1. A 2-day course on POWER ELECTRONICS AND APPLICATIONS (DC Motor Drives) Universiti Putra Malaysia August, Dr. Nik Rumzi Nik Idris Department of Energy Conversion Universiti Teknologi Malaysia

2. Contents Introduction Trends in DC drives DC motors
Modeling of Converters and DC motor
Phase-controlled Rectifier
DC-DC converter (Switch-mode)
Modeling of DC motor
Closed-loop speed control
Cascade Control Structure
Closed-loop speed control - an example
Torque loop
Speed loop
Summary

3. 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
AC will eventually replace DC – at a slow rate
DC strong presence – easy control – huge numbers

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

5. DC Motors Introduction Rotor: armature windings Stator: field windings
Current in
Current out
Stator: field windings
Rotor: armature windings
Mechanical commutator
Large machine employs compensation windings

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

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

8. 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 Te 
Maximum
Torque capability
Armature voltage control
Field flux control
base

9. 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)

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

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

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

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

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

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

16. 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

17. 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

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

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

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

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

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

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

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

25. Modeling of Converters and DC motor
DC motor – small signal model
Te = kt ia
ee = kt 
Extract the dc and ac components by introducing small perturbations in Vt, ia, ea, Te, TL and m
ac components
dc components

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

27. Modeling of Converters and DC motor
DC motor – small signal model + -

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

29. 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

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

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

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

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

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

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

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

37. 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