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Controlled-Rectifier Fed Drive
Armature
Field
ECE 442 Power Electronics
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Chopper-Fed Drive
Armature
Field
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Equivalent Circuit of a Separately-Excited
DC Motor
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v f  Rf i f  Lf
di f
dt
di
va  Ra ia  La a  eg
dt
eg  K vi f
Td  K t i f ia
d
Td  J
 B  TL
dt
ECE 442 Power Electronics
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Steady  state
d
0
dt
Vf  Rf I f
E g  K v I f
Va  Ra I a  Eg
Va  Ra I a  K v I f
Td  K t I f I a
Td  B  TL
Pd  Td 
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Solve for the motor speed
Va  Ra I a  K v I f
Va  I a Ra Va  I a Ra


Kv I f
 Vf 
Kv 

R 
 f 
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Control of the motor speed
• Control armature
voltage, Va
– Voltage control
• Control the field
current, If
– Current control
• Control the armature
current, Ia
Va  I a Ra

 Vf 
Kv 

R 
 f 
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Magnetization Characteristic
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Characteristics of Separately-Excited Motors
Use armature
voltage control
Use field-current control
Rated
speed
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Equivalent circuit of a DC Series Motor
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Eg  K v I a
Va  ( Ra  R f ) I a  Eg
Va  ( Ra  R f ) I a  K v I f
Td  Kt I a I f
Td  B  TL

Va  ( Ra  R f ) I a
Kv I f
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a
a
f
a
g
V

(
R

R
)
I

K

I
a
a
f
a
v
Controlling the Motor Speed
• Control the armature
voltage, Va
Td  Kt I a I f
Td  B  TL

Va  ( Ra  R f ) I a
Kv I f
• Control the armature
current, Ia
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Characteristics of DC Series motors
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Operating Modes
• In variable-speed applications, a dc motor
may be operating in one or more of the
following “Modes”
•
•
•
•
Motoring
Regenerative braking
Dynamic braking
Plugging
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Motoring Mode
“Back emf”, Eg is < supply voltage Va
Both Ia and If are positive
Developed torque meets load demand
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Regenerative Braking Mode
Motor acts as a Generator
Eg becomes > supply Voltage Va
Ia becomes negative
Kinetic energy of the motor is returned to the supply
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Dynamic Braking
Replace the supply voltage with a resistor
Power dissipated in the resistor rather than given back
to the source
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Plugging
Reverse the armature terminals while running
Va and Eg act in the same direction
Ia is reversed, producing “braking” torque
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Four-Quadrant Operation
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Single-Phase DC Drive
Change Va by changing firing angle 
Inductor Lm is a “smoothing” Inductor to prevent
discontinuous current
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Armature Reversal
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Field Reversal
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Single-Phase Half-Wave Converter Drive
Vm
(1  cos  a )
2
0  a  
Va 
Vf 
Vm

(1  cos  f )
0 f 
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Quadrant of Operation
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Waveform Summary
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Single-Phase Semiconverter Drives
Va 
Vm

(1  cos  a )
0  a  
Vf 
Vm

(1  cos  f )
0 f 
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Quadrant(s) of Operation
Semiconverter – one quadrant converter
One polarity of output voltage and current
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Waveform Summary
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Single-Phase Full-Converter Drives
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Quadrant(s) of Operation
Full converter – two quadrant converter
Output voltage polarity can be positive or negative
Output current has one polarity
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Waveform Summary
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Single-Phase Dual-Converter Drives
Dual converter can operate in all four quadrants
Output voltage and current can be positive or negative
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