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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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 Kia 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 KI 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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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 KI 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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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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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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