powerpointsdc motor drives

1、PowerElectronicsChap. 13DC Motor DrivesChap.13DC Motor DrivesOutlinesu Introductionu Equivalent Circuit of DC Motorsu Permanent-magnet DC Motorsu DC Motors with A Separately Excited Field Windingu DC Servo Drivesu Adjustable-sped DC Drivesu SummaryChap.13DC Motor DrivesIntroduction In the past few y

2、ears, the use of ac motor drivesinthespeedandpositioncontrolapplicationsisincreasingduetotheirsimplestructure, efficiency,low andmaintenance,highreliability,highhigh power density.Maintenance costHighLowACMotorInitial costLowHighDCMotorChap.13DC Motor DrivesEquivalent Circuit of DC MotorsPermanent-

3、magnetestablish the field fluxField Windingf= k f I ffArmature Windinghandles the electric powerRotorDCMotorStatorChap.13DC Motor DrivesEquivalent Circuit of DC MotorsWith field winding ff= k f I fPermanent-magnetChap.13DC Motor DrivesEquivalent Circuit of DC Motors The electromagnetic torque is pro

4、duced by the interaction between the field flux and the armature current.= ktf f iaTem A back-emf is produced by the rotation of armature conductors at a speed in the presence of a field flux.ea= kef f wmElectrical PowerPe= eaia= kef f wmiaMechanical PowerPm= wmTem= ktf f wmiaP= P = kNmV In steady s

5、tate,k A Wb Wbrad/s emteChap.13DC Motor DrivesEquivalent Circuit of DC MotorsdiaV= e+ R i+ LtaaaadtJ= J dwm+ Bw+ TT(t)emmWLdtTotal equivalent JTotal equivalent BEquivalent load torqueTWLinertiadampingChap.13DC Motor DrivesEquivalent Circuitof DC Motors= ktf f iaea= kef f wmTemGeneratorGenerator High

6、 performance drives may operate in all four quadrantsChap.13DC Motor DrivesPermanent-magnet DC Motors Permanent magnets on the stator produce a constant field fluxff Electromagnetic torque.= ktf f= kT IaTemkT Back-emfEa= kEwm= kef fkE Voltage equationVt= Ea + Ra IaEquivalent circuitChap.12Introducti

7、on to Motor DrivesPermanent-magnet DC Motors The speed of a load with arbitrary torque can be controlled bycontrolling Vin a permanent-magnet dc motor with a constant f f.t1- Ra Limitation: the maximum speed is limited to the rated speed.w=(VT)mtemkkETTorque-speed characteristicContinuous torque-spe

8、ed capabilityChap.13DC Motor DrivesDC Motors with A Separately Excited Field Winding The limitations of permanent-magnet DC motors can beovercome by using a separately excited field winding to adjust f f .= VfIfRfRa1w=(V-T)mk ftk femeftfEquivalent circuitChap.13DC Motor DrivesDC Motors with A Separa

9、tely Excited Field WindingConstant torque controlConstant power control= ktf f iaTemea= kef f wm In the field weakening region, the speed may be exceeded by 50-100% of its rated value, depending on the motor design.Chap.13DC Motor DrivesDC Servo Drives If it were not for the disadvantages of having

10、a commutator and brushes, the dc motor would be ideally suited for servodrives, because the instantaneous torquecan be controlled= ktf f ialinearly by controlling the armature current.Tem13.5.1 Transfer function modelThree-loop control of dc servo drivesChap.13DC Motor DrivesDC Servo Drives13.5.1 Tr

11、ansfer function modeld(Di)Dv= De+ R Di+ Ltaaaaadt Equations for analyzing small-signal dynamic performance= kDwDeaEmDTem= kT Dia+ J d (Dwm )= DT+ BDwDTemWLmdtVt (s) = Ea (s) + (Ra + sLa )Ia (s)Ea (s) = kEwm (s)Tem (s) = kT Ia (s)Tem (s) = TWL (s) + (B + sJ )wm (s)wm (s) = sqm (s) Take the Laplace tr

12、ansformChap.13DC Motor DrivesDC Servo Drives13.5.1 Transfer function modelVt (s) = Ea (s) + (Ra + sLa )Ia (s)Ea (s) = kEwm (s) Take the Laplace transformTem (s) = kT Ia (s)Tem (s) = TWL (s) + (B + sJ )wm (s)wm (s) = sqm (s)Chap.13DC Motor DrivesDC Servo Drives13.5.1 Transfer function modelInputs The

13、 superposition principle yields,Ra + sLakTw(s) =V (s) -T(s)m(R+ sL )(sJ + B) + kkt(R+ sL )(sJ + B) + kWLkaaTEaaTEChap.13DC Motor DrivesDC Servo Drives13.5.1 Transfer function modelRa + sLakTw(s) =V (s) -T(s)mtWL(R+ sL )(sJ + B) + k+ sL )(sJ + B) + kk(RkaaTEaaTE Two closed-loop transfer functionsG (s

14、) = wm (s)kT=1(R+ sL )(sJ + B) + kkV (s)taaTETWL ( s)=0G (s) = wm (s)Ra + sLa=2(R+ sL )(sJ + B) + kkT(s)WLaaTEVt (s)=0Chap.13DC Motor DrivesDC Servo Drives13.5.1 Transfer function model Neglect the friction term by setting B = 0 and consider the motor without load J = Jm,kT1G (s) =1L JR J(R+ sL) + k

15、ksJ am + s am +1)2k(smaaTEEkkkkTETE Define the following constant,=La= Ra JmElectrical time constantMechanical time ttmconstantekkRTEa Using time constants yields,G (s) = wm (s) =1(s2tt+ st1+1)V (s)ktEmemChap.13DC Motor DrivesDC Servo Drives13.5.1 Transfer function model In general, tmte . To replac

16、e st m1by s(t m +te ) ,1G (s) = wm (s) =k(s2tt+ stk(st+1)(st1+1)+1)V (s)tEmemEme11w(s)mk(st+1)Em1V (s)(st+1)teChap.13DC Motor DrivesDC Servo Drives13.5.1 Transfer function model In general, tmte . To replace st m1by s(t m +te ) ,1G (s) = wm (s) =k(s2tt+ stk(st+1)(st1+1)+1)V (s)tEmemEme111w(s)V (s)mk

17、(st+1) (stt+1)EmeChap.13DC Motor DrivesDC Servo Drives13.5.1 Transfer function modelt e The electrical time constantdetermines how quickly thearmature current builds up in response to a step change DVt in the terminal voltage, where the rotor speed is assumed to be constant.d(Di)Dv= De+ R Di+ Ltaaaa

18、adtDea = kE DwmChap.13DC Motor DrivesDC Servo Drives The mechanical time constant tmdetermines how quicklythe speed builds up in response to a step change DVtin the terminal voltage, provided that the electrical time constant t e is assumed to be negligible and the armature current can change instan

19、taneously.Vt (s)w(s)mk(st+1)EmDvtDw(1- e-t tm )(t)mkEChap.13DC Motor DrivesDC Servo Drives13.5.2 Power electronic converterA power electronic converter supplying a dc motor should have the following capabilities:u The converter should allow both its output voltage and current to reverse in order to

20、realize four-quadrant operation.u The converter should be able to operate in a current- controlled mode by holding the current at its maximum acceptable value during fast acceleration and deceleration.u For accurate control of position, the average voltage output of the converter should vary linearl

21、y with its control input, independent of the load on the motor.u The converter output should respond as quickly as possible to its control input.Chap.13DC Motor DrivesDC Servo Drives13.5.2 Power electronic converter A full-bridge switch-mode dc-dc converter produces a four- quadrant controllable dc

22、output for dc motor drives.= kcVcontrolVtChap.13DC Motor DrivesDC Servo Drives13.5.3 Ripple in the armature current The peak-to-peak ripple in the armature current caused by the switch-mode dc-dc converter impacts on the torque pulsations and heating of the motor.vt (t) = Vt + vr (t)Ripple component

23、s v (t), i (t)tri(t) = I+ i(t)aar The armature circuit equation:dir (t)V+ v (t) = E+ R I+ i (t) + Ltraaaradtdir (t)V= E+ R Iv (t) = R i (t) + Ltaaara radtChap.13DC Motor DrivesDC Servo Drives The ripple current is primarily determined by the armature inductance.dir (t)v(t)Lradt Theripple voltage is

24、maximum when the average outputvoltage is zero and all switches operate at equal duty ratiosPWM bipolar voltage switchingPWM unipolar voltage switchingVdVd(DI=(DI=)p-pmaxp-pmax2Lf8LfasasChap.13DC Motor DrivesDC Servo Drives13.5.4 Control of servo drives The current-limiting circuit operates only whe

25、n the drivecurrent tries to exceed an acceptable limit fast accelerations and decelerations.duringIa,maxNo internal current control loopChap.13DC Motor DrivesDC Servo Drives13.5.4 Control of servo drives To improve the dynamic response in high-performance servo drives, and internal current loop is u

26、sed to control the armature current and torque directly.i* Iaa,maxWith internal current control loopChap.13DC Motor DrivesDC Servo Drives13.5.5 Nonlinearity due to blanking time The blanking time of PWM dc-dc converters causes output voltage errors.Chap.13DC Motor DrivesDC Servo Drives13.5.5 Nonline

27、arity due to blanking time The blanking time of PWM dc-dc converters causes output voltage errors. The effects due to blanking time on the drive performance can be minimized by using the current-controlled mode converters.Chap.13DC Motor DrivesAdjustable-speed DC Drives Unlike servo drives, the resp

28、onse time to speed and torque commands is not as critical in adjustable-speed drives.13.6.1 Switch-mode dc-dc converter= ktf f iaea= kef f wmTem If a four-quadrant operation is needed and a switch-mode converter is utilized, then the full-bridge converter is used.Chap.13DC Motor DrivesAdjustable-spe

29、ed DC Drives13.6.1 Switch-mode dc-dc converter If the speed does not have to reverse but braking is needed, then the two-quadrant converter can be used.= ktf f iaea= kef f wmTemMotoringBrakingTwo-quadrant operationChap.13DC Motor DrivesAdjustable-speed DC Drives13.6.1 Switch-mode dc-dc converter If

30、the speed does not have to reverse but braking is needed, then the two-quadrant converter can be used.= ktf f iaea= kef f wmTemMotoringBrakingTwo-quadrant operationChap.13DC Motor DrivesAdjustable-speed DC Drives13.6.1 Switch-mode dc-dc converter If the speed does not have to reverse but braking is

31、needed, then the two-quadrant converter can be used.= ktf f iaea= kef f wmTemMotoringBrakingTwo-quadrant operationChap.13DC Motor DrivesAdjustable-speed DC Drives13.6.1 Switch-mode dc-dc converter For a single-quadrant operation where the speed remains unidirectional and braking is not required, the

32、 step-down converter can be used.Single-quadrant operationChap.13DC Motor DrivesAdjustable-speed DC Drives13.6.2 Line-frequency controlled converter In large power adjustable-speed dc drives, it may be economical to utilize a line-frequency controlled converter (phase-controlled converter).Chap.13DC

33、 Motor DrivesAdjustable-speed DC Drives13.6.2 Line-frequency controlled converter A disadvantage of the line-frequency converters is the longer response to the speed control signals (due to fire delay angle), compared to high frequency switch-mode dc- dc converters.Chap.13DC Motor DrivesAdjustable-s

34、peed DC Drives13.6.2 Line-frequency controlled converterTwo back-to-back connected thyristorFour-quadrant operationconvertersChap.13DC Motor DrivesAdjustable-speed DC Drives13.6.2 Line-frequency controlled converterTwo back-to-back connected thyristorFour-quadrant operationconvertersOne phase-contro

35、lled converter together with two pairs of contactors.Chap.13DC Motor DrivesAdjustable-speed DC Drives13.6.2 Line-frequency controlled converterTwo back-to-back connected thyristorFour-quadrant operationconvertersOne phase-controlled converter together with two pairs of contactors.Chap.13DC Motor Dri

36、vesAdjustable-speed DC Drives13.6.2 Line-frequency controlled converterTwo back-to-back connected thyristorFour-quadrant operationconvertersOne phase-controlled converter together with two pairs of contactors.Chap.13DC Motor DrivesAdjustable-speed DC Drives13.6.4 Control of adjustable-speed drives A

37、 ddtlimiter allows the speed command to change slowly,thus preventing the rotor current from exceeding its rating.Open-loop speed controlChap.13DC Motor DrivesAdjustable-speed DC Drives13.6.5 Field weakening in adjustable- speed dc motor drivesThe operation higher than the rated speed of the dc moto

38、rcan be realized by reducing the field flux f f.A line-frequencycontrolled converter is normally used to control the field winding.I fthroughChap.13DC Motor DrivesAdjustable-speed DC Drives13.6.6 Power factor of the line current Both the diode rectifier and the phase-controlled rectifiers draw line

39、currents that consist of large harmonics. These harmonics cause the power factor of operation to be poor.Drive capability of dc motorsLine-frequency converter driveFor constant load torque, thisline frequency converter results in a very poor displacement angle at low speeds.Chap.13DC Motor DrivesAdjustable-speed DC Drives13.6.6 Power factor of the line current Both the diode rectifier and the phase-controlled rectifiers draw line currents that consist of large harmonics. These harmonics cause the power factor o