The loadability of electric machines is above all determined by temperature limits. In this work, thermal network models, suitable for totally enclosed fan cooled (TEFC) induction motors, are used to predict the machine temperatures. Two specific models are suggested: one with 107 nodes, which primarily is intended as a design tool, and one with 7 nodes, for increased simplicity. The steady-state solution method is valid in a broad speed-range and for variable load. Time-dependent solutions are obtained as a function of four input variables: frequency, stator voltage, torque and ambient temperature.

/pdf/thermal-modelling-of-small-cage-induction-motors-3bikf8grfj.pdf

Thermal Modelling of Small Cage Induction Motors

The theory presented in the paper attempts to explain the origins of load losses in detail. It includes an analysis of the losses produced, at any given value of slip, by current flow through the rotor iron between two adjacent bars, owing to the imperfect insulation of the squirrel cage. The theory also includes a calculation of the eddy-current and hysteresis losses in the rotor teeth, caused by the high-frequency harmonic fluxes that penetrate them. It shows how eddy currents in the iron surface can reduce the value of the differential harmonic leakage of a bar, thus allowing more harmonic bar current to flow, and hence more I2R losses to be produced. It discusses how the effective skew differs from its geometrical value, depending upon the width of the rotor bars. The theoretical formulas developed are used to predict the torque/slip characteristics of three 7½hp squirrel-cage induction motors, and the results show good agreement with the experimental curves, thus verifying the validity of the theoretical argument. A simplified theory of interbar currents is finally given, and the effect of skew and slot combination on the magnitude of the load losses, under normal operating conditions, is discussed.

/pdf/origins-of-load-losses-in-induction-motors-with-cast-3wsfiadz7f.pdf

Origins of load losses in induction motors with cast aluminium rotors

The principles and operation of a unique, terminal properties-based speed and position sensing approach for induction machines are demonstrated. A closed-loop nonlinear observer is proposed which uses an analytical model to calculate, in real time, the mechanical states of rotor speed and position given the torque command to the system and the induction motor terminal voltages and line currents. The uniqueness of this observer is that it uses inherent machine magnetic saliency and winding asymmetry to develop a magnetic encoder model of the machine itself. The error between the measured and calculated spatial harmonic EMFs is then used to drive the observer to accurate position and speed estimates in the presence of model parameter errors. Experimental data are included which show harmonic flux terms identified, measured and compared with an analytical model using a slip-ring induction motor with the rotor windings open and short circuited. >

Application of nonlinear observers for rotor position detection on an induction motor using machine voltages and currents

A study of the literature reveals that there is still considerable confusion on various aspects of stray losses, particularly with regard to their definition and origin. Accordingly the theoretical information is presented in a rationalised and systematic way to obtain a fully comprehensive approach, based on classifying stray losses into two groups, namely those due to main flux variations (stray no-load losses) and those due to leakage fluxes (stray load losses). In this way three and six main basic types of stray losses can be recognised under the two headings, respectively. Having identified the physical origin of these stray losses, their effect on motor design parameters can be ascertained qualitatively, and available methods of quantitative calculation, which are mostly semiempirical, can then be judged.A critical appraisal of two synthetic test methods for stray load losses, the reverse-rotation and d.c./a.c.-short-circuit tests, is made in the light of basic theory, and the fundamental deficiencies of these tests, particularly the reverse-rotation test, are demonstrated theoretically and by tests carried out on a number of medium-to-large induction motors ranging from 400hp, 4pole, to 1700hp, 8pole. The importance of insulating squirrel cages to avoid circulating-current losses deduced theoretically is confirmed by experiment.Additionally, a direct investigation into squirrel-cage circulating-current losses on a 1500hp 4pole motor shows how such losses can be controlled by a suitable current-displacement rotor design; these additional losses being directly measurable at no load on this machine with open stator slots.Consideration is also given to the fact that modern, high-efficiency (particularly totally enclosed) machine designs cannot cater thermally for substantially increased losses, e.g. excessive stray losses of any kind. There is therefore already an overriding and definite safeguard that the declaration of efficiency to BS 269 is substantially correct, or, more specifically, that there cannot be any major error in the value of the declared losses. However, a modification of the fixed allowance for stray load losses is suggested.From the theoretical and practical investigations carried out, it is concluded that making a nominal allowance for stray load losses is at present the only reasonable and practicable method of declaring efficiency.

Survey of basic stray losses in squirrel-cage induction motors

The definition, origin and measurement of stray load losses in induction machines have already been addressed in a companion paper. This paper completes the study by examining the mechanisms for prediction of those losses as well as some techniques for reduction. It also suggests some directions for future work.

Philip L. Alger

Papers

Induced High-Frequency Currents in Squirrel-Cage Windings