Tesla and the Induction Motor

Abstract

Communications TESLA AND THE INDUCTION MOTOR To the Editor: I read with interest Ronald Kline’s article in the April 1987 issue of Technology and Culture and found his account of the parallel develop­ ment of the induction motor and its “engineering science” well written and informative, for the most part. However, I noticed certain technical misconceptions that appear to have led to inaccurate histor­ ical conclusions. Specifically, Kline misrepresents Tesla’s role in the development of the induction motor and its theory, and this misrep­ resentation appears to be based on an incorrect analysis of the motors described in Tesla’s U.S. patent no. 381,968 (referred to hereafter as ’968). Contrary to Kline’s assertion on page 290 (and repeated on page 299) that ’968 was Tesla’s “first patent for the induction motor,” the motors disclosed in that patent are not induction motors at all. Induction motors depend for their operation on the interaction between an electromagnetic field produced by currents fed to the motor’s “primary” coils from an outside source and an electromag­ netic field produced by currents induced in “secondary” coils or conductors by the rotation (“slip”) of the primary field relative thereto. The motors in ’968 clearly do not depend for their operation on such induced currents; they have no operative secondary coils or conductors. In fact, the elements of these motors which do interact with the generator-fed coils are designed to minimize induced cur­ rents, since these would merely dissipate power in the form of heat. Moreover, there is no slip between their generator-fed electromag­ netic fields and any interacting motor element. The motors are, in fact, synchronous motors—motors whose rotors revolve at the same speed as their revolving electromagnetic fields. Specifically, the motors of figures 9, 13, and 15 are either “hyster­ esis” or “reluctance” types, depending on which of the described, alternate shapes is chosen for their rotors. In either case, they depend Permission to reprint a communication printed in this section may be obtained only from the author. 1013 1014 KendallJ. Dood/Leland I. Anderson!Ronald R. Kline for their operation on the condition that the rotating electromagnetic fields of their stator coils induce magnetic poles {not electric currents) in their ferromagnetic rotors. The motor of figure 10 is simply an inside-out version of the motors of figures 9 and 15. Here the coils rotate while the interacting ferromagnetic material remains station­ ary. Finally, the motor of figure 17, though of neither the hysteresis nor the reluctance type, is also a synchronous motor. It has sets of coils on both the rotor and stator, with both sets being energized directly from the same generator source. Here again, no induced currents contribute to the operation of the motor. A more significant error, in consideration of Kline’s overall argu­ ment, is his assertion that the motor of figure 10 “worked exactly like the one Ferraris built” (p. 290). The Ferraris motor illustrated in figure 3 of Kline’s article is neither a hysteresis motor nor a reluctance motor. In fact, it is not even a synchronous motor. The difference between the Ferraris motor and the Tesla motor can best be seen in Tesla’s motor of figure 13 of ’968. Both motors have salient poles (the difference in their number is due merely to Tesla’s choice of threephase energization as opposed to Ferraris’s two-phase), but Ferraris’s rotor is made of copper, a highly conductive, nonmagnetic material, whereas Tesla’s is made of iron or steel, a highly magnetic but less conductive material made even less conductive by being laminated. Thus, the rotating electromagnetic field of Ferraris’s motor cannot possibly induce ferromagnetic poles in his rotor as it does in Tesla’s; but it does readily induce an electric current that, in turn, establishes the rotor’s electromagnetic field—which it cannot do in Tesla’s. The strength of the induced current is proportional to the slip between the field and the rotor and would, of course, disappear entirely if the rotor and the field were forced to rotate at the same speed. Hence, Tesla’s ’968 patent is not “evidence...