2.6.3 Characteristic of Compound-wound motor connection common use aids speed falls opposes speed increases ratio series-to-shunt field ampere-turns

Transcription

1 .6.3 Chrcteristic of Compound-wound motor A compound-wound motor hs both series nd shunt field winding, (i.e. one winding in series nd one in prllel with the rmture circuit), by vrying the number of turns on the series nd shunt windings nd the directions of the mgnetic fields produced by these windings (ssisting or opposing), fmilies of (c/s) my be obtined to suit lmost ll pplictions. There re two common types of compound motor connection, the long-shunt connection nd short-shunt connection. And there re two different types of compound motors in common use, they re the cumultive compound motor nd the differentil compound motor. In the cumultive compound motor, the field produced by the series winding ids the field produced by the shunt winding. The speed of this motor flls more rpidly with incresing current thn does tht of the shunt motor becuse the field increses. In the differentil compound motor, the flux from the series winding opposes the flux from the shunt winding. The field flux, therefore, decreses with incresing lod current. Becuse the flux decreses, the speed my increses with incresing lod. Depending on the rtio of the series-to-shunt field mpere-turns, the motor speed my increses very rpidly. Fig.(.5) 57

2 The torque-speed (c/s) of cumultively compound D.C motor In the cumultive compounded D.C. motor, there is component of flux which is constnt nd nther component which is proportionl to its rmture current (nd thus to its lod). Therefore, the cumultively compounded motor hs higher strting torque thn shunt motor (whose flux is constnt) but lower strting torque thn series motor (whose entire flux is proportionl to rmture current). At light lods, the series field hs very smll effect, so the motor behves pproximtely s shunt D.C. motor. As the lod gets very lrge, the series flux becomes quite importnt nd the torque-speed curve begins to look like series motor's (c/s). A comprison of the torque-speed (c/s) of ech of these type of mchines is shown in figure (.6). The torque-speed (c/s) of differentilly compound D.C motor In differentilly compound D.C. motor, the shunt mgneto motive force nd series mgneto motive force subtrct from ech other. This mens tht s the lod on the motor increses, I increses nd the flux in the motor decreses. But s the flux decreses, the speed of the motor increses. This speed increses cuses nther increses in lod, which further increses I, further decresing the flux, nd incresing the speed gin. The result is tht differentilly compounded motor is unstble nd tends to run wy. It is so bd tht differentilly compounded motor is unsuitble for ny ppliction. Cumultively n compound n Shunt Series T Figure (.6 T 58

3 .7 D.C Motor Strter If D.C motor whose rmture is sttionry is switched directly to its supply voltge, it is likely tht the fuses protecting the motor will burn out. Becuse the rmture resistnce is smll, frequently being less thn one ohm. Thus, dditionl resistnce must be dded to the rmture circuit t the instnt of closing the switch to strt the motor. As the speed of the motor increses. The rmture conductors re cutting flux nd generted voltge, cting in opposition to the pplied voltge, is produced, which limits the flow of rmture current. Thus the vlue of the dditionl rmture resistnce cn then be reduced. When t norml running speed, the generted e.m.f. is such tht no dditionl resistnce is required in the rmture circuit. To chieve this vrying resistnce in the rmture circuit on strting D.C motor strter is used, s shown in fig.(.7). The strting hndle is moved slowly in clockwise direction to strt the motor. For shunt-wound motor, the field winding is connected to stud () or (M) vi sliding contct on the strting hndle. To give mximum field current hence mximum flux, hence mximum torque on strting, since T φ.i. Fig.(.7) 59

4 .8 Speed Control of D.C Motor.8. Shunt-Wound Motor The speed of shunt-wound D.C motor, n, is proportionl to ( V I. R ). The speed is vried either by vrying the vlue of flux, (φ ), or φ by vrying the vlue of ( R ). The former is chieved by using vrible resistor in series with the field winding, s shown in fig.(.8) nd such resistor is clled the shunt field regultor. As the vlue of resistnce of the shunt field regultor is incresed, the vlue of the field current, ( I ), is decresed. This results in decrese in the vlue of flux, ( φ ), nd hence n increse in the speed, since n. Thus only speeds bove tht φ given without shunt field regultor cn be obtined by this method. Speeds below those given by ( V I. R ) re obtined by incresing φ the resistnce in the rmture circuit, s shown in fig.(.8), where ( R R) V I + n φ Since resistor (R) is in series with the rmture, it crriers the full rmture current nd results in lrge power loss in lrge motors where considerble speed reduction is required for long periods. f Fig.(.8) 60

5 .8. Series-Wound Motor The speed control of series-wound motors is chieved using either () field resistnce, or (b) rmture resistnce techniques. () The speed of D.C series-wound motor is given by : V IR n = K φ Where (K) is constnt, (V) is the terminl voltge, ( R) is the combined resistnce of the rmture nd series field nd (φ ) is the flux. Thus, reduction in flux results in n increse in speed. This is chieved by putting vrible resistnce in prllel with the field winding nd reducing the field current, nd hence flux, for given vlue of supply current. A circuit digrm of this rrngement is shown in fig. (.9). A vrible resistor connected in prllel with the series-wound field to control speed is clled diverter speeds bove those given with no diverter re obtined by this method. Fig.(.9) 6

6 (b) speed below norml re obtined by connecting vrible resistor is series with the field winding nd rmture circuit, s shown in fig.(.0). This effectively increses the vlue of (R) in the eqution. V I. R n = K φ And thus reduces the speed. Since the dditionl resistor crries the full supply current, lrge power loss is ssocited with lrge motors in which considerble speed reduction is required for long periods. Fig.(.0) xmple (.) A D.C motor hs speed of (900 r.p.m) when connected to (460 V) supply. Find the pproximte vlue of the speed of the motor when connected to (00 V) supply, ssuming the flux decreses by (30%) nd neglecting the rmture volt drop? Solution: b = Kφ b = Kφ.n.n φ = φ φ 0.3 φ = 0. 7φ Now b b φ 900 = 0.7. φ n n = 559 r.p.m 6

7 xmple (.): A series motor hs n rmture resistnce of (0. Ω) nd series field resistnce of (0.3 Ω). It is connected to (40 V) supply nd t prticulr lod runs t (440 r.p.m) when drwing (5 A) from the supply. () Determine the bck e.m.f t this lod. (b) Clculte the speed of motor when the lod is chnged such tht the current is incresed to (30 A). Assume tht this cses doubling of flux. Solution: () t initil lod, is given by b ( R R ) = V I + f b = 40-5(0.+0.3) = 3.5 Volt. (b) When the current is incresed to (30 A), the bck e.m.f. is given by. b ( R R ) = V I + =40 3o(0.+0.3) =5 volt Now bck e.m.f Thus b b i.e. φ. n =. φ. n 3.5 φ 440 = 5 φ n b φ. n n = = r.p.m 3.5 f ******************************************** 63

8 xmple (.3); A series motor runs t (800 r.p.m) when the voltge is (400 V) nd the current is (5 A). The rmture resistnce is (0.4 Ω) nd the series field resistnce is (0. Ω). Determine the resistnce to be connected in series to reduce the speed to (600 r.p.m) with sme current. Solution: t (800 r.p.m) = V I( R + R b f = 400 5(0.4+0.) =385 volt ) t (600 r.p.m), since the current is unchnged, the flux is unchnged. Thus b φ n, or b n, nd ( 385)( 600) b = = volt ( 800) And ( R + R R) b = V I f =400-5( R) Rerrnging gives R = = From which, extr series resistnce, R= i.e., R=3.85 Ω b b n = n thus the ddition of series resistnce of (3.85 Ω) hs reduced the speed from (800 r.p.m) to (600 r.p.m). ************************************************* 64

9 xmple (.4): On full-lod (300 V) series motor tkes (90 A) nd runs t (900 r.p.m) the rmture resistnce is (0. Ω) nd the series winding resistnce is (50 mω). Determine the speed when developing full lod torque but with (0. Ω) diverter in prllel with the field winding. (ssume tht the flux is proportionl to the field current). Solution: t (300 V) ( R R ) = V I + b f I X ( ) = =86.5 Volts With the (0. Ω) diverter in prllel with ( ) R f The equivlent resistnt R = = 0. 04Ω By current division, current I X 0. 8I, I X 0. = I = I X = 0.8I Torque, T I φ nd for full lod torque I φ = I φ Since flux is proportionl to field current φ I nd φ 0. 8I Then ( 90)(90) = ( I )(0.8I ), Hence = V I R ) ( b + R ( 90) I nd I 00. 6A = 0.8 = ( )= 85.9 Volts = Bck e.m.f., b φ. n from which b b φ n = φ n I n = 0.8 I n new speed n = r.p.m = 65

10 .9 The efficiency of D.C. motor is given by. It ws stted in section (.4), tht the efficiency of D.C. mchine output power fficiency, η = 00 0 input power 0 Also, the totl losses = I R + I V C (for shunt motor) nd, f + totl losses= I R + C (for series motor), where C is the sum of the iron, friction nd windge losses, R is the totl resistnce for series motor R = ( R + R f ) for motor, the input power=vi nd the output power=vi-losses hence, VI I R I f V C η = 00 0 (for shunt motor) 0 VI VI IR C = 00 0 VI 0 η (for series motor) The efficiency of motor is mximum when the lod is such tht I R = I V C (for shunt motor), I R = C (for series motor) f + xmple (.5): A 50 V series motor drws current of 40 A. The rmture resistnce is 0.5 Ω nd the field resistnce is 0.05 Ω. Determine the mximum efficiency of the motor. Solution: For series motor VI I η = ( R + R ) VI f C 00 VI I ( R ) + R f For mximum efficiency, η = 00 VI (50)(40) (40) ( ) = 00 = (50)(40)

11 .0 D.C Stepping Motors D.C stepping motors re unique D.C motors tht re used to control utomtic industril processing equipment. D.C motors of this type re fund in numericlly controlled mchines nd robotic system used by industry. They re very efficient nd develop high torque the stepping motor is used primrily to chnge electricl pulses into rotry motion tht cn be used to produce mechnicl movements. The shft of D.C stepping motor rottes specific number of mechnicl degrees with ech incoming pulse of electricl energy. The mount of rotry movement or ngulr displcement produced by ech pulse cn be repeted precisely with ech succeeding pulse from the drive source. The resulting output of this device is used to ccurtely locte or position utomtic process mchinery. Fig.(.0) 67

12 . lectromechnicl power control equipment There re so mny types of electromechnicl power control equipment used tody tht it is lmost impossible to discuss ech type. However, some of the very importnt types will be discussed in the following prgrphs... Relys Relys represent one of the most widely used control devices vilble tody. The electromgnet of rely contins sttionry core. Mounted close to one end of the core is movble piece of mgnetic mteril clled the rmture. When the coil is ctivted electriclly, it produces mgnetic field in the metl core. The rmture is then ttrcted to the core, which in turn produces mechnicl motion. When the coil is de-energized, the rmture is returned to its originl position by spring ction. Figure (.) shows simplified digrm of the construction of rely tht is used to control. Common Norml Closed (N.C) Norml Open (N.O) Fig.(.) 68

13 Relys use smll mount of current to crete n electromgnetic field tht is strong enough to ttrct the rmture. When the rmture is ttrcted, it either opens or closes the contcts. The contcts then either turn (on) or (off) circuits tht re using lrge mounts of current. There re two types of contcts used in conjunction with most relys. Normlly open (N.O) nd normlly close (N.C). The (N.O) contcts remin open when the rely coil is de-energized, nd re closed only when the rely is energized. The (N.C) contcts remin closed when the rely is de-energized, nd re open only when the coil is energized. Appliction D.C Motor Reversing The direction of rottion of permnent-mgnet D.C motor cn be reversed by reversing the two power line s shown in fig.(.). Forwrd Reverse Fig.(.) 69

14 .. Solenoids A solenoid, shown in fig.(.3) is n electromgnetic coil with movble core tht is constructed of mgnetic. The core, or plunger, is sometimes ttched to n externl spring. This spring cuses the plunger to remin in fixed position until moved by the electromgnetic field tht is creted by current through the coil. This externl spring lso cuses the core or plunger to return to its originl position when the coil is de-energized. Solenoid re used for vriety of control pplictions. Mny gs nd fuel oil furnces use solenoid vlves to utomticlly turn the fuel supply (on) or (off) upon demnd. Dishwshers used one or more solenoids to control the flow of wter. Fig.(.3) 70