Chapter 2
Chapter 2
Chapter 2
MEK 10103 ELECTRIC DRIVES AND APPLICATIONS By: Engr. Dr. Kok Boon Ching (JEK 2011)
OUTLINES
Introduction to DC drives Types of DC motors and their performance T f t d th i f Starting of DC motors Braking of DC motors Speed control of DC motors
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Stator
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Armature mmf produces flux which distorts main flux produced by field Mechanical commutator is use to maintain armature current direction
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VF = I F RF
In steady state,
VT = I A RA + E A
Three possible methods of speed control: - Armature voltage VT - Armature resistance RA - Field flux
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VT RA TE K E ( K E ) 2
VT
RATE (k E )
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VT = m ( K E ) + I A RA
VT
VT ,rated
Constant TL
I A RA
base
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VT RA TE K E ( K E ) 2
VT K E
TL
RA
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m =
TL
VT RA TE K E ( K E ) 2
VT K E
base
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Speed-torque characteristic
m =
=
VT RA KE KF I A KE KF VT KE KF RA 1 TE K E K F fl
Tfl Tstart TE
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TE
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STARTING OF DC MOTORS
Main concern during starting is the starting current limit limit. At standstill, back emf is zero and the only resistance opposing flow of current is the armature circuit resistance, which is quite small. 2 methods for limiting the starting current:
armature voltage control variable resistance controller in full supply condition.
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STARTING OF DC MOTORS
Example: A 50 hp, 250V motor has RA of 0.06 , and full load current less than 200 A, the starting current is IA = (VT EA)/ RA = (250V 0V)/ 0.06 ( ) = 4167A.
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BRAKING OF DC MOTORS
An electric drive operates in three modes:
Steady-state Acceleration including starting Deceleration including stopping/ braking
Deceleration occurs when TL > TE. Mechanical brake or electric braking are used.
Electric Braking El t i B ki m Motoring M t i A
B m1 m2 C
TE
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BRAKING OF DC MOTORS
During electric braking, motor current tends to exceed the safe limit limit. For short period braking, maximum current is allowed to exceed the rated current. The higher the braking torque, the greater the deceleration. Three types of electric b ki Th t f l t i braking:
Regenerative braking Dynamic braking Plugging or reverse voltage braking
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field flux cannot be increased further due to saturation effect. increase speed higher than rated rated. decrease V using variable voltage source.
Regenerative braking is not possible in series DC motor as speed increases, flux decreases.
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Series
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Motoring TE
Series
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Motoring
RB1
TE
0 Series
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Armature resistance control (RA , m ) Field flux (or field voltage) control ( , m ) Armature voltage control (VT , m )
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m =
R + Radd VT A TE K E ( K E )2
TL
VT K E
Radd
TE
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m =
VT RA TE K E ( K E ) 2
TL
VT K E
TE
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But it only can provide the speed control below base ( t d) speed as th armature voltage b (rated) d the t lt cannot be allowed to exceed rated value.
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WARD-LEONARD SCHEMES
AC Motor
drive
DC Gen
DC Motor
Advantages: -Regenerative braking capability -Efficient 4-quadrant operation -Power factor improvement -AC supply is dynamically decoupled from load -Uninterruptible supply
Drawbacks: -High initial cost -Low efficiency -Frequent maintenance -More noise -Need large floor area
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AC
DC Motor
Uncontrolled rectifier
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CONTROLLED RECTIFIERS
Single-phase fully-controlled rectifier Single-phase half-controlled Single phase half controlled rectifier Three-phase fully-controlled rectifier Three-phase half-controlled rectifier
Va or m
Ia or Tm
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