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In order to precisely define the harmonic content of the main flux and stator voltage of an induction motor that is fed by a high-frequency carrier in addition to the normal set of stator voltages, this paper performs the analytical calculation of the zero-sequence components of the air-gap flux and stator voltage. The calculations are made by defining suitable winding functions to derive the self- and mutual inductances and by adopting a new inverse air-gap function to model the variation of the saturation level of the machine produced by the additional magnetomotive force. Analysis of the calculated expressions that are confirmed by experimental tests shows that the proposed analytical technique can be suitably applied to quantify the effects of high-frequency carriers on the air-gap flux and offers useful hints to understand sensorless control mechanisms. Index Terms--High-frequency modeling, induction machine, saturation, sensorless control.

Recent articles demonstrate that sensorless drives for induction motors can operate at zero-electrical and mechanical frequencies with the aid of high-frequency signal injection techniques and processing the zero-sequence voltage signals. The paper presents some practical implementation issues affecting the performance of such a technique and shows how to improve the quality of the estimated angle. All the assessments are experimentally proved and have a general value that can be extended to other sensorless techniques injecting high-frequency signal. Index Terms--AC machines, high-frequency injection, sensorless control.

In this paper, the rotor position of a synchronous reluctance motor is estimated exploiting the natural saliency of the machine through an addition of a small high-frequency sinusoidal voltage to the stator voltage reference. Due to rotor saliency, a stator voltage component is generated orthogonal to the added signal, whose amplitude depends on the phase displacement between the d-axis and the direction of the injected signal. A robust rotor position tracking can be accomplished by adjusting the direction of the carrier voltage in order to minimize the amplitude of the generated orthogonal voltage. The proposed technique allows zero-speed operation, does not require additional voltage or current sensors, and can easily correct the influence of the armature reaction. Experimental results confirm the validity of the proposed sensorless technique. Index Terms--Rotor position tracking, sensorless control, synchronous reluctance motors (SRMs).

The paper presents a method to compute the air-gap flux position in induction motors at very low including zero-stator frequency. A low-frequency (50 / 100 Hz) sinusoidal stationary signal is added to the normal stator variables to provide the machine with a suitable permanent excitation. Such an additional excitation modulates the saturation level of the magnetic core of the machine according to the angular position of the air-gap flux. As a result, a new zero-sequence stator-voltage component is generated that contains useful information about the position of the air-gap flux unaffected by load variation. Such a zero-sequence voltage can be easily employed to provide a wide bandwidth measurement of the air-gap flux position. A key feature of the proposed approach is that a low-frequency (0 / 5 Hz) signal is demodulated, thus avoiding any drawback featured by previous sensorless techniques operating with high-frequency signal injection. Index Terms--Carrier frequency signals, modulation of saturation, sensorless control, zero frequency flux, zero speed control.