Direct vector control

The direct measurement technique was popular in the earlier stages due to the sensitivity of the estimation techniques to motor parameter variations.This technique also provided instantaneous and well damped control of torque. The earlier forms of this type of control involved the measurement of the air-gap flux. 

Field sensing devices utilizing Hall effect were installed at two or more places within the air-gap of the motor to measure the air-gap flux. The mechanical and thermal stresses on these semiconductor devices caused problems in measurement. The rotor slot harmonics are filtered and only the fundamental components are extracted. A detailed description of the placement of sensing coils for air-gap flux measurement has been presented in. Amorphous micro core sensors were also used to detect the magnetic field and to sense the torque and rotor currents.Various schemes have been proposed for sensing of air-gap flux. A method for flux control independent of stator resistance variation based on the magnetizing current is presented in. This measurement technique performs satisfactorily up to 1 Hz but the performance degrades at lower speeds. Another method for measurement of the air-gap flux used the tapped stator windings of the motor. An air-gap flux sensing method was proposed using search coils and providing an open loop flux estimator using the predictor corrector algorithm. The major disadvantage of air-gap flux oriented control is that the system becomes unstable above a certain critical load. The problem of instability is overcome by using the rotor flux instead of the air-gap flux. Since the rotor flux is not directly accessible, the measurement techniques are even more complex A torque drive vector control using the rotor flux orientation presented problems with the integrator adjustment for obtaining the field angle using LEM transducers. Another method which measures the stator and rotor flux using Halls sensors to obtain the field angle and rotor flux, achieved four quadrant operations for fast torque response .

The measurement of the rotor flux is dependent on the leakage inductance. A method using the measurement of the stator flux has been proposed to overcome this effect.The maximum torque capability of this orientation was found to be equal to that of a properly tuned flux feedforward vector control system. The limitation of the stator flux is that for low speed operation, it is difficult to obtain a good performance. 

This is due to the fact that the stator resistance has to be known or estimated in order to calculate the stator flux from the terminal voltages. 

The rotor flux measurement technique is the most widely used for the flux feedback vector control strategy. It can be noted that the major disadvantage of the various flux measurements using different methods is the deterioration of performance at low speeds, i.e. 3-5% of the rated speed.

Also, the delay time in signal processing affects the dynamic behavior of the drive system because it results in stator current coupling. This effect can be reduced by employing simplified decoupling circuits for the machine and the inverter.