Indirect vector control

Despite the strong incentives such as the absence of transducers and wide speed range operation, acceptance of the flux feedforward technique has been slow due to its dependence on the rotor parameters. With the advent of various parameter adaptation schemes to alleviate this problem, it has gained more attention and is
widely used for different industrial applications. A nonlinear model for the saturated induction machines is also derived to take into account the saturation effects on the parameters. The rotor flux is estimated from the stator currents and rotor speeds, or from line voltage and current measurements. The field angle is obtained from the induction motor model in synchronously rotating reference frames.

The components of the rotor flux are also estimated using the state observer technique. In another method, the flux is estimated using the state observer technique by having the rotor speed and the DC link voltage as measured quantities. This method for the current source inverter generates the commanded currents from which the rotor flux and the electromagnetic torque are estimated. A dual observer technique is employed which simplifies the speed controller design. Torque control is achieved by using a predictive observer and employing separate torque and flux feedback loops.

An analytic method was presented to detect the spatial position and magnitude of the rotor flux based on machine structure using voltage sensors. Another approach which calculates the speed from the structure of the motor and the measured stator phase voltages is presented. This method is independent. performance at low speeds, methods is the deteiodely for the flux feedder to calculate the stator flux from the terminal volt of electromagnetic parameter variation but is not valid for low speed operation. The effects of the machine structure on the torque pulsations were studied and the slot rotor structure is found to be undesirable for vector control due to problems at low frequencies. Vector control for special motor structures such as the twin stator induction motor has been proposed. Air-gap flux control for a double cage motor with a current controlled inverter drive was proposed. This provides deep bar compensation to avoid rotor resistance sensitivity and improve the transient performance. Current control for a doubly fed induction motor was proposed in. Vector control has been employed for spindle applications with wide constant power range using dual winding motor. A larger operation range before employing field weakening is achieved by winding changeover.