Power Electronics and Drives (Version 3-2003): Dr. Zainal Salam, UTMJB 1 DC Motor Drive General Concept Speed Control SCR Drives Switched-mode DC Drives

Power Electronics and Drives (Version 3-2003): Dr. Zainal Salam, UTMJB

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

Td

J

B

TL

+

+

+


5

Operation

• When a separately excited motor is excited by a field current of if and an armature current of ia flows in the circuit, the motor develops a back emf and 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

v

fvg

ff

ga

aaaa

ff

fffff

K

iKe

LR

edt

diLiRv

LRdt

diLiRv


7

Basic torque equation

)(kg.mmotor theof inertia:

(N.m) torqueload:

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

where

:i.e. inertia, and

friction 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

TBdt

dJT

iKT

KK

iiKT

L

Ld

atd

vt

aftd


8

Steady-state operation

fvaagaaa

fvg

fff

IKRIERIV

IKE

RIV

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

dd

Laftd

fv

a

a

a

fv

aaa

TP

TBIIKT

IK

V

I

R

IK

RIV

: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

IK

RIV

IKRIV

IK

RIV

RIVIK

IKRIERIV


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:base

– the speed which correspond to the rated Va, rated Ia and rated If.

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

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

• Constant Power region ( base, )– 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.

base

Torque

Power


14

Four quadrant operation

1

23

4

A

C

D

TORQUE

SPEED

FORWARDMOTORING

REVERSEMOTORING

REVERSEGENERATING

B

FORWARDGENERATING

vaeg

ia

ia = +; Te = +va = +; = +

ia = ; Te = va = + ; m= +

m

Te

eg va

ia = ; Te = va = ; m=

ia

ia = +; Te = +va = ; =

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.


19

Basic single-phase drive

fm

f

ga

gaa

am

a

VV

ER

EVI

VV

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

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

KIRKI

T

V

KIRKI

TV

IKIT

KIVV

VRIV

VV

KI

32.62

60

20025.22

5.2

60

24022cos

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 orqueconstsnt t a hasmotor DC excited saperatelyA

1

1


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 ohm

Field resistance: Rf =175 ohm

Motor constant: KV =1.5 V/A-rad/sAssume the inductance of the armature and field circuit is large enough to ensure continuous and ripple-free currents. If the delay angle of the armature converter (a) is 45 degrees and the required armature current is 30A,

• a) Calculate the developed torque, Td. • b) Speed of the motor, (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

afvd

f

ff

fm

f

58.5530235.15.1

235.1175

216

2160cos24022

cos2

)(

0. maximum, iscurrent fiels theSince


23

Example 3 (cont)

oLLm

aa

aLLm

a

aaga

g

aaag

oa

LLma

a

aaag

fv

g

V

V

VV

AlsoVRIEV

VE

VVRIVE

VV

V

RIVEIK

E

4.132

240233

)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


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

IK

V

R

EVI

DVT

tVdt

TV

Tt

on

: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

(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 D2.

Q1

Q2Q3

Q4

Torque

+va

–

T2

D2

D1

T1


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


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 T3 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

Documents

Power Electronics and Drives (Version 3-2003): Dr. Zainal Salam, UTMJB 1 DC Motor Drive General Concept Speed Control SCR Drives Switched-mode DC Drives