Direct Torque Control and Sensor-Less Control of Induction Machine

Introduction

The application of vector control method for variable speed induction motor drives has been described in chapter 3. Generally, a closed loop vector control scheme results in a complex control structure as it consists of the following components;

• 1 PID controller for motor flux and toque

• 1 Current and/or voltage decoupling network

• 1 Complex coordinate transformation

• 1 Two axis to three axis transformation

• 1 voltage or current modulator

• 1 Flux and torque estimator

• 1 PID speed controller

In a direct torque control system introduced by Takahashi and Depenbrock independently in 1986, the first five components are replaced by two hysteresis comparators and a selection table^1 − 3. This method therefore, results in highly simplified control structure compared to vector control. In the vector control scheme, it is assumed that the controllable power source can force any desired wave shape and value of current into the stator winding. But in practical circuits an inverter can produce only seven discrete space vector values of the actuating variable. In most cases none of these values is exactly equal to the desired instantaneous value of the space vector. Although by using high switching frequency in a PWM inverter the desired curves of the actuating variable can be sufficiently approximated. However, for high power drives, the switching frequency can not be more than 200-300 Hz due to economic reasons. Thus, in high power drives, it is very difficult to apply a current wave shape of desired magnitude and shape. The vector control therefore, can not provide very fast control required in many drives.

In direct torque control instantaneous values of torque and flux are calculated from primary variables and controlled independently by using an optimum switching table. The controllers for a direct torque control drive do not require complex coordinate transformation essential in all vector controlled drives. Instead the decoupling of non-linear ac motor structure is obtained by the use of ‘on –off’ control of inverter switches. The voltage vector is selected from the inverter feeding the motor with the help of hysteresis controllers.