dc motor drives 2007 ppt

8/10/2019 Dc Motor Drives 2007 Ppt

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DC MOTOR DRIVES(MEP 1422)

Dr. Nik Rumzi Nik Idris

Department of Energy Conversion

FKE, UTM


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Contents Introduction

Trends in DC drives

Principles of DC motor drives

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


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INTRODUCTION

DC DRIVES: Electric drives that use DC motorsas the prime movers

Dominates variable speed applications beforePE converters were introduced

DC motor: industry workhorse for decades

Will AC drive replaces DC drive ?

Predicted 30 years ago

AC will eventually replace DCat a slow rate

DC strong presenceeasy controlhuge numbers


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Introduction

DC Motors

Several limitations:

Advantage: Precise torque and speed controlwithout sophisticated electronics

Regular Maintenance Expensive

Heavy Speed limitations

Sparking


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Introduction

DC Motors - 2 pole

Stator

Rotor


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Introduction

DC Motors - 2 pole

Mechanical commutator to maintain armature current direction

X

X

X

X

X

Armature mmf produces

flux which distorts main

flux produce by field

Armature reaction


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Introduction

Flux at one side of the pole may saturate

Zero flux region shifted

Flux saturation, effective flux per pole decreases

Large machine employs compensation windings and interpoles

Armature mmf distorts field flux

Armature reaction


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Introduction

at ikTe Electric torque

Ea ke Armature back e.m.f.

Lf Rf

if

aa

aat edt

diLiRv

+

ea

_

LaRa

ia+

Vt

_

+

Vf

_

dt

diLiRv ffff


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Introduction

aaat EIRV In steady state,

2Tea

T

t

k

TR

k

V

Therefore steady state speed is given by,

Three possible methods of speed control:

Field flux

Armature voltage VtArmature resistance Ra

aaaat edt

diLiRV

Armature circuit:


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Introduction

2Tea

T

t

k

TR

k

V

Te

TLT

t

k

V

Vt

Varying Vt

Requires variable DC supply


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Introduction

2Tea

T

t

k

TR

k

V

Te

TLT

t

k

V

Vt

Varying Vt



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Te

Varying Vt


TL

T

eaTt

k

TR)k(V

Introduction

Constant TL


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Vt

Introduction

aaTt RI)k(V

aaRI

rated,tV

base

Varying Vt

Constant TL

T

eaTt

k

TR)k(V


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Introduction

2Tea

T

t

k

TR

k

V

Te

Ra

TL

T

t

k

V

Varying Ra

Simple control

Losses in external resistor


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Introduction

2Tea

T

t

k

TR

k

V

Te

TL

T

t

k

V

Varying

Not possible for PM motor

Maximum torque capability reduces


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Introduction

For wide range of speed control

0 to base armature voltage, above basefield flux reduction

Armature voltage control : retain maximum torque capability

Field flux control (i.e. flux reduced) : reduce maximum torque capability

Te

Maximum

Torque capability

Armature voltage control

Field flux control

base


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Introduction

Te

Maximum

Torque capability

base


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Introduction

Te

Constant powerConstant torque

base

0 to base armature voltage, above basefield flux reduction

P= EaIa,max= kaIa,max

Pmax

Pmax = EaIa,max= kabaseIa,max

1/

P


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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)


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Phase-controlled rectifier (ACDC)

T

Q1Q2

Q3 Q4

3-phase

supply

+

Vt

ia


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Phase-controlled rectifier

Q1Q2

Q3 Q4

T

3-phase

supply

3-

phase

supply

+

Vt



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Phase-controlled rectifier

Q1Q2

Q3 Q4

T

F1

F2

R1

R2

+ Va -

3-phase

supply



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Phase-controlled rectifier (continuous current)

Firing circuitfiring angle control

Establish relation between vcand Vt

firing

circuit

current

controller

controlled

rectifier

+

Vt

vciref +

-



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Firing angle control

180

v

vcos

V2V

t

cm

a

ct v

180

v180

v

v

t

c

linear firing angle control

cosvv sc

Cosine-wave crossing control

s

cm

av

vV2V

http://localhost/var/www/apps/conversion/tmp/scratch_6/cosine_crossing.jpghttp://localhost/var/www/apps/conversion/tmp/scratch_6/cosine_crossing.jpghttp://localhost/var/www/apps/conversion/tmp/scratch_6/cosine_crossing.jpghttp://localhost/var/www/apps/conversion/tmp/scratch_6/cosine_crossing.jpg


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Steady state: linear gain amplifier

Cosine wavecrossing method


Transient: sampler with zero order hold

T

GH(s)

converter

T 10 ms for 1-phase 50 Hz system

3.33 ms for 3-phase 50 Hz system


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0.3 0.31 0.32 0.33 0.34 0.35 0.36-400

-200

0

200

400

0.3 0.31 0.32 0.33 0.34 0.35 0.36-10

-5

0

5

10


Td

Td Delay in average output voltage generation

010 ms for 50 Hz single phase system

Output

voltage

Cosine-wave

crossing

Control

signal



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Model simplified to linear gain if bandwidth

(e.g. current loop) much lower than sampling

frequency

Low bandwidthlimited applications

Low frequency voltage ripple high currentripple undesirable



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Switchmode converters

Q1Q2

Q3 Q4

T

+

Vt-

T1



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+

Vt

-

T1

D1

T2

D2

Q1Q2

Q3 Q4

T

Q1 T1 and D2

Q2 D1 and T2



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Q1Q2

Q3 Q4

T

+ Vt

-T1

D1

T2D2

D3

D4

T3

T4



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Switching at high frequency

Reduces current ripple

Increases control bandwidth

Suitable for high performance applications



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Switchmode converters - modeling

+

Vdc

Vdc

vc

vtri

q

0

1q

when vc> vtri, upper switch ON

when vc< vtri, lower switch ON



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tri

onTt

ttri T

tdtq

T

1d

tri

vc

q

Ttri

d

Switchmode convertersaveraged model


dc

dT

0dc

tri

t dVdtVT

1V

tri

Vdc Vt


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Vtri,p-Vtri,pvc

d

1

0

0.5

p,tri

c

V2

v5.0d

c

p,tri

dcdct v

V2

VV5.0V

Switchmode convertersaveraged model



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Switchmode converterssmall signal model


)s(vV2

V)s(V c

p,tri

dct

)s(vV

V)s(V c

p,tri

dct

2-quadrant converter

4-quadrant converter


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DC motorseparately excited or permanent magnet


Extract the dc and ac components by introducing small

perturbations in Vt, ia, ea, Te, TLand m

aa

aaat edt

diLRiv

Te= kt ia ee= kt

dt

dJTT mle

a

a

aaat

e~

dt

i~

dLRi

~v~

)i~

(kT~

aEe

)~(ke~ Ee

dt

)~(dJ~BT

~T~

Le

ac components

aaat ERIV

aEe IkT

Ee kE

)(BTT Le

dc components


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DC motorsmall signal model


Perform Laplace Transformation on ac components

aa

aaat e~

dt

i~

dLRi

~v~

)i~

(kT~

aEe

)~(ke~ Ee

dt)~(dJ~BT~T~ Le

Vt(s) = Ia(s)Ra+ LasIa + Ea(s)

Te(s) = kEIa(s)

Ea(s) = kE(s)

Te(s) = TL(s) + B(s) + sJ(s)


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DC motorsmall signal model


Tkaa sLR

1

)s(Tl

)s(Te

sJB

1

Ek

)s(Ia )s()s(Va+

-

-

+


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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

convertertorque

controllerspeed

controller

position

controller+

-

+

-

+

-

tacho

Motor* T**

kT


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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


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Closed-loop speed controlan example

OBJECTIVES: Fast responselarge bandwidth

Minimum overshoot

good phase margin (>65o)

Zero steady state errorvery 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


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Ra = 2 La = 5.2 mH

J = 152 x 106kg.m2B = 1 x104kg.m2/sec

kt = 0.1

Nm/A

ke = 0.1

V/(rad/s)

Vd= 60 V Vtri= 5 V

fs= 33kHz

Permanent magnet motors parameters

Closed-loop speed controlan example

PI controllers Switching signals from comparison

of vcand triangular waveform


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Torque controller design

Tc

vtri

+

Vdc

q

q

+

kt

Torque

controller

Tkaa sLR

1

)s(Tl

)s(Te

sJB

1

Ek

)s(Ia )s()s(Va+

-

-

+

Torque

controller

Converter

peak,tri

dc

V

V)s(Te

-

+

DC motor


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Bode Diagram

Frequency (rad/sec)

-50

0

50

100

150From: Input Point To: Output Point

M

agnitude(dB)

10-2

10-1

100

101

102

103

104

105

-90

-45

0

45

90

Phas

e(deg)


Torque controller design

Open-loop gain

compensated

compensated

kpT= 90

kiT= 18000


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Speed controller design

Assume torque loop unity gain for speed bandwidth


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Bode Diagram

Frequency (Hz)

-50

0

50

100

150From: Input Point To: Output Point

M

agnitude(dB)

10-2

10-1

100

101

102

103

104

-180

-135

-90

-45

0

Phase

(deg)


Speed controller

Open-loop gain

compensated

kps= 0.2

kis= 0.14

compensated


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Large Signal Simulation results

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45-40

-20

0

20

40

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45-2

-1

0

1

2

Speed

Torque


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CLOSED-LOOP SPEED CONTROL DESIGN EXAMPLE

SUMMARY

Power electronics convertersto obtain variable armature voltage

Phase controlled rectifiersmall bandwidthlarge rippleSwitch-mode DC-DC converterlarge bandwidthsmall ripple

Controller design based on linear small signal model

Power converters - averaged model

DC motorseparately 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)

dc motor drives 2007 ppt

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