the level of high strting current my be mintined for quite some time so s to cuse serious dmge to the brush/commuttor nd to the rmture winding. Also the source should be cpble of supplying this burst of lrge current. The other lods lredy connected to the sme source, would experience dip in the terminl voltge, every time D.C motor is ttempted to strt with full voltge. This dip in supply voltge is cused due to sudden rise in voltge drop in the source's internl resistnce. The durtion for which this drop in voltge will persist once gin depends on inerti (size) of the motor. Hence, for smll D.C motors extr precution my not be necessry during strting s lrge strting current will very quickly die down becuse of fst rise in the bck emf. However, for lrge motor, strter is to be used during strting A simple strter To limit the strting current, suitble externl resistnce R ext is connected in series (Figure V 39.2()) with the rmture so tht I st = R ext +r. At the time of strting, to hve sufficient strting torque, field current is mximized by keeping the externl field resistnce R f, to zero vlue. As the motor picks up speed, the vlue of R ext is grdully decresed to zero so tht during running no externl resistnce remins in the rmture circuit. But ech time one hs to restrt the motor, the externl rmture resistnce must be set to mximum vlue by moving the jockey mnully. Imgine, the motor to be running with R ext = 0 (Figure 39.2(b)). st V r

8 Now if the supply goes off (due to some problem in the supply side or due to lod shedding), motor will come to stop. All on sudden, let us imgine, supply is restored. This is then nothing but full voltge strting. In other words, one should be constntly lert to set the resistnce to mximum vlue whenever the motor comes to stop. This is one mjor limittion of simple rheosttic strter point strter A 3-point strter is extensively used to strt D.C shunt motor. It not only overcomes the difficulty of plin resistnce strter, but lso provides dditionl protective fetures such s over lod protection nd no volt protection. The digrm of 3-point strter connected to shunt motor is shown in figure Although, the circuit looks bit clumsy t first glnce, the bsic working principle is sme s tht of plin resistnce strter. The strter is shown enclosed within the dotted rectngulr box hving three terminls mrked s A, L nd F for externl connections. Terminl A is connected to one rmture terminl Al of the motor. Terminl F is connected to one field terminl F1 of the motor nd terminl L is connected to one supply terminl s shown. F2 terminl of field coil is connected to A2 through n externl vrible field resistnce nd the common point connected to supply (-ve). The externl rmtures resistnces consist of severl resistnces connected in series nd re shown in the form of n rc. The junctions of the resistnces re brought out s terminls (clled studs) nd mrked s 1,2, Just beneth the resistnces, continuous copper strip lso in the form of n rc is present. There is hndle which cn be moved in the clockwise direction ginst the spring tension. The spring tension keeps the hndle in the OFF position when no one ttempts to move it. Now let us trce the circuit from terminl L (supply + ve). The wire from L psses through smll electro mgnet clled OLRC, (the function of which we shll discuss little lter) nd enters through the hndle shown by dshed lines. Ner the end of the hndle two copper strips re firmly connected with the wire. The furthest strip is shown circulr shped nd the other strip is shown to be rectngulr. When the hndle is moved to the right, the circulr strip of the hndle will mke contcts with resistnce terminls 1, 2 etc. progressively. On the other hnd, the

9 rectngulr strip will mke contct with the continuous rc copper strip. The other end of this strip is brought s terminl F fter going through n electromgnet coil (clled NVRC). Terminl F is finlly connected to motor field terminl Fl. Working principle Let us explin the opertion of the strter. Initilly the hndle is in the OFF position. Neither rmture nor the field of the motor gets supply. Now the hndle is moved to stud number 1. In this position rmture nd ll the resistnces in series gets connected to the supply. Field coil gets full supply s the rectngulr strip mkes contct with rc copper strip. As the mchine picks up speed hndle is moved further to stud number 2. In this position the externl resistnce in the rmture circuit is less s the first resistnce is left out. Field however, continues to get full voltge by virtue of the continuous rc strip. Continuing in this wy, ll resistnces will be left out when stud number 12 (ON) is reched. In this position, the electromgnet (NVRC) will ttrct the soft iron piece ttched to the hndle. Even if the opertor removes his hnd from the hndle, it will still remin in the ON position s spring restoring force will be blnced by the force of ttrction between NVRC nd the soft iron piece of the hndle. The no volt relese coil (NVRC) crries sme current s tht of the field coil. In cse supply voltge goes off, field coil current will decrese to zero. Hence NVRC will be deenergised nd will not be ble to exert ny force on the soft iron piece of the hndle. Restoring force of the spring will bring the hndle bck in the OFF position. The strter lso provides over lod protection for the motor. The other electromgnet, OLRC overlod relese coil long with soft iron piece kept under it, is used to chieve this. The current flowing through OLRC is the line current I L drwn by the motor. As the motor is loded, I hence I L increses. Therefore, I L is mesure of loding of the motor. Suppose we wnt tht the motor should not be over loded beyond rted current. Now gp between the electromgnet

10 nd the soft iron piece is so djusted tht for I I, the iron piece will not be pulled up. However, if IL I rted force of ttrction will be sufficient to pull up iron piece. This upwrd movement of the iron piece of OLRC is utilized to de-energize NVRC. To the iron copper strip (Δ shped in figure) is ttched. During over loding condition, this copper strip will lso move up nd put short circuit between two terminls B nd C. Crefully note tht B nd C re nothing but the two ends of the NVRC. In other words, when over lod occurs short circuit pth is creted cross the NVRC. Hence NVRC will not crry ny current now nd gets deenergised. The moment it gets deenergised, spring ction will bring the hndle in the OFF position thereby disconnecting the motor from the supply. Three point strter hs one disdvntge. If we wnt to run the mchine t higher speed (bove rted speed) by field wekening (i.e., by reducing field current), the strength of NVRC mgnet my become so wek tht it will fil to hold the hndle in the ON position nd the spring ction will bring it bck in the OFF position. Thus we find tht flse disconnection of the motor tkes plce even when there is neither over lod nor ny sudden disruption of supply Speed control of shunt motor We know tht the speed of shunt motor is given by: L V -I r n= kφ where, V is the voltge pplied cross the rmture nd φ is the flux per pole nd is proportionl to the field current I f. As explined erlier, rmture current I is decided by the mechnicl lod present on the shft. Therefore, by vrying V nd I f we cn vry n. For fixed supply voltge nd the motor connected s shunt we cn vry V by controlling n externl resistnce connected in series with the rmture. I f of course cn be vried by controlling externl field resistnce R f connected with the field circuit. Thus for.shunt motor we hve essentilly two methods for controlling speed, nmely by: 1. vrying rmture resistnce. 2. vrying field resistnce Speed control by vrying rmture resistnce The inherent rmture resistnce r being smll, speed n versus rmture current I chrcteristic will be stright line with smll negtive slope s shown in figure In the discussion to follow we shll not disturb the field current from its rted vlue. At no lod (i.e., I = 0) speed is highest nd 0 = V V n kφ = kφ. Note tht for shunt motor voltge pplied to the field nd rmture circuit re sme nd equl to the supply voltge V. However, s the motor is loded, I r drop increses mking speed little less thn the no lod speed n 0. For well designed shunt motor this drop in speed is smll nd bout 3 to 5% with respect to no lod speed. This drop in speed from no lod to full lod condition expressed s percentge of no lod speed is clled the inherent speed regultion of the motor. rted n-n0 Inherent % speed regultion = 100 n 0

11 n Speed n 0 = no lod speed n = full lod speed n 0 n Speed Armture current Figure 39.4: Speed vs. rmture current chrcteristic. I T e Torque (rted) Figure 39.5: Speed vs. torque chrcteristic. It is for this reson, d.c shunt motor is sid to be prcticlly constnt speed motor (with no externl rmture resistnce connected) since speed drops by smll mount from no lod to full lod condition. Since e T=kφ I, for constnt φ opertion, T e becomes simply proportionl to I. Therefore, speed vs. torque chrcteristic is lso similr to speed vs. rmture current chrcteristic s shown in figure The slope of the n vs I or n vs T e chrcteristic cn be modified by delibertely connecting externl resistnce r ext in the rmture circuit. One cn get fmily of speed vs. rmture curves s shown in figures 39.6 nd 39.7 for vrious vlues of r ext. From these chrcteristic it cn be explined how speed control is chieved. Let us ssume tht the lod torque T L is constnt nd field current is lso kept constnt. Therefore, since stedy stte opertion demnds T e = T L, T e = kφ I too will remin constnt; which mens I will not chnge. Suppose r ext = 0, then t rted lod torque, operting point will be t C nd motor speed will be n. If dditionl resistnce r ext1 is introduced in the rmture circuit, new stedy stte operting speed will be n 1 corresponding to the operting point D. In this wy one cn get speed of n 2 corresponding to the operting point E, when r ext2 is introduced in the rmture circuit. This sme lod torque is supplied t vrious speed. Vrition of the speed is smooth nd speed will decrese smoothly if r ext is incresed. Obviously, this method is suitble for controlling speed below the bse speed nd for supplying constnt rted lod torque which ensures rted rmture current lwys. Although, this method provides smooth wide rnge speed control (from bse speed down to zero speed), hs serious drw bck since energy loss tkes plce in the externl resistnce r ext reducing the efficiency of the motor.

12 Speed control by vrying field current In this method field circuit resistnce is vried to control the speed of d.c shunt motor. Let us rewrite.the bsic eqution to understnd the method. n V-Ir = kφ If we vry I f, flux φ will chnge, hence speed will vry. To chnge I f n externl resistnce is connected in series with the field windings. The field coil produces rted flux when no externl resistnce is connected nd rted voltge is pplied cross field coil. It should be understood tht we cn only decrese flux from its rted vlue by dding externl resistnce. Thus the speed of the motor will rise s we decrese the field current nd speed control bove the bse speed will be chieved. Speed versus rmture current chrcteristic is shown in figure 39.8 for two flux vlues φ nd φ 1. Since φ < φ, the no lod speed ' 1 n o for flux vlue φ 1 is more thn the no lod speed n o corresponding to φ. However, this method will not be suitble for constnt lod torque.

13 To mke this point cler, let us ssume tht the lod torque is constnt t rted vlue. So from the initil stedy condition, we hve T =T = 1 kφ I. If lod torque remins constnt nd flux L rted e rted is reduced to φ 1, new rmture current in the stedy stte is obtined from kφ 1I 1 =TL rted. Therefore new rmture current is φ I 1 = I rted φ 1 φ But the frction, > 1; hence new rmture current will be greter thn the rted rmture φ 1 current nd the motor will be overloded. This method therefore, will be suitble for lod whose torque demnd decreses with the rise in speed keeping the output power constnt s shown in figure Obviously this method is bsed on flux wekening of the min field. Therefore t higher speed min flux my become so wekened, tht rmture rection effect will be more pronounced cusing problem in commuttion.

14 Speed control by rmture voltge vrition In this method of speed control, rmture is supplied from seprte vrible d.c voltge source, while the field is seprtely excited with fixed rted voltge s shown in figure Here the V rmture resistnce nd field current re not vried. Since the no lod speed n 0 =, the speed versus I chrcteristic will shift prllely s shown in figure for different vlues of V. kφ As flux remins constnt, this method is suitble for constnt torque lods. In wy rmture voltge control method is similr to tht of rmture resistnce control method except tht the former one is much superior s no extr power loss tkes plce in the rmture circuit. Armture voltge control method is dopted for controlling speed from bse speed down to very smll speed s one should not pply cross the rmture voltge which is higher thn the rted voltge Wrd Leonrd method: combintion of V nd I f control In this scheme, both field nd rmture control re integrted s shown in figure Arrngement for field control is rther simple. One hs to simply connect n pproprite rheostt in the field circuit for this purpose. However, in the pre power electronic er, obtining vrible d.c supply ws not esy nd seprtely excited d.c genertor ws used to supply the motor rmture. Obviously to run this genertor, prime mover is required. A 3-phse induction motor is used s the prime mover which is supplied from 3-phse supply. By controlling the

15 field current of the genertor, the generted emf, hence V cn be vried. The potentil divider connection uses two rheostts in prllel to fcilitte reversl of genertor field current. First the induction motor is strted with genertor field current zero (by djusting the jockey positions of the rheostts). Field supply of the motor is switched on with motor field rheostt set to zero. The pplied voltge to the motor V, cn now be grdully incresed to the rted vlue by slowly incresing the genertor field current. In this scheme, no strter is required for the d.c motor s the pplied voltge to the rmture is grdully incresed. To control the speed of the d.c motor below bse speed by rmture voltge, excittion of the d.c genertor is vried, while to control the speed bove bse speed field current of the d.c motor is vried mintining constnt V. Reversl of direction of rottion of the motor cn be obtined by djusting jockeys of the genertor field rheostts. Although, wide rnge smooth speed control is chieved, the cost involved is rther high s we require one dditionl d.c genertor nd 3-phse induction motor of similr rting s tht of the d.c motor whose speed is intended to be controlled. In present dy, vrible d.c supply cn esily be obtined from.c supply by using controlled rectifiers thus voiding the use of dditionl induction motor nd genertor set to implement Wrd leonrd method Series motor In this motor the field winding is connected in series with the rmture nd the combintion is supplied with d.c voltge s depicted in figure Unlike shunt motor, here field current is not independent of rmture current. In fct, field nd rmture currents re equl i.e., I f = I. Now torque produced in d.c motor is: T φ I Ι f I I 2 before sturtion sets in i.e., φ I fter sturtion sets in t lrge I I

16 Since torque is proportionl to the squre of the rmture current, strting torque of series motor is quite high compred to similrly rted d.c shunt motor Chrcteristics of series motor Torque vs. rmture current chrcteristic 2 Since T I in the liner zone nd T I in the sturtion zone, the T vs. I chrcteristic is s shown in figure speed vs. rmture current From the KVL eqution of the motor, the reltion between speed nd rmture current cn be obtined s follows: V = I( r+r se) +E b = Ir+kφ n V-Ir or, n = kφ V-Ir In the liner zone n = k'i V r = k'i k' V-Ir In the sturtion zone n = k'φ The reltionship is inverse in nture mking speed dngerously high s I 0. Remember tht the vlue of I, is mesure of degree of loding. Therefore, series motor should never be operted under no lod condition. Unlike shunt motor, series motor hs no finite no lod speed. Speed versus rmture current chrcteristic is shown in figure nvsi:side: b. st

17 speed vs. torque chrcteristic Since I T in the liner zone, the reltionship between speed nd torque is V r k'' T k' k'' nd k' represent pproprite constnts to tke into ccount the proportionlity tht exist between current, torque nd flux in the liner zone. This reltion is lso inverse in nture indicting once gin tht t light lod or no lod ( T 0) condition; series motor speed pproches dngerously high vlue. The chrcteristic is shown in figure For this reson, series motor is never connected to mechnicl lod through belt drive. If belt snps, the motor becomes unloded nd s consequence speed goes up unrestricted cusing mechnicl dmges to the motor.

18 39.7 Speed control of series motor Speed control below bse speed For constnt lod torque, stedy rmture current remins constnt, hence flux lso remins constnt. Since the mchine resistnce r+r se is quite smll, the bck emf E b is pproximtely equl to the rmture terminl voltge V. Therefore, speed is proportionl to V. If V is reduced, speed too will be reduced. This V cn be controlled either by connecting externl resistnce in series or by chnging the supply voltge. Series-prllel connection of motors If for drive two or more (even number) of identicl motors re used (s in trction), the motors my be suitbly connected to hve different pplied voltges cross the motors for controlling speed. In series connection of the motors shown in figure 39.17, the pplied voltge cross ech motor is V/2 while in prllel connection shown in figure 39.18, the pplied voltge cross ech motor is V. The bck emf in the former cse will be pproximtely hlf thn tht in the ltter cse. For sme rmture current in both the cses (which mens flux per pole is sme), speed will be hlf in series connection compred to prllel connection.

19 Speed control bove bse speed Flux or field current control is dopted to control speed bove the bse speed. In series motor, independent control of field current is not so obvious s rmture nd field coils re in series. However, this cn be chieved by the following methods: 1. Using diverter resistnce connected cross the field coil. In this method shown in figure 39.19, portion of the rmture current is diverted through the diverter resistnce. So field current is now not equl to the rmture current; in fct it is less thn the rmture current. Flux wekening thus cused, rises the speed of the motor. 2. Chnging number of turns of field coil provided with tpings. In this cse shown figure 39.20, rmture nd field currents re sme. However provision is kept to chnge the number of turns of the field coil. When number of turns chnges, field mmf NseI f chnges, chnging the flux hence speed of the motor. 3. Connecting field coils wound over ech pole in series or in. prllel. Generlly the field terminls of d.c mchine re brought out fter connecting the field coils (wound over ech pole) in series. Consider 4 pole series motor where there will be 4 individul coils plced over the poles. If the terminls of the individul coils re brought out, then there exist severl options for connecting them. The four coils could be connected in series s in figure 39.21; the 4 coils could be connected in prllel or prllel combintion of 2 in series nd other 2 in series s shown in figure n figure For series connection of the coils (figure 39.21) flux produced is proportionl to I nd

20 I for series-prllel connection (figure 39.22) flux produced is proportionl to 2. Therefore, for sme rmture current I, flux will be doubled in the second cse nd nturlly speed will be pproximtely doubled s bck emf in both the cses is close to supply voltge V. Thus control of speed in the rtio of 1:2 is possible for series prllel connection. In similr wy, reder cn work out the vrition of speed possible between (i) ll coils connected in series nd (ii) ll coils connected in prllel Brking of d.c shunt motor: bsic ide It is often necessry in mny pplictions to stop running motor rther quickly. We know tht ny moving or rotting object cquires kinetic energy. Therefore, how fst we cn bring the object to rest will depend essentilly upon how quickly we cn extrct its kinetic energy nd mke rrngement to dissipte tht energy somewhere else. If you stop pedling your bicycle, it will eventully come to stop eventully fter moving quite some distnce. The initil kinetic energy stored, in this cse dissiptes s het in the friction of the rod. However, to mke the stopping fster, brke is pplied with the help of rubber brke shoes on the rim of the wheels. Thus stored K.E now gets two wys of getting dissipted, one t the wheel-brke shoe interfce (where most of the energy is dissipted) nd the other t the rod-tier interfce. This is good method no doubt, but regulr mintennce of brke shoes due to wer nd ter is necessry. If motor is simply disconnected from supply it will eventully come to stop no doubt, but will tke longer time prticulrly for lrge motors hving high rottionl inerti. Becuse here the stored energy hs to dissipte minly through bering friction nd wind friction. The sitution cn be improved, by forcing the motor to operte s genertor during brking. The ide cn be understood remembering tht in motor mode electromgnetic torque cts long the

21 direction of rottion while in genertor the electromgnetic torque cts in the opposite direction of rottion. Thus by forcing the mchine to operte s genertor during the brking period, torque opposite to the direction of rottion will be imposed on the shft, thereby helping the mchine to come to stop quickly. During brking ction, the initil K.E stored in the rotor is either dissipted in n externl resistnce or fed bck to the supply or both Rheosttic brking Consider d.c shunt motor operting from d.c supply with the switch S connected to position 1 s shown in figure S is single pole double throw switch nd cn be connected either to position 1 or to position 2. One end of n externl resistnce R b is connected to position 2 of the switch S s shown. Let with S in position 1, motor runs t n rpm, drwing n rmture current I nd the bck emf is E b = kφ n. Note the polrity of E b which, s usul for motor mode in opposition with the supply voltge. Also note T e nd n hve sme clock wise direction. + I S 2 I ppl y I f r n r ppl y I f φ r n D.C su + - I T e E b R b D.C su + - I T e E b R b Figure 39.23: Mchine opertes s motor Now if S is suddenly thrown to position 2 t t = 0, the rmture gets disconnected from the supply nd terminted by R b with field coil remins energized from the supply. Since speed of the rotor cn not chnge instntneously, the bck emf vlue E b is still mintined with sme polrity previling t t = 0 -. Thus t t = 0 +, rmture current will be I = E b /(r + R b ) nd with reversed direction compred to direction previling during motor mode t t = 0 -. Obviously for t > 0, the mchine is operting s genertor dissipting power to R b nd now the electromgnetic torque T e must ct in the opposite direction to tht of n since I hs chnged direction but φ hs not (recll T e φ I ). As time psses fter switching, n decreses reducing K.E nd s consequence both E b nd I decrese. In other words vlue of brking torque will be highest t t = 0 +, nd it decreses progressively nd becoming zero when the mchine finlly come to stop Plugging or dynmic brking Figure 39.24: Mchine opertes s genertor during brking This method of brking cn be understood by referring to figures nd Here S is double pole double throw switch. For usul motoring mode, S is connected to positions 1 nd 1'. Across terminls 2 nd 2', series combintion of n externl resistnce R b nd supply voltge with polrity s indicted is connected. However, during motor mode this prt of the circuit remins inctive.

22 + 1 S 2 I R b S 2 I R b - D.C supply, V I f φ + - r I T e E b n D.C supply, V D.C supply, V I f φ + - r I T e E b n D.C supply, V - Figure 39.25: Mchine opertes s motor 1' S 2' + - 1' S 2' Figure 39.26: Mchine opertes s genertor during brking (plugging). To initite brking, the switch is thrown to position 2 nd 2' t t = 0, thereby disconnecting the rmture from the left hnd supply. Here t t = 0 +, the rmture current will be I = (E b + V)/(r + R b ) s E b nd the right hnd supply voltge hve dditive polrities by virtue of the connection. Here lso I reverses direction producing T e in opposite direction to n. I decreses s E b decreses with time s speed decreses. However, I cn not become zero t ny time due to presence of supply V. So unlike rheosttic brking, substntil mgnitude of brking torque previls. Hence stopping of the motor is expected to be much fster then rheosttic breking. But wht hppens, if S continuous to be in position 1' nd 2' even fter zero speed hs been ttined? The nswer is rther simple, the mchine will strt picking up speed in the reverse direction operting s motor. So cre should be tken to disconnect the right hnd supply, the moment rmture speed becomes zero Regenertive brking A mchine operting s motor my go into regenertive brking mode if its speed becomes sufficiently high so s to mke bck emf greter thn the supply voltge i.e., E b > V. Obviously under this condition the direction of I will reverse imposing torque which is opposite to the direction of rottion. The sitution is explined in figures nd The norml motor opertion is shown in figure where rmture motoring current I is drwn from the supply nd s usul E b < V. Since E b = kφ n 1. The question is how speed on its own become lrge enough to mke E b < V cusing regenertive brking. Such sitution my occur in prctice when the mechnicl lod itself becomes ctive. Imgine the d.c motor is coupled to the wheel of locomotive which is moving long plin trck without ny grdient s shown in figure Mchine is running s motor t speed of n 1 rpm. However, when the trck hs downwrd grdient (shown in figure 39.28), component of grvittionl force long the trck lso ppers which will try to ccelerte the motor nd my increse its speed to n 2 such tht E b = kφ n 2 > V. In such scenrio, direction of I reverses, feeding power bck to supply. Regenertive brking here will not stop the motor but will help to rrest rise of dngerously high speed. +

23 + D.C su ppl y - I f I φ + - r I T e E b n 1 Electric loco n 1 Plin trck without grdient E b < V Figure 39.27: Mchine opertes s motor + I D.C supply I f φ + - r I T e E b n Electric loco n 1 - Trck with grdient E b > V Figure 39.28: Mchine enters regenertive brking mode Tick the correct nswer 1. A 200 V, 1000 rpm, d.c shunt motor hs n rmture resistnce of 0.8 Ω nd its rted rmture current is 20 A. Rtio of rmture strting current to rted current with full voltge strting will be: (A) 1 V (B) 12.5 V (C) 25 V (D) 16 V 2. A 200 V, 1000 rpm, d.c shunt motor hs n rmture resistnce of 0.8 Ω nd found to run from 200 V supply stedily t 950 rpm with bck emf of 190 V. The rmture current is: (A) A (B) 10 A (C) 250 A (D) 12.5 A 3. A d.c 220 V, shunt motor hs n rmture resistnce of 1 Ω nd field circuit resistnce of 150 Ω. While running stedily from 220 V supply, its bck emf is found to be 209 V. The motor is drwing line current of: (A) 11 A (B) A (C) A (D) 9.53 A 4. A 220 V, d.c shunt motor hs r = 0.8 Ω nd drws n rmture current of 20 A while supplying constnt lod torque. If flux is suddenly reduced by 10%, then immeditely the rmture current will become:

24 (A) 45.5 A nd the new stedy stte rmture current will be 22.2 A. (B) 20 A nd the new stedy stte rmture current will be 22.2 A. (C) 22.2 A nd the new stedy stte rmture current will be 45.5 A. (D) 20 A nd the new stedy stte rmture current will be 25 A. 5. A 220 V, d.c shunt motor hs r = 0.8 Ω nd drws n rmture current of 20 A while supplying constnt lod torque. If 4.2 Ω resistnce is inserted in the rmture circuit suddenly, then immeditely the rmture current will become: (A) 20 A nd the new stedy stte rmture current will be 3.2 A. (B) 3.2 A nd the new stedy stte rmture current will be 20 A. (C) 47.2 A nd the new stedy stte rmture current will be 3.2 A. (D) 3.2 A nd the new stedy stte rmture current will be 47.2 A. 6. A seprtely excited 220 V, d.c genertor hs r = 0.6 Ω nd while supplying constnt lod torque, drws n rmture current of 30 A t rted voltge. If rmture supply voltge is reduced by 20%, the new stedy stte rmture current will be: (A) 24 A (B) 6 A (C) 30 A (D) 36 A 7. A 250 V, d.c shunt motor hving negligible rmture resistnce runs t 1000 rpm t rted voltge. If the supply voltge is reduced by 25%, new stedy stte speed of the motor will be bout: (A) 750 rpm (B) 250 rpm (C) 1000 rpm (D) 1250 rpm Solve the following 1. A d.c motor tkes n rmture current of 50A t 220V. The resistnce of the rmture is 0.2Ω. The motor hs 6 poles nd the rmture is lp wound with 430 conductors. The flux per pole is 0.03Wb. Clculte the speed t which the motor is running nd the electromgnetic torque developed. 2. A 10KW, 250V, 1200 rpm d.c shunt motor hs full lod efficiency of 80%, r = 0.2Ω nd R f = 125Ω. The mchine is initilly operting t full lod condition developing full lod torque. i. Wht extr resistnce should be inserted is the rmture circuit if the motor speed is to be reduced to 960 rpm? ii. Wht dditionl resistnce is to be inserted in the field circuit in order to rise the speed to 1300 rpm? Note tht for both prts (i) nd (ii) the initil condition is the sme full lod condition s stted in the first prgrph nd lod torque remins constnt throughout. Effect of sturtion nd rmture rection my be neglected.