Timothy M. Rowan

A New Synchronous Current Regulator and an Analysis of Current-Regulated PWM Inverters

Detailed models are presented for the stationary and synchronous sine-triangle current regulators. Analytical and test results are compared for purposes of model verification and regulator evaluation. The results demonstrate the limitations of the two most often used current regulators and the robustness of the synchronous current regulator. The stationary sine-triangle and hysteretic current regulators are shown to have steady-state characteristics that depend on slip, operating frequency, and motor impedance. In contrast the synchronous regulator, because it lacks these dependencies, exhibits ideal steady-state current regulator characteristics without sacrificing bandwidth. Moreover, the complexities traditionally associated with the synchronous regulator are overcome with a simple equivalent implementation.

A simple on-line adaptation for indirect field orientation of an induction machine

Three simple methods of adapting for rotor time constant changes of an indirect field-oriented induction machine are proposed. These methods utilize voltages as field-oriented reference models in the basic model reference adaptive control (MRAC) structure. Each of these methods offers some implementation or performance advantage over previously proposed MRAC methods and other adaption techniques. The proposed methods are analyzed along with other previously proposed MRAC methods, and the advantages and limitations of each are discussed. Experimental results are presented for the proposed methods with the most advantages.<<ETX>>

A Quantitative Analysis of Induction Motor Performance Improvement by SCR Voltage Control

Minimum input power and maximum efficiency operation occur at characteristic slip values which can be realized for any induction motor operating at part load by properly adjusting the amplitude of the applied stator terminal voltages. These two criteria are shown to yield perceptibly different results when the motor is driven from a silicon-controlled rectifier (SCR) voltage controller. In addition, it is demonstrated that a constant power factor controller results in an operating regime which is substantially poorer than operation at either minimum input power or maximum efficiency. It is further shown that minimum stator current and minimum power factor angle criteria yield results which are closer to the ideal than the constant power factor controller.

A new flux and stator resistance identifier for AC drive systems

The effect of stator resistance on AC drive performance is analyzed for flux vector and indirect field-oriented controllers. A new technique-the back electromagnetic force (BEMF) detector-for reducing the adverse effects of stator resistance on field-oriented control is presented and evaluated through simulation and experimental results. The BEMF detector is shown to reduce the impact of the stator resistance variations and also provide an estimate of the stator resistance. The detector is compatible with most control strategies and with or without position feedback.

Operation of Naturally Sampled Current Regulators in the Transition Mode

A fundamental component model of current-regulated naturally sampled PWM inverters operating in the pulse-dropping mode is proposed. Analytical and test results are compared for purposes of model verification and regulator evaluation. The results demonstrate the problems with the classical (stationary reference frame) current regulator that occur in this region of operation. In contrast, the synchronous reference frame regulator is shown to have ideal steady-state response in this region.

A frequency based determination of the transient inductance and rotor resistance for field commissioning purposes

The paper presents a deterministic method of estimating the transient inductance and rotor resistance of an induction machine for field commissioning purposes. The parameter values obtained by the method are compared with those for a transient method of commissioning. Skin effects inherent with the transient test are documented. Experimental results for a wide range of machine sizes are included for both methods. The quality of field orientation is established for each method through experimental testing.

Indirect field-oriented control of an induction motor in the field-weakening region

A self-organized field-oriented induction machine controller that addresses problems encountered in the field-weakening region is presented. The field-oriented controller (FOC) is based on model reference adaptive principles but relies on samples of the state of the machine to determine control inputs. The proposed controller is compared with other approaches to field-weakening operation, and the advantages and limitations of each are discussed. Experimental results are presented for the classical and the self-organized approaches to field-weakening operation. >

Field-oriented control of an induction machine in the field-weakening region with DC-link and load disturbance rejection

A method of actively maintaining voltage margin in field-oriented induction machine controllers is proposed. A voltage-margin controller is developed that rejects DC-link and load disturbances, such that current regulation and field orientation is maintained. In addition, the voltage-margin controller coupled with rotor-flux-oriented control is shown to provide maximum torque capability equivalent to stator-flux-oriented control.

Operation of PWM voltage source-inverters in the overmodulation region

Pulse width modulated (PWM) inverters experience a reduction in gain when overmodulation occurs. The pulse dropping or transition region is examined for continuous and discontinuous modulation strategies. Transition region characteristics for a number of modulation strategies are introduced. The effect of the transition region on field oriented control (FOC) is presented. The adverse effects of bus disturbances on current regulated AC inverters, while in the transition region, are demonstrated by experimental results. The problems encountered are the consequence of the reduced gain of the PWM inverter regardless of the PWM strategy. A compensated modulation technique (CMT) adaptable to continuous and discontinuous modulators eliminates the voltage error and transitions to six-step operation without inducing a voltage transient. The CMT applies to voltage and current regulated PWM inverters employing most of the modern switching strategies. Experimental results presented in the paper demonstrate the CMTs smooth transition to six-step and the improved performance a CMT-PWM algorithm provides.

A simplified inverter model for on-line control and simulation

A simplified fundamental component model for sine triangle comparison pulse width modulated (PWM) inverters operating in the transition region is presented. The model is evaluated by comparing it to detailed inverter simulations and through experimental verification. Results from a real-time control application show that the model compensates for the reduced inverter gain due to bus voltage variation and operation in the pulse dropping region. Application of the model to high-performance drives is explored.<<ETX>>