Dc Motor Drives Fantastic

DC MOTOR DRIVES(MEP 1422)

Dr. Nik Rumzi Nik Idris

Department of Energy Conversion

FKE, UTM

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

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

Introduction

DC Motors

• Several limitations:

• Advantage: Precise torque and speed control without sophisticated electronics

• Regular Maintenance • Expensive

• Heavy • Speed limitations

• Sparking

Current in

Current out

Stator: field windings

Rotor: armature windings

Introduction

DC Motors

•Mechanical commutator

•Large machine employs compensation windings

Introduction

at ikTe Electric torque

Ea ke Armature back e.m.f.

Lf Rf

if

aa

aat edtdi

LiRv

+

ea

_

LaRa

ia+

Vt

_

+

Vf

_

dtdi

LiRv ffff

Introduction

aaat EIRV In steady state,

2T

ea

T

t

k

TRkV

Therefore speed is given by,

Three possible methods of speed control:

Field fluxArmature voltage Vt

Armature resistance Ra

aa

aat edtdi

LiRV

Armature circuit:

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

Te

MaximumTorque capability

Armature voltage controlField flux control

base

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)

Modeling of Converters and DC motor

Phase-controlled rectifier (AC–DC)

T

Q1Q2

Q3 Q4

3-phasesupply

+

Vt

ia

Phase-controlled rectifier

Q1Q2

Q3 Q4

T

3-phasesupply

3-phasesupply

+

Vt


Phase-controlled rectifier

Q1Q2

Q3 Q4

T

F1

F2

R1

R2+ Va -

3-phasesupply


Phase-controlled rectifier (continuous current)

• Firing circuit –firing angle control

Establish relation between vc and Vt

firingcircuit

currentcontroller

controlled rectifier

+

Vt

–

vciref+

-



• Firing angle control

180vv

cosV

Vt

cma

ct v

180v

180vv

t

c

linear firing angle control

cosvv sc

Cosine-wave crossing control

s

cma v

vVV



•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 – 3.33 ms for 3-phase 50 Hz system

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 0 – 10 ms for 50 Hz single phase system

Outputvoltage

Cosine-wave crossing

Control signal



• 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


Switch–mode converters

Q1Q2

Q3 Q4

T

+Vt

-

T1



+Vt

-

T1D1

T2

D2

Q1Q2

Q3 Q4

T

Q1 T1 and D2

Q2 D1 and T2



Q1Q2

Q3 Q4

T+ Vt -

T1D1

T2D2

D3

D4

T3

T4



• Switching at high frequency

Reduces current ripple

Increases control bandwidth

• Suitable for high performance applications


Switch–mode converters - modeling

+

Vdc

−

Vdc

vc

vtri

q

0

1q

when vc > vtri, upper switch ON

when vc < vtri, lower switch ON


tri

onTt

ttri Tt

dtqT1

dtri

vc

q

Ttri

d

Switch–mode converters – averaged model


dc

dT

0dc

trit dVdtV

T1

Vtri

Vdc Vt

Vtri,p-Vtri,pvc

d

1

0

0.5

p,tri

c

V2v

5.0d

cp,tri

dcdct v

V2V

V5.0V

Switch–mode converters – averaged model


DC motor – small signal model


Extract the dc and ac components by introducing small perturbations in Vt, ia, ea, Te, TL and m

aa

aaat edtdi

LRiv

Te = kt ia ee = kt

dtd

JTT mle

aa

aaat e~dti~

dLRi

~v~

)i~(kT

~aEe

)~(ke~ Ee

dt)~(d

J~BT~

T~

Le

ac components

aaat ERIV

aEe IkT

Ee kE

)(BTT Le

dc components



Perform Laplace Transformation on ac components

aa

aaat e~dti~

dLRi

~v~

)i~(kT

~aEe

)~(ke~ Ee

dt)~(d

J~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)



Tkaa sLR

1

)s(Tl

)s(Te

sJB1

Ek

)s(Ia )s()s(Va

+-

-

+

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

convertertorquecontroller

speedcontroller

positioncontroller

+

-

+

-

+

-

tacho

Motor* T**

kT


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


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


Ra = 2 La = 5.2 mH

J = 152 x 10–6 kg.m2B = 1 x10–4 kg.m2/sec

kt = 0.1 Nm/Ake = 0.1 V/(rad/s)

Vd = 60 V Vtri = 5 V

fs = 33 kHz

Permanent magnet motor’s parameters

Closed-loop speed control – an example

• PI controllers • Switching signals from comparison of vc and triangular waveform


Torque controller design

Tc

vtri

+

Vdc

−

q

q

+

–

kt

Torque controller

Tkaa sLR

1

)s(Tl

)s(Te

sJB1

Ek

)s(Ia )s()s(Va

+-

-

+

Torquecontroller

Converter

peak,tri

dc

VV)s(Te

-+

DC motor

Bode Diagram

Frequency (rad/sec)

-50

0

50

100

150From: Input Point To: Output Point

Mag

nitu

de (

dB)

10-2

10-1

100

101

102

103

104

105

-90

-45

0

45

90

Pha

se (

deg)


Torque controller design Open-loop gain

compensated

compensated

kpT= 90

kiT= 18000


Speed controller design

Assume torque loop unity gain for speed bandwidth << Torque bandwidth

1Speedcontroller sJB

1

* T* T

–

+

Torque loop

Bode Diagram

Frequency (Hz)

-50

0

50

100

150From: Input Point To: Output Point

Mag

nitu

de (

dB)

10-2

10-1

100

101

102

103

104

-180

-135

-90

-45

0

Pha

se (

deg)


Speed controllerOpen-loop gain

compensated

kps= 0.2

kis= 0.14

compensated


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

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)

Documents

Dc Motor Drives Fantastic