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Among the electro-mechanical devices transforming energy from electrical to mechanical, the squirrel cage induction motor can be surely considered a workhorse of the industry due to its robustness, low cost, and good performance when directly fed by the ac grid. Being the most influencing motor topology in terms of energy consumption, optimizing the efficiency of squirrel cage induction motors could lead to a great impact toward the reduction of the human environmental footprint. The induction motor design aided by finite element analysis presents significant challenges because an accurate performance prediction requires a considerable computational burden. This article makes use of an innovative fast and accurate performance evaluation method embedded into an automatic design procedure to optimize different rotor slot geometries. After introducing the performance estimation approach, its advantages and limits are discussed comparing its prediction with the experimental tests carried out on an off-the-shelf induction motor. Different rotor cage structures with increasing geometrical complexity are then optimized in terms of starting and rated performance adopting the same design optimization process, the same stator geometry and constituent materials. The analysis of the optimal solutions shows how it is possible to improve the rated efficiency without compromising other performance indexes. The presented results can be used as general design guidelines of squirrel cage induction motors for industrial applications. [ABSTRACT FROM AUTHOR]

A detailed magnetic reluctance-based computational model of an induction machine, into which a traditional rotor-flux-orientated vector control scheme has been grafted, is used to examine the influence of saturation of both main and leakage flux paths upon vector-controlled drive performance. Individual machine teeth, windings, and conductors are incorporated, as is pseudo-3-D modeling of skew. The cross-saturation effect is therefore determined from the basic dimensional and winding design data of the machine. Effective compensation methods can then be derived for practical implementation. Direct rotor flux orientation and magnitude control is used to ensure that inaccuracies in a machine-parameter-based vector control scheme are not reflected in the results for cross saturation. Results for a 15-kW four-pole induction motor with rotor skewed one stator slot pitch show that skew leakage flux is predominantly responsible for cross saturation. The skew leakage flux is shown to heavily saturate one end of the machine and to reduce the air gap flux density at the other. At 200% rated load current, this equates to an 11% reduction in direct axis flux if [i.sub.sd] is kept constant. To compensate for this reduction, a 27% increase in [i.sub.sd] would be required. Index Terms---Cross coupling, induction machines, skew, vector control.

This paper presents a novel low-intrusion load and efficiency evaluation method for in-service induction motors based on vibration measurements. This method enhances the traditional vibration analysis by providing motor load and efficiency information in addition to the mechanical health information. The application is in multimotor plants, where individual motor monitoring is too expensive to implement yet where motor operating conditions need to be known to work at improved plant efficiency. The vibration signature of the machine, measured by an accelerometer and processed by fast Fourier transform (FFT) is used to extract frequencies defining shaft speed and the supply frequency. This data, in conjunction with the basic motor performance data, enables the determination of the actual load and indirectly, the efficiency which the motor is operating at. For motors supplied form a variable-speed drive (VSD), the loss segregation method is used to yield the motor losses indirectly and thus, the efficiency. [ABSTRACT FROM AUTHOR]