This paper presents a viable transducerless rotor position and velocity estimation scheme for PWM inverter driven induction, synchronous, and reluctance machines with the capability of providing robust and accurate dynamic estimation independent of operating point, including zero and very high speeds, light and heavy loading. The injection of a balanced three-phase high frequency signal (500 to 2 kHz) generated by the inverter, followed by appropriate signal demodulation and processing combined with a closed-loop observer, enable the tracking of rotor magnetic saliencies from the machine terminals. Although rotor magnetic saliency is inherent within reluctance machines, and most synchronous machines, saliency in the induction machine is introduced via a modulation of the rotor slot leakage with minimal detrimental effects on the machine performance. Experimental verification for the induction machine is included. >

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Transducerless position and velocity estimation in induction and salient AC machines

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 main objective of this paper is to present the "INFORM" method based on real-time inductance measurements for sensorless control of AC drives.

Sensorless control of AC machines at low speed and standstill based on the "INFORM" method

The authors present a new sensorless induction motor drive. The drive uses an adaptive flux observer for speed estimation and the discrete-time direct torque control technique for torque and stator flux control. The adaptive flux observer uses a mechanical model to improve the behavior during speed transients. The estimated stator flux of the adaptive observer is used in the discrete-time direct torque control method to provide fast torque response combined with torque ripple free operation over the whole speed range. The sensorless drive system is capable of working from very low speed to high speed and exhibits good dynamic and steady state performance. The authors demonstrate the steady state and dynamic performance of the proposed sensorless drive using simulation and experimental results.

Speed sensorless direct torque control of induction motors using an adaptive flux observer

Due to the increasing availability of improved power electronics and digital processors at reduced costs, there has been a trend to seek higher performance from electric machine systems through the design of more sophisticated control systems software. There exist significant challenges in the search for improved control system designs, however, since: the dynamics of most electric machine systems exhibit significant nonlinearities; not all state variables are necessarily measured; and the parameters of the system can vary significantly from their nominal values. In recent years, a wide range of nonlinear methods for feedback control, state estimation, and parameter identification have emerged, and some of these results are reviewed and summarized. >

Nonlinear control of electric machines: an overview

It is well-documented that back-EMF MRAS-based speed sensorless induction motor drives can suffer from instability problems especially at low-speed operation, and that MRAS schemes using flux estimation suffer from drift problems associated with direct integration. Sensorless IM drives are also sensitive to stator resistance mismatch, particularly at low speeds. Novel stable MRAS speed and stator resistance estimators based on back-EMF quantities, and which are capable of being operated simultaneously, are proposed. Full details of the theoretical foundation and of the design procedure are given. The stability of the estimators is assured by appropriate location of the closed-loop poles of the estimators. Extensive simulation and experimental investigations have been performed to verify the stability of the proposed estimators in the motoring, generating and braking modes of operation.

A stable back-EMF MRAS-based sensorless low-speed induction motor drive insensitive to stator resistance variation

The authors describe a model reference adaptive control system for speed control of the vector-controlled inverter induction motor drive without a speed sensor. The rotor speed is estimated with a full-order adaptive observer and is used as the feedback signal for the vector and speed controls. In order to estimate the speed accurately at low speeds, the motor stator resistance is also identified at the same time to correct the mismatched resistance value used in the observer. The resulting system is verified to be hyperstable. As a result, the drive can be operated in a wide range of speed, and especially, it can be operated stably at the state of zero speed. Experiments also show that the scheme is effective. >

Adaptive-speed identification scheme for a vector-controlled speed sensorless inverter-induction motor drive

A new current-commutating method applying the action of the mutually coupled inductances is described and is termed the coupled reactor commutating (CRC) method. With this method a unique self-excited controlled current type inverter is constructed. The CRC inverter not only gives an efficient voltage adjustment function, but it also liberates men from the task of treating reactive powers. An example of the application to the induction motor drives is described in which an overall efficiency as high as 85 percent has been reached. The troublesome problem of parasitic torque pulsation in the low-revolution range, which is inherent in the controlled current type inverters, is also resolved by the torque-smoothing current control method with the CRC inverter.

A New Controlled Current Type Inverter with Improved Performance

A new technique for a PWM inverter is described. By using a triangular wave of saturable amplitude, adjustment of the amplitude of the multiple reference waves can be avoided. Two typical circuits for inducing saturable triangular waves are illustrated, and a new strategy is proposed which uses the line-to-line voltages of a three-phase ac source as reference wave instead of the usual phase voltages. Two approaches which improve the performance of the PWM inverter and largely simplify the control circuit are developed. One depresses the occurrence of subharmonics without making use of the usual synchronization of the carrier wave with the reference waves; the other is a method for reducing the switchings of the power devices. A simple purely digital control circuit with improved performance has been constructed according to the new technique, and model PWM patterns for constant V/F control as well as their harmonic content are given.

New PWM Technique Using a Triangular Carrier Wave of Saturable Amplitude

The problem of reducing the pulsating torque in current source inverter induction motor systems is investigated. Two typical approaches presented in recent publications are analyzed by making clear 1) the correlation among the current harmonics and the torque harmonics, and 2) the conceptual contradiction existing between these two approaches. By synthesizing the analyzed results, fundamental principles for treating the current harmonics and torque harmonics are established. In addition, the method for evaluating the degree of torque ripple is described, along with a proposed criterion for torque ripple content. By applying this criterion to these two approaches, objective estimates of degree of torque pulsations, which were unfeasible so far, are now made possible.

The Principles of Eliminating Pulsating Torque in Current Source Inverter Induction Motor Systems

A coupled-reactor-commutating sinusoidal-wave current-source inverter is described. The inverter consists of two coupled-reactor-commutating inverters. By superposing the output currents of these two inverters, output currents of a 180° conducting angle are formed, and are controlled in order to achieve a sinusoidal waveform. The operation and the control scheme of the inverter are described, and experimental results are presented. Compared with the usual PWM voltage-source inverters, the on-off frequency of each chopper may be decreased almost to one tenth of that of the PWM inverter.

Tung Hai Chin

Papers

Adaptive-speed identification scheme for a vector-controlled speed sensorless inverter-induction motor drive

A New Controlled Current Type Inverter with Improved Performance

New PWM Technique Using a Triangular Carrier Wave of Saturable Amplitude

The Principles of Eliminating Pulsating Torque in Current Source Inverter Induction Motor Systems

A Coupled-Reactor-Commutating Sinusoidal-Wave Inverter