A sensorless vector control scheme for induction motors using a space phasor based current hysteresis controller

The increasing use of AC machines compared to DC motors in electrical drive applications has several reasons. A very important advantage of AC machines is their simple construction. However, AC drives often need mechanical sensors (tachometers, position encoders) for field orientation. In many applications these sensors reduce robustness and increase costs of a drive considerably. The objective of this paper is to present the practical implementation of a sensorless vector control scheme for induction motors using a space phasor based current hysteresis controller. This scheme measures the stator current hysteresis error direction to determine the rotor flux, position, during inverter zero vector states. This measurement of the rotor flux position is done indirectly by sensing the motor back emf, which is orthogonal to the rotor flux position. The basic idea is to measure the current hysteresis error direction of the stator current on the application of an inverter zero vector i.e. "short circuit at machine terminals". A generalization is possible by a combination of two (different) active states of the inverter. By a linear transformation of the electrical equations for both inverter states, a mathematical zero state is constructed. The use of this virtual short circuit is necessary at higher speeds when the inverter output voltage is fully utilized. However at low speeds the drop across the stator resistance is comparable to the motor hack emf, to overcome this a simple parameter adaptation scheme is also presented here. The proposed scheme enables a smooth transition to six-step mode of operation. The whole scheme is implemented using a space phasor based current hysteresis controller, in which the inverter voltage vectors close to the machine back emf vector is selected, without measuring the back emf vector. The back emf vector is sensed indirectly from the direction of the current error space phasor. A single chip solution is arrived at by using the TMS320F240 DSP from Texas Instruments.