Classic AC motors: The elusive aspect of the induction motor principle

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. At first glance, the simple experiments depicted in Figs. 1-16C and 1-17 appear to lead to contradictory interpretations. For, in one case, the conducting object obligingly follows the moving magnetic field, whereas it eagerly flies away in the other case. This is a dilemma that must be resolved to avoid confusion in understanding real induction motors.

A subtle feature of experiment Fig. 1-16C must be taken into account. Although object X does, indeed, follow the moving magnetic field, it can only do so at a slower speed. This might not be obvious in such a primitively arranged demonstration, for it’s conceivable that X might move at 90 or 95 percent of the rate at which the magnet is moved. However, this inherent speed lag is predicated on electrical, not mechanical considerations such as friction or inertia, although such “loading” could produce additional speed retardation.

The important consequence of the above statements is that the relative motion of X is opposite to the direction of the moving magnetic field. Thus, both experiments demonstrate the same phenomenon—there is no contradiction between cause and effect in them. A corollary of these matters can provide further insight: If object X in Fig. 1-16C could attain the same speed as the moving field, the relative motion b the two would be zero, and no eddy currents would be induced in X.

But, without these induced currents, there would also be no associated magnetic field to interact with the field from the moving magnet. This being the case, X would no longer experience any physical force, i.e., “motor action” would cease. Significantly, actual induction motors can approach, but cannot attain synchronous speed (the speed of the rotating magnetic field provided by the stator windings). For sake of simplicity; these matters were not dealt with in section 1.

All of the foregoing experiments and allusions can be neatly summed up by Lenz’s law, which tells us that an induced magnetic field, such as from eddy currents, must oppose the inducing magnetic field. The translation of this fundamental law into the hardware of an induction motor is made easier with the notion of relative motion. Notice that X would develop eddy currents, but no lateral motion if the magnet were merely moved up and down in the vertical plane. Similarly, a single- phase induction motor not provided with means to produce a rotating electromagnetic field, would not develop any starting torque.

The situation described in Fig. 1-16C also illustrates the principle of the linear induction motor used in propulsion of high-speed trains, where relative motion occurs in the continuous horizontal plane, rather than in a circular pattern. Interestingly, the interaction between the inducing and the induced magnetic fields can also be used to levitate the trains so that they have no physical contact with guiding rails.

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