WELCOME TO THE LECTURE ON DC MOTOR
Force on conductor If conductor is plced in mgnetic field nd current is llowed to flow through the conductor, the conductor will experience mechnicl force. N S
Electric Motor
A Two Pole DC Motor
A Four Pole DC Motor
Armture of DC Motor
Fleming's Left-Hnd Rule
Direction of Force in Conductor
Direction of Force in Conductor
Digrm of Simple DC Motor
Motor Principle An electric motor is mchine which converts electricl energy into mechnicl energy. Principle: It is bsed on the principle tht when current-crrying conductor is plced in mgnetic field, it experiences mechnicl force whose direction is given by Fleming s Left-hnd Rule nd whose mgnitude is given by Force, F = B I l newton Where B is the mgnetic field in wb/m^2. I is the current in mperes nd l is the length of the coil in meter.
DC Motor Opertion
Current in DC Motor
Mgnetic Field in DC Motor
Force in DC Motor
Torque in DC Motor
Electric Motor
Commuttor
Comprison of Genertor nd Motor Action Fundmentl lws of nture is no energy conversion from one form to nother is possible until there is some one to oppose the conversion. Mgnetic Drg for Genertor Bck e.m.f. for Motor
DC Motor Types Importnt DC motor types: Shunt Motors Series Motors Compound Motors
Counter e.m.f or Bck e.m.f. of DC Motor When the motor rmture rottes, the conductors lso rottes nd hence cut the flux. According to the lws of Electromgnetic Induction, e.m.f. is induced in them whose direction is found by Fleming s Right-hnd Rule, is in opposition of the pplied voltge. Becuse of its opposing direction, it is clled s counter e.m.f. or bck e.m.f.
Equivlent Circuit of DC Motor V = E + I R + b V brush
Voltge Eqution of DC Motor Multiplying both sides by I we get, VI = E b I + I VI = Electricl Input to the rmture, E b I = Electricl Equivlent of mechnicl power developed in the rmture, I 2 R = Cu loss in rmture. Motor Efficiency:It is the rtio of power developed by the rmture to its input, i.e. E I b = VI E b 2 V R
Condition For Mximum Power The gross mechnicl power developed by motor is: For Mximum Power Condition: m R I VI P 2 = ( ) 2 R I VI d dp m = R I E V b + = ( ) 2 2 0 2 V R I R I V R I V R I VI di di = = = = 2 2 V E V E V R I E V b b b = + = + =
Torque T = F Torque Newton-metre(N-m) where, F= Force, r = rdius of the circumference. Power developed = wtt where, =Angulr velocity in rdin/second Power developed= r ( F r) 2πN 2πN ω T ω wtt 2πN ω= rd / s 2πN 60 So, P= T 60 2π P= NT 60 NT P= 9.55
Torques in A DC Motor There re Two Torques in dc motor: Armture Torque Shft Torque
Power developed in rmture = Armture Torque( ) where N is in r.p.m. Also, electricl converted into mechnicl power in the rmture is = So, T T T 2πN 60 2πN EbI T = EbI T = 60 2πN ( Also, E ZN P b =Φ )volt A Therfore, T = ΦZ 2π 2πN 60 P A = 0.159ΦZ =ΦZN I P A I P A T N in rps I N-m E b I wtt
Armture Torque cont. P T = 0. 159ΦZ A I From the bove eqution,we find tht, I In the cse of Series Motor:Since field winding crry full rmture current: In the cse of Shunt Motor: Φ T Φ I T I Φ 2 is prcticlly constnt.so, T I
Shft Torque( ) T sh The Torque which is vilble for doing useful work is known s shft torque,. T sh 2πN Output= Tsh 60 Output T sh = 2 π N 60 Output Tsh = 9.55 N ( T ) Lost Torque:The differences iron nd friction losses of the motor. T sh is known s lost torque nd is due to
Exmples Ex-1: A 220-V d.c. mchine hs n rmture resistnce of 0.5Ω. If the full-lod rmture current is 20A, find the induced e.m.f. when the mchine cts s (i) genertor (ii) motor.
Exmples cont. Ex-2: A 440-V shunt motor hs n rmture resistnce of 0.8Ω nd field resistnce of 200Ω. Determine the bck e.m.f. when giving n output of 7.46kW t 85 percent efficiency.
Exmples cont. Ex-3: A d.c. motor tkes n rmture current of 110A t 480V.The rmture circuit resistnce is 0.2Ω. The mchine hs 6-poles nd the rmture is lp-connected with 864 conductors. The flux per pole is 0.05Wb. Clculte (i) the speed, (ii) the gross torque developed by the rmture.
Exmples cont. Ex-4: Determine the developed torque nd shft torque of 220V,4-poles series motor with 800 conductors wve-connected supplying lod of 8.2kW by tking 45A from the mins. The flux per pole is 25mWb nd its rmture circuit resistnce is 0.6Ω.
For Series Motor Speed of A DC Motor cont N 1 I 1 Φ 1 N 2 I 2 Φ 2 = Speed in the 1 st cse = rmture current in the 1 st cse = flux/pole in the 1 st cse = Speed in the 2 nd cse = rmture current in the 2 nd cse = flux/pole in the 2 nd cse We cn write using the speed eqution: E E 1 nd 1 b 2 N N2 b Φ Φ 1 2 N2 Eb2 Φ1 = N1 Eb 1 Φ 2 N2 Eb2 I Prior to the sturtion of mgnetic poles: Φ I = N For Shunt Motor 1 Eb 1 I Using the speed eqution: N 2 E b 2 Φ 1 = If Φ,then 2 = Φ 1 N E Φ 1 b1 2 1 2 N 2 = N 1 E E b 2 b1
Speed Regultion The speed regultion is the chnge in speed when the lod on the motor is reduced from rted vlue to zero, expressed s percent of the rted lod speed. N. L. speed F. L. speed F. L. speed Speed regultion= 100= 100 dn N
Torque nd Speed of A DC Motor N Increse in flux decreses the speed but increses the torque. It cnnot be hppened T = K V Φ I I Φ Becuse torque lwys tend to produce rottion.hence, if torque increses, motor speed must increse rther thn decrese. If torque is decresed by decresing the field current the following sequences re found: NΦ 1. Bck e.m.f. E b = drops instntly, the speed remining constnt K becuse of the inerti of hevy rmture. ( V E b) 2. Due to decrese of E, I is incresed becuse of I = b R 3. A smll decrese of Φ is more thn counterblnced by lrge increse of I which mens net increse of torque, T. 4. This increse in produces n increse in motor speed. T = K E Φ b R
Motor Chrcteristics Torque nd rmture current i.e. I chrcteristic.its known s Electricl chrcteristic. Speed nd rmture current i.e. chrcteristic. Speed nd torque: i.e. T chrcteristic.it is lso know s Mechnicl chrcteristic. T Φ I N nd N N T E b Φ I
Chrcteristics of A Series Motor T chrcteristic:. Here, Φ I. So, I N I N T chrcteristic: N E Φ b chrcteristic:it is found from the bove two chrcteristics. T I 2 T Tsh N N Current, I Current, I Torque, T
Chrcteristics of A Shunt Motor T Φ Φ I chrcteristic: is prcticlly constnt. So, N I chrcteristic: is prcticlly constnt. So, N E b N chrcteristic:it is found from the bove two chrcteristics. T T I T Tsh N N Current, I Current, I Torque,T
Chrcteristics of Compound Motor 1) Cumultive-compound Motors 2) Differentil-compound Motors
Summry Type of motor Chrcteristics Applictions Shunt Approximtely constnt speed, Adjustble speed, Medium strting torque(up to 1.5 F.L. torque) For driving constnt speed line shfting Lthes Centrifugl Pumps Mchine tools Blowers nd fns Reciprocting pumps Series Vrible speed, Adjustble vrying For trction work i.e. speed, High strting torque Electric locomotives Rpid trnsit systems Trolley crs etc. Crnes nd hoists Conveyors Compound Vrible speed, Adjustble vrying speed, High strting torque For intermittent high torque lods For shers nd punches Elevtors Conveyors Hevy Plners Rolling mills; Ice Mchines; Printing presses; Air Compressors
Losses nd Efficiency Sme s dc genertor 1) Copper Losses 2) Mgnetic Losses 3) Mechnicl Losses The condition for mximum power developed V by the rmture is: I R = E b = The condition for mximum efficiency is tht rmture Cu losses re equl to constnt losses. 2
Power Stges A B C Electricl Input =VI Wtt Cu Losses Driving Power in Armture = wtt E b I Iron nd Friction Losses Motor Output in Wtt Overll efficiency, η c = C A Electricl efficiency, η e = B A Mechnicl efficiency, η m = C B
Exmples