lesson 17 332a · 2015-06-17 · lesson 17 332a.pptx 10 example 17-1: a 440 v series motor draws a...
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ET 332a
Dc Motors, Generators and Energy Conversion Devices
1Lesson 17 332a.pptx
Learning Objectives
� Identify common connections of dc motors� Explain how to connected various dc motors to reverse
their rotation.� Identify the circuit model of a series dc motor and use
model equations to solve problems
2Lesson 17 332a.pptx
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Other Dc Motor Connections
Lesson 17 332a.pptx 3
Series dc motor- field excitation comes from armature current.
VT
RfRa
Eaload
+
Series field
Field current equalsarmature current
Armature
Field winding constructed of few turns of large diameter wire
Lesson 17 332a.pptx 4
Other Dc Motor Connections
Compound motor - series and shunt windings in the same machine combine to make the field flux.
load
VTRfRa
Ea
+Rs
Series field
Armature
Shuntfield
Type of compound motors
Commulatively compounded motors - series and shuntmmfs add to the flux produced in the field
Differentially compounded motors - series and shuntmmfs subtract - unstable connection.
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Reversing Dc Motors
Lesson 17 332a.pptx 5
Reverse the polarity of either the armature or the field connections. Changes the direction of armature/field flux which reverses the direction of the developed torque.
Reversing direction of Series or Compound motor
General theory - shunt and separately excited motors
Same as other types- take care to avoid creating differentially compounded motor. series and shunt fields must change polarity together.
In series motor must only change the polarity of the field or armature. Reversing terminal voltage will reverse both field and armature polarity
Reversing Connections for Series and
Compound Dc Motors
Lesson 17 332a.pptx 6
Series Motors
Reverse connection
VT
Series field++
Forward connection
+
VT
+Ia flowsinto negativeterminal
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Lesson 17 332a.pptx 7
Reversing Connections for Series and Compound Dc Motors
Compound Motors
Reverse connection
Series field
Shuntfield
+
+
Forward connection
Shuntfield
+
+
Series Dc Motor Model and Equations
Lesson 17 332a.pptx 8
Racir
Rs
VT
Ia
Circuit Model Linear operation equations –assumes nomagnetic saturation.
General Equations
afTD
pGa
IIkT
nkE
⋅⋅=
Φ⋅⋅=
aciraTa
pG
aciraT
RIVE
k
RIVn
⋅−=
Φ⋅
⋅−=
In series motor, Ia = If so above equations become
2
aTD
pGa
IkT
nkE
⋅=
Φ⋅⋅=
)RR(IVE
k
)RR(IVn
saciraTa
pG
saciraT
+⋅−=
Φ⋅
+⋅−=Torque
proportional to Ia squared
Include series field R
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Lesson 17 332a.pptx 9
Series Dc Motor Model and Equations
Torque and speed characteristics of series motor
2
aTD IkT ⋅=
pG
saciraT
k
)RR(IVn
Φ⋅
+⋅−=
For linear operation, Φp proportional to Ia
As load torque decreases developed torque decreases causing Ia to decrease This decreases field flux. Motor speed, n, increases with decreasing field flux.
TD ΦpIa
n
Series motors must operate with direct coupled mechanical loads to prevent centrifugal damage due to high speeds.
Series Dc Motor Example-Speed and Torque
Calculations
Lesson 17 332a.pptx 10
Example 17-1: A 440 V series motor draws a current of 100 A at a speed of 1000 rpm The total armature circuit resistance is 0.11 ohms and the field resistance is 0.09 ohms. Assuming a linear magnetizing, curve calculate the speed and the torque when the current drops to 50 A.
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Example 17-1 Solution (1)
Lesson 17 332a.pptx 11
Information from problem statement
Racir = 0.11 ohms
Rs = 0.09 ohms
VT= 440 V
Ia = 100 A
n1 =1000 rpm
Lesson 17 332a.pptx 12
Example 17-1 Solution (2)
Find the motor speed at reduced armature current
Answer
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Lesson 17 332a.pptx 13
Example 17-1 Solution (3)
Torque is proportional to Ia2
Find TD1, initial developed torque
Answer
As load torque decreases developed torque decreases causing speed increase
ET 332a
Dc Motors, Generators and Energy Conversion Devices
Lesson 17 332a.pptx 14