speed control of dc motors by dr. ungku anisa ungku amirulddin department of electrical power...
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Speed Control of DC MotorsByDr. Ungku Anisa Ungku AmirulddinDepartment of Electrical Power EngineeringCollege of Engineering
Dr. Ungku Anisa, July 2008 1EEEB283 – Electrical Machines & Drives
DC Drives OutlineIntroduction to DC DrivesSeparately Excited DC MotorSpeed Control MethodsSpeed Control StrategyOperating ModesReferences
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 2
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
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 3
IntroductionSome limitations:
High maintenance (commutators & brushes)ExpensiveSpeed limitationsSparking
Commonly used DC motorsSeparately excitedSeries (mostly for traction applications)
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 4
Separately Excited DC Motor
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Lf Rf
if
aa
aaaa edt
diLiRv
+
ea
_
LaRa
ia+
vt
_
+
vf
_
dt
diLiRv f
ffff
afte iiKT Electromagnetic torque
fva iKe Armature back e.m.f.
Kt = torque constantKv = voltage constant (V/A-rad/s)Kv = Kt
Separately Excited DC MotorMotor is connected to a
load.Therefore,
whereTL= load torque
J = load inertia (kg/m2)
B = viscous friction coefficient (Nm/rad/s)
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 6
Le TBdt
dJT
Separately Excited DC Motor – Steady State Condition
Time derivatives = 0. Therefore, (1)
(2)
(3)
(4)The developed power
(5)
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 7
Lafte TBIIKT
fff IRV
fva IKE
fvaa
aaaa
IKIR
EIRV
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 current If (by changing field resistance Rf) flux
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 8
e
fv
a
fv
a
ft
e
fv
a
fv
a
fv
aaa
TIK
R
IK
V
IK
T
IK
R
IK
V
IK
IRV
2
Te
fv
a
IK
V intercept
2fv
a
IK
R slope
Speed Control Methods – Va control
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 9
e
fv
a
fv
a TIK
R
IK
V2
Te
Va↓
TL
Requires variable DC supply
fv
a
IK
V
Speed Control Methods – Ra control
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 10
Te
Ra ↑
TL Simple controlLosses in external resistor Rarely used.
fv
a
IK
V
2fv
a
IK
R slope
e
fv
a
fv
a TIK
R
IK
V2
Speed Control Methods – control
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 11
Te
If ↓
fv
a
IK
V
2fv
a
IK
R slope
TL
Not possible for PM motor
Normally employed for speed above base speed
e
fv
a
fv
a TIK
R
IK
V2
Speed Control Strategy for Separately Excited DC Motor
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 12
Base speed base = Speed at rated Va, If and Ia
= 0 to base speed control by Va
> base speed control by flux weakening (, i.e. If ↓)T
Va control controlbase
Speed Control Strategy for Separately Excited DC Motor
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 13
= 0 to base speed control by Va > base speed control by flux weakening ()T Ia For maximum torque capability, Ia = Ia maxPd = EaIa = (KvIf)Ia = constant when > base
in order to go beyond base, If (1/)
Per unit quantities
Va1.0Ia
If, Te,
Va control controlbase
Speed Control Strategy
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 14
Torque and power relations below and beyond base
Per unit quantities
Va1.0Ia
If, Te,
Va control controlbase
P, T
Te Te = KvIf Ia
P =TeP
constant powerconstant torque
Operating ModesMotoringBack EMF Ea < Va
Ia and If are positiveMotor develops
torque to meet load demand (i.e. Te =TL )
Dr. Ungku Anisa, July 2008 15EEEB283 – Electrical Machines & Drives
Operating ModesRegenerative BreakingMotor 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
Dr. Ungku Anisa, July 2008 16EEEB283 – Electrical Machines & Drives
Operating ModesDynamic BreakingSimilar to
regenerative breaking
But Va removed, replaced by Rb
Kinetic energy of motor is dissipated in Rb (i.e. machine works as generator)
Dr. Ungku Anisa, July 2008 17EEEB283 – Electrical Machines & Drives
Operating ModesPluggingSupply voltage Va is
reversedVa assists Ea in
forcing Ia in reverse direction
Rb connected in series to limit current
Dr. Ungku Anisa, July 2008 18EEEB283 – Electrical Machines & Drives
Operating Modes - Four Quadrant Operation
Dr. Ungku Anisa, July 2008 EEEB283 – Electrical Machines & Drives 19
Q1+Va , +Ea +
+Ia +TPower = +ve
Q4-Va , -Ea -
+Ia +TPower = -ve
Q2+Va , +Ea +
-Ia -TPower = -ve
Q3-Va , -Ea -
-Ia -TPower = +ve
ReferencesRashid, M.H, Power Electronics: Circuit, Devices and
Applictions, 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.
Dr. Ungku Anisa, July 2008 20EEEB283 – Electrical Machines & Drives