Chapter 2B SpeedControl DcMotor
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Transcript of Chapter 2B SpeedControl DcMotor
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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
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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
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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
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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
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Speed Control Methods – Va control
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eaa T
K
R
K
V2
Te
Va↓
TL
Requires variable DC supply
K
Va
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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
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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
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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
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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
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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.
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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
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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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