Electric Drives
description
Transcript of Electric Drives
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GENERAL ELECTRICAL DRIVES
What is electrical drives?Components of electrical drivesAdvantages of electrical drives
DC drives Vs AC drivesTorque equationsLoad torque profilesFour quadrant operation
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DC DRIVES Vs AC DRIVES
DC drives:
Advantage in control unit
Disadvantage in motor
AC Drives:
Advantage in motor
Disadvantage in control unit
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General Torque EquationTranslational (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 TT Deceleration
0 Le TT Constant 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 onlythe load torque but also dynamic torque, ( Jd/dt ).
During deceleration, the dynamic torque, ( Jd/dt ), hasa negative sign. Therefore, it assists the motortorque, Te.
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Torque Equation: Graphical
Te
Forwardrunning
Speed
Forwardbraking
Reverseacc.
Reverserunning
Reversebraking
Forwardacc.
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4Q OPERATION
SPEED
TORQUE
I
III
II
IV
TeTe
Te Te
FMFB
RMRB
F: FORWARD R: REVERSE M : MOTORING B: BRAKING
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4Q OPERATION: LIFT SYSTEM
Counterweight Cage
Motor
Positive speed
Negative torque
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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 edtdi
LiRv
+
ea
_
LaRa
ia+
Vt
_
+
Vf
_
dtdi
LiRv ffff
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Torque-speed characteristics
aaaa EIRV In steady state,
T
ae
T
a
k
VT
k
R 2
Therefore speed is given by,
Three possible methods of speed control:
Armature resistance RaField flux Armature voltage Va
aa
aaa edt
diLiRV
Armature circuit:
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Torque-speed characteristics of DC motor
Torque
Speed
MaximumloadTorque
No load speed
Separately excited DC motors have goodspeed regulation.
Full load speed
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Motor speed control function
1. Resistance in armature circuit When a resistance is inserted in the armature circuit, the speed drop ω increases and the motor speed decreases.
2. Terminal voltage (armature voltage) Reducing the armature voltage Vt of the motor reduces the motor speed
3. Field flux (field voltage) Reducing the field voltage reduces the flux Φ and the motor speed increases
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Torque
Speed
MaximumTorque
Ra increasing
• Power loss in Ra• Does not maintain maximum torque capability• Poor speed regulation
Controlling Speed by adding resistor (armature circuit)
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Controlling Speed by adding resistor (armature circuit)
holding
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EXAMPLE
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DC Motor Speed Control
Torque
Speed
MaximumTorque
By Changing Armature voltage
Trated
Va increasing
• good speed regulation• maintain maximum torque capability
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DC Motor Speed Control
holding
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EXAMPLE
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DC Motor Speed Control
Torque
Speed
MaximumTorque
Flux Decreasing
Trated
• Slow transient response• Does not maintain maximum torque capability
By Decreasing Flux
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EXAMPLE
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Speed control of DC Motors
Below base speed: Armature voltage control (retain maximum torque capability)
Above base speed: Field weakening (i.e. flux reduced) (Trading-off torque capability for speed)
Torque
speed
Line of MaximumTorque Limitation
Armature voltage controlField flux control
base
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SOLID STATE CONTROL
Rectifier (AC to DC)
1. Half wave drive
2. Full wave drive
3. Effects of freewheeling
Chopper (DC to DC)
1. First quadrant
2. Second quadrant
3. Two quadrant
4. Four quadrant
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RECTIFICATION HALF-WAVE (SINGLE PHASE)
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RECTIFICATION HALF-WAVE (SINGLE PHASE)
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EXAMPLE 6.4
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RECTIFICATION FULL-WAVE (SINGLE PHASE)
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RECTIFICATION FULL-WAVE (SINGLE PHASE)
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Example 6.5
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RECTIFICATION FIRST QUARDRANT (THREE PHASE)
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Field Control Armature Control
RECTIFICATION TWO QUADRANT (THREE PHASE)
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RECTIFICATION TWO QUARDRANT (THREE PHASE)
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Converter 1 (ON) Converter 2 (ON)
RECTIFICATION TWO QUARDRANT (THREE PHASE)
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Phase-controlled rectifier (AC–DC)
T
Q1Q2
Q3 Q4
3-phase Or 1-phasesupply
+
Vt
ia
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Phase-controlled rectifier: 4Q Operation
Q1Q2
Q3 Q4
T
1 or 3-phasesupply
1 or 3-phasesupply
+
Vt
Methods of Armature Voltage Control
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Phase-controlled rectifier : 4Q Operation
Q1Q2
Q3 Q4
T
F1
F2
R1
R2+ Va -
3-phasesupply
AN ALTERNATIVE WAY
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SOLID STATE CONTROL
Rectifier (AC to DC)
1. Half wave drive
2. Full wave drive
3. Effects of freewheeling
Chopper (DC to DC)
1. First quadrant
2. Second quadrant
3. Two quadrant
4. Four quadrant
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DC-DC Chopper Circuit
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Switch–mode converters: 1Q Converter
Q1Q2
Q3 Q4
T
+Vt
-
T1
Converters For DC motor Drives
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Switch–mode converters: 2Q Converter
+Vt
-
T1D2
T2
D1
Q1Q2
Q3 Q4
T
Q1 T1 or D1
Q2 D2 or T2
Converters For DC motor Drives
Ia
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4 Quadrant DC-DC Control
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Switch–mode converters: 4Q Converter
Q1Q2
Q3 Q4
T+ Vt -
T1D1
T2D2
D3
D4
T3
T4
Converters For DC motor Drives
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Switch–mode converters
• Switching at high frequency
Reduces current ripple
Increases control bandwidth
• Suitable for high performance applications
Advantages of Switch mode converters