DC MOTOR DRIVES (MEP 1523)

1. DC MOTOR DRIVES(MEP 1523) Dr. Nik Rumzi Nik Idris Department of Energy Conversion FKE, UTM

2. INTRODUCTION • DC DRIVES: Electric drives that use DC motors as the prime movers • DC motor: industry workhorse for decades • Dominates variable speed applications before PE converters were introduced • Will AC drive replaces DC drive ? • Predicted 30 years ago • DC strong presence – easy control – huge numbers • AC will eventually replace DC – at a slow rate

3. Introduction DC Motors • Advantage: Precise torque and speed control without sophisticated electronics • Several limitations: • Regular Maintenance • Expensive • Heavy • Speed limitations • Sparking

4. Rotor PM Stator Introduction DC Motors - 2 pole: permanent magnet excitation

5. Rotor Stator Introduction DC Motors - 2 pole: wound stator excitation

6. X X X X X Introduction Armature reaction DC Motors - 2 pole Armature mmf produces flux which distorts main flux produce by field • Mechanical commutator to maintain armature current direction

7. Armature mmf distorts field flux  Large machine employs compensation windings and interpoles Introduction Armature reaction Flux at one side of the pole may saturate Zero flux region shifted Flux saturation, effective flux per pole decreases

8. Introduction Armature reaction Field flux Armature flux Resultant flux

9. Introduction DC Motors

10. Ra Lf Rf La ia + ea _ + Vt _ if + Vf _ di = + f v R i L f f f dt Electric torque Armature back e.m.f. Introduction

11. Armature circuit: In steady state, Therefore steady state speed is given by, Three possible methods of speed control: Field flux Armature voltage Vt Armature resistance Ra Introduction

12. Varying Vt  TL Vt↓ Te Introduction Requires variable DC supply

13. Varying Ra Ra↑ Introduction  TL Te Simple control Losses in external resistor

14. Varying  ↓ Introduction  TL Te Not possible for PM motor Maximum torque capability reduces

15. Armature voltage control Field flux control Te Maximum Torque capability  base Introduction Armature voltage control : retain maximum torque capability Field flux control (i.e. flux reduced) : reduce maximum torque capability For wide range of speed control 0 to base  armature voltage, above base  field flux reduction

16. Te Maximum Torque capability  base Introduction

17. Pmax Constant torque Constant power  base Introduction P Te 0 to base  armature voltage, above base  field flux reduction P= EaIa,max = kaIa,max Pmax = EaIa,max = kabaseIa,max    1/

18. MODELING OF CONVERTERS AND DC MOTOR POWER ELECTRONICS CONVERTERS Used to obtain variable armature voltage • Efficient • Ideal : lossless • Phase-controlled rectifiers (AC  DC) • DC-DC switch-mode converters(DC  DC)

19. ia  + Vt  3-phase supply Q1 Q2 Q3 Q4 T Modeling of Converters and DC motor Phase-controlled rectifier (AC–DC)

20. + Vt  3-phase supply 3-phase supply  Q1 Q2 Q3 Q4 T Modeling of Converters and DC motor Phase-controlled rectifier

21. R1 F1 3-phase supply + Va - F2 R2  Q1 Q2 Q3 Q4 T Modeling of Converters and DC motor Phase-controlled rectifier

22. Firing circuit –firing angle control •  Establish relation between vc and Vt + Vt – + vc iref  controlled rectifier current controller firing circuit - Modeling of Converters and DC motor Phase-controlled rectifier (continuous current)

23. linear firing angle control Cosine-wave crossing control Modeling of Converters and DC motor Phase-controlled rectifier (continuous current) • Firing angle control

24. Steady state: linear gain amplifier • Cosine wave–crossing method • Transient: sampler with zero order hold converter T GH(s) T – 10 ms for 1-phase 50 Hz system – 3.33 ms for 3-phase 50 Hz system Modeling of Converters and DC motor Phase-controlled rectifier (continuous current)

25. Modeling of Converters and DC motor Phase-controlled rectifier (continuous current) Output voltage Control signal Td Cosine-wave crossing Td – Delay in average output voltage generation 0 – 10 ms for 50 Hz single phase system

26. Modeling of Converters and DC motor Phase-controlled rectifier (continuous current) • Model simplified to linear gain if bandwidth (e.g. current loop) much lower than sampling frequency •  Low bandwidth – limited applications • Low frequency voltage ripple  high current ripple  undesirable

27.  T1 Q1 + Vt - Q2 Q3 Q4 T Modeling of Converters and DC motor Switch–mode converters

28.  T1 Q1 Q2 D1 Q3 Q4 T + Vt - T2 D2 Q1  T1 and D2 Q2  D1 and T2 Modeling of Converters and DC motor Switch–mode converters

29.  D1 Q1 D3 Q2 T1 T3 + Vt - Q3 Q4 T T4 T2 D2 D4 Modeling of Converters and DC motor Switch–mode converters

30. Modeling of Converters and DC motor Switch–mode converters • Switching at high frequency  Reduces current ripple  Increases control bandwidth • Suitable for high performance applications

31. + Vdc − Vdc vtri q vc when vc > vtri, upper switch ON when vc < vtri, lower switch ON Modeling of Converters and DC motor Switch–mode converters - modeling

32. Ttri vc q d Vdc Vt Modeling of Converters and DC motor Switch–mode converters – averaged model

33. d 1 0.5 0 vc -Vtri,p Vtri,p Modeling of Converters and DC motor Switch–mode converters – averaged model

34. Modeling of Converters and DC motor Switch–mode converters – small signal model 2-quadrant converter 4-quadrant converter

35. Te = kt ia ee = kt  ac components dc components Modeling of Converters and DC motor DC motor – separately excited or permanent magnet Extract the dc and ac components by introducing small perturbations in Vt, ia, ea, Te, TL and m

36. Vt(s) = Ia(s)Ra + LasIa + Ea(s) Te(s) = kEIa(s) Ea(s) = kE(s) Te(s) = TL(s) + B(s) + sJ(s) Modeling of Converters and DC motor DC motor – small signal model Perform Laplace Transformation on ac components

37. Modeling of Converters and DC motor DC motor – small signal model

38. Modeling of Converters and DC motor DC motor – small signal model: Block diagram transformation

39. torque controller position controller speed controller converter + - + - + - T* * * Motor tacho • The control variable of inner loop (e.g. torque) can be limited by limiting its reference value • It is flexible – outer loop can be readily added or removed depending on the control requirements kT 1/s CLOSED-LOOP SPEED CONTROL Cascade control structure

40. CLOSED-LOOP SPEED CONTROL Design procedure in cascade control structure • Inner loop (current or torque loop) the fastest – largest bandwidth • The outer most loop (position loop) the slowest – smallest bandwidth • Design starts from torque loop proceed towards outer loops

41. OBJECTIVES: • Fast response – large bandwidth • Minimum overshoot • good phase margin (>65o) BODE PLOTS • Zero steady state error – very large DC gain METHOD • Obtain linear small signal model • Design controllers based on linear small signal model • Perform large signal simulation for controllers verification CLOSED-LOOP SPEED CONTROL Closed-loop speed control – an example

42. Permanent magnet motor’s parameters Ra = 2  La = 5.2 mH B = 1 x10–4 kg.m2/sec J = 152 x 10–6 kg.m2 ke = 0.1 V/(rad/s) kt = 0.1 Nm/A Vd = 60 V Vtri = 5 V fs = 33 kHz CLOSED-LOOP SPEED CONTROL Closed-loop speed control – an example • PI controllers • Switching signals from comparison of vc and triangular waveform

43. q vtri Torque controller + Vdc − Tc + – DC motor Converter kt - Torque controller + + + - - q CLOSED-LOOP SPEED CONTROL Torque controller design

44. kpT= 90 kiT= 18000 CLOSED-LOOP SPEED CONTROL Torque controller design Open-loop gain compensated compensated

45. * T* T + Speed controller  1 – Torque loop CLOSED-LOOP SPEED CONTROL Speed controller design Assume torque loop unity gain for speed bandwidth << Torque bandwidth

46. kps= 0.2 kis= 0.14 CLOSED-LOOP SPEED CONTROL Speed controller Open-loop gain compensated compensated

47. CLOSED-LOOP SPEED CONTROL Large Signal Simulation results Speed Torque

48. CLOSED-LOOP SPEED CONTROL – DESIGN EXAMPLE SUMMARY Speed control by: armature voltage (0 b) and field flux (b) Power electronics converters – to obtain variable armature voltage Phase controlled rectifier – small bandwidth – large ripple Switch-mode DC-DC converter – large bandwidth – small ripple Controller design based on linear small signal model Power converters - averaged model DC motor – separately excited or permanent magnet Closed-loop speed control design based on Bode plots Verify with large signal simulation