This document discusses the characteristics and operation of DC motors. It describes the construction and components of DC motors, including that the armature is usually located on the rotor and the field is on the stator. It also discusses separately excited DC motors and series DC motors, how their speed and torque can be controlled, and their applications.

1. DC Motor Characteristics
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Institute of Technology Bandung

2. DC Motor Construction
va
vf
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3. DC Motor Features
• Depending on the connection between armature and
field windings, a dc motor can be classified as separately
excited and series excited dc motors.
• In small dc motors, the field can be permanent magnet.
• The armature is usually located on the rotor and the field
is on the stator.
• Due to the action of commutator and brush, the mmf
generated by the armature current is perpendicular to
the flux generated by field winding.
• At present, most of dc motor applications are replaced
by ac motors.
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4. Separately Excited DC Motor
ia Ra La
Tl
Lf
va Ea J
Rf Te
vf if
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5. General Expressions of
Separately Excited DC Motor
dia
va  Ra ia  La  ea
dt
ea  K
di f
vf  Rf if  Lf
dt
   i f 
d
Te  Kia  Tl  B  J
dt
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6. Separately Excited DC Motor
Tl (s )
_
 1 I a (s )  1  (s )
Va (s )
Ra  sLa Te (s ) sJ
_
E a (s )
The system is
nonlinear
I f (s )
V f (s )  (s )
1  i f  K
R f  sL f
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7. Constant-Field Block Diagram of DC Motors
Tl (s )
_
 1 I a (s )  1  (s )
Va (s ) K
Ra  sLa Te (s ) sJ
_
E a (s )
K
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8. Motor Characteristics
• The system is linear when the field is
constant.
• Under this condition, the torque is
proportional to armature current.
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9. Transfer Function
K sLa  Ra
 (s)  V (s)  Tl ( s )
s JLa  sJRa  K  s JLa  sJRa  K 
2 2 a 2 2
 Ra 

K / 
JLa / JLa
Va ( s ) 
s 

 La 
/ J
 Tl ( s )
2 2
 K    K 
s  s Ra / La   
2
s  s Ra / La   
2 
 JLa   JL 
   a 
 2
o / JLa s    / J T ( s)
Va ( s )  2
s  s  o
2
s  s  o 2 l
  Ra / La
K
o 
JLa
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10. Steady-State Expressions
Va  Ra I a  K
Va K
Ia  
Ra Ra
Te  Tl  KI a
Va Ra Va Ra
  Ia   Te
K K  K K 2
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11. Speed Control
• The speed is proportional to armature voltage.
• The speed can also be controlled by armature
resistance. The armature resistance can also be
used to limit the starting current.
• The starting current is independent to the flux or
field current.
• The field can be reduced to increase the speed
at the expense of reduced torque/armature-
current.
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12. Armature Resistance Control


Va Ra

V
Ia  a 
Ra
T
Ra
K K K K 2 e
Te
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13. Armature Voltage Control

Va
Te
Va Ra V Ra
  Ia  a  T
K K K K 2 e
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14. Field Control


Va Ra V Ra
  Ia  a  T
K K K K 2 e
Te
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15. Control of Separately Excited DC Motor
Constant Torque Constant Power
Va If
Te  KI a
P  Te

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16. Control Characteristics
• Below base speed, the speed is controlled by
the armature voltage at maximum field current
(to maintain maximum torque/armature-current
or constant torque capability).
• Above base-speed, the speed can only be
increased by reducing the field current (constant
power capability). The maximum speed is limited
by the mechanical capability and armature
reaction.
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17. Four-Quadrant Operation

Te
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18. Braking of DC Motors
• Dynamic braking : The kinetic energy is
dissipated to the braking resistor.
• Counter current braking or plugging: The kinetic
energy is dissipated in the machine and current-
limiting resistor. The current can be so high with
braking method.
• Regenerative braking. The kinetic energy is sent
back to the source. This method can only be
used when the source is able to receive the
regenerated energy.
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19. Control of Separately Excited DC Motor
AC source
M G M
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20. Ward-Leonard Characteristics
• Fully four-quadrant operation is possible.
• Below base speed, the motor field current is
constant at maximum value and the speed is
controlled by the generator field current.
• Above base speed, the motor speed is
increased by reducing the motor field current.
• If the ac motor is a synchronous motor, the ac
power factor is controllable.
• No harmonics are generated.
• The system is large, expensive, less responsive,
and needs a lot of maintenance.
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21. Series DC Motors
• This motor is commonly used in dc electric
traction.
• This motor can be used as ac comutator
motor.
• This motor can be operated as a universal
motor.
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22. Steady-State Characteristics
• Speed can be controlled by
Ia  I f controlling the armature
voltage.
Assume   K f I a • Speed can also be controlled
Te  Tl  KK f I a
2 by using the armature
resistance and field diverter.
Va  Ra I a Va Ra • In electric traction, series
  
K KK f I a KK f parallel connection is also
used to control the speed.
Va Ra
  • A low frequency ac supply is
KK f Te KK f desirable if the motor is
operated as a series ac
commutator motor.
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23. Braking Methods
• Mechanical Braking
• Counter plug Te  Kia
• Dynamic braking
• Regenerative Braking
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24. Counter Plug
va  E
ia 
Ra
Te   Kia
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25. Dynamic Braking
E
ia 
Rb  Ra
Te   Kia
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26. Regenerative Braking
E  va
ia 
Ra
Te   Kia
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27. The End