The analysis and design of the phase-locked loop (PLL) system is presented for the phase tracking system of the three phase utility interface inverters. The dynamic behavior of the closed loop PLL system is investigated in both continuous and discrete-time domains, and the optimization method is considered for the second order PLL system. In particular, the performance of the three phase PLL system is analyzed in the distorted utility conditions such as the phase unbalancing, harmonics, and offset caused by the nonlinear load conditions and measurement errors. The tracking errors under these distorted utility conditions are also derived. The phase tracking system is implemented in a digital manner using a digital signal processor (DSP) to verify the analytic results. The design considerations for the phase tracking system are deduced from the analytic and experimental results.

A phase tracking system for three phase utility interface inverters

A new saturation model for induction machines that can be easily extended to other types of AC machines is presented. It is shown that saturation is responsible for the generation of flux space harmonic components traveling in the air gap with the same synchronous speed as the fundamental flux component, with the third being the dominant harmonic component. Superposition of the effects of the fundamental and third harmonic components of the air gap flux is utilized in order to model the saturation of the ferromagnetic parts of the machine. The concept of winding functions is used to derive the inductance terms relating to both stator and rotor winding components. In this approach, the air gap length is assumed to be variable, being a function of the position and level of the air gap flux. Terminal and torque values for steady and transient states are obtainable from the proposed model, with experimental results showing that the model proposed predicts spatial saturation effects with good accuracy. >

Modelling of saturated AC machines including air gap flux harmonic components

In a conventional rotor flux oriented induction motor drive, the flux reference is usually made proportional to the inverse of the rotor speed for field weakening operation. The authors indicate, however, that such a variation cannot maintain optimal (maximum) torque capability maintain optimal (maximum) torque capability of the machine over the entire speed range. It is further shown that nearly optimal torque capability can be achieved in a stator flux oriented system when the stator flux reference is varied as the inverse of the rotor speed. >

Selection of the flux reference for induction machine drives in the field weakening region

A loss model controller (LMC) for determining the optimal air-gap flux that minimizes the losses in scalar controlled induction motor drives is presented. The suggested LMC is simple, and its implementation does not affect significantly the cost and the complexity of the drive. Although the conception of the suggested LMC is based on the loss model of the induction motor, it is shown that its realization does not require knowledge of the loss model.

Loss minimization in induction motor adjustable-speed drives

This paper presents a new loss-model-based controller for an induction motor drive. Among the many loss minimization algorithms (LMA) for an induction motor, a loss-model-based approach has the advantages of fast response and high accuracy. However, the performance of the loss-model controller (LMC) depends on the accuracy of the modeling of the motor drive and losses. In the development of the loss model, there is always a tradeoff between accuracy and complexity. This paper presents a new LMC to determine an optimum flux level for the efficiency optimization of the vector-controlled induction motor drive. An induction motor (IM) model in d-q coordinates is referenced to the rotor magnetizing current. This transformation results in no leakage inductance on the rotor side, thus the decomposition into d-q components in the steady-state motor model can be utilized in deriving the motor loss model. The suggested LMC is simple, but leakage inductances are not omitted. The complete closed loop vector control of the proposed LMC-based IM drive is successfully implemented in real-time using digital signal processor board DS 1104 for a laboratory 1/3 hp motor. The effectiveness of the proposed scheme is demonstrated through simulation and experimental results.

New Online Loss-Minimization-Based Control of an Induction Motor Drive

A new type of converter for variable-reluctance-machine (VRM) drives is described. In this converter topology, the energy extracted from an ongoing phase is stored in a dump capacitor. The energy stored is consequently used either to quickly turn on the next ongoing phase or to energize the conducting phase frequently during the conduction interval instead of being returned to the supply as for a conventional C-dump circuit. Since, the additional switch used to pass the energy to the C-dump capacitor is switched under a relatively-low-voltage condition and its switching frequency is relatively low, the rating of the additional switch is modest. The major advantage os this converter is that it uses a low-switching device/phase ratio while achieving better performance characteristics than the other topologies. >

A modified C-dump converter for variable-reluctance machines

A new method of efficiency maximization that utilizes sensing of the third-harmonic component of air-gap flux is proposed. This signal is used to determine the resulting instantaneous position of the fundamental component of the air gap flux and, consequently, the torque- and flux-producing components of the stator current. In addition, the third harmonic signal is also used to determine the rotor speed. Hence, the output power of the machine can be calculated with only a single sensor wire attached to the neutral point of the machine. The flux-producing component can be readily adjusted to produce the minimum input power for a fixed amount of output power (fixed speed). >

Simple efficiency maximizer for an adjustable frequency induction motor drive

Voltage unbalance or sag conditions generated by the line excitation can cause the input rectifier stage of an adjustable-speed drive (ASD) to enter a single-phase rectifier operation. This degradation of the input power quality can have a significant negative impact on the induction machine performance characteristics, but the presence of an LC filter in the drive's input rectifier stage can be used to attenuate these undesired effects. The purpose of this paper is to investigate the impact of the inductor placement in the ASD topology on the drive's performance under voltage unbalance or sag conditions. More specifically, the relative advantages of choosing either a dc-link choke inductor or three ac line inductors are discussed using a combination of closed-form analysis and simulations. The results of first-order sizing calculations show that a dc-link choke inductor may offer some volume and mass advantages over three separate ac line inductors for the same ASD performance under unbalanced voltage conditions. Experimental results using a 5-hp ASD confirm the key analytical performance predictions

Impact of Inductor Placement on the Performance of Adjustable-Speed Drives Under Input Voltage Unbalance and Sag Conditions

Active rectifiers using pulsewidth-modulation voltage-source converters are popular choices for adjustable-speed drives to provide high input power quality. However, input current harmonics are often generated as a result of input voltage source disturbances. Since the frequencies of the fifth-, seventh-, and higher-order harmonics can easily exceed the current controller bandwidth in high-power applications, the elimination of these harmonic disturbances imposes difficult control challenges. Cost-effective observer-based harmonic estimation and mitigation methods are introduced in this paper that compensate delay effects caused by the digital signal processing. These delays can significantly limit the current controller bandwidth if not compensated. Analytical, simulation, and experimental results are developed to illustrate the effectiveness of the new harmonic estimation and control techniques.

Input Harmonic Estimation and Control Methods in Active Rectifiers

The usual method of induction motor torque control uses the indirect field orientation principle in which the rotor speed is sensed and slip frequency is added to form the stator impressed frequency. Unfortunately, the rotor resistance varies as the motor heats up under load thereby changing the rotor time constant which has a deleterious effect on the torque response. In this paper two new field oriented control schemes are presented which employ rotor end ring current detection and thereby remove the dependence of the controller accuracy on temperature so that the controller is entirely independent of rotor time constant variations. The field orientation schemes do not require an incremental encoder for rotor position sensing. The motor torque can be accurately controlled even down to zero speed operation. >

V. Blasko

Papers

A modified C-dump converter for variable-reluctance machines

Simple efficiency maximizer for an adjustable frequency induction motor drive

Impact of Inductor Placement on the Performance of Adjustable-Speed Drives Under Input Voltage Unbalance and Sag Conditions

Input Harmonic Estimation and Control Methods in Active Rectifiers

Field oriented control of induction machines employing rotor end ring current detection