DC motors

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