Download presentation

Presentation is loading. Please wait.

Published byRemington Wolfington Modified about 1 year ago

1
A 2-day course on POWER ELECTRONICS AND APPLICATIONS ( DC Motor Drives) Universiti Putra Malaysia August, 2004 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 Dominates variable speed applications before PE converters were introduced DC motor: industry workhorse for decades 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 Several limitations: Advantage: Precise torque and speed control without sophisticated electronics Regular MaintenanceExpensive HeavySpeed limitations Sparking

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

6
Introduction Electric torque Armature back e.m.f. LfLf RfRf ifif +ea_+ea_ LaLa RaRa iaia +Vt_+Vt_ +Vf_+Vf_ dt di LiRv f fff

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

8
Introduction For wide range of speed control 0 to base armature voltage, above base field flux reduction Armature voltage control : retain maximum torque capability Field flux control (i.e. flux reduced) : reduce maximum torque capability TeTe Maximum Torque capability Armature voltage control Field flux control base

9
MODELING OF CONVERTERS AND DC MOTOR Used to obtain variable armature voltage POWER ELECTRONICS CONVERTERS 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) T Q1 Q2 Q3Q4 3-phase supply +Vt+Vt iaia

11
Phase-controlled rectifier Q1 Q2 Q3Q4 T 3-phase supply 3- phase supply +Vt+Vt Modeling of Converters and DC motor

12
Phase-controlled rectifier Q1 Q2 Q3Q4 T F1 F2 R1 R2 + V a - 3-phase supply Modeling of Converters and DC motor

13
Phase-controlled rectifier (continuous current) Firing circuit –firing angle control Establish relation between v c and V t firing circuit current controller controlled rectifier +Vt–+Vt– vcvc i ref + - Modeling of Converters and DC motor

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

15
Phase-controlled rectifier (continuous current) Steady state: linear gain amplifier Cosine wave–crossing method Modeling of Converters and DC motor Transient: sampler with zero order hold T G H (s) converter T – 10 ms for 1-phase 50 Hz system – 3.33 ms for 3-phase 50 Hz system

16
Phase-controlled rectifier (continuous current) TdTd T d – Delay in average output voltage generation 0 – 10 ms for 50 Hz single phase system Output voltage Cosine-wave crossing Control signal Modeling of Converters and DC motor

17
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 Modeling of Converters and DC motor

18
Switch–mode converters Q1 Q2 Q3Q4 T +Vt-+Vt- T1 Modeling of Converters and DC motor

19
Switch–mode converters +Vt-+Vt- T1 D1 T2 D2 Q1 Q2 Q3Q4 T Q1 T1 and D2 Q2 D1 and T2 Modeling of Converters and DC motor

20
Switch–mode converters Q1 Q2 Q3Q4 T + V t - T1 D1 T2 D2 D3 D4 T3 T4 Modeling of Converters and DC motor

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

22
Switch–mode converters - modeling + V dc − V dc vcvc v tri q when v c > v tri, upper switch ON when v c < v tri, lower switch ON Modeling of Converters and DC motor

23
vcvc q T tri d Switch–mode converters – averaged model Modeling of Converters and DC motor V dc VtVt

24
V tri,p -V tri,p vcvc d Switch–mode converters – averaged model Modeling of Converters and DC motor

25
DC motor – small signal model Modeling of Converters and DC motor Extract the dc and ac components by introducing small perturbations in V t, i a, e a, T e, T L and m T e = k t i a e e = k t ac components dc components

26
DC motor – small signal model Modeling of Converters and DC motor Perform Laplace Transformation on ac components V t (s) = I a (s)R a + L a sIa + E a (s) T e (s) = k E I a (s) E a (s) = k E (s) T e (s) = T L (s) + B (s) + sJ (s)

27
DC motor – small signal model Modeling of Converters and DC motor

28
CLOSED-LOOP SPEED CONTROL Cascade control structure 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 1/s converter torque controller speed controller position controller tacho Motor ** T* ** k T

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 (>65 o ) 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 Ra = 2 La = 5.2 mH J = 152 x 10 –6 kg.m 2 B = 1 x10 –4 kg.m 2 /sec k t = 0.1 Nm/A k e = 0.1 V/(rad/s) V d = 60 VV tri = 5 V f s = 33 kHz Permanent magnet motor’s parameters Closed-loop speed control – an example PI controllers Switching signals from comparison of v c and triangular waveform

32
CLOSED-LOOP SPEED CONTROL Torque controller design TcTc v tri + V dc − q q + – ktkt Torque controller Torque controller Converter - + DC motor

33
CLOSED-LOOP SPEED CONTROL Torque controller design Open-loop gain compensated k pT = 90 k iT = 18000

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

35
CLOSED-LOOP SPEED CONTROL Speed controller Open-loop gain compensated k ps = 0.2 k is = 0.14 compensated

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

37
CLOSED-LOOP SPEED CONTROL – DESIGN EXAMPLE SUMMARY 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 Speed control by: armature voltage (0 b ) and field flux ( b )

Similar presentations

© 2017 SlidePlayer.com Inc.

All rights reserved.

Ads by Google