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ECE 442 Power Electronics 1
Cycloconverter
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ECE 442 Power Electronics 2
Transformer Polarity as shown, S1 and S2 ON
+
-
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ECE 442 Power Electronics 3
Transformer Polarity as shown, S3 and S4 ON
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+
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ECE 442 Power Electronics 4
Transformer Polarity as shown, S3 and S4 ON
+
-
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ECE 442 Power Electronics 5
Transformer Polarity as shown, S1 and S2 ON
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+
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ECE 442 Power Electronics 6
S1S2 S3S4 S3S4 S1S2 vL
vxformer
vL has a lower frequency than vxformer
S1S2 S3S4 S3S4 S1S2 S1S2 S3S4
Look At The Voltages
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ECE 442 Power Electronics 7
Replace The Load With A Battery
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ECE 442 Power Electronics 8
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Turn S1 and S2 ON
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ECE 442 Power Electronics 9
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Turn S3and S4 ON
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LECTURE 29
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• DC DRIVES: Electric drives that use DC motors as the prime
movers
• Dominates variable speed applications before PE converters
were introduced
• DC motor: industry workhorse for decades
• Will AC drive replaces DC drive ?
– Predicted 30 years ago
– AC will eventually replace DC – at a slow rate
– DC strong presence – easy control – huge numbers
DC DRIVES
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DC Motors
• Limitations:
• Advantage: simple torque and speed control without
sophisticated electronics
• Regular Maintenance • Expensive motor
• Heavy motor • Sparking
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General Torque Equation
Translational (linear) motion:
dt
dJT
Rotational motion:
dt
dvMF
F : Force (Nm) M : Mass (Kg ) v : velocity (m/s)
T : Torque (Nm) J : Moment of Inertia (Kgm2 ) : angular velocity
( rad/s )
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Torque Equation: Motor drives
dt
dJTTor
dt
dJTT LeLe
0 Le TT Acceleration
0 Le TTDeceleration
0 Le TTConstant speed
Te : motor torque (Nm) TL : Load torque (Nm)
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…continue
Drive accelerates or decelerates depending on whether Te is
greater or less than TL
During acceleration, motor must supply not only the load torque
but also dynamic torque, ( Jd/dt ).
During deceleration, the dynamic torque, ( Jd/dt ), has a
negative sign. Therefore, it assists the motor torque, Te.
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Torque Equation: Graphical
Te
Forward running
Speed
Forward braking
Reverse acc.
Reverse running
Reverse braking
Forward acc.
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Load Torque
Load torque, TL, is complex, depending on applications.
SPEED
TORQUE
TL = k2
TL = k
TL = k
In general:
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DC MOTOR DRIVES
Principle of operation
Torque-speed characteristic
Methods of speed control
Armature voltage control
Variable voltage source
Phase-controlled Rectifier
Switch-mode converter (Chopper)
1Q-Converter
2Q-Converter
4Q-Converter
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Current in
Current out
Stator: field windings
Rotor: armature windings
Principle of Operation
DC Motors
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Equivalent circuit of DC motor
at ikTe Electromagnetic torque
Ea ke Armature back e.m.f.
Lf Rf
if
aa
aat edt
diLiRv
+ ea _
La Ra
ia + Vt _
+ Vf _
dt
di L i R v f f f f
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NPTEL LINK
•
https://www.youtube.com/watch?v=1Auay7ja2oY&list=PL3A9EE4428285E166
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