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CLOSED LOOP CONTROL OF DC DRIVES
CLOSED LOOP CONTROL OF DC DRIVES
Controller Requirements
Fast tracking capability
Less steady state error
Robust to load disturbance during steady state and
transient conditions.
• Methods of speed control in DC drives
DC Motor Speed Control
Torque
Speed
MaximumTorque
By Changing Ra
Ra increasing
• Power loss in Ra• Does not maintain maximum torque capability• Poor speed regulation
DC Motor Speed Control
Torque
Speed
MaximumTorque
Flux Decreasing
Trated
• Slow transient response• Does not maintain maximum torque capability
By Decreasing Flux
DC Motor Speed Control
Torque
Speed
MaximumTorque
By Changing Armature voltage
Trated
Va increasing
• good speed regulation• maintain maximum torque capability
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
Armature and field control
In practice, for speeds less than the base speed (rated), the armature current and field currents are maintained at fixed values (hence constant torque operation), and the armature voltage controls the speed.
For speeds higher than the base speed, the armature voltage is maintained at rated value, and the field current is varied to control the speed. However, this way the power developed Pd is maintained constant. This mode is referred to as “field weakening” operation.
Block diagram of closed loop control
Controller Design Procedure
Controller Design Procedure- Contd.
Controller Design Procedure- Contd.
Transfer Function of Motor
• Transfer Function of Converters
CURRENT CONTROL FOR DC DRIVES
To derive transfer function of converter, control techniques should be explained
Current Control for Chopper
PWM Controller
Hysteresis Controller
Comparison
Transfer function of chopper drive
Three-phase Fully controlled Rectifier- transfer function
Introduction to PID Controllers for deriving the transfer function of current&
speed controllers
About Kp value
About Ki value
About Kd value
Overall block diagram
Simulation
