K.D. Hurst

Sensorless speed measurement using current harmonic spectral estimation in induction machine drives

This paper proposes a sensorless speed measurement scheme which improves the performance of transducerless induction machine drives, especially for low frequency operation. Speed-related harmonics which arise from rotor slotting and eccentricity are analyzed using digital signal processing. These current harmonics exist at any nonzero speed and are independent of time-varying parameters such as stator winding resistance. A spectral estimation technique combines multiple current harmonics to determine the rotor speed with more accuracy and less sensitivity to noise than analog filtering methods or the fast Fourier transform. An on-line initialization routine determines machine-specific parameters required for slot harmonic calculations. This speed detector, which has been verified at frequencies as low as 1 Hz, can provide robust, parameter-independent information for parameter tuning or as an input to a sensorless flux observer for a field-oriented drive. The performance of the algorithm is demonstrated over a wide range of inverter frequencies and load conditions. >

Zero-speed tacholess IM torque control: simply a matter of stator voltage integration

Most industry experts agree that the next generation of commercial drives will include some sort of sensorless torque control. Achieving a modest level of control in the very-low-speed range greatly increases the competitive value of the drive and expands its range of applications. After reviewing many of the previously presented sensorless control methods, this paper shows that the simple method of stator-flux orientation can provide zero-speed torque control equally as well as more complex approaches, which rely on elaborate mathematical models to improve the operating range. This paper experimentally demonstrates that, by using only a slight amount of low-pass filtering in the integration of the stator voltage, a reasonable flux estimate can be obtained for stator-flux-field-oriented torque control. With this simple method, adequate torque control has been demonstrated over a wide range of speed and load conditions, and even at zero rotor speed with moderate or heavy load torque.

A thermal monitoring and parameter tuning scheme for induction machines

This paper presents a method for tracking the stator and rotor temperatures in three-phase induction machines. The proposed algorithm can be used to predict insulation failure in the machine and to tune the parameters of a thermal model for overload protection. Microprocessor-based thermal protection has become widely available as a viable replacement for the traditional, mechanical trip element. If the thermal condition of the machine changes, however, existing microprocessor-based devices have no means of recognizing or correcting the built-in model. The proposed scheme enables the protection unit to adapt to changes in the thermal conductance between the rotor core and the ambient atmosphere, which could be caused by a broken fan, clogged air vents, or other factors that affect machine cooling. The algorithm compares one rotor speed estimate derived from terminal measurements to a more accurate estimate obtained by digital analysis of the rotor slot ripple. The error between the two estimates provides a corrective input to the thermal model. By continuing this process over a period of time after startup, the thermal time constant of the rotor can be derived. Additionally, by combining this approach with trend analysis, a supervisory algorithm can detect incipient failures in the electrical integrity of the machine.

A self-tuning, closed-loop flux observer for sensorless torque control of standard induction machines

This paper proposes a self-tuning, closed-loop flux observer which provides field-oriented torque control for induction machines without a tachometer. The observer algorithm combines the dual methods of calculating flux from the slip relation and from back-EMF estimation in order to provide a closed-loop topology. At low speeds, the observer also depends on a model of the mechanical system. The observer accuracy and robustness is augmented by a parameter-independent, accurate speed detector which analyzes magnetic saliency harmonics in the stator current. The harmonic detection scheme provides accurate rotor speed updates during steady-state operation down to 1 Hz source frequency. This additional speed information is used to tune the mechanical system parameters and the rotor resistance. The tuned observer exhibits improved dynamic performance, accurate steady-state speed control and an extended range of control near zero speed. The algorithm requires no special machine modifications and can be implemented on most existing low and medium performance drives. The closed-loop nature of the flux observer, combined with the harmonic detection scheme, provides flux and speed error feedback which significantly increases the robustness or sensorless control across the entire speed range. >

Zero-speed tacho-less IM torque control: simply a matter of stator voltage integration

Most industry experts agree that the next generation of commercial drives will include some sort of sensorless torque control. Achieving a modest level of control in the very low-speed range in fact greatly increases the competitive value of the drive and expands its range of applications. After reviewing many of the previously presented sensorless control methods, this paper shows that the simple method of stator flux orientation can provide zero speed torque control equally as well as more complex approaches which rely on elaborate mathematical models to improve the operating range. This paper experimentally demonstrates that by using only a slight amount of low-pass filtering in the integration of the stator voltage, a reasonable flux estimate can be obtained for stator flux field oriented torque control. With this simple method adequate torque control can be achieved even at zero rotor speed.

Speed sensorless field-oriented control of induction machines using current harmonic spectral estimation

This paper proposes a direct field-oriented controller for induction machine drives based on a new sensorless speed measurement scheme. Current harmonics which arise from rotor slotting and eccentricity are analyzed using digital signal processing techniques. An on-line initialization algorithm determines slot harmonic parameters which depend on machine structural characteristics. A digital spectral estimation algorithm then analyzes multiple current harmonics to produce accurate results at any nonzero speed, independent of time-varying motor parameters. This robust speed information is used to tune the parameters of a mechanical model of the motor and the load. A speed observer based on the mechanical model provides a speed signal to the field-oriented control algorithm. A controller utilizing this tachometer-less observer has the advantage over other sensorless schemes of being able to obtain high accuracy speed information down to very low speeds. This is due to the fact that this scheme does not rely on flux (voltage) measurement, which is significantly degraded at low speed. The algorithm applies to a broad class of induction motors and requires no special machine modifications.<<ETX>>

A self-tuning thermal protection scheme for induction machines

This paper proposes a new method for improving the performance of modern thermal protection devices for induction machines. Microcomputer-based thermal models have begun replacing the traditional electromechanical elements for thermal overload protection in induction motor drives, because they more reliably estimate temperatures in the machine and provide the user with greater flexibility in responding to overload conditions. In a totally enclosed fan-cooled induction motor, however, cooling conditions can change because of a broken fan or clogged air vents, leading to inaccurate temperature estimation by the software model. This paper proposes a scheme which updates the parameters of the thermal model in the event that they change. The tuning scheme is driven by the difference between two speed estimates; one is based on the slip relation and the other on parameter-independent saliency harmonics in the current spectrum. As the rotor resistance changes, the slip relation speed estimate produces an error which drives the tuning mechanism. Since the rotor resistance is proportional to the rotor temperature, this provides a temperature curve from which the thermal time constant of the rotor can be derived. An assumed thermal model of the machine is then updated to account for any change in the thermal time constant. Unusual temperature increases under normal operation can also be detected.