11/21/2012
Overview
• Introduction
• Shunt Motor
• Compound Motor
26-Shunt and Compound DC Motors
Text Chapter 6.6-6.9
ECEGR 450
Electromechanical Energy Conversion
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Dr. Louie
Shunt Motor
Shunt Motor
• Field winding and armature windings are
connected in parallel
• An external variable resistance may be present to
start or control the speed of the motor
Shunt Motor: field and armature windings are connected in
parallel




Rfw: field winding resistance (Ohm)
Rfx: adjustable field winding resistance (Ohm)
Ra: armature resistance (Ohm)
Xf: field winding reactance (Ohm) Rax
Ra
ia
+
-
Ea if
iL
Rfw
jXf
+
-
Vs
Rfx
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Dr. Louie
Shunt Motor Torque
Shunt Motor
• For a fixed voltage
source, the flux is
constant
• Therefore
iL
Rax
K a P
Td  Kia
Ra
ia
• Torque is proportional to
armature current
+
-
Ea if
torque (Nm)
with
armature
reaction
Td  KaPia
K
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Rfw
jXf
+
-
Vs
armature current (A)
Rfx
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Shunt Motor Model
Shunt Motor
• Under a given load, circuit analysis yields
• Flux produced by the field winding in a shunt
motor is constant
• As load on the motor increases:
Ea  vs  iaR a (assuming Rax is 0 after starting)
• Recall that
Ea  K aP m
m 
vs  iaR a
K a P
Ra
ia
+
-
Ea if
 Armature current increases
 Due to voltage drop across Ra, the back emf
decreases
 Ignoring armature reaction, the speed decreases
due to
iL
Rax
• Therefore:
Rfw
jXf
+
-
Vs
m 
vs  iaR a
KaP
Rfx
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Dr. Louie
Shunt Motor
vs  iaR a
K a P
mnL
vs
K a P
m  mnL 
Shunt Motor
• Armature reaction increases with armature
current
mnL
 Flux is decreased
 Speed is increased
speed (rpm)
m 
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iaR a
K a P
Since the armature resistance is low, the
speed of the motor is nearly constant
• Increase in speed may be less than, greater than
or equal to the drop in speed due to the
increased armature current
armature current (A)
How does this compare to a series motor?
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Shunt Motor
Shunt Motor
• Including armature reaction
Torque-speed relationship is found from:
Td  K aPia
Ignoring armature
reaction
Speed
ia 
vs mK aP

Ra
Ra
Td 
Armature Current
unique point of maximum
developed power
vs  iaR a
K a P
2 2
vsK aP mK ap

Ra
Ra
torque (Nm), power (W)
m 
mnL
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power
torque
speed (rpm)
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Series Motor
Recall for a series
motor:
• Observations:
torque
 Torque decreases linearly with speed
 Speed increases as the load decreases
 Power developed is maximum when:
torque (Nm), power (w)
High torque at
low speeds
Shunt Motor
Pd,max 
Nearly constant
power operation
region
speed (rpm)
v2s
4R a
Low torque at
high speeds
power
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Compound Motor
Compound Motor
• A series winding may be added to a shunt motor
to create a compound motor
• Hybrid of the characteristics of shunt and series
motors
• Two types:
 Cumulative
• Flux set up by series winding is additive with flux set
up by shunt winding P  sh  k fia
 Differential
• Flux set up by series winding is subtractive with flux
set up by shunt winding P  sh  k fia
is
is
Series winding
if
Shunt winding
S
Cumulative
(mmfs add)
Differential
(mmfs subtract)
Ra
ia
-
ea if
Long-shunt compound motor: shunt winding is
placed directly across the voltage source
Rs
jXs
jXf
Ra
ia
Rfw
+
+
-
jXs
Rax
Rfw
+
vs
+
-
if
Ea
jXf
-
vs
Rfx
Rfx
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Compound Motor
Short-shunt compound motor: series winding is
placed before shunt winding
Rs
Shunt winding
S
Compound Motor
is
Series winding
if
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Rax
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Compound Motor
Torque vs Armature Current
As load increases for a long-shunt cumulative
compound motor:
cumulative
 Total flux increases due to increase in series
winding current P  sh  k fia
 Increase flux with the increased armature current
increases the torque at a greater rate than in a
shunt motor
torque (Nm)
shunt
Td  K aiash  K ak fia2
 Increased flux with increased voltage drops in the
armature and field windings slows down the motor
more rapidly than in a shunt motor
m 
armature current (A)
vs  ia(R a  R s )
K a(sh  k fia )
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differential
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Speed vs Armature Current
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Compound Motor
• Observations:
 Cumulative compound motor
• Definite no-load speed (compare this with a series
motor)
• High starting torque if load is increased
• Used in applications that require high torque and
prone to sudden load changes (elevators, cranes,
etc)
speed (rpm)
differential
shunt
 Differential compound motor
cumulative
• Possibility for damaging high speeds to be obtained if
flux in series winding approaches that in shunt
winding
armature current (A)
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Summary
• Shunt DC motors have field winding placed in
shunt with voltage source
 Near-constant speed operation
• Compound DC motors are hybrids of shunt and
series motors, each with distinct characteristics
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