Air-gap Flux Density Harmonics Research Articles

EFFECT OF SATURATION THIRD HARMONIC ON THE PERFORMANCE OF SQUIRREL-CAGE INDUCTION MACHINES

ABSTRACT This paper contributes to the understanding of the behavior of the saturation third harmonic and how energy is converted from fundamental to third harmonic form in a squirrel-cage induction machine in me presence of magnetic saturation. It is shown that the power transferred via the third harmonic airgap flux is accounted for by a change in the magnitude and phase-shift in the fundamental airgap flux. A procedure is proposed to link the rotor third harmonic circuit quantities to a set of equivalent fundamental component circuit quantities which leads to a modified equivalent circuit for the induction machine.

Torque harmonics minimisation in permanent magnet synchronous motor with back EMF estimation

A predictive current control in the synchronous reference frame with the back EMF estimation using the previous voltages and currents is presented. To reduce the torque harmonics produced by the air gap flux harmonics, the compensated d-axis and q-axis currents from the estimated back EMFs are employed. The effectiveness of the proposed control scheme is compared to the conventional one through simulation and experiment. >

Transient analysis of cage-induction motors using finite-elements

A method for the analysis of induction motor transients using the two-dimensional finite-element method is described. Unlike previously published work the method does not use an eddy-current formulation to account for the currents induced in the rotor bars. It relies on the decomposition of the rotor currents into harmonic distributions and the use of auxiliary circuit equations to take the time-stepping into account. The adoption of this technique yields some distinct advantages. Firstly, time-stepping is limited to the circuit model alone, thereby allowing an optimum step size to be selected at all times. Secondly, the use of a magnetostatic finite-element formulation makes it possible to use nonlinear solutions of comparatively low accuracy, thus bringing substantial savings in computer resources. Finally, the extensive use of circuit techniques also allows the effect of axial skew and spatially distributed airgap flux harmonics to be incorporated. >

Segmented-rotor disc motor

The steady-state synchronous behaviour of a large-airgap, single-sided segmented-rotor disc machine is investigated theoretically and experimentally. Two methods of calculating normal forces using Maxwell's stresses are described. Their predictions are compared with measured results. A two-axis model is shown to give acceptable results for large-airgap machines but the use of the harmonic airgap flux density is preferred for small airgaps. Equivalence is drawn between Maxwell stress and energy methods in an Appendix. Prediction of the synchronous performance of a prototype motor by existing methods of analysis is shown to be possible provided that an airgap fringe factor is included in calculation of the direct and quadrature axis reactances. Experimental and theoretical results for torque, current, power factor and efficiency are presented and compared. The pull-in capability of a reluctance motor was found to deteriorate when operated with a large airgap, and ways of overcoming this problem are suggested.

D-C Dynamic Braking of Squirrel-Cage Induction Motors [includes discussion

A rational method is given for estimating the torque speed curves of squirrel-cage motors during dynamic braking. An expression for average torque and one for peak torque are also developed. Results of the use of these are compared to results by previously published methods, with generally favorable conclusions. It is shown that squirrel-cage copper losses resulting from air-gap flux harmonics are much reduced at speeds below one-tenth of synchronous speed, with a consequent reduction of apparent rotor resistance. It is also inferred that inductor-generator action accounts for 20 to 30 per cent of the torque in the region of maximum torque. This paper is presented because no such rational method has yet been published and also because it is felt that the subject is timely in view of the increasing requirement of accuracy in the engineering of industrial controls.