Chopper Fed Dc Motor

INTRODUCTION During the nineteenth century, when motive supplying was dc, dc moves were employ extensively to rive designer direct from the dc seed. The travel zip could be varied by adjusting line of products afoot(predicate) by a rheostat. That was an open gyrate assert. close of the selects were immutable f number and the characteristics could not be matched with a job requirement. A vast development in the dc drives system took place when the ward Leonard mark off System was introduced in the 1980s. The system was aim-generator system to suffer function to the drive drive.The supply power uncommitted was still dc and dc travel was practised to drive the dc generator set at a more or less ceaseless induce. Afterwards when the ac power system came into worldly concern and became popular, ac beat backs were developed and became attractive owing to their constructional simplicity, callosity and lower initial as well as maintenance cost. Machine requiring variable quicken drives use the ward Leonard System employing ac motors driving dc motors at a constant uppering. In the mid-fifties electronic came into existence and brought about remarkable approach in the fixity work system.The open- loop manual bid system was replaced by close loop victualsback simpleness, which resulted in breakd answer and better accuracy. initially, flub diodes and ignitrons were developed and ac to dc converters were used to instruction dc motors. The advent of thyristors capable of handling crowing accredited has revolutionized the field of voltaic power control. Thyratrons, ignitrons, hydrargyrum waiver rectifiers, magnetic amplifiers and motor generator sets save all been replaced by secanture state circuits employing semi-conductor diodes and thyristors.Thyristor controlled drives employing some(pre titulary) ac and dc motors find wide applications in industry as variable hotfoot drives. In the 1960s ac power was converted into d c power for direct control of drive motors with solid state devices ( extravagantly power te diodes and silicon controlled rectifiers). Initially saturable reactors were use in conjunction with power silicon rectifiers for dc drives. Of late solid state circuits apply semi- conductor diodes and thyristors argon becoming popular for compulsive the speed of ac and dc otors and ar increasingly replacing the t raditional electric power control circuit based on thyratrons, ignitrons, mercury arc rectifiers, magnetic amplifiers, motor-generator sets, and so onas compared to the electric and electro- automatonlike systems of speed control. The electronic system has higher(prenominal) accuracy, salienter reliability, and quick response and also has higher efficiency as in that location is no I2R losses and moving parts. Moreover four-quadrant speed control is possible to meet precise high standards. All electronic circuits control the speed of the motor by controlling either, ?The potentiality apply to the motor armature or ? The field afoot(predicate) or ? two of the above DC motors lot be point from dc supply if available or from ac supply, by and by it has been converted to dc supply with the supporter of rectifiers which can be either half beckon or bountiful wave and either controlled ( by variable the conduction angle of the thyristors used) or loose. AC motors can be run on the ac supply or from dc supply, after it has been converted into ac supply with the help of inverters (opposite of rectifiers).As express above, the average output potential residuum of a thyristors controlled rectifiers by changing its conduction angle and hence the armature potency can be adjusted to control its speed. When run on a dc supply, the armature dc potential drop can be changed with the help of uncontrolled rectifiers (using altogether diodes and not thyristors). The dc electric potentials so captureed can be thus chopped with the help of a thyris tors whirlybird circuit. In this method of speed control of a dc motor, available ac supply is initial rectified into dc supply using uncontrolled rectifiers. The supply is then filtered and smooth ended dc output is supplied to the thyristors eggbeater.It allows dc to flow through for the date long ton and then disconnects for the epoch Toff. This musical rhythm is repeated. During supply-on conclusion (i. e. for the time finish Ton) the dc motor gets supply and accelerates. During the supply off period Toff (i. e. for the time period Toff) in that location is no supply to the motor and the motor decelerates till the next on cycle begins. If the cycles repeated continuously at a decided frequency and the elements of the cycle are maintained in a fixed relationship, the motor will then turn at a constant electric potential crossways the motor will be,V0 = (V*Ton)/(Ton +Toff) = (V*Ton)/T = f*V*Ton The dc potential drop across the motor can be control by varying the Time Ratio Control (TRC) which whitethorn be accomplished by, ? Varying the duration of the on-time, Ton keeping the total time period, T or frequency, f constant ? Keeping the on- time, Ton constant and varying the frequency, f. ? Varying both. The Variable dc potency below the supply dc voltage is do available to the dc motor and thereof, the motor speed available is below base speed.For automatic control of speed, both contemporary feedback and speed feedback is used. BRIEF countersign ON CHOPPER A dc eggwhisk is a static device used to obtain variable dc voltage from a source of constant dc voltage. The dc eggbeater offers great efficiency, fleet response, smooth control, lower maintenance, small size, etc. Solid state cleaver collect to motley advantages are widely used in the barrage fire operated vehicles, traction motor control, control of a life-size number of dc motors from a common dc bus with a considerable improvement of power factor.PRINCIPLE OF CHOPPER OPER ATION A chopper is a thyristors on/ off switch that connects elongate to and disconnects it from the supply and produces a chopped load voltage from a constant input voltage. The chopper is stand for by a thyristors (SCR). It is triggered periodically and is kept conducting for a period Ton and is blocked for a period Toff. During the period Ton, when the chopper is on, the supply terminals is connected to the load terminals. And during the interval Toff when the chopper is off, load up-to-date flows from the freewheeling diode Df.So, the load terminals are victimize circuited by Df and load voltage is therefore nix during Toff. Hence the chopper dc voltage is produced at the load terminals. Now, the average load voltage, Eo is given by Eo = Edc*? ? =Duty Cycle=(Ton/Toff) Or, Eo = Edc*(Ton/T) T=Ton + Toff So the voltage can be varied by varying the duty cycle, ? of the chopper. CLASSIFICATION OF CHOPPER Power semiconductor unit devices are used in chopper circuits are uni-dire ctional device. A chopper can however operate in both of the four quadrants by an grant ar footslogment of semiconductor devices.These characteristics of their operation in any of the four quadrants form the stern of their circleification as, 1. Type-A or First Quadrant eggwhisk 2. Type-B or atomic number 42 Quadrant chop shot. 3. Type-C or Two Quadrant Type-A Chopper 4. Type-D or Two Quadrant or Type-B Chopper 5. Type-E or Four Quadrant Chopper. PERFORMANCE comparison OF DC MOTORS The kindred circuit and on its basis the performance equation of a separately-excited dc beat back and series dc motor are presented below. ? severally-excited dc motorThe equivalent circuit of a separately-excited dc motor coupled with a load at a lower place steady state condition is shown in the form 4. 1. The load crookedness, TL opposes the electro-magnetic contortion, Te. For the field circuit, Vf = If*rf For the armature circuit, Vt = Ia + Ia*ra Motor back potential drop or armatur e emf, Ea=Ka ? Ia=Km? m (4. 1) Te=ka ? Ia = KmIa Also, Te = D wm + TL where, rf= bowl circuit electrical resistance in ohm, Ia=Armature online in A, Vt=Motor terminal voltage in V, ra=Armature circuit resistance in ohm, Km=Ka ?=Torque constant in Nm/A*emf constant in V-sec/rad, m=Angular speed of motor in rad/sec, D=Viscous friction constant in Nm-sec/rad. Electromagnetic power, P=wmTe watts From equation (1), Ea=Kmwm=Vt-Iara Or wm= (Vt Iara)/Km= (Vt Iara)/Ka ? (4. 1) So it is seen from equation (4. 2) that speed can be controlled by varying, ? Armature terminal voltage, Vt This method is known as Armature-voltage control. urge on below base speed is obtained by this method. ? Field flux, ? This method is known as Field flux control. focal ratio above base speed is obtained by this method. ? DC serial MotorIn a dc series motor, field turn of events is in series with the armature circuit. It is designed to carry the rated armature current. The fig. shows the equivalent ci rcuit of a dc series motor driving load with load torque, TL. For the armature circuit, Vt = Ea + Ia ( ra+ rs ) .. (4. 3) Te = Ka ? Ia For no saturation in the magnetic circuit, ? = CIa Hence, Te = KaCIa2 = KIa2 Also, Ea = Ka ? wm = KaCIawm = KIawm From eqn (4. 3), Vt = KIawm + Ia (ra + rs) = Ia Kwm + (ra + rs) Or, speed wm = (Vt/ KIa) (ra +rs)/K . (4. 4) where, rs = Series field resistance in ohm,K = KaC = constant in Nm/A2 or in V-sec/ A- rad. CLOSED LOOP check OF CHOPPER FED DC MOTOR For possible purposes motors are required to operate at craved speed with low losses to meet the craved load torque characteristics which depends on the armature current. Suppose a motor is operating at a especial(a) speed an suddenly a load is applied, the speed falls and the motors takes time to come up to the coveted speed . but a speed feed back with an inner current loop provides faster response to any disturbance in speed domination ,load torque and supply voltage.Another flat coat for the requirement of feedback loop in dc drives is that, the armature of a outsized motor represents very small resistivity which when supplied with nominal voltage would result in an unjustified current of up to 10 times the nominal value. Under normal conditions, this is prevented by the induced armature voltage, E which cancels most of the applied voltage, Va so that only the difference is driving the armature current, Ia. But under transient conditions or steady state over load of the motor, there is always a danger of excessive currents payable to sudden torque demand and rapidly changing armature voltage or speed . t is therefore important to provide a fast current or torque limit to protect the motor, the power supply and the load. This is best realized by feedback control establishing an put inive safe guard against electrical and mechanical stresses. In it the output of the speed control, Ec is applied to the current limiter which sets the reference current, Ia (refere nce) for the current loop. the armature current is sensed by a current sensor, after being filtered by an active filter to acquire ripples which is then compared with the reference current, Ia (ref. the illusion current is neat through a current controller whose output, Vc adjusts the ignition angle of the chopper and brings the motor speed to the craved value. Any positive speed fault caused by an increased in either speed command or load torque demand can produce a high reference current, Ia (ref) the motor accelerates to correct the speed error and finally settles down feather at any reference current, Ia(ref) which makes the motor torque equal to the load torque resultant in a speed error closed to zero.For any large positive speed error, current limiter saturates and limits the reference currents, Ia (ref) to a maximum value, Ia (max) the speed error is then corrected at the maximum allowable armature current ,Ia(max)until the speed error becomes small and the current lim iters comes out of the saturation . normally ,the speed error is corrected with the Ia less than the maximum permissible armature current, Ia max. For speeds below the base speeds, the field error, Ef is large and the field controller saturates thereby applying the maximum ield voltage and current. The speed control from zero to base speed is normally make at the maximum field by armature voltage control. When the speed is closed to the base speed, Va is almost dear the rated value and field controller comes out of saturation. The speed control above base speed is slackly done by field weakening at the rated armature voltage. In the field control loop, the back emf Eb is compared with a reference voltage , Eb (ref) the value of which is mainly between 0. 85 to 0. 95 of the rated armature voltage.For a speed command above the base speed, the speed error causes a higher value of Va then motor accelerates, back emf , Eb increases and field error, Ef decreases. The field current whe n decreases and the motor speed continue to increase until it reaches the coveted speed. In this mode of operation, the drive responds slowly due to large field time constant. A full converter is generally used in the field because it has the ability to reverse the voltage thereby cut the field current much faster as compared to the semi converter.MODELING AND OBSERVATIONS ? imitate using Matlab ? DC Motor with Load Parameter given emf = 220v Current = 6. 2A Ra = 4 ohm La = 0. 072H amphetamine = 1470 rpm J = 0. 0607 kg-m2 Kb= 1. 26v/rad/sec Bt =0. 0869N-m/rad/sec Parameters calculated Ta=La/Ra= 0. 02sec Tm=j/Bt =0. 7sec K1 = Bt/KB2 + Ra Bt =0. 0449 -1/T1 1/T2 =-1/2Bt/J +Ra/La + sqre1/4(Bi/J + Ra/La)2-(Kb2 + Ra Bt ) /JLa T1 = 0. 1077sec T2 = 0. 0208sec Tm = J/Bt = 0. 7sec DC MotorTransfer function I(s)/V(s) =k1(1+sTm)/ (1+sT1)(1+sT2)= 0. 032s+0. 045/0. 002s2+0. 4s+1 wm(s)/I(s)= Kb/Bt(1+sTm) =14. 5/(1+0. 75) Converter Transfer function Kr =1. 35V/Vcm =1. 35*230/10 =31. 05V/v Tr = 1/12*Fs = 1/12*50 =0. 00166sec T. F = kr/(1+sTr) =31. 05/(1+0. 00166s) Design of Current Controller Tc=T2=0. 0208sec K = T1/2Tr = 0. 1077/2*0. 00166 = 32. 43 Kc = KTc/k1HCKrTm =32. 43 *0. 0208/0. 0449*1*31. 05*0. 7 =0. 69 Transfer function Gc(s) = Kc(1+sTc)/sTc = 0. 69(1+0. 0208s)/0. 0208s = 0. 69 + 0. 0143s/0. 0208s Current Loop I(s)/I*(s) = Ki/(1+sTi) Ti = T3/1+ kfi Ki = kfi/Hc(1+ kfi) Kfi = KcKrKiTmHc/Tc Kfi = 0. 9*31. 05*0. 0449*0. 7*1/0. 0208 Kfi = 32. 44 Ki = Kfi/HC(1+ Kfi) Kfi = 32. 44/1*(1+32. 44) Kfi = 0. 97 Ti = T3/(1+ Kfi) = T1+Tr/(1+ Kfi) = 0. 1077+0. 00166/1+32. 44 = 0. 0032sec Speed controller Design T4 = Ti + Tw K2 = Ki Kb Hw /Bt Tm K2 = 0. 97*1. 26*1/0. 0869*0. 7 K2 = 20. 092 KS = 1/(2 Kt T4) KS = 1/2*20. 092*0. 0032 KS = 7. 77 Ts = 4T4 =4*0. 0032 Ts = 0. 0128 Transfer function T. F = KS (1+sTS)/sTS = 7. 77(1+0. 0128s)/0. 0128s = (7. 77 + 0. 0994)/0. 0128s ? Modeling using PSIM Parameters Given Source (Vdc) = 800V Transistor (npn) intensity level Voltage = 0 Initia l position = 0 Current oarlock = 1 Gating Block (G) oftenness = 50Hz Number of Points = 2 Switching Points = 0 180 junction rectifier (D) Diode Voltage Drop = 0 Initial Position = 0 Current glad = 0 inductor (L) Inductance = 0. 01 Initial Current = 0A Current ease up = 0 Capacitor (C) Capacitance = 0. 00005F Initial capacitive Voltage = 0V Current Flag = 0 DC MOTOR (DCM) Ra = 0. 055 ohm La = 0. 01H Rf = 55 ohm Lf = 0. 02H MI = 0. 2 Vt = 440V Ia = 80A If = 4A n = 1500rpm Torque Flag = 0 Master/Slave Flag = 1 Field Source = 400VSpeed Sensor (Ws) wear = 1 Simulation Control Time tonicity = 1e-005 Total Time = 0. 02 Print Time = 0 Print Step = 1 Load Flag = 0 Slave Flag = 0 industrial APPLICATIONS DC drives are highly versatile vigour conversion devices. It can meet the demand of load up requiring high starting, accelerating and decelerating torques. At the resembling time dc drives are easily adaptable for wide range of speed control and quick reversal. So, in industrial ap plication where accurate control of speed and / or torque is required chopper controlled dc drives are unrivalled.Therefore, chopper controlled dc motors are universally employed in steel and aluminum mills, power shovels, electric elevators, railway locomotives and large earth moving equipments. Uses o Various Chopper Controlled DC drives with reasons Types of DC drives Applications Advantages hi-fi speed control can be done. Separately Excited dc drives Used in make-up mills, steel rolling Variation of speed from very high to low value can be mills, diesel-electric propulsion of done ships, etc. starting line torque is very high upto 500%. level best short operating torque is upto 400%. Speed enactment is widely variable. It is very high Series dc drive Used in hoists, cranes, conveyors, at no load. trolley-cars, electric locomotives, Speed control by series field. etc. Used in lathes, centrifugal pumps, beginning torque is medium, unremarkably limited to 250% by Constant speed dc electrical shunt drive reciprocating pumps, fans, blowers, a starting resistance but whitethorn be increased. conveyors, spinning and weaving Maximum momentary operating torque is usually limited machines, etc. to about 200% by commutation. Speed regulation is about 5-10 %. Speed increases about 200% by field control and decreases by armature voltage control. Starting torque is medium, usually limited to 250% by a starting resistance but may be increased. Maximum momentary operating torque is usually limited Adjustable speed dc shunt drives Used for application requiring to about 200% by commutation. adjustable speed control, either constant torque or constant output. SCOPE OF MODIFICATIONS Chopper controlled dc drives are widely used in hoists, cranes, elevators, shears, crushers, conveyor, portmanteau mills, punch presses, air compressors, ice making machines, tractions, etc.So these drives should be modified in the sideline ways to make them more efficient and accurate, 1. The chopper controlled dc motors should be made with large diameter armatures and large pole size of reduced height. 2. The pairing as well as the main and commutating poles should be well laminated to reduce the eddy current essence and to improve the commutation. 3. Large numbers of commutator bars should be used to reduce the voltage between the commutator segments and to improve the commutation. 4.The commutator should be made larger in ball club to provide extra insulation to withstand large and rapid voltage fluctuations. 5. Compensating windings should be used in large motors to reduce the armature reactions effects. 6. The current densities used for the armature and Interpol windings should be reduced as compared to the conventional dc motors of the same frame size and rating in dress to reduce the effect of heating of armature and Interpol. 7. Low inertia armature should be employed for improving the response . 8.Split brushes of hot commutating quality should be used for reducing the effect of transformer voltage in the coil undergoing commutations. 9. Better class of insulation should be used to allow higher temperature rise and dissipation of more losses from a given frame. 10. Now a days chopper controlled dc drives are widely used in the automobile industries. So, it should have high efficiency and accuracy, luminance weight, low maintenance cost. BIBLIOGRAPHY 1. Electric Drives Ramakrishnan, Prentice star sign India. 2. Power Electronics P. S. Bimbhra, Khanna Publishers. 3. Software MATLAB 6. 5 and PSIM.