UNIT-I DRIVE CHARACTERISTICS

Transcription

1 Eectrica Drives: UNI-I DRIVE CHARACERISICS Motion contro is required in arge nuber of industria and doestic appications ike transportation systes, roing is, paper achines, textie is, achine toos, fans, pups, robots, washing achines etc. Systes epoyed for otion contro are caed DRIVES, and ay epoy any of prie overs such as diese or petro engines, gas or stea turbines, stea engines, hydrauic otors and eectric otors, for suppying echanica energy for otion contro. Drives epoying eectric otors are known as ELECRICAL DRIVES. An ELECRIC DRIVE can be defined as an eectroechanica device for converting eectrica energy into echanica energy to ipart otion to different achines and echaniss for various kinds of process contro. Cassification of Eectric Drives According to Mode of Operation Continuous duty drives Short tie duty drives Interittent duty drives According to Means of Contro Manua Sei autoatic Autoatic According to Nuber of achines Individua drive Group drive Muti-otor drive According to Dynaics and ransients Uncontroed transient period Controed transient period

2 According to Methods of Speed Contro Reversibe and non-reversibe uncontroed constant speed. Reversibe and non-reversibe step speed contro. Variabe position contro. Reversibe and non-reversibe sooth speed contro. Advantages of Eectrica Drive 1. hey have fexibe contro characteristics. he steady state and dynaic characteristics of eectric drives can be shaped to satisfy the oad requireents.. Drives can be provided with autoatic faut detection systes. Prograabe ogic controer and coputers can be epoyed to autoaticay contro the drive operations in a desired sequence. 3. hey are avaiabe in wide range of torque, speed and power. 4. hey are adaptabe to aost any operating conditions such as exposive and radioactive environents 5. It can operate in a the four quadrants of speed-torque pane 6. hey can be started instanty and can iediatey be fuy oaded 7. Contro gear requireent for speed contro, starting and braking is usuay sipe and easy to operate. Choice (or) Seection of Eectrica Drives Choice of an eectric drive depends on a nuber of factors. Soe of the iportant factors are. 1. Steady State Operating conditions requireents Nature of speed torque characteristics, speed reguation, speed range, efficiency, duty cyce, quadrants of operation, speed fuctuations if any, ratings etc. ransient operation requireents Vaues of acceeration and deceeration, starting, braking and reversing perforance. 3. Requireents reated to the source ypes of source and its capacity, agnitude of votage, votage fuctuations, power factor, haronics and their effect on other oads, abiity to accept regenerative power 4. Capita and running cost, aintenance needs ife. 5. Space and weight restriction if any. 6. Environent and ocation. 7. Reiabiity.

3 Group Eectric Drive his drive consists of a singe otor, which drives one or ore ine shafts supported on bearings. he ine shaft ay be fitted with either pueys and bets or gears, by eans of which a group of achines or echaniss ay be operated. It is aso soe ties caed as SHAF DRIVES. Advantages A singe arge otor can be used instead of nuber of sa otors Disadvantages here is no fexibiity. If the singe otor used deveops faut, the whoe process wi be stopped. Individua Eectric Drive In this drive each individua achine is driven by a separate otor. his otor aso iparts otion to various parts of the achine. Muti Motor Eectric Drive In this drive syste, there are severa drives, each of which serves to actuate one of the working parts of the drive echaniss. E.g.: Copicated eta cutting achine toos Paper aking industries, Roing achines etc. Genera Eectric Drive Syste Bock diagra of an eectric drive syste is shown in the figure beow. Source Power Moduator Motor Load Contro Unit Sensing Unit (Input Coand)

4 A odern variabe speed eectrica drive syste has the foowing coponents Eectrica achines and oads Power Moduator Sources Contro unit Sensing unit Eectrica Machines Most coony used eectrica achines for speed contro appications are the foowing DC Machines Shunt, series, copound, separatey excited DC otors and switched reuctance achines. AC Machines Induction, wound rotor, synchronous, PM synchronous and synchronous reuctance achines. Specia Machines Brush ess DC otors, stepper otors, switched reuctance otors are used. Power Moduators Functions: Moduates fow of power fro the source to the otor in such a anner that otor is iparted speed-torque characteristics required by the oad During transient operation, such as starting, braking and speed reversa, it restricts source and otor currents with in perissibe iits. It converts eectrica energy of the source in the for of suitabe to the otor Seects the ode of operation of the otor (i.e.) Motoring and Braking. ypes of Power Moduators In the eectric drive syste, the power oduators can be any one of the foowing Controed rectifiers (ac to dc converters) Inverters (dc to ac converters) AC votage controers (AC to AC converters) DC choppers (DC to DC converters) Cyco converters (Frequency conversion)

5 Eectrica Sources Very ow power drives are generay fed fro singe phase sources. Rest of the drives is powered fro a 3 phase source. Low and ediu power otors are fed fro a 400v suppy. For higher ratings, otors ay be rated at 3.3KV, 6.6KV and 11 KV. Soe drives are powered fro battery. Sensing Unit Speed Sensing (Fro Motor) orque Sensing Position Sensing Current sensing and Votage Sensing fro Lines or fro otor terinas Fro Load orque sensing eperature Sensing Contro Unit Contro unit for a power oduator are provided in the contro unit. It atches the otor and power converter to eet the oad requireents. Cassification of Eectrica Drives Another ain cassification of eectric drive is DC drive AC drive Coparison between DC and AC drives DC DRIVES AC DRIVES he power circuit and contro circuit he power circuit and contro circuit are is sipe and inexpensive copex It requires frequent aintenance Less Maintenance he coutator akes the otor hese probes are not there in these otors buky, costy and heavy and are inexpensive, particuary squirre cage induction otors Fast response and wide speed range In soid state contro the speed range is wide

6 of contro, can be achieved soothy by conventiona and soid state contro Speed and design ratings are iited due to coutations and conventiona ethod is stepped and iited Speed and design ratings have upper iits Appications Paper is Ceent Mis extie is Sugar Mis Stee Mis Eectric raction Petrocheica Industries Eectrica Vehices Dynaics of Motor Load Syste Fundaentas of orque Equations A otor generay drives a oad (Machines) through soe transission syste. Whie otor aways rotates, the oad ay rotate or undergo a transationa otion. Load speed ay be different fro that of otor, and if the oad has any parts, their speed ay be different and whie soe parts rotate others ay go through a transationa otion. Equivaent rotationa syste of otor and oad is shown in the figure. Motor Load Notations Used: J = Moent of inertia of otor oad syste referred to the otor shaft = Instantaneous anguar veocity of otor shaft, rad/sec. = Instantaneous vaue of deveoped otor torque, N- kg

7 = Instantaneous vaue of oad torque, referred to the otor shaft N- Load torque incudes friction and wind age torque of otor. Motor-oad syste shown in figure can be described by the foowing fundaenta torque equation. = d dt d dj ( J ) = J + (1) dt dt Equation (1) is appicabe to variabe inertia drives such as ine winders, ree drives, Industria robots. dj For drives with constant inertia = 0 dt = d + J dt () Equation () shows that torque deveoped by otor is counter baanced by oad torque and a d dynaic torque J dt ony during the transient operations. d. orque coponent J dt is caed dynaic torque because it is present Note: Energy associated with dynaic torque J d dt is stored in the for of kinetic energy given J by. Cassification of Load orques: Various oad torques can be cassified into broad categories. Active oad torques Passive oad torques Load torques which has the potentia to drive the otor under equiibriu conditions are caed active oad torques. Such oad torques usuay retain their sign when the drive rotation is changed (reversed) Eg: orque due to force of gravity orque due tension orque due to copression and torsion etc

8 Load torques which aways oppose the otion and change their sign on the reversa of otion are caed passive oad torques Eg: orque due to friction, cutting etc. Coponents of Load orques: he oad torque can be further divided in to foowing coponents (i) Friction orque ( F ) Friction wi be present at the otor shaft and aso in various parts of the oad. F is the equivaent vaue of various friction torques referred to the otor shaft. (ii) Windage orque ( W ) When otor runs, wind generates a torque opposing the otion. his is known as windage torque. (iii) orque required to do usefu echanica work. Nature of this torque depends upon particuar appication. It ay be constant and independent of speed. It ay be soe function of speed, it ay be tie invariant or tie variant, its nature ay aso change with the oad s ode of operation. Vaue of friction torque with speed is shown in figure beow F

9 Its vaue at stand sti is uch higher than its vaue sighty above zero speed. Friction at zero speed is caed stiction or static friction. In order to start the drive the otor shoud at east exceed stiction. Friction torque can aso be resoved into three coponents Speed v c s orque Coponent v varies ineary with speed is caed VISCOUS friction and is given by v = B Where B is viscous friction co-efficient. Another coponent C, which is independent of speed, is known as COULOMB friction. hird coponent s accounts for additiona torque present at stand sti. Since s is present ony at stand sti it is not taken into account in the dynaic anaysis. Windage torque, W which is proportiona to speed squared is given by w = C C is a constant Fro the above discussions, for finite speed = L + B + + C C

10 Characteristics of Different types of Loads One of the essentia requireents in the section of a particuar type of otor for driving a achine is the atching of speed-torque characteristics of the given drive unit and that of the otor. herefore the knowedge of how the oad torque varies with speed of the driven achine is necessary. Different types of oads exhibit different speed torque characteristics. However, ost of the industria oads can be cassified into the foowing four categories. Constant torque type oad orque proportiona to speed (Generator ype oad) orque proportiona to square of the speed (Fan type oad) orque inversey proportiona to speed (Constant power type oad) Constant orque characteristics: Most of the working achines that have echanica nature of work ike shaping, cutting, grinding or shearing, require constant torque irrespective of speed. Siiary cranes during the hoisting and conveyors handing constant weight of ateria per unit tie aso exhibit this type of characteristics. Speed L =K orque orque Proportiona to speed: Separatey excited dc generators connected to a constant resistance oad, eddy current brakes have speed torque characteristics given by =k

11 Speed L orque orque proportiona to square of the speed: Another type of oad et in practice is the one in which oad torque is proportiona to the square of the speed.eg Fans rotary pups, copressors and ship propeers. Speed L = K orque orque Inversey proportiona to speed: Certain types of athes, boring achines, iing achines, stee i coier and eectric traction oad exhibit hyperboic speed-torque characteristics

12 Speed L α 1 orque Muti quadrant Operation: For consideration of uti quadrant operation of drives, it is usefu to estabish suitabe conventions about the signs of torque and speed. A otor operates in two odes Motoring and braking. In otoring, it converts eectrica energy into echanica energy, which supports its otion. In braking it works as a generator converting echanica energy into eectrica energy and thus opposes the otion. Motor can provide otoring and braking operations for both forward and reverse directions. Figure shows the torque and speed co-ordinates for both forward and reverse otions. Power deveoped by a otor is given by the product of speed and torque. For otoring operations power deveoped is positive and for braking operations power deveoped is negative. Speed Forward Braking II Forward Motoring I orque III Reverse Motoring IV Reverse Braking

13 In quadrant I, deveoped power is positive, hence achine works as a otor suppying echanica energy. Operation in quadrant I is therefore caed Forward Motoring. In quadrant II, power deveoped is negative. Hence, achine works under braking opposing the otion. herefore operation in quadrant II is known as forward braking. Siiary operation in quadrant III and IV can be identified as reverse otoring and reverse braking since speed in these quadrants is negative. For better understanding of the above notations, et us consider operation of hoist in four quadrants as shown in the figure. Direction of otor and oad torques and direction of speed are arked by arrows. Motion Motion Counter weight Epty Cage II I Counter weight Loaded Cage Motion III IV Motion Epty Cage Loaded Cage Counter weight Counter weight Load orque with epty cage Load orque with oaded cage

14 A hoist consists of a rope wound on a dru couped to the otor shaft one end of the rope is tied to a cage which is used to transport an or ateria fro one eve to another eve. Other end of the rope has a counter weight. Weight of the counter weight is chosen to be higher than the weight of epty cage but ower than of a fuy oaded cage. Forward direction of otor speed wi be one which gives upward otion of the cage. Load torque ine in quadrants I and IV represents speed-torque characteristics of the oaded hoist. his torque is the difference of torques due to oaded hoist and counter weight. he oad torque in quadrants II and III is the speed torque characteristics for an epty hoist. his torque is the difference of torques due to counter weight and the epty hoist. Its sigh is negative because the counter weight is aways higher than that of an epty cage. he quadrant I operation of a hoist requires oveent of cage upward, which corresponds to the positive otor speed which is in counter cockwise direction here. his otion wi be obtained if the otor products positive torque in CCW direction equa to the agnitude of oad torque L1. Since deveoped power is positive, this is forward otoring operation. Quadrant IV is obtained when a oaded cage is owered. Since the weight of the oaded cage is higher than that of the counter weight.it is abe to overcoe due to gravity itsef. In order to iit the cage within a safe vaue, otor ust produce a positive torque equa to L in anticockwise direction. As both power and speed are negative, drive is operating in reverse braking operation. Operation in quadrant II is obtained when an epty cage is oved up. Since a counter weigh is heavier than an epty cage, its abe to pu it up. In order to iit the speed within a safe vaue, otor ust produce a braking torque equa to L in cockwise direction. Since speed is positive and deveoped power is negative, it s forward braking operation. Operation in quadrant III is obtained when an epty cage is owered. Since an epty cage has a esser weight than a counter weight, the otor shoud produce a torque in CW direction. Since speed is negative and deveoped power is positive, this is reverse otoring operation. Steady State Stabiity: Equiibriu speed of otor-oad syste can be obtained when otor torque equas the oad torque. Eectric drive syste wi operate in steady state at this speed, provided it is the speed of stabe state equiibriu. Concept of steady state stabiity has been deveoped to readiy evauate the stabiity of an equiibriu point fro the steady state speed torque curves of the otor and oad syste.

15 In ost of the eectrica drives, the eectrica tie constant of the otor is negigibe copared with the echanica tie constant. During transient condition, eectrica otor can be assued to be in eectrica equiibriu ipying that steady state speed torque curves are aso appicabe to the transient state operation. Now, consider the steady state equiibriu point A shown in figure beow L A shift A M orque he equiibriu point wi be tered as stabe state when the operation wi be restored to it after a sa departure fro it due to disturbance in the otor or oad. Due to disturbance a reduction of in speed at new speed, eectrica otor torque is greater than the oad torque, consequenty otor wi acceerate and operation wi be restores to point A. siiary an increase in speed caused by a disturbance wi ake oad torque greater than the otor torque, resuting into deceeration and restoring of operation to point A. Now consider equiibriu point B which is obtained when the sae otor drives another oad as shown in the figure. A decrease in speed causes the oad torque to becoe greater than the otor torque, eectric drive deceerates and operating point oves away fro point B. Siiary when working at point B and increase in speed wi ake otor torque greater than the oad torque, which wi ove the operating point away fro point B

16 B L shift A M orque Fro the above discussions, an equiibriu point wi be stabe when an increase in speed causes oadtorque to exceed the otor torque. (i.e.) When at equiibriu point foowing conditions is satisfied. dl d > d d (1) Inequaity in the above equation can be derived by an aternative approach. Let a sa perturbation in speed, resuts in perturbation in and respectivey. herefore the genera oadtorque equation becoes and = he genera equation is ( + ) = ( + ) + = = Subtracting (3) fro () and rearranging d J dt + d + J dt = ( + ) Jd + dt Jd d + + J () dt dt (3) (4)

17 Fro sa perturbations, the speed torque curves of the otor and oad can be assued to be straight ines, thus Where d d and d d d = (5) d d = (6) d are respectivey sopes of the steady state speed torque curves of otor and oad at operating point under considerations. Substituting (5) and (6) in (4) we get, d J dt d + d d = d 0 (7) his is a first order inear differentia equation. If initia deviation in speed at t=0 be ( ) 0 then the soution of equation (7) is An operating point wi be stabe when = 1 d J d ( ) exp t (8) d 0 d happen exponentia ter in equation (8) shoud be negative. approaches zero as t approaches infinity. For this to Basics of Regenerative Braking In the regenerative braking operation, the otor operates as generator, whie it is sti connected to the suppy. Here, the otor speed is greater than the synchronous speed. Mechanica energy is converted into eectrica energy, part of which is returned to the suppy and rest of the energy is ast as heat in the winding and bearings of eectrica achines pass soothy fro otoring region to generating region, when over driven by the oad. An exape of regenerative braking is shown in the figure beow. Here an eectric otor is driving a troey bus in the uphi and downhi direction. he gravity force can be resoved into two coponents in the uphi direction. One is perpendicuar to the oad surface (F) and another one is parae to the road surface F. he parae force pus the otor towards botto of the hi. If we negect the rotationa osses, the otor ust produce force F opposite to F to ove the bus in the uphi direction.

18 F F F F Down Hi Uphi F F his operation is indicated as shown in the figure beow in the first quadrant. Here the power fow is fro the otor to oad. DOWN HILL Power Fow Speed Power Fow UPHILL L Speed M M LOAD M LOAD L Speed M orque

19 Now we consider that the sae bus is traveing in down hi, the gravitationa force doesn t change its direction but the oad torque pushes the otor towards the botto of the hi. he otor produces a torque in the reverse direction because the direction of the otor torque is aways opposite to the direction of the oad torque. Here the otor is sti in the sae direction on both sides of the hi. his is known as regenerative braking. he energy is exchange under regenerative braking operation is power fows fro echanica oad to source. Hence, the oad is driving the achine and the achine is generating eectric power that is returned to the suppy. Regenerative braking of Induction otor: An induction otor is subjected to regenerative braking, if the otor rotates in the sae direction as that of the stator agnetic fied, but with a speed greater than the synchronous speed. Such a state occurs during any one of the foowing process. Downward otion of a oaded hoisting echanis During fux weakening ode of operation of IM. Under regenerative braking ode, the achine acts as an induction generator. he induction generator generates eectric power and this power is fed back to the suppy. his achine takes ony the reactive power for excitation. he speed torque characteristic of the otor for regenerative braking is shown in the figure. Braking Speed Motoring orque

20 Regenerative Braking for DC otor: In regenerative braking of dc otor, generated energy is suppied to the source. For this the foowing condition is to be satisfied. E > V and I a shoud be negative Speed Motoring Braking orque Modes of Operation: An eectrica drive operates in three odes: We know that Steady state Acceeration incuding Starting Deceeration incuding Stopping d = + J dt According to the above expression the steady state operation takes pace when otor torque equas the oad torque. he steady state operation for a given speed is reaized by adjustent of steady state otor speed torque curve such that the otor and oad torques are equa at this speed. Change in speed is achieved by varying the steady state otor speed torque curve so that otor torque equas the oad torque at the new desired speed. In the figure shown beow when the otor paraeters are adjusted to provide speed torque curve 1, drive runs at the desired speed 1. Speed is changed to when the otor paraeters are adjusted to provide speed torque curve. When oad torque opposes otion, the otor works as a otor operating in quadrant I or III depending on the direction of rotation. When the oad is active it can reverse its sign and act to assist the otion. Steady state operation for such a case can be obtained by adding a echanica brake which wi produce a torque in

21 a direction to oppose the otion. he steady state operation is obtained at a speed for which braking torque equa the oad torque. Drive operates in quadrant II or IV depending upon the rotation. 1 1 orque Acceeration and Deceeration odes are transient odes. Drive operates in acceeration ode whenever an increase in its speed is required. For this otor speed torque curve ust be changed so that otor torque exceeds the oad torque. ie taken for a given change in speed depends on inertia of otor oad syste and the aount by which otor torque exceeds the oad torque. Increase in otor torque is accopanied by an increase in otor current. Care ust be taken to restrict the otor current with in a vaue which is safe for both otor and power oduator. In appications invoving acceeration periods of ong duration, current ust not be aowed to exceed the rated vaue. When acceeration periods are of short duration a current higher than the rated vaue is aowed during acceeration. In cosed oop drives requiring fast response, otor current ay be intentionay forced to the axiu vaue in order to achieve high acceeration. Figure shown beow shows the transition fro operating point A at speed 1 to operating point B at a higher speed, when the otor torque is hed constant during acceeration. he path consists of AD 1 E 1 B. In the figure beow, 1 to 5 are otor speed torque curves. Starting is a specia case of acceeration where a speed change fro 0 to a desired speed takes pace. A points entioned in reation to acceeration are appicabe to starting. he axiu current aowed shoud not ony be safe for otor and power oduator but drop in source votage caused due to it shoud aso be in acceptabe iits. In soe appications the otor shoud acceerate soothy, without any jerk. his is achieved when the starting torque can be increased step essy fro its zero vaue. Such a start is known as soft start.

22 M M 1 B E 1 D 3 M 1 Deceeration 3 A D 1 M 3 E 3 E 4 5 C - Motor operation in deceeration ode is required when a decrease in its speed is required. According to the equation d = + J, deceeration occurs when oad torque exceeds the otor torque. In dt those appications where oad torque is aways present with substantia agnitude, enough deceeration can be achieved by sipy reducing the otor torque to zero. In those appications where oad torque ay not aways have substantia aount or where sipy reducing the otor torque to zero does not provide enough deceeration, echanica brakes ay be used to produce the required agnitude of deceeration. Aternativey, eectric braking ay be epoyed. Now both otor and the oad torque oppose the otion, thus producing arger deceeration. During eectric braking otor current tends to exceed the safe iit. Appropriate changes are ade to ensure that the current is restricted within the safe iit. Figure shown above shows paths foowed during transition fro point A at speed 1 to a point C at a ower speed 3.When deceeration is carried out using eectric braking at a constant braking torque, the operating point oves aong the path AD 3 E 3 C. When sufficient oad torque is present or when echanica braking is used the operation takes pace aong the path AD E C. Stopping is a specia case of deceeration where the speed of a running otor is changed to zero.

23 Probes: A otor having a suitabe contro circuit deveops a torque by the reationship M = a + b, where a and b are positive constants. his otor is used to drive a oad whose torque is expressed as L = c + d, where c and d are positive constants. he tota inertia of the rotating asses is J. Soution: a) Deterine the reations aongst the constants a, b, c and d in order that the otor can start together with the oad and have an equiibriu operating speed? b) Cacuate the equiibriu operating speed? c) Wi the drive be stabe at this speed? d) Deterine the initia acceeration of the drive? e) Deterine the axiu acceeration of the drive? a) At = 0, M =b and L =d Hence the otor can start with the oad ony if b > d M = L at equiibriu speed i. e. a+ b= c + d i. e. c a ( b d) = 0 a± a + 4c Hence= c In order that is finite a + Sign before the radica a + 4c a > c i. e. a i. e. 4c ( b d) sign before the radia a + 4c c > 0 ( b d) > a + 4c( b d) ( b d) < 0 ( b d) + 4c ( b d) wi give a wi give a > 0, which is positive as ong as positive ony if i.e c<0, which is not true, since c is given to be a positive constant. Hence the + sign true before the radia ony wi give a positive finite equiibriu speed. If a + 4c( b d) >0 a+ b) Equiibriu speed = a + 4c c ( b d)

24 d c) L d = c and M = a d d If the equiibriu speed has to be stabe dl dm > i.e.c> a d d fro the answer to (b), we have c= a+ a + 4c ( b d) which wi be aways> a Hence, the equiibriu operating speed deterined earier is a stabe point of operation of drive. d d) Acceerating torque J = M L dt Initiay M =b and L =d herefore, initia acceeration = b d J d e) Acceerating torque J = M L dt = a c + b d d a c + b d herefore, acceeration A = = dt J his wi be axiu at a speed when da = 0 d a c = 0 J a = c Substituting this speed at which the acceeration is in the genera expression for acceeration, A ax = a = ( a c) ( a ) + 4c 4cJ 4c J ( b d) + b d we get axiu,