dc drives

DC MOTOR DRIVES

• 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 beforePE 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 φ= Electric torque

φω= 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

φ−

φ=ω

Therefore speed is given by,

Three possible methods of speed control:

Field fluxArmature voltage VtArmature resistance Ra

aa

aat edtdi

LiRV ++=

Armature circuit:

Introduction

For wide range of speed control 0 to ωbase → armature voltage, above ωbase → field flux reduction

Armature voltage control : retain maximum torque capability

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

Te

ω

MaximumTorque capability

Armature voltage controlField flux control

ωbase

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

ω

3-phasesupply

+

Vt

−

ia

Phase-controlled rectifier

Q1Q2

Q3 Q4

ω

T

3-phasesupply

3-phasesupply

+

Vt

−−


Phase-controlled rectifier

Q1Q2

Q3 Q4

ω

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

α+

Vt

–

vciref +

-



• Firing angle control

π

= 180vv

cosV

Vt

cma

α= ct v

180v

180vv

t

c=α

linear firing angle control

α= cosvv sc

Cosine-wave crossing control

s

cma v

vVV

π=



•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 generation0 – 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

ω

T

+Vt-

T1



+Vt-

T1D1

T2

D2

Q1Q2

Q3 Q4

ω

T

Q1 → T1 and D2

Q2 → D1 and T2



Q1Q2

Q3 Q4

ω

T+ Vt -

T1 D1

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 modelModeling of Converters and DC motor

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 modelModeling of Converters and DC motor

DC motor – small signal model


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

aa

aaat edtdi

LRiv ++=

Te = kt ia ee = kt ω

dtd

JTT mle

ω+=

aa

aaat e~dti~

dLRi

~v~ ++=

)i~(kT

~aEe =

)~(ke~ Ee ω=

dt)~(d

J~BT~

T~

Leω+ω+=

ac components

aaat ERIV +=

aEe IkT =

ω= Ee kE

)(BTT Le ω+=

dc components



Perform Laplace Transformation on ac components

aa

aaat e~dti~

dLRi

~v~ ++=

)i~(kT

~aEe =

)~(ke~ Ee ω=

dt)~(d

J~BT~

T~

Leω+ω+=

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

Te(s) = kEIa(s)

Ea(s) = kEω(s)

Te(s) = TL(s) + Bω(s) + sJω(s)



Tkaa sLR

1+

)s(Tl

)s(Te

sJB1+

Ek

)s(Ia )s(ω)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

Motorθθ* T*ωω*

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 modelMETHOD

• Design controllers based on linear small signal model

• Perform large signal simulation for controllers verification


Ra = 2 Ω 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(ω)s(Va

+-

-

+Torquecontroller

Converter

peak,tri

dc

VV)s(Te

-+

DC motor

Bode Diagram

Frequency (rad/s ec)

-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+

ω* T* T ω

–

+

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 →ωb) and field flux (ωb↑)

1

DC Motor Drive

• General Concept• Speed Control• SCR Drives• Switched-mode DC Drives

2

DC Motor

• Advantages of DC motor:– Ease of control– Deliver high starting torque– Near-linear performance

• Disadvantages:– High maintenance– Large and expensive (compared to induction

motor)– Not suitable for high-speed operation due to

commutator and brushes– Not suitable in explosive or very clean

environment

3

DC Motor Drives

• The DC drive is relatively simple and cheap (compared to induction motor drives). But DC motor itself is more expensive.

• Due to the numerous disadvantages of DC motor (esp. maintenance), it is getting less popular, particularly in high power applications.

• For low power applications the cost of DC motor plus drives is still economical.

• For servo application, DC drives is still popular because of good dynamic response and ease of control.

• Future Trend? Not so bright prospect for DC, esp. in high power drives.

4

Separately Excited DC Motor

• The field windings is used to excite the field flux.

• Armature current is supplied to the rotor via brush and commutator for the mechanical work.

• Interaction of field flux and armature current in the rotor produces torque.

va, Va

vf, If

La

Ra

Lf

Rf

ia, Ia if, If

Eg

ω

Td

J

B

TL

+

+

+

−

−

−

5

Operation

• When a separately excited motor is excited by a field current of if and an armature current of iaflows in the circuit, the motor develops a back emfand a torque to balance the load torque at a particular speed.

• The if is independent of the ia .Each windings are supplied separately. Any change in the armature current has no effect on the field current.

• The if is normally much less than the ia.

6

Field and armature equations

rad/sec)(in speedmotor theis and

rad/s)-V/A(in constant agemotor volt theis

:as expressed is voltage,speed asknown also is which emf,back motor The

ly.respective inductor, and

resistor armature theare and where

:current armature ousInstantane

lyrespective inductor, and

resistor field theare and where

:current field ousInstantane

ω

ω

v

fvg

ff

ga

aaaa

ff

fffff

K

iKe

LR

edtdi

LiRv

LRdt

diLiRv

=

++=

+=

7

Basic torque equation

)(kg.mmotor theof inertia:

(N.m) torqueload: )(N.m/rad/s constant,friction viscous:

where

:i.e. inertia, andfriction theplus torqueload the toequal bemust

torquedeveloped theoperation, normalFor

:as written isit Sometimes

rad/s)-V/A(in

constant. torque theis )( where

:ismotor by the develped torqueThe

2J

T

B

TBdtd

JT

iKT

KK

iiKT

L

Ld

atd

vt

aftd

++=

=

=

=

ωω

φ

8

Steady-state operation

fvaagaaa

fvg

fff

IKRIERIV

IKE

RIV

ω

ω

+=+=

=

=

circuit armature The

:bygiven is emfback The

circuit, fieldFor

saturated.not ismotor theAssuming zero. is sderivative time,operations state-steadyUnder

Rf

Ra

Ia

La

If

Eg

Lf

+

−

+ +

−

Va

−Va

9

Steady-state torque and speed

ω

ω

ω

ω

dd

Laftd

fv

a

a

a

fv

aaa

TP

TBIIKT

IKV

I

R

IKRIV

=

+==

=

−=

:ispower required The

:is torquedeveloped The

tage.supply vol on theonly depends speedmotor theconstant,kept iscurrent field theif isThat

small, is i.e. loaded,lightly ismotor the

or when usual), is(which valuesmall a is If

:derivedeasily becan speedmotor The

10

Torque and speed control

• From the derivation, several important facts can be deduced for steady-state operation of DC motor.

• For a fixed field current, or flux (If) , the torque demand can be satisfied by varying the armature current (Ia).

• The motor speed can be varied by:– controlling Va (voltage control)– controlling Vf (field control)

• These observations leads to the application of variable DC voltage to control the speed and torque of DC motor.

11

Example 1• Consider a 500V, 10kW , 20A rated- DC motor

with armature resistance of 1 ohm. When supplied at 500V, the UNLOADED motor runs at 1040 rev/min, drawing a current of 0.8A (ideally current is zero at no-load).

– Estimate the full load speed at rated values – Estimate the no-load speed at 250V.

rad/sec)strictly equation thisreality,in :(Note

rev/min 51948.0

)1(8.0250

250V,at voltageand load-noAt

rev/min 100048.0

)1(20500

value,rated and load fullAt

48.01040

)1(8.0500

=−=−=

+=

=−=−=

=−=−=

+=+=

fv

aaa

fvaaa

fv

aaafl

aaafv

fvaagaaa

IKRIV

IKRIV

IKRIV

RIVIK

IKRIERIV

ω

ω

ω

ω

ω

12

Variable speed operation

• Family of steady-state torque speed curves for a range of armature voltage can be drawn as above.

• The speed of DC motor can simply be set by applying the correct voltage.

• Note that speed variation from no-load to full load (rated) can be quite small. It depends on the armature resistance.

500 Speed (rev/min)

Torque

1000750250

500V375V250V125V

Rated torque)

13

Base Speed and Field-weakening

• Base speed:ωbase– the speed which correspond to the rated Va, rated Ia

and rated If.

• Constant Torque region (ω < ωbase, )– Ia and If are maintained constant to met torque

demand. Va is varied to control the speed. Power increases with speed.

• Constant Power region (ω > ωbase, )– Va is maintained at the rated value and if is reduced to

increase speed . However, the power developed by the motor (= torque x speed) remains constant. Known as field weakening.

ωbaseω

Torque

Power

14

Four quadrant operation

1

23

4

A

C

D

TORQUE

SPEED

FORWARDMOTORING

REVERSEMOTORING

REVERSEGENERATING

B

FORWARDGENERATING

vaeg

ia

ia = +; Te = +va = +; ω = +

ia = − ; Te = −va = + ; ω m= +

ω m

Te

eg va

ia = − ; Te = −va = − ; ω m= −

ia

ia = +; Te = +va = −; ω = −

ia

vaegeg va

ia

15

Regenerative Braking (in Q2)

• Say the motor running at position A. Suddenly va is reduced (below eg). The current ia will reverse direction.Operating point is shifted to B.

• Since ia is negative, torque Te is negative.

• Power is also negative, which implies power is “generated” back to the supply.

• In other words, during the deceleration phase, kinetic energy from the motor and load inertia is returned to the supply.

• This is known as regenerative braking-an efficient way to brake a motor. Widely employ in electric vehicle and electric trains. If we wish the motor to operate continuously at position B, the machine have to be driven by mechanical source.

• The mechanical source is a “prime mover”. • We must force the prime mover it to run faster so

that the generated eg will be greater than va.

16

Drive types

• SCR “phase-angle controlled” drive – By changing the firing angle, variable DC

output voltage can be obtained.– Single phase (low power) and three phase (high

and very high power) supply can be used– The line current is unidirectional, but the output

voltage can reverse polarity. Hence 2- quadrant operation is inherently possible.

– 4-quadrant is also possible using “two sets” of controlled rectifiers.

• Switched-mode drive– Using switched mode DC-DC converter. Dc

voltage is varied by duty cycle.– Mainly used for low to medium power range.– Single-quadrant converter (buck): 1- quadrant– Half bridge: 2-quadrant– Full bridge: 4-quadrant operation

17

Thyristor/SCR drives

• Mains operated.

• Variable DC voltages are obtained from SCR firing angle control.

• Slow response.

• Normally field rectifier have much lower ratings than the armature rectifier. It is only used to establish the flux.

Speedreference

Three/single phase supply Single phase supply

ControlandSCRfiring

Currentsensor TachometerCurrent

Speed

M

T

18

Continuous/Discontinuous current

• The key reason for successful DC drive operation is due to the large armature inductance La.

• Large La allows for almost constant armature current (with small ripple) due to “current filtering effect of L”. (Refer to notes on Rectifier).

• Average value of the ripple current is zero. No significant effect on the torque.

• If La is not large enough, or when the motor is lightly loaded, or if supply is single phase (half-wave), discontinuous current may occur.

• Effect of discontinuous current: Output voltage of rectifier rises; motor speed goes higher. In open-loop operation the speed is poorly regulated.

• Worthwhile to add extra inductance in series with the armature inductance.

9

Basic single-phase drive

fm

f

ga

gaa

am

a

VV

ER

EVI

VV

απ

απ

cos2

: voltageField

emfback theis ;

:iscurrent (DC) Armature

cos2

:is voltagearmature current, continuousFor

=

−=

=

+vs

_

Ia

Ta1

Ta2

Ta3

+

+

−

Va

Ra

Ta4

La

Eg −

If

+vs

_

Tf1

Tf2

Tf3+

−

Lf

Tf4

Lf

Vf

ARMATURE FIELD

20

Basic three-phase drive

fm

f

Ea

Eaa

aLLm

a

VV

VR

VVI

VV

απ

απ

cos2

:fieldfor used is phase single If

emfback theis ;

:current (DC) Armature

cos3

: voltageArmature

,

=

−=

= −

Ia

Ta1

+

+

−

Va

Ra

La

Eg−

Ta3

Ta2

Ta6

_ vcn +

n_ vbn

+

_ van +

Ta5

Ta4

+

−

Lf

Lf

Vf

If

ARMATURE FIELD

21

Example 2

o

fafm

a

faf

am

af

fEE

Eaaa

am

a

f

V

KIRKIT

V

KIRKITV

IKIT

KIVV

VRIV

VV

KI

32.62

60200

25.225.2

6024022

cos

2cos

cos2

and5.2

i.e emf,back theis Where

And

cos2

current, continuousFor

.continuous iscurrent theAssumerpm. 200at opearte motor to for the angle g triggerinthe

Calculate ohm. 2 is resistance armature theand 2.5motor theofconstant field The supply. ac 240V a toconnected

converter wave-full sby driven ismotor The Nm. 60 ofload orqueconstant t a hasmotor DC excited saperatelyA

1

1

=

××+

×

=

+

=

+

=

=

==

+=

=

=

−

−

ππ

ωπα

ωαπ

ωω

απ

22

Example 3A rectifier-DC motor drive is supplied by a three-phase, full controlled SCR bridge 240Vrms/50Hz per-phase. The field is supplied by a single-phase 240V rms/50Hz, with uncontrolled diode bridge rectifier. The field current is set as maximum as possible.The separately excited DC motor characteristics is given as follows:Armature resistance:Ra = 0.3 ohmField resistance: Rf =175 ohmMotor constant: KV =1.5 V/A-rad/sAssume the inductance of the armature and field circuit is largeenough to ensure continuous and ripple-free currents. If the delay angle of the armature converter (αa) is 45 degrees and the required armature current is 30A,

• a) Calculate the developed torque, Td.• b) Speed of the motor, ω (rad/s)• c) If the polarity of the field current is reversed, the motor

back emf will reverse. For the same armature current of 30A, determine the required delay angle of the armature converter.

NmIIKT

AV

R

VI

VV

Va

afvdf

ff

fm

f

58.5530235.15.1

235.1175

216

2160cos24022

cos2

)(

0. maximum, iscurrent fiels theSince

=××==

===

=×==

=

πα

π

α

23

Example 3 (cont)

oLLm

aa

aLLm

a

aaga

g

aaag

oa

LLma

a

aaag

fv

g

VV

VV

AlsoVRIEV

VE

VVRIVE

VV

V

RIVEIK

E

4.132240233

)5.378(cos

3cos

cos3

,3.3783.0303.387

and3.387

thenreversed, is field ofpolarity theNow (c)

sec/rad 06.209235.15.1

3.3873.3873.0303.396

3.39645cos240233

cos3

, 45 with phase-by three supplied is armature The

speedMotor (b)

1

,

1

,

,

o

=

××

−×=

×=

=

−=×+−=+=

−=

=×

=

=×−=−==×××==

=−=

=

−

−

−

−

−

ππα

απ

ω

πα

π

α

ω

24

Reversal

• DC motor in inherently bi-directional. Hence no-problem to reverse the direction. It can be a motor or generator.

• But the rectifier is unidirectional, because the SCR are unidirectional devices.

• However, if the rectifier is fully controlled, it can be operated to become negative DC voltage, by making firing angle greater than 90 degrees,

• Reversal can be achieved by:– armature reversal using contactors (2-

quadrant)– field reversal using contactors (2-quadrant)– double converter (full 4-quadrants)

25

Reversal using armature or field contactors

FIELD

DRIVE REVERSING USING ARMATURE OR FIELD CONTACTORS

CONTACTOR

CONTACTOR AT THE ARMATURESIDE (SINGLE PHASE SYSTEM)

Vs

Va Eg Va Eg Va Eg

1

2

1

2

CONTACTOR AT

1POSITION

(MOTORING)

CONTACTOR AT

2POSITION

(BRAKING/GENERATION)

CONTACTOR AT

2POSITION

(RESERVE)

26

Reversing using double converters

converter 1 converter 2

FIELD

Vs

Principle of reversal

Practical circuit

27

Switched–mode DC drives

• Supply is DC (maybe from rectified-filtered AC, or some other DC sources).

• DC-DC converters (coppers) are used.

• suitable for applications requiring position control or fast response, for example in servo applications, robotics, etc.

• Normally operate at high frequency– the average output voltage response is

significantly faster – the armature current ripple is relatively less

than the controlled rectifier

• In terms of quadrant of operations, 3 possible configurations are possible: – single quadrant,– two–quadrant – and four–quadrant

28

Single-quadrant drive

• Unidirectional speed. Braking not required.

fv

a

a

gaa

ont

a

IKV

R

EVI

DVT

tVdt

TV

Tt

on

=

−=

===

<<

ω

:as edapproximat becan speed and

;

:iscurrent (DC) Armature

1

:statesteady at voltagearmature The ,0For

0

ia

va

ton T

iaTorque ( ia)a)

Q4 Q1

Q2Q3

ω (va)

29

2 Quadrant DC drives

• FORWARD MOTORING (T1 and D2 operate)– T1 on: The supply is connected to motor terminal.– T1 off: The armature current freewheels through

D2.– Va (hence speed) is determined by the duty ratio.

• REGENERATION (T2 and D1 operate)– T2 on: motor acts as a generator– T2 off:, the motor acting as a generator returns

energy to the supply through D1.

Q1

Q2Q3

Q4

Torque

ω

+va

–

T2 D2

D1T1

30

4 Quadrant DC drives

• A full-bridge DC-DC converter is used.

+ va –

T1 T3

T2T4

D1

D4

D3

D2

Q1

Q2Q3

Q4

Torque

ω

31

4-quadrant: Forward motoring

• T1 and T2 operate; T3 and T4 off.

• T1 and T2 turn on together: the supply voltage appear across the motor terminal. Armature current rises.

• T1 and T2 turn off: the armature current decay through D3 and D4

+ va –

T1 T3

T2T4

D1

D4

D3

D2

32

Regeneration

• T1, T2 and T3 turned off.

• When T4 is turned on, the armature current rises through T4 and D2.

• When T4 is turned off, the motor, acting as a generator, returns energy to the supply through D1 and D2.

+ va –

T1 T3

T2T4

D1

D4

D3

D2

33

Reverse motoring

• T3 and T4 operate; T1 and T2 off.

• When T3 and T4 are on together, the armature current rises and flows in reverse direction.

• Hence the motor rotates in reverse direction.

• When T3 and T4 turn off, the armature current decays through D1 and D2.

+ va –

T1 T3

T2T4

D1

D4

D3

D2

34

Reverse generation

• T1, T3 and T4 are off.

• When T1 is on, the armature current rises through T2 and D4.

• When Q2 is turned off, the armature current falls and the motor returns energy to the supply through D3 and D4.

+ va –

T1 T3

T2T4

D1

D4

D3

D2

DC Motor Drives 13-1

DC-Motor Drives

•These drives continue to be used


DC-Motor Structure

• With permanent magnets or a wound field

. DC Motor Drives 13-3

DC-Motor Equivalent Circuit

• The mechanical system can also be represented as an electrical circuit

. DC Motor Drives 13-4

Four-Quadrant Operation of DC-Motor Drives

• High performance drives may operate in all four quadrants


DC-Motor Drive Torque-Speed Characteristics and Capabilities

• With permanent magnets


DC-Motor Drive Capabilities

• Separately-Excited field


Controlling Torque, Speed and Position

• Cascaded control is commonly used


Small-Signal Representation of DC Machines

• Around a steady state operating point


Electrical Time-Constant of the DC Machine

• The speed is assumed constant


Mechanical Time-Constant of the DC Machine

• The load-torque is assumed constant


DC-Motor Drive: Four-Quadrant Capability

• If a diode-rectifier is used, the energy recovered during regenerative braking is dissipated in a resistor


Ripple in the Armature Current

• Bi-polar and uni-polar voltage switchings


Control of Servo Drives

• A concise coverage is presented in “Electric Drives: An Integrative Approach” by N. Mohan (www.MNPERE.com)


Effect of Blanking Time

• Non-linearity is introduced


Converters for Limited Operational Capabilities

• Two switches for 2-quadrant operation and only one switch for 1-quadrant operation


Line-Controlled Converters for DC Drives

• Large low-frequency ripple in the dc output of converters


Four Quadrant Operation using Line Converters

• Two options to achieve 4-quadrant operation


Effect of Discontinuous Current Conduction

• Speed goes up unless it is controlled


Open-Loop Speed Control

• Adequate for general-purpose applications


DC Drive Characteristics and Capabilities

• Line current in switch-mode and line-converter drives

dc drives

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