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