Speed Control of DC Motors

Compiled ByDr Qamar Saeed

Dr. Ungku Anisa Ungku AmirulddinDepartment of Electrical Power EngineeringCollege of Engineering

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DC Drives OutlineIntroduction to DC DrivesSeparately Excited DC MotorSpeed Control MethodsSpeed Control StrategyOperating ModesReferences

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IntroductionDC Drives – Electric drives employing DC

motors as prime moversDominated variable speed applications before

introduction of Power Electronic convertersStill popular even after Power ElectronicsAdvantage: Precise torque and speed control

without sophisticated electronicsApplications: Rolling mills, hoists, traction, cranes

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IntroductionSome limitations:

High maintenance (commutators & brushes)ExpensiveSpeed limitationsSparking

Commonly used DC motorsSeparately excitedSeries (mostly for traction applications)

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Separately Excited DC Motor

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Lf Rf

if+

ea

_

LaRa

ia+

vt

_

+

vf

_

abae iKiKT Electromagnetic torque

ba KKe Armature back e.m.f.

Armaturecircuit

Fieldcircuit

aa

aaaa edt

diLiRv

dt

diLiRv f

ffff

Separately Excited DC MotorMotor is connected to a

load.Therefore,

whereTL= load torque

J = load inertia (kgm2)

B = viscous friction coefficient (Nm/rad/s)

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Le TBdt

dJT

Separately Excited DC Motor – Steady State Condition

Time derivatives = 0. Therefore, (1)

(2)

(3)

(4)The developed power

(5)

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Laabe TBIKIKT

fff IRV

KKE ba

KIR

EIRV

aa

aaaa

ed TP

Speed Control Methods for Separately Excited DC MotorFrom equation (3),

Three possible methods for speed control:Armature voltage Va

Armature resistance Ra

Field flux (by changing field resistance Rf)

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eaa

eaaaaa

TK

R

K

V

K

T

K

R

K

V

K

IRV

2

Te

K

Va intercept

2K

Ra slope

Speed Control Methods – Va control

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eaa T

K

R

K

V2

Te

Va↓

TL

Requires variable DC supply

K

Va

Speed Control Methods – Ra control

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eaa T

K

R

K

V2

Te

Ra ↑

TL Simple controlLosses in external resistor Rarely used.

K

Va 2K

Ra slope

Speed Control Methods – control

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eaa T

K

R

K

V2

Te

↓

K

Va

2K

Ra slope

TL

Not possible for PM motor

Normally employed for speed above base speed

Speed Control Strategy for Separately Excited DC Motor

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Base speed base = Speed at rated Va, If and Ia

= 0 to base speed control by Va

> base speed control by flux weakening ()T

Va control controlbase

Speed Control Strategy for Separately Excited DC Motor

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= 0 to base speed control by Va > base speed control by flux weakening ( )T Ia For maximum torque capability, Ia = Ia maxPd = EaIa = (K)Ia = constant when > base

in order to go beyond base, (1/)

Per unit quantities

Va1.0Ia

If, Te,

Va control controlbase

Speed Control Strategy

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Torque and power relations below and beyond base

Per unit quantities

Va1.0Ia

If, Te,

Va control controlbase

P, T

Te Te = KIa

P =K P

constant powerconstant torque

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Series Motor Speed

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Speed Control of DC Motor – Example- 5-11 (G. K . Dubey)A 220 V, 500 A, 600 rpm separately excited motor has armature and field resistance of 0.02 and 10 respectively. The load torque is given the expression

TL = 2000 – 2N,

where N is the speed in rpm. Speeds below the rated are obtained by armature voltage control and speeds above the rated are obtained by field control.

i) Calculate motor terminal voltage and armature current when the speed is 450 rpm.ii) Calculate field winding voltage and armature current when the speed is 750 rpm. Assume the rated field voltage is the same as the rated armature voltage.

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Speed Control of DC Motor

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Operating ModesMotoringBack EMF Ea < Va

Ia and If are positive

Motor develops torque to meet load demand (i.e. Te =TL )

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BRAKING

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In braking, the motor works as a generator developing a negative torque which opposes the motion. It is of three types: Regenerative braking; Dynamic or rheostatic braking; and Plugging or reverse voltage braking.

Operating ModesRegenerative BrakingMotor acts as generatorDevelops Ea > Va

Ia negative (flows back to source)

If positiveMachine slows down

until Ea = Va

Used only when there are enough loads to absorb regenerated power

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Regenerative Braking

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Operating ModesDynamic BrakingSimilar to

regenerative breaking

But Va removed, replaced by Rb

Kinetic energy of motor is dissipated in Rb (i.e. machine works as generator)

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Dynamic Braking

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Dynamic Braking

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Operating ModesPluggingSupply voltage Va is

reversedAssists Ea in forcing

Ia in reverse direction

Rb connected in series to limit current

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Operating Modes - Four Quadrant Operation

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Q1+Va , +Ea +

+Ia +TPower = +ve

Q4-Va , -Ea -

+Ia +TPower = -ve

Q2+Va , +Ea +

-Ia -TPower = -ve

Q3-Va , -Ea -

-Ia -TPower = +ve

Note: In the figure, Eg = Ea

ReferencesChapman, S. J., Electric Machinery Fundamentals, McGraw

Hill, New York, 2005.Rashid, M.H, Power Electronics: Circuit, Devices and

Applications, 3rd ed., Pearson, New-Jersey, 2004.Dubey, G.K., Fundamentals of Electric Drives, 2nd ed., Alpha

Science Int. Ltd., UK, 2001.Nik Idris, N. R., Short Course Notes on Electrical Drives,

UNITEN/UTM, 2008.Ahmad Azli, N., Short Course Notes on Electrical Drives,

UNITEN/UTM, 2008.

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