Frost maintains forests and grasslands as alternate states in a montane tropical forest–grassland mosaic; but alien tree invasion and warming can disrupt this balance

1. Forest–grassland mosaics, with abrupt boundaries between the two vegetation types, occur across the globe. Fire and herbivory are widely considered primary drivers that maintain these mosaics by limiting tree establishment in grasslands, while edaphic factors and frosts are generally considered to be secondary factors that reinforce these effects. However, the relative importance of these drivers likely varies across systems. In particular, although frost is known to occur in many montane tropical mosaics, experimental evidence for its role as a driving factor is limited. 2. We used replicated in situ transplant and warming experiments to examine the role of microclimate (frost and freezing temperatures) and soil in influencing germination and seedling survival of both native forest trees and alien invasive Acacia trees in grasslands of a tropical montane forest–grassland mosaic in the Western Ghats of southern India. 3. Seed germination of both native and alien tree species was higher in grasslands regardless of soil type, indicating that germination was not the limiting stage to tree establishment. However, irrespective of soil type, native seedlings in grasslands incurred high mortality following winter frosts and freezing temperatures relative to native seedlings in adjoining forests where freezing temperatures did not occur. Seedling survival through the tropical winter was thus a primary limitation to native tree establishment in grasslands. In contrast, alien Acacia seedlings in grasslands incurred much lower levels of winter mortality. Experimental night‐time warming in grasslands significantly enhanced over‐winter survival of all tree seedlings, but increases were much greater for alien Acacia than for native tree seedlings. 4. Synthesis. Our results provide evidence for a primary role for frost and freezing temperatures in limiting tree establishment in grasslands of this tropical montane forest–grassland mosaic. Future increases in temperature are likely to release trees from this limitation and favour tree expansion into grasslands, with rates of expansion of non‐native Acacia likely to be much greater than that of native trees. We suggest that studies of frost limitation to plant establishment are needed across a range of tropical ecosystems to re‐evaluate the general importance of frost as a driver of vegetation transitions in the tropics

On-Line Identification of Winding Resistances and Load Torque in Induction Machines

Rotor and stator resistances along with load torque are typically uncertain quantities in induction machines. The machine heating makes the winding resistances vary during operation whereas the load torque strictly depends on the application. All those variables need to be on-line estimated to improve the drive performances and in particular to minimize the power loss at steady state. A new adaptive observer is designed in this brief. It is able to exponentially estimate the motor fluxes and to identify the aforementioned critical parameters from stator currents/voltages and rotor speed measurements. In contrast to other solutions proposed in the literature, rotor and stator resistances are not estimated on the same time scale. New insights on the behavior of an intuitively inspired observer are thus given through a detailed stability proof, which does not rely on linearization arguments around constant operating conditions. Persistency of excitation conditions, which only depend on exogenous signals to the estimation error system, are analyzed in detail and a clear physical interpretation is presented. Key features of the proposed solution are the overall simplicity of the estimation scheme, the low dimension of the regressor matrix (being exactly related to the number of unmeasured or uncertain quantities) and the exponential convergence to zero of the estimation errors. Simulations confirm the correctness of all the mathematical derivations. Experimental results show the effectiveness of the proposed approach in implementing an advanced version of the indirect field oriented control scheme: the uncertain rotor flux modulus reference that minimizes the power loss at steady state can be actually estimated and imposed. \ua9 2013 IEEE

Adaptive Flux Observer for Induction Machines with On-Line Estimation of Stator and Rotor Resistances

A nonlinear adaptive control algorithm is proposed. It is based on a two time-scale observer that allows the on-line identification of three critical uncertain parameters, i.e., load torque and motor stator and rotor resistances. In addition, the set-point of the rotor flux is adjusted in order to minimize the Joule losses and to improve the power efficiency. Experimental results illustrate the feasibility of the developed approach

A Control Scheme with Energy Saving and DC-Link Disturbance Rejection for Electric Vehicle

A control scheme for induction motor that integrates within a coherent solution some of the features that are commonly required to an electric vehicle drive is presented. The main features of the proposed control scheme are the capability to exploit the maximum torque in the whole speed range, a weak dependence on the motor parameters, a good robustness against the variations of the dc-link voltage and, whenever possible, the maximum efficiency. The effectiveness of the control scheme is confirmed by experimental tests

Robust Control Schemes for Induction Motor Drives for Electric Vehicle Applications

Three control schemes for the operation of induction motors in the field-weakening region are compared and assessed in terms of performance and complexity. These three control schemes fully utilize the maximum available voltage and current at any speed, and thus provide the maximum possible torque in the entire field weakening region. For the comparison, the three control schemes were implemented on the same experimental platform, i.e. the same Digital Signal Processor (DSP) board, power inverter and motor drive. In this way, it is possible to judge not only the performance of each solution, but also its requirements in terms of computational time, tuning complexity, parameter knowledge and stability of operation

Le politiche comunitarie e nazionali

In questo capitolo del volume vengono descritte le politiche nazionali ed internazionali relative alla politica agricola comunitaria, analizzando nel dettaglio l'applicazione della revisione a medio termine della Pac

Stator Flux vector Control of Induction Motor Drives in the Field-Weakening Region

A torque control scheme that utilizes the stator flux components as control variables, applied to a speed-sensorless induction motor drive, is presented. The proposed scheme allows the motor to exploit the maximum torque capability in the whole speed range, and shows a reduced dependence from the motor parameters. It is based on a control algorithm that decreases the d-component of the stator flux as soon as the voltage corresponding to the maximum torque exceeds the available voltage. The control scheme produces a smooth transition into and out of the field weakening region and preserves the torque dynamic. The feasibility of the field weakening technique is confirmed by computer simulations and experimental tests