A sensorless finite-state predictive torque control (FS-PTC) strategy uses stator current, estimated stator and rotor flux, and estimated rotor speed to predict stator flux and torque. Direct application of measured stator currents and using a noisy estimated speed in the prediction model degrade the steady-state performance in terms of higher current total harmonic distortion (THD), torque ripple, and flux ripple, particularly at low speeds. This paper proposes an extended Kalman filter (EKF)-based, which is a promising state observer, improved prediction model of sensorless FS-PTC for induction motor drives. The EKF has been used to estimate rotor speed, rotor/stator flux, and stator currents accurately. The estimated stator currents, instead of measured currents, are fed back to the prediction model, and thus, small stator current THD is confirmed. Depending on the commanded speed, either the rotor current model or the open-loop stator voltage model is proposed for the EKF to achieve better performance in a wide speed range, including the field-weakening region. The proposed control system has been verified experimentally, and excellent torque and flux responses, robustness, and stable operation at lower and higher speeds have been achieved.

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A Speed-Sensorless FS-PTC of Induction Motors Using Extended Kalman Filters

This paper presents a simple overmodulation method employed in direct torque control (DTC) constant switching frequency (CSF) controller of induction machines. The proposed overmodulation method is utilized to extend a constant torque region and hence produce high torque capability in field-weakening region with six-step operation. It will be shown that the overmodulation operation using the DTC-CSF scheme can be established by controlling the stator flux locus from circular to the hexagonal shape. This is achieved by modifying the flux error status produced from the flux hysteresis controller before it is fed to the lookup table. The main benefit of the proposed method is its simplicity since it requires only a minor modification to the conventional DTC hysteresis-based structure and does not require a space-vector modulator.

A Wide-Speed High Torque Capability Utilizing Overmodulation Strategy in DTC of Induction Machines With Constant Switching Frequency Controller

Four sensorless control schemes for the operation of induction motors in the field-weakening region are compared and assessed in terms of performance and complexity. These four control schemes fully utilize the maximum available voltage and current and can produce the maximum possible torque in the entire field-weakening region. For comparison, the four control schemes are implemented on the same experimental platform, i.e., the same DSP board, power inverter, and motor drive. In this way, it is possible to assess not only the performance of each solution, but also its requirements in terms of computational time, tuning complexity, parameter knowledge, and stability of operation.

A Comparison of Four Robust Control Schemes for Field-Weakening Operation of Induction Motors

This paper presents a novel field-weakening strategy combined with a modified discrete speed-adaptive full-order observer to improve the performance of speed-sensorless induction machine drives in the field-weakening region. The robust field-weakening controller regulates the field current and the clamp of torque current according to the machine operating mode. Compared with the conventional inverse proportion to speed method and optimal flux level calculation method, the proposed controller shows the superiorities over maximum torque selection and parameter robustness. Moreover, a discrete speed-adaptive full-order observer is designed to estimate the rotor speed. However, when the machine operates at high speed, two poles of the conventional forward Euler approximation-based observer are outside the unit circle, which leads to system instability. To solve the conflict between stability, accuracy, and computational burden, a modified Euler approximation is used to discretize the observer. A comparative study of different approximations substantiates the presented approximation. The effectiveness of the proposed solution is demonstrated experimentally on an industrial induction machine drive.

Speed-Sensorless Induction Machine Control in the Field-Weakening Region Using Discrete Speed-Adaptive Full-Order Observer

This paper proposes a vector-controlled squirrel cage induction motor drive with synchronized sinusoidal pulse width modulation (PWM) for traction applications. In traction applications, switching loss in insulated-gate bipolar transistors is very important, and hence, a switching frequency of few hundred hertz is used in these devices. A triangular carrier comparison-based synchronized sinusoidal PWM for medium-voltage inverters with low switching frequency (less than 500 Hz) is proposed in this paper. To have very good dynamic behavior of the synchronization scheme, the triangular carrier is generated from the instantaneous voltage references in a phase-locked manner. A synchronous carrier-based overmodulation scheme that ensures linearity between the reference voltage and the fundamental motor terminal voltage is also proposed. An inverse gain-based linearization method is used to match the reference and inverter output voltage fundamental. This overmodulation strategy fails at the zone of high values of modulation index and a reference modification approach is used in that zone. The proposed PWM and overmodulation schemes integrate methods of field weakening and harmonic current estimation available in the literature and propose a consolidated scheme, which ensures good dynamic response with wide speed variation for rotor flux-oriented induction motor drive. The scheme is experimentally verified and the results are presented.

Rotor Flux-Oriented Control of Induction Motor With Synchronized Sinusoidal PWM for Traction Application

In this paper,a new flux-weakening control method based on rotor flux orientation is presented to improve the current tracking capability and output characteristics of the motor torque,and the method is implemented in digital ac spindle driver system.Using Labview virtual oscilloscope to observe the real-time experimental data,the experimental results show that the method can effectively improve the dynamic response of motor as well as the output characteristics of the motor torque.

Yang Kong-liang

Papers

Maximum Torque Control of an Induction Machine in the Field Weakening Region