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MotorTorque Rotation
Mechanical Power
Electrical Power
DRIVE
What the Drive Does ?
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Electric
SupplyStarter Motor
Belt /
Gear
Conventional Technology
Electric
SupplyMotor Machine
Power
Converter
Drive
Requirement of Modern Machines
Control over Torque & Speed of the motor ?
Control over Torque & Speed of the motor ?
NO !
YES !
Machine
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Speed
Torque
T2
Speed
Torque
T1T1
Torque requirement
Speed Variation Load Variation
T2
NN1 N2
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Few expectations from the Drive
Variable speed at the motor shaft maintaining torque producing
capability of the motor
Speed regulation
Controlled acceleration & deceleration
Quick & safe reversal and braking
Current limiting
Protection against abnormal conditions
Auto-optimization
Easy fault diagnostics
Communication capable Energy Saving
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Speed
time
Reqd.
Speed
Controlled Acceleration & Deceleration
t1
Ramp-up
time
t2
Ramp down time
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Coasting
Braking or Quick Stop
Stop alongRamp-down time
Stopping & Braking of Motors
N
time
stop
OFF2
OFF1
OFF3
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Speed Control of Three Phase Induction Motor
120 f
PNs =Synchronous Speed =
Ns f
Nr < Ns , Hence % Slip =Ns
X 100
Normally 4 to 6 % Slip is common for all practicalapplications. And we say that induction motor is constant
speed motor.
Ns-Nr
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Why we require variable voltage ?
Torque is proportional to Flux and Armature
current .
T x I Vf
Thus if we reduce only frequency ,then flux will also
reduce.Therefore we need to reduce voltage proportional
to reduction in frequency.
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General block diagram of Inverter .
Rectifier Inverter
+
-
3 Constant VoltageConstant Frequency
3 Variable Voltage
Variable Frequency
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Rectifier Section
Uncontrolled Bridge Rectifier
R Y B
+
-
Output DC VoltageInput AC Voltage
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Converting DC in to AC
S2
S3
S1
S4
+ / -
- / +
LOAD
Periodic reversal of polarity occurs by switchingS1S4 and S2S3 switches
DC Input
AC Output
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Transistorized Inverter Block Diagram
Rectifier
DC Link Inverter
Supply
C
+
-
C
Motor
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Pulse Width Modulated Waveform
0V
Time
Pulse Width Modulation
Voltage
Current
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Pulse Width Modulated Waveform
Voltage going to the inverter is chopped rectangular
shape
Pulse Width is varying .
PWM voltage is of high frequency chopped pulses.
Due to motor inductance the current flowing through
winding will be of sinusoidal nature
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Basic Induction Motor Characteristics with V/f control
Speed
Normal Operating
Point
Torque Speed Characteristic of An Induction Motor
Slip
Pull out Torque
Torque
Variable frequency operation
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Block diagram of V/f control
RFG Modulator
M
Nref
Current limiting
ControllerSlip Compensation
fref
InverterBridge
f
V
- +
V/f characteristics block
f
v
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V/f Control is called as open loop control.
Vector control operation is performed because it
has following advantages.
Speed Accuracy
Torque Accuracy
Speed range
Dynamic Performance
Performance at low speed
Shock load operation
Vector Control
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Vector Control
How vector control works ?
In AC Motor the stator winding current
sets the Flux and torque, therefore it is difficult
to control Flux and Torque separately.In drive above independent control of Flux and Torque
producing components is achieved with the help of
high speed microprocessor and the same is called as
Vector Control.
Iload
Iflux
Itotal
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Vector Control
Block diagram of Vector Control operation
RFG N I I
I
Gating
Unit Inv
Motor Model
M
w
I
RefI
I w Ref
Coordinate converter
Iw
I
Slip
Speed Controller
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