68 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH 2009 Behavior of the Three-Phase Induction Motor With Spiral Sheet Rotor Ramon Mujal Rosas, Oriol Boix Aragon` es, Senior Member, IEEE, Xavier Colom Fajula, and Alejandro Rol´ an Blanco Abstract—Improvements in torque at low currents using a rotor with spiral sheets are analyzed. Several rotors and stators have been built combining different constructive and mechanical char- acteristics of the related elements: inertias, constructive materials, geometrical shapes of the sheets, and geometrical disposition of the sheets. These different types of motors have been simulated using computer-aided tools and then tested in the laboratory. Fi- nally, four stators (1000, 1500, 1500-type A, and 3000 r/min), with the same constructive parameters, have been simulated and tested with the following rotors types: solid rotor, solid rotor with diamag- netic rings, drag cup, and simple and double squirrel cage rotor; the results have been compared to those obtained with the seven variants of the spiral sheet rotor presented in this paper. Index Terms—Electric machines, special machine, spiral sheet rotor, three-phase asynchronous motor. I. INTRODUCTION T HE ROTORS of conventional asynchronous motors are formed by magnetic sheets packed above the shaft of the machine. The rotating magnetic field created by the stator in- duces currents parallel to the shaft and thus upright to the rotor sheets [1]. These currents cannot flow between the sheets if they are electrically isolated, requiring the intervention of con- ventional squirrel cage rings to close the electric circuit so that the rotor currents can circulate. Typical configurations of three-phase asynchronous motors [2] are shown in Fig. 1 and described in the following points. 1) Single cage winding with high resistance and minimal inductance. 2) Deep slot cage, with low resistance and progressive reac- tance with the slip. 3) Low resistance and low reactance slot. 4) Finally, there are additional configurations of double and triple cage combining characteristics of the previous dis- positions, with different resistance and reactance values for each cage. In all of these dispositions, currents go through the cage con- ductors [3], while magnetic field goes through the sheets. The performance of these motors depends on the magnetic field, the current, and the distance from the conductor to the shaft. Manuscript received December 4, 2007; revised February 6, 2008. First published January 20, 2009; current version published February 19, 2009. This work was supported by the “Ministerio de Ciencia y Tecnolog´ ıa de Espa˜ na” under DPI 2004-03180 Research Project. Paper no. TEC-00476-2007. R. M. Rosas, O. B. Aragon` es, and A. R. Blanco are with the Department of Electrical Engineering, Technical University of Catalonia, 08222 Terrassa, Spain (e-mail: mujal@ee.upc.edu; boix@ee.upc.edu; alejandro.rolan@upc.edu). X. C. Fajula is with the Department of Chemical Engineering, Technical Uni- versity of Catalonia, 08222 Terrassa, Spain (e-mail: xavier.colom@upc.edu). Digital Object Identifier 10.1109/TEC.2008.2005283 Fig. 1. Typical configurations of three-phase asynchronous motors. TABLE I MAIN MECHANICAL CHARACTERISTICS OF ROTORS II. PROTOTYPES DESCRIPTION As an example, the basic mechanical characteristics of some tested rotors are presented in Table I [3]. These rotors used have the same dimensions and are assembled to stators of 1000, 1500, and 3000 r/min, the latter having the same constructive parameters. III. THREE-PHASE ASYNCHRONOUS TORQUE MOTOR The so-called torque-motor [5] is a three-phase asynchronous motor with the stator built in a classical way, but its rotor is constructed by a hard sheet iron cylinder (with small hysteresis cycle). With this kind of rotor, a higher useful section for flux circulation is obtained, but one of the drawbacks of this motor is the fact that the lines of magnetic fields go through its core reaching considerable depths, inducing an electromotive force (EMF) that makes it weaker. A current circulating at greater depth has lower participation in the generation of torque, owing to the following causes. 1) Lower distance between the currents and the shaft of the motor. 2) The higher reactance results in lower power factor al- though the losses in the copper are the same as if they circulate on the periphery. 0885-8969/$25.00 © 2009 IEEE